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Boats and foils comparison


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11 minutes ago, Sidecar said:

You have the blue horizontal arrow the wrong way round....

Get some sleep.

No. No.  I drew it as I meant to draw it.  But I think I might see my confusion.  I believe that regardless of which blue force I increase on a foiling AC75, the result will be to pivot around the intersection of the two red lines. The "axis of rotation".  

But, I think you are telling me that this is wrong. If I apply force on the foil arrow it will lift the boat up.  If I apply force to the sail arrow it will scoot the boat sideways.  But in neither case will the boat pivot around the axis.  Is this your contention?

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Just a few interesting bits of the straight-line performances from today: Upwind /Downwind VMGs - race 1: Upwind /Downwind VMGs - race 2: Same story in both races actually.

Thanks to weta27's pics I have created an approximation of NZ's "BFB v2" foil. Main points: Foil area is almost the same, possibly even a smidge larger. Flaps have increased in area as

OK, it sounds like there's some interest in this topic, so here goes.   Any engineering effort starts by defining the requirements.  From this figure, it looks like the average foil area is 1.64

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10 minutes ago, Sidecar said:

You have the blue horizontal arrow the wrong way round.... And you are missing a downward vertical blue arrow representing the weight of the boat. 

Get some sleep.

Your diagram is also missing a horizontal blue arrow representing water resistance at or near foil lift which is opposite to the wind pressure above.

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6 minutes ago, Blitzkrieg9 said:

No. No.  I drew it as I meant to draw it.  But I think I might see my confusion.  I believe that regardless of which blue force I increase on a foiling AC75, the result will be to pivot around the intersection of the two red lines. The "axis of rotation".  

But, I think you are telling me that this is wrong. If I apply force on the foil arrow it will lift the boat up.  If I apply force to the sail arrow it will scoot the boat sideways.  But in neither case will the boat pivot around the axis

You drew it as you meant to, but it does not reflect reality

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7 minutes ago, Blitzkrieg9 said:

No. No.  I drew it as I meant to draw it.  But I think I might see my confusion.  I believe that regardless of which blue force I increase on a foiling AC75, the result will be to pivot around the intersection of the two red lines. The "axis of rotation".  

But, I think you are telling me that this is wrong. If I apply force on the foil arrow it will lift the boat up.  If I apply force to the sail arrow it will scoot the boat sideways.  But in neither case will the boat pivot around the axis

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

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3 minutes ago, kiwin said:

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

Okay.   Hmmm...  I think I'm catching on...  I'm close to grasping this.  Thanks for your input!!!

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9 minutes ago, kiwin said:

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

Okay. I might have had the eureka moment. I have two simple scenarios and I need two simple answers. 

1) An AC75 is sitting still in the water. Neptune pushes up on one foil. What happens?  

2) an AC75 is foiling along at 30 knots. Neptune pushes up on the leeward foil. What happens?

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16 minutes ago, amc said:

In the absence of anything else 

1. boat capsizes 

2. boat lifts off and flies, drifting faster sideways until air pressure resistance negates heeling moment and then the Neptune force and RM couple will capsize the AC75 to windward.

Fixed.

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Yes.

And your “Neptune” force will only ever be limited to the mass of the boat, because any more than that and the foils will lift and breech and lose lift.

It is almost self regulating, but get it wrong and you have seen the results.

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7 minutes ago, Blitzkrieg9 said:

"Drifting faster sideways"

To leeward?

The reality is that this is a very complex set of forces, if you change any one then a whole bunch of other things must change with it or boat capsizes 

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Also you are close to understanding what a knife edge balancing act sailing one of these monsters is. I presume that like some cars I have driven that they are basically trying to kill you. Yes I survived a 70's  TVR.

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2 hours ago, Blitzkrieg9 said:

This just can't be true.  The leeward foil is not the center of gravity/ mass.  If Neptune were to rise from the ocean and exert 20,000 lbs of upward force on the leeward foil of an AC75 in flight, it wouldn't just lift up the whole boat. Rather, it would capsize the boat to windward. 

 

I don't think it's helpful to consider the effects of a single theoretical force. Excess lift on the leeward foil greater than the weight of the boat would simply lift the boat up, it wouldn't necessarily capsize it until it leaves the water, at which point leeway resistance is lost so the boat will start moving sideways and could just continue to rise as RM and capsize moment remain in balance. But pitch likely isn't balanced (no rudder up/down once out of the water) so rather than capsize to windward, the boat will probably pitch forward and pitchpole, or go bow up and pitchpole backwards. It all depends on where the centre of lift is in regard to the boat's centre of gravity.

The sail produces a force that pushes the boat forwards at an angle that we resolve into the linear and rotational forces described by Max. The unhelpful components are balanced out (drag, lift, capsizing moment, etc.) so that what's left over goes to forward motion at a speed that is limited by drag. Increasing or reducing any single force means rebalancing, if you go outside the ability of the model to deal with the change, you reach a point of inflection or a discontinuity beyond which the results are meaningless.

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3 hours ago, Blitzkrieg9 said:

Okay.  So...  let's assume the foil is magically directly in the middle of the boat under the center of gravity.  Increasing lift would just make the boat fly higher.  I got that. 

BUT, the foil isn't in the middle. It's way off to one side.  So, increasing the lift way out there should have the same effect as rolling the boat around its axis, right?  So, how does increasing lift (fZ) on the leeward foil NOT increase righting moment?  

On an airplane, if I increase the lift on just one wing, doesn't that make the plane roll?

You can take any point you want as the moment reference center and balance the moments about that.  If there is a force that is unknown, then that is a good place for the moment reference center because the arm then is zero and the moment from the unknown (or variable) force is zero.

So consider taking moments about the center of effort of the leeward foil.  Now it doesn't matter what the lift on the foil is.  The righting moment about this reference center comes from the weight of the boat to windward of the immersed foil.  That is largely set by the Design Rule.  And it is independent of the design or operation of the leeward foil.  This righting moment will dictate the sail trim. 

As long as the moment about the leeward foil is balanced, you can vary the lift on the leeward foil and all it will do is accelerate the boat up or down.

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1 hour ago, Sidecar said:

The entire boat would “fly” (very rapidly) upwards..... kept in balance by the heeling (and driving) force of wind on the the sails.

Sleep well, and don’t have any beer for breakfast.....

@Sidecar i went back and reread a bunch of comments. Thanks again to you and @erdband @MaxHugenand a few others.  I think I have a much greater understanding (I'm still naive, but no longer ignorant per se)...  

I believe my initial assertion is still correct insofaras an increased upward lift on the leeward foil WILL mathematically result in a greater righting moment.  Let's call this "Posit A".  

But my error was not understanding the insignificance of "Posit A".  

In fact, the forces from every direction are so great on a foiling AC75 (at say, 35kn) that in reality increasing the Fz vector at the leeward foil will miraculously just plain lift up the ENTIRE vessel and barely affects the righting moment at all.  (I.e. the center of gravity is basically on the foil itself).

This is my current understanding. 

 

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10 minutes ago, Blitzkrieg9 said:

@Sidecar i went back and reread a bunch of comments. Thanks again to you and @erdband @MaxHugenand a few others.  I think I have a much greater understanding (I'm still naive, but no longer ignorant per se)...  

I believe my initial assertion is still correct insofaras an increased upward lift on the leeward foil WILL mathematically result in a greater righting moment.  Let's call this "Posit A".  

But my error was not understanding the insignificance of "Posit A".  

In fact, the forces from every direction are so great on a foiling AC75 (at say, 35kn) that in reality increasing the Fz vector at the leeward foil will miraculously just plain lift up the ENTIRE vessel and barely affects the righting moment at all.  (I.e. the center of gravity is basically on the foil itself).

This is my current understanding. 

 

The COG cannot be on the foil itself or there is no righting moment. The distance of the COG from the lee foil is what creates the righting moment

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I am sorry, I am trying to help. You seem to constantly conflate the COG with the pivot point. They are not the same thing. When in flight the boat operates around the centre of lateral & vertical resistance. This is roughly the centroid of the leeward foil, this being the only part in the water.

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23 minutes ago, kiwin said:

I am sorry, I am trying to help. You seem to constantly conflate the COG with the pivot point. They are not the same thing. When in flight the boat operates around the centre of lateral & vertical resistance. This is roughly the centroid of the leeward foil, this being the only part in the water.

You're 100% right. I'm struggling to differentiate between the vertical component of the  COG and the pivot point.  To my mind, these are on the same vertical axis. 

Plus, I do not understand the concept of "the centroid of the leeward foil".  I must needs further my research and understanding. Thank you!

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When the boat is foiling it is supported by the leeward foil and the rudder. If you draw a line from the rudder through the lee foil, then this is the actual pivot around which the rig and ballast operate. Note that this is not parallel to the centre line. Also note that the rig exerts a force which is forward as well as to leeward. This certainly results in trying to press the nose of the boat down, so the rudder foils may well be pulling the stern down to compensate and increasing righting moment.

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8 minutes ago, kiwin said:

When the boat is foiling it is supported by the leeward foil and the rudder. If you draw a line from the rudder through the lee foil, then this is the actual pivot around which the rig and ballast operate. Note that this is not parallel to the centre line. Also note that the rig exerts a force which is forward as well as to leeward. This certainly results in trying to press the nose of the boat down, so the rudder foils may well be pulling the stern down to compensate and increasing righting moment.

Same as planes, the elevator actually holds the tail down. The centre of gravity is in front of the centre of lift. As witnessed by a stalled plane falling nose first

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Great discussion here. Thanks to all from a dedicated lurker.

Just wondering if variable incidence of the rudder foil has been considered in RM discussion. A change in AoA of the rudder foil through a change in rudder vertical axis angle would have a much bigger effect than roll through differential flap angle on the arm foil. It was very interesting to watch the different wakes from the rudders at different times. They are certainly availing of negative lift from the rudder foil.

Also, and this may have been covered earlier, the change in height to the CoE through ride height adjustment will have as significant an effect on RM as the cant angle effect discussed above. The higher the ride height the more RM required. It was noticeable that ETNZ flew lower more often than anyone else.

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21 minutes ago, SpudH said:

Great discussion here. Thanks to all from a dedicated lurker.

Just wondering if variable incidence of the rudder foil has been considered in RM discussion. A change in AoA of the rudder foil through a change in rudder vertical axis angle would have a much bigger effect than roll through differential flap angle on the arm foil. It was very interesting to watch the different wakes from the rudders at different times. They are certainly availing of negative lift from the rudder foil.

Also, and this may have been covered earlier, the change in height to the CoE through ride height adjustment will have as significant an effect on RM as the cant angle effect discussed above. The higher the ride height the more RM required. It was noticeable that ETNZ flew lower more often than anyone else.

G'day "lurker"! Yes, when you get down to the "fine detail", the ± force from the rudder foil also has to be accounted for. Until you get to higher boat speeds, that force is a positive Lift, which actually reduces RM a bit.

The rudder foil has to balance the Longitudinal moments of sail force, as well as adjust the main foil AoA... these are all interconnected. Changes in rudder foil force also have to be compensated by the main foil, so that total vertical lift balances the total boat+crew mass.

Ride height has only a small influence on RM.  Flying lower does allow the foil to be canted further, and that does move the centre of vertical lift outboard, but the difference between say 20° and 22° cant is not much. The extra cant is more helpful in increasing Lateral force.

IMO :)

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9 hours ago, Blitzkrieg9 said:

Please, no...  let's not "complicate further".  :) :) :)  i still can't grasp the basics. 

Seriously tho, thanks!  I'm about 6 beers deep so I just scrolled straight part your dissertation.  But I will read it about 4 times tomorrow. 

Hahaha... welcome to the rabbit hole. We've probably all been there at one time or another.   I've asked the wabbit to explain this in simpler terms... let's see how he does. Carry on, wabbit :

A force is measured in Newtons, and 1000 Newtons is a kilo-Newton (kN). Let's say the boat+crew exerts a downward force of 80kN.  We'll use this in the examples.

A see-saw is a good place to start. A moment is a turning (rotational) force. In Figure 1,  Forces A & B are the same, as are their distances from the axis. As these forces are turning in opposite directions and cancel each other out, the see-saw is balanced.

In Fig 2, by halving Force B but doubling it's distance to the axis, the turning forces of the see-saw are still equal and balanced.

1283042344_RabbitsMoments1.thumb.JPG.544714d03c566dcbfed59593142c06f9.JPG

Since a moment is only concerned with forces that rotate around an axis, we're not interested in the Total up and down forces when calculating moments. You can see that these are not the same in Figures 1 and 2.

For moments, the "distance" is measured perpendicular to the direction of the force, from the force's centre of effort (CE) to the chosen axis.

In Fig 3 and 4, the turning forces have remained the same, but Force B has been rotated. So the turning forces - moments - are exactly the same in Figures 1 through to 4.
1151184437_RabbitsMoments2.thumb.JPG.7c8ea2f848543624faf4da99d644512b.JPG

Figure 5 just gives the forces some "boaty" names - "Heeling Force" and the "Boat Mass" which provides the Righting Moment. These turning forces haven't changed, and are still balanced.

How can you increase the sail force to increase boat speed? The turning forces still have to balance, so if the Heeling Force increases, the Distance to the axis must decrease, as shown in Figures 6 and 7.

1274447681_RabbitsMoments3.thumb.JPG.39385c33944688706a76b9bcd8008fac.JPG

How do the moments stack up if the axis is moved to the centre of the boat? If we assume the CoG of the boat is also on the centreline for simplicity,  Figure 8 illustrates that the Boat Mass is no longer a turning moment, and instead the Vertical Lift from the foil must be accounted for, and becomes the Righting Moment.

So if the foil Force is increased, then the Heeling Force can also increase, and the moments can still be balanced.      Yes...   but NO.

679432573_RabbitsMoments4.thumb.JPG.449d3bf85faff83a91a757b0a0aae4e6.JPG

The problem is that the "directional" forces, which are Up versus Down in this case, also have to balance at the same time.

Now we're not interested if the forces are turning, only if their direction is Up or Down. In Figure 9, since the Heeling Force is sideways, it becomes irrelevant. So only the downward Boat Mass versus the Lifting Force are assessed. As long as the Up force (Vertical Lift) from the foil equals the Down force of the boat mass, the boat will not rise or sink as it's balanced.

Ignoring other factors, the Vertical Lift cannot exceed the Down force of the Boat Mass, or the boat will simply rise until the foil breaches the surface and loses lift.

Well, how did the wabbit do - does he get a carrot?          592341560_RabbitsMoments5.JPG.1d109e45f5abcaeefd7bbe6c7957d839.JPG

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5 hours ago, SpudH said:

Great discussion here. Thanks to all from a dedicated lurker.

Just wondering if variable incidence of the rudder foil has been considered in RM discussion. A change in AoA of the rudder foil through a change in rudder vertical axis angle would have a much bigger effect than roll through differential flap angle on the arm foil. It was very interesting to watch the different wakes from the rudders at different times. They are certainly availing of negative lift from the rudder foil.

Also, and this may have been covered earlier, the change in height to the CoE through ride height adjustment will have as significant an effect on RM as the cant angle effect discussed above. The higher the ride height the more RM required. It was noticeable that ETNZ flew lower more often than anyone else.

Yes, when I made my VPP, I included rudder vertical force, too. Rudder downforce increases righting moment, but it also increases drag. Initially, I always had downforce on the rudder, but as I played with parameters like location of CG and pitch angle of the boat, I found that upwind performance was best when the rudder produced a slight vertical lift. Reaching angles (mark rounding) however require a lot of rudder downforce, and downwind as well depending on wind strength.

Rudder and foil vertical forces:

rud_foil_z.png.82cc1dc8f7d1ac97f767a382ef43c992.png

(As the rudder produces more downforce, the foil needs to produce more vertical lift)

Roll and pitch moments:

heel_pitch_ms.png.a702c68326c58af78968c64461f26570.png

 

 It's still far from ideal, and there's a lot to optimize here, because any change in lift / drag ratios of the foil or rudder horizontal would change where the optimum performance is.

 

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Ask yourself one simple question.

 

If they could change out foils each day for different conditions what would the foil be each time.

Me thinks big in low winds and small in high winds as they go from a shortage of lift to a surplus of lift. Its all about drag.

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While there is a link between wind speed and boat speed, I think what boat speed you do changes the foil size.  Most of the time you are travelling fast (even in low wind speeds) so small foils all the time and very small for very fast?  It is only for lift off that you need large foils and if you can solve that using other boat changes then better because it is a very small time in the whole race picture?

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3 hours ago, Kiwing said:

 It is only for lift off that you need large foils and if you can solve that using other boat changes then better because it is a very small time in the whole race picture?

I still think that hull aerodynamics had a role to play.... And I think TR overall, did it best, resulting in the smallest foils.

Over in the “New sailing twin skin setup” thread, I opined:

“For me the mast stump was about lowering the deck to minimise the frontal area facing 40 to 50 knots AWS. Plus channeling the airflow, which in itself means that the (additional) sail area [created] low down is even more effective.

..... air (and water) pushed away sideways is wasted energy. Best is to push it under. TR’s heavily flared bows best pushes air under the flat sections to help create an air cushion when close to the water, assisting lift off. Once it is up and away, the effect is minimised if not lost, but the job has been done, which means you need smaller foils for lift off, which pays back at higher speeds. TR, relative to LR, pushes less air overall out of the way, and more of it under than sideways.”

DF78887E-B74D-44B0-A6FC-1D8B989EF344.thumb.jpeg.25fc71eef8f0f13a6cf6a4814680f8a9.jpeg

We all can understand that “hull lift” can reduce RM max, but if it is only really effective at lift off and in soft conditions, when the hull underside is sufficiently close to the water, it must help. It also would help to keep “flight”steady, hull too high, the effect is negligible/non existent due to increasingly excessive cross flow, hull drops low and it increasingly kicks in. With a canoe central underbody (plus keel runner underneath) half a tunnel hull is enough for an advantage, because a full tunnel hull is unachievable under the rules?

BTW, did anyone else also notice that LR add on their own keel runner late in the day?

I also think TR possibly could have been improved if they had extended their canoe skeg right up to the rudder like LR did....

And then there is this:

 

A73F5921-3C5E-45AA-9FDF-C0A712AF0271.jpeg

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On 3/19/2021 at 5:34 AM, MaxHugen said:

You need to study moments, like with the YT link and others with some exercises you can try.

You can place the "axis of rotation" on the boat wherever you like, but you have to work out which forces to include and their distances from the chosen axis.

It's "convenient" to use the leeward foil as the axis, because for all but the most involved calcs, you can ignore that foil's lift and mass forces.

If you used a central position on the hull as your axis, you would then need to include the leeward foil's forces, but now you have only about half of the boat mass contributing to RM, while the other half adds to Heeling moment and opposes the foil Lift.

 

On 3/19/2021 at 7:43 AM, kiwin said:

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

 

On 3/19/2021 at 10:50 AM, kiwin said:

When the boat is foiling it is supported by the leeward foil and the rudder. If you draw a line from the rudder through the lee foil, then this is the actual pivot around which the rig and ballast operate. Note that this is not parallel to the centre line. Also note that the rig exerts a force which is forward as well as to leeward. This certainly results in trying to press the nose of the boat down, so the rudder foils may well be pulling the stern down to compensate and increasing righting moment.

I wanted to thank the general level of input here on this forum, especially to @erdb & @Basiliscus for their insight and input.

I was getting frustrated however with the lack of acknowledgement on the curious nature of these beasts being a three legged stool (outboard foil arms and rear but centreline mounted rudder foil) and it is only in @kiwin post above (@ 10.50am) that someone finally hints at how dynamic this attribute makes these craft when you consider how small changes in Roll (Heel) will have on both Nose Up or Down attitude, as well as changing AOA on foil surfaces simultaneously. So Roll can be affected by Flap angle, foil arm cant, mainsheet or jibsheet trim, wind pressure on (or off), compettitors windwash, and rudder flap angle - to name just a few of the inputs that could set the whole show off kilter.

Think about how the Axis being offset from the boats centreline makes the chase for dynamic stability just so much more complex than the Foilng Cats or even Canting Keels ever had........ Its a wonder that these boats were ever capable of being flown so accurately and repeatedly and in close proximity to each other. Bravo to the concept, Wonderment at the real matchracing that we witnessed and now finally we will have a Mea Culpa to all the naysayers that were writing such crap when this rule was first mooted? You know who you are, and we will not forget......

So Add that to your mix of going down the rabbit hole if you want a truly disturbed nights sleep......

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2 hours ago, oatsandbeans said:

Ok so if you were building a AC boat for the next cup what foil design would you use? Answers of “just copy ETNZ foils” are not allowed as sure as hell they won’t be using them next time round! 

The fastest ones right from the beginning and then learn how to sail them.

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10 minutes ago, I ride bikes said:

The fastest ones right from the beginning and then learn how to sail them.

These boats are a set of compromises.  My belief is you must learn to get the sails working to maximum low speed grunt.  Then you have to design some minimum foils that will get you up.  Once foiling all the drag factors start being most important. Mostly the sails IMHO.

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Max, coming out of a tack the 75 takes some time to reach the maximum speed it can achieve on the new tack. Initially the AWA is high and AWS low, however the heeling moment remains constant. As the speed increases the power available from the wind increases, hence to maintain constant heeling moment the sails have to be eased or twisted. twist is best as it lowers the CofE and increases the forward force. Lowering the CofE requires a higher cant angle to avoid side loads on the strut. My guess is that AofA of the strut is measured and the cant is increased to keep it near zero as the boat accelerates

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3 hours ago, alanjs said:

Max, coming out of a tack the 75 takes some time to reach the maximum speed it can achieve on the new tack. Initially the AWA is high and AWS low, however the heeling moment remains constant. As the speed increases the power available from the wind increases, hence to maintain constant heeling moment the sails have to be eased or twisted. twist is best as it lowers the CofE and increases the forward force. Lowering the CofE requires a higher cant angle to avoid side loads on the strut. My guess is that AofA of the strut is measured and the cant is increased to keep it near zero as the boat accelerates

The image you referenced is not typical of cant angles, I posted it as I suspected at the time that they may have had a hydraulic pressure leak, although I don't know if that was actually the case.

I'd recommend you have a look at @dorox website at  https://ac36.herokuapp.com/stats_app  where you can select foil arm cant angle stats for any race of any of the AC series.

FYI, to get the foil angle from the reported foil arm angle , subtract 42°.

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9 hours ago, oatsandbeans said:

Ok so if you were building a AC boat for the next cup what foil design would you use? Answers of “just copy ETNZ foils” are not allowed as sure as hell they won’t be using them next time round! 

There is no easy answer to this, as the fastest configuration at one TWS is not the same as at a higher TWS etc.  ETNZ has shown that with good control systems the narrow foils work OK at lower TWS.

At higher TWS there are two major problems (besides plenty of others too).

Drag. At high boat speeds, they carry too much sail. So they have to twist off the top to reduce heel by lowering the CE, but the top section still contributes to drag with no benefit. Allowing a mainsail that has a shorter luff for high wind speeds may be a possibility.

Foils comprise around 50% of total drag. Allowing teams to select foils on the day for the TWS will definitely help, as they could use higher speed foils that are not only narrower but also thinner to minimise drag, whilst using thicker high-lift foils at low wind speeds. Or could a brilliant designer invent a foil that would vary foil profile?

Cavitation. If the drag issues can be addressed, foil cavitation becomes a problem. This can occur with any foil, although at different speed depending on profile. @Basiliscus did a 6 part series which explains this, starting at Part 1.

Again relying on a brilliant designer, perhaps a foil can be designed that provides sufficient sub-cavitation lift for take-off at high TWS, then "morphs" into a ventilating super-cavitating section?

 

Another possibility may be by changing the boat design to dynamically increase RM. The latest boats in the IMOCA class use a curved design with their surface piercing foils. As they heel, the foils are lowered, and the curved design moves the centre of lift further outboard. Is this even a possibility for the AC75?

But all this may be wishful thinking from the rabbit hole.  :rolleyes:

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The thickness is driven by other than hydrodynamic considerations.  On the AC50, the foil span could be allowed to extend very far inboard, and the foil wings were cantilevered, so the foil thickness was driven by structural strength and stiffness.  On the AC75, the foil span is much more limited, and the T foil results in a much smaller unsupported span.  But the ballast and foil control requirements drive the thickness.

I think in a Version 2 AC75 you'll see more convergence of the designs.  Since the foil arms are supplied items, I think it's unlikely there will be significant changes to the Design Rule with regard to foil size or weight, as that would change the specifications for the production arms.

Atmospheric pressure is greater than vapor pressure (or else the oceans would boil) so ventilating the top of a foil results in a greater loss of lift than if the top of the foil is cavitated.  I don't see intentionally ventilating the foils is a good way to go.

As for morphing foils, you can view the flap as a form of morphing.  The Design Rule is pretty restrictive with regard to other means of morphing the foils.

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2 hours ago, Basiliscus said:

I don't see intentionally ventilating the foils is a good way to go.

Understood.  I was thinking more in terms of ventilating the cavitation bubble of a super-cavitating foil, as Peter Larsen did.

Perhaps an A75 v2 might include some mods to the Design Rules? And/or allowing a change of foils within a few hours of a race...

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3 hours ago, MaxHugen said:

Drag. At high boat speeds, they carry too much sail. So they have to twist off the top to reduce heel by lowering the CE, but the top section still contributes to drag with no benefit. Allowing a mainsail that has a shorter luff for high wind speeds may be a possibility.

Could be a Rule change for AC75 V2 which allows for light and heavy weather rigs. Shorter mast and smaller flatter cut sails for the heavy weather version.

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6 minutes ago, Sailbydate said:

Could be a Rule change for AC75 V2 which allows for light and heavy weather rigs. Shorter mast and smaller flatter cut sails for the heavy weather version.

That sounds like a sensible move, assuming they reduce the 120 hr pre-race measurement rule.

It will be interesting to see how this evolves. :)

Ps: SailGP took this route, with a segment of the wing sail that could be omitted, reducing wing from 24m to 18m in height.

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2 minutes ago, coercivity said:

can the flaps be adjusted in opposite directions so as generate rotational force.

Yes, they could use differential flap settings. However, the downside is additional drag.

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15 minutes ago, MaxHugen said:

That sounds like a sensible move, assuming they reduce the 120 hr pre-race measurement rule.

It will be interesting to see how this evolves. :)

Ps: SailGP took this route, with a segment of the wing sail that could be omitted, reducing wing from 24m to 18m in height.

I think also, a shorter rig would also improve jib efficiency (by closing the slot, slightly) in the AC75.

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But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds  Then you have less drag as the surplus lift of the current foils is not there. 

As the speed runs up the the lift rises by the square of the Velocity so they must be killing lift with the flaps inducing even more drag. The nose down attitude is to reduce the fixed AOA of the foil without to much flap.

If you had a perfect foil for say [email protected] it would be less than half the current area but they would be stuck on the water in all winds other than north of 20Knts 

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2 hours ago, Sailbydate said:

I think also, a shorter rig would also improve jib efficiency (by closing the slot, slightly) in the AC75.

Not so sure about that. The jibs on the AC75 can already "close the slot" with their traveller systems, probably well past what is optimal.

There's a good discussion on the slot, with helpful diagrams, by A. Gentry on p14 of A Review of Modern Sail Theory.

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3 hours ago, coercivity said:

But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds  Then you have less drag as the surplus lift of the current foils is not there. 

As the speed runs up the the lift rises by the square of the Velocity so they must be killing lift with the flaps inducing even more drag. The nose down attitude is to reduce the fixed AOA of the foil without to much flap.

If you had a perfect foil for say [email protected] it would be less than half the current area but they would be stuck on the water in all winds other than north of 20Knts 

At 30 knots boat speed, NZ's foil, allowing for 0.5° of leeway and 22° cant, would have 0° AoA and ~ flap to produce ~70kN of vertical force (Fz).

This is based on a foil profile that is producing a CL of ~0.47, and a L/D ratio of 59.

At 40 knots, the flap angle is changed to -1.7° for the same Fz.   CL dropped to 0.26 and L/D to 37.

As you can see, modest changes in flap angle considerably change the CL, and maintains the required Fz without changing NZ's foil area of 1.32 m^2.

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3 hours ago, coercivity said:

But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds

I'm not sure this works. Having both foils in the water can indeed double the lift, but only at the cost of cancelling all the righting moment.

So as soon the hull leaves the water, the boat would have no righting moment at all. Perhaps changing the windward flaps from positive to nuetral/negative lift at the moment the hull lifts would work?

But could these boats get to take off speed with only the righting moment of the hull? Again perhaps quick adjustments of flaps may help, but it sounds like making an already very dynamic transition even more so.

If possible, it would be a good thing!

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8 hours ago, MaxHugen said:

Understood.  I was thinking more in terms of ventilating the cavitation bubble of a super-cavitating foil, as Peter Larsen did.

Perhaps an A75 v2 might include some mods to the Design Rules? And/or allowing a change of foils within a few hours of a race...

Ventilating the top of a supercavitating foil will reduce the lift.

If you are referring to Sailrocket, I believe that boat used a base-ventilated foil.  That is different from a supercavitating foil because the upper surface is wetted.  Ventilating the base of such a foil reduces the drag compared to a cavitated base because the lower pressure of the cavitated flow pulls back more on the foil than does atmospheric pressure.  

Base ventilated foils are subject to the same pressure limitations as sub- or trans-cavitating foils.  However, the flat "rooftop" portion of the pressure distribution can extend further aft because the pressure doesn't need to recover to ambient pressure at the trailing edge.  Instead, it only needs to come down to atmospheric pressure.  

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8 hours ago, coercivity said:

But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds  Then you have less drag as the surplus lift of the current foils is not there. 

...

The Design Rule and the Foil Control System design were specifically intended to prevent the windward foil from pulling down to create righting moment.  When the windward foil is in the water and lifting upward, the righting moment is reduced.

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4 minutes ago, Basiliscus said:

Ventilating the top of a supercavitating foil will reduce the lift.

If you are referring to Sailrocket, I believe that boat used a base-ventilated foil.  That is different from a supercavitating foil because the upper surface is wetted.  Ventilating the base of such a foil reduces the drag compared to a cavitated base because the lower pressure of the cavitated flow pulls back more on the foil than does atmospheric pressure.  

Base ventilated foils are subject to the same pressure limitations as sub- or trans-cavitating foils.  However, the flat "rooftop" portion of the pressure distribution can extend further aft because the pressure doesn't need to recover to ambient pressure at the trailing edge.  Instead, it only needs to come down to atmospheric pressure.  

OK.  So does cavitation only occur at the top and bottom edges of the flat "base" in a base-ventilated foil?

And does "super-cavitating" refer to a foil that employs cavitation on only the upper surface, but from the LE?

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Cavitation occurs anywhere the local pressure drops below the vapor pressure.  Sharp edges cause a local pressure peak that initiates flow separation and cavitation.  A base ventilated foil is designed to maintain attached flow back to the blunt trailing edge.

The definition of a supercavitating foil is one in which the vapor cavity starts forward on the foil and extends aft beyond the foil.  They typically have a sharp leading edge that results in the cavity beginning there, so the whole upper surface is covered by vapor. 

There are several reasons for wanting to do this.  One is to avoid the erosion that occurs when bubbles collapse near the foil before they reach the trailing edge.  Another is to reduce the skin friction of the upper surface by not having it wetted.  The sharp leading edge also provides consistent and predictable characteristics compared to allowing cavitation to start at a variety of locations depending on the operating conditions.  The sharp leading edge also leads to clean sheet cavitation as opposed to cloud cavitation.  So most supercavitating foil sections are more or less wedge-shaped.

I suppose a foil that had a flat base, leading to a cavitated bubble extending into the wake with wetted top and bottom surfaces, could be considered a supercavitating foil because the cavity closes behind the foil.  But that is kind of the worst of all worlds from a drag standpoint.

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15 hours ago, MaxHugen said:

........
Drag. At high boat speeds, they carry too much sail. So they have to twist off the top to reduce heel by lowering the CE, but the top section still contributes to drag with no benefit.
..........

@MaxHugen We have a picture of TNZ with the top of the sail inverted providing RM.  So they can use far more power (which I think we agree they have?) to overcome increased drag from cavitation etc??
I don't know why this is not considered and evaluated more?  It seems the analysts on here have written the inverted top off for some reason?

Am I missing something?

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7 hours ago, Basiliscus said:

Cavitation occurs anywhere the local pressure drops below the vapor pressure.  Sharp edges cause a local pressure peak that initiates flow separation and cavitation.  A base ventilated foil is designed to maintain attached flow back to the blunt trailing edge.

The definition of a supercavitating foil is one in which the vapor cavity starts forward on the foil and extends aft beyond the foil.  They typically have a sharp leading edge that results in the cavity beginning there, so the whole upper surface is covered by vapor. 

There are several reasons for wanting to do this.  One is to avoid the erosion that occurs when bubbles collapse near the foil before they reach the trailing edge.  Another is to reduce the skin friction of the upper surface by not having it wetted.  The sharp leading edge also provides consistent and predictable characteristics compared to allowing cavitation to start at a variety of locations depending on the operating conditions.  The sharp leading edge also leads to clean sheet cavitation as opposed to cloud cavitation.  So most supercavitating foil sections are more or less wedge-shaped.

I suppose a foil that had a flat base, leading to a cavitated bubble extending into the wake with wetted top and bottom surfaces, could be considered a supercavitating foil because the cavity closes behind the foil.  But that is kind of the worst of all worlds from a drag standpoint.

Does chord length (and Re) have any effect on the onset of cavitation or on how cavitation affects drag?

Just wondering if the real benefit of the T foil may have been the simpler flap mechanism allowing reduced thickness and therefore chord length -> less adverse effects from cavitation.

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4 minutes ago, erdb said:

Does chord length (and Re) have any effect on the onset of cavitation or on how cavitation affects drag?

Just wondering if the real benefit of the T foil may have been the simpler flap mechanism allowing reduced thickness and therefore chord length -> less adverse effects from cavitation.

Bloody good thought.

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6 hours ago, Kiwing said:

@MaxHugen We have a picture of TNZ with the top of the sail inverted providing RM.  So they can use far more power (which I think we agree they have?) to overcome increased drag from cavitation etc??
I don't know why this is not considered and evaluated more?  It seems the analysts on here have written the inverted top off for some reason?

Am I missing something?

Have you a link to to that pic, or is it just the one where the entire top half is luffing?

I've kept an eye on every overhead shot during races with at last mid strength wind and haven't seen any inversion. Nor on the Volvo Ocean 65s during in-port races etc in strong wind conditions.

At this point I'm inclined to disregard inverted sails.

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1 hour ago, erdb said:

Does chord length (and Re) have any effect on the onset of cavitation or on how cavitation affects drag?

Just wondering if the real benefit of the T foil may have been the simpler flap mechanism allowing reduced thickness and therefore chord length -> less adverse effects from cavitation.

A reduced thickness definitely helps, as it can have a flatter top surface and more even low pressure distribution. Probably more important than chord length.

A profile I tried has a decent distribution over the top, but may be too thin for structural reasons. It would also help if it produced some more lift from the aft half of the lower surface, but that increases drag significantly.

This profile is at 60 knots with a decent CL of 0.64 and L/D of 37, but the Cpmin is rather low at -0.52, so I have to check if this is still below the water vapour pressure.

image.png.36050d50fb0a0d8b8d93e5829b88d02f.png

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1 hour ago, erdb said:

Does chord length (and Re) have any effect on the onset of cavitation or on how cavitation affects drag?

Just wondering if the real benefit of the T foil may have been the simpler flap mechanism allowing reduced thickness and therefore chord length -> less adverse effects from cavitation.

Reynolds number has to do with the boundary layer and cavitation is not fundamentally a boundary layer phenomenon (although it can affect the boundary layer).

Chord length, per se, does not affect cavitation - only the pressures do.  However, for a given physical thickness, it is easier to design a foil with a higher cavitation onset speed if the chord is larger because then the thickness ratio is smaller.  More chord for the same span also means more area and less loading, which makes it easier to avoid cavitation at takeoff.  So while chord by itself does not directly affect cavitation, a longer thinner section will help.

 

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6 minutes ago, MaxHugen said:

A reduced thickness definitely helps, as it can have a flatter top surface and more even low pressure distribution. Probably more important than chord length.

A profile I tried has a decent distribution over the top, but may be too thin for structural reasons. It would also help if it produced some more lift from the aft half of the lower surface, but that increases drag significantly.

This profile is at 60 knots with a decent CL of 0.64 and L/D of 37, but the Cpmin is rather low at -0.52, so I have to check if this is still below the water vapour pressure.

image.png.36050d50fb0a0d8b8d93e5829b88d02f.png

If you want to use Xfoil to design a base ventilated section, take a look at this thread in which Prof. Mark Drela responded to my first attempt at a base ventilated section with his own approach and design.  He has some good advice on how to set some Xfoil parameters to better represent the cavity.

Here is the design he came up with.  It could be good for 65 to 80 kt (with sweep)!

bv02.thumb.png.3867606f9d0e6d0fb91c8ee80e4a7323.png

 

bv02.txt

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2 minutes ago, Basiliscus said:

If you want to use Xfoil to design a base ventilated section, take a look at this thread in which Prof. Mark Drela responded to my first attempt at a base ventilated section with his own approach and design.  He has some good advice on how to set some Xfoil parameters to better represent the cavity.

Here is the design he came up with.  It could be good for 65 to 80 kt (with sweep)!

bv02.thumb.png.3867606f9d0e6d0fb91c8ee80e4a7323.png

 

bv02.txt 7.66 kB · 0 downloads

Thanks, will check the links out! :)

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56 minutes ago, Basiliscus said:

If you want to use Xfoil to design a base ventilated section, take a look at this thread in which Prof. Mark Drela responded to my first attempt at a base ventilated section with his own approach and design.  He has some good advice on how to set some Xfoil parameters to better represent the cavity.

Here is the design he came up with.  It could be good for 65 to 80 kts

 

I've been searching for a while, but I haven't found any info on what the "Kdl parameter" is, in relation to XFoil or XFLR5.

Could you provide me with some info - or a source - pls?

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1 hour ago, MaxHugen said:

I've been searching for a while, but I haven't found any info on what the "Kdl parameter" is, in relation to XFoil or XFLR5.

Could you provide me with some info - or a source - pls?

I had to get Dr. Drela to help me out finding it, too, in that thread to which I linked!  

In Xfoil, it's in OPER->VPAR->LAG.  Then step through the parameters.  Kdl is the last (third) one.  (You have to use OPER->VPAR->? to get the menu that shows the LAG command.)

As to what it is, here is what Dr. Drela had to say:

"Re: Ventilation bubble modeling.
You can reduce the turbulent wake dissipation via the Kdl parameter in the VPAR menu, LAG command. Increasing Kdl from the default 0.9 value will make the wake recirculation bubble mix out more slowly, which I think better mimics a ventilation bubble. Second PDF corresponds to Kdl=4.0 which gives a much longer bubble."

''Klag and Kdl are two different parameters.

Klag = 5.6 always.
Kdl = 0.9 for a normal wake. Make a lot bigger for a non-dissipative wake, which is closest to a base ventilation bubble."

"Now that I think about it, a ventilated base bubble open to the atmosphere must have Cp=0 or nearly so. In reality, the bubble will assume whatever shape it has to in order to achieve this Cp=0 over its length.

The closest way to simulate such a bubble in XFOIL is to essentially turn off the wake viscous dissipation with a very large Kdl (I'm using 8.0 now), and shape the rear of the airfoil to get very nearly Cp=0 in the near wake.

I played around with this idea, and came up with the attached BV02. It's heavily based on transonic airfoil technology, with a deep center "spar" section with little loading, and loading in the front and back. The L/D is a ridiculously huge 20:1. This is even with a generous 6.8% thickness, and min(Cp) = -0.18, which should be good to 65 knots, or 80 knots with some modest sweep. Yowsah!"

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3 hours ago, Basiliscus said:

I had to get Dr. Drela to help me out finding it, too, in that thread to which I linked!  

In Xfoil, it's in OPER->VPAR->LAG.  Then step through the parameters.  Kdl is the last (third) one.  (You have to use OPER->VPAR->? to get the menu that shows the LAG command.)

As to what it is, here is what Dr. Drela had to say:

"Re: Ventilation bubble modeling.
You can reduce the turbulent wake dissipation via the Kdl parameter in the VPAR menu, LAG command. Increasing Kdl from the default 0.9 value will make the wake recirculation bubble mix out more slowly, which I think better mimics a ventilation bubble. Second PDF corresponds to Kdl=4.0 which gives a much longer bubble."

''Klag and Kdl are two different parameters.

Klag = 5.6 always.
Kdl = 0.9 for a normal wake. Make a lot bigger for a non-dissipative wake, which is closest to a base ventilation bubble."

"Now that I think about it, a ventilated base bubble open to the atmosphere must have Cp=0 or nearly so. In reality, the bubble will assume whatever shape it has to in order to achieve this Cp=0 over its length.

The closest way to simulate such a bubble in XFOIL is to essentially turn off the wake viscous dissipation with a very large Kdl (I'm using 8.0 now), and shape the rear of the airfoil to get very nearly Cp=0 in the near wake.

I played around with this idea, and came up with the attached BV02. It's heavily based on transonic airfoil technology, with a deep center "spar" section with little loading, and loading in the front and back. The L/D is a ridiculously huge 20:1. This is even with a generous 6.8% thickness, and min(Cp) = -0.18, which should be good to 65 knots, or 80 knots with some modest sweep. Yowsah!"

Thanks for contacting Dr Drela!  I noticed in his tests with BV03 that increasing Kdl to 4.0 slightly reduced CL, but there was a major reduction in drag - 22.5% less!

If I understand all this correctly, due to the small difference in pressure between the bubble and the free flow, the rate of dissipation is low, so the bubble wake is longer.

Although the pressure variation is higher over the surface of the foil, it is still relatively low, so turbulence influences the boundary layer less, hence less surface drag and the much lower Cd?

Makes me wonder if a profile that started cavitation at the LE and created a very thin "layer" over the top surface of the foil would reduce drag even further, without drastically reducing lift. Or maybe the lift can be obtained primarily from the lower surface, although I guess this would bump up surface drag there, perhaps negating the reduction on the top surface?

 

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The problem with base drag is you can get any answer you want, depending on just what the base pressure is.  Using the wake lag parameters in Xfoil to model a ventilated wake is really a  kluge as the ventilated foil is way outside of the fundamental assumptions behind Xfoil's methods.  I wouldn't put much stock in the drag numbers you get.  I think the lift and pressure distribution results are probably more accurate.  So Xfoil is a good way to design the section shape, but you're going to need a more sophisticated CFD method to get a good estimate of performance.

There have been supercavitating sections with the contour of the upper surface near the edge of the cavitation bubble.  The upper surface pressure will be at vapor pressure, of course, and will remain so as the angle of attack increases from the ideal angle of attack.  I don't know what the drag implications are of cavitated flow vs a wetted surface without cavitation.  Sailrocket reported a lot of pitting of the surface of their base ventilated foil, so it must have been experiencing some form of cavitation, possibly bubble cavitation in addition to the air ventilating the base.

I believe you'll see these kinds of sections have a very narrow range of operation.  That may be acceptable for a record-breaker that only needs to be concerned with performance near the speed record, but for a course-racing yacht performing brilliantly when it gets just the right conditions while being a dog otherwise is not the way to win regattas.  To have a high probability of win, you'd rather win by a little in nearly any condition rather than win big in only some conditions.

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22 hours ago, MaxHugen said:

Have you a link to to that pic, or is it just the one where the entire top half is luffing?

I've kept an eye on every overhead shot during races with at last mid strength wind and haven't seen any inversion. Nor on the Volvo Ocean 65s during in-port races etc in strong wind conditions.

At this point I'm inclined to disregard inverted sails.

He has posted an image on the twin skins thread. Just looks like over powered luffing to me. Not buying into an increase of RM due to reversed forces.

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3 hours ago, The Advocate said:

He has posted an image on the twin skins thread. Just looks like over powered luffing to me. Not buying into an increase of RM due to reversed forces.

Yes, saw the post. That was the image I also alluded to.

@Mikko Brummer posted this excellent pic that he took with a masthead mounted camera on a Finn. It showed an incredible "S" curve in the upper section, which the Finn sailors do in fact use for negative force in high wind strengths for RM, possibly way beyond their original design limits. Mikko was talking about sailing the Finn in 30 knots TWS, IIRC !

image.png.f74ac274a066c7c48408e3c849ebd31a.png

I mocked up an S shaped profile and tried it in XFoil:

image.png.42bf3ff5ff25ea54196f9f9560399d1c.png

Even this rough attempt produced a surprisingly good CL of 0.44 and L/D of 52, so would certainly contribute a lot to RM.

However, the AC75's mast rotation would play havoc with using this technique. In the above pic, imagine the forward ~10% pointing "up" - ie rotated clockwise. That would be the angle of mast rotation as used lower down in the "power" section of the sail.

I haven't seen a vid or pic showing this being tried by the AC75 while racing, but then there's been a dearth  of race days with strong winds, so maybe?

There's a difference between active luffing for RM like in the Finns, and just, well, luffing...

IMO :rolleyes:

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36 minutes ago, MaxHugen said:

There's a difference between active luffing for RM like in the Finns, and just, well, luffing...

IMO :rolleyes:

Yes. I suspect some tend to see what they want to see. 

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On 3/23/2021 at 2:54 AM, Basiliscus said:

The problem with base drag is you can get any answer you want, depending on just what the base pressure is.  Using the wake lag parameters in Xfoil to model a ventilated wake is really a  kluge as the ventilated foil is way outside of the fundamental assumptions behind Xfoil's methods.  I wouldn't put much stock in the drag numbers you get.  I think the lift and pressure distribution results are probably more accurate. .... I believe you'll see these kinds of sections have a very narrow range of operation.

Yes, I'm going nowhere fast down this track.

I reverted to using your methods for reducing the onset of cavitation. My latest design can get to 50 knots without Cpmin reaching the water vapour pressure threshold - if I did the calcs correctly - but at just over 6% width it's probably not feasible from an engineering standpoint.

Tried a different way of hinging a flap, hoping to improve lift for such a thin foil. Thinking along the lines of a wingsail and the slot between the wing and flap... do you think there's any merit in pursuing this?

image.thumb.png.7d0f6ee8b82950c5c721d9af261fd6fe.png

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Yes, there's a lot of merit in considering a slotted flap.  INEOS actually used one on on their AC75.  

I'd put the pivot location in a different place, however.  It would be better to put it under the leading edge of the flap.  That is the classic location for a Fowler flap.  It moves the flap aft as it rotates down, increasing the planform area.  

You're going to need something other than Xfoil to design a slotted flap because Xfoil can only handle single element sections.  The only free code I know for slotted flaps is Javafoil.  There are some things that Javafoil can't do, like predict stalling by means of wake bursting, but maximum lift is probably not a big issue for a hydrofoil that is constrained by cavitation.

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8 hours ago, Basiliscus said:

Yes, there's a lot of merit in considering a slotted flap.  INEOS actually used one on on their AC75.  

I'd put the pivot location in a different place, however.  It would be better to put it under the leading edge of the flap.  That is the classic location for a Fowler flap.  It moves the flap aft as it rotates down, increasing the planform area.  

You're going to need something other than Xfoil to design a slotted flap because Xfoil can only handle single element sections.  The only free code I know for slotted flaps is Javafoil.  There are some things that Javafoil can't do, like predict stalling by means of wake bursting, but maximum lift is probably not a big issue for a hydrofoil that is constrained by cavitation.

I wonder if it was the W foil where Ineos tried the slot, it has some unusual protusions on the underside which could be offset pivot points. My attempt to get some aft movement didn't work out really, will have a look at the Fowler flap mechanism. Being restricted to one rotation axis makes it difficult.

Reading the JavaFoil manual to see how to use a multi-element profile.

An ongoing issue I have is trying to convert a concept done in SVG to a set of x-y coordinates.  XFLR5 has a very basic design function, but with only 12 splines it's difficult - and sometimes impossible - to create the foil profile there.

Do you know of any way to convert SVG to coords, or a different spline-based app that can export to x-y coords?

image.png.1af9d719bbb5a1b88edbd039c8978e78.png

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1 hour ago, MaxHugen said:

I wonder if it was the W foil where Ineos tried the slot, it has some unusual protusions on the underside which could be offset pivot points. My attempt to get some aft movement didn't work out really, will have a look at the Fowler flap mechanism. Being restricted to one rotation axis makes it difficult.

Reading the JavaFoil manual to see how to use a multi-element profile.

An ongoing issue I have is trying to convert a concept done in SVG to a set of x-y coordinates.  XFLR5 has a very basic design function, but with only 12 splines it's difficult - and sometimes impossible - to create the foil profile there.

Do you know of any way to convert SVG to coords, or a different spline-based app that can export to x-y coords?

image.png.1af9d719bbb5a1b88edbd039c8978e78.png

Max here is a photo of a Fowler flap hinge setup. 

67DA03E2-6BDE-4311-8337-3DA2B90AD0EE.jpeg

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32 minutes ago, amc said:

Max here is a photo of a Fowler flap hinge setup. 

67DA03E2-6BDE-4311-8337-3DA2B90AD0EE.jpeg

Thanks. That sure does look like the setup on Ineos's W foil ! 

Looks like a nasty source of drag, possibly creating vortex as well?

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2 hours ago, MaxHugen said:

...

An ongoing issue I have is trying to convert a concept done in SVG to a set of x-y coordinates.  XFLR5 has a very basic design function, but with only 12 splines it's difficult - and sometimes impossible - to create the foil profile there.

...

I don't use XFLR5, so I can't help you there.  But here's how I generate coordinates for Xfoil.  It's perhaps an overly manual process, but it's not so onerous that I've been driven to automate it more fully.

My CAD program is Rhino.  I can import a vector PDF or an SVG.  If I have a graphic format it can't import, like a raster PDF or JPEG, then I often use Inkscape to trace it and create an SVG.

Once I have the shape defined as curves in CAD, I use Rhino's Convert To LInes command to convert the spline into a polygon.   Then I extract the points and export them as a points text file.  I find it helps to export the top and bottom contours separately.  I then read the point files into Excel. 

In Excel I normalize the coordinates by the chord length and sort them back to front on the upper surface and front to back on the lower surface.  (It's because of the sort that it helps to export the two surfaces separately.)  Then I delete everything but the normalized coordinates and save as a text file.  The final step is to use a text editor to change the tab characters to spaces because Xfoil hates tabs.

I load the coordinates into Xfoil and execute PANE at the top level to interpolate for the desired number of coordinates.  Then I'll get a quick pressure distribution to see how it looks.  This is often quite oscillatory due to errors in the original coordinates.  I use Xfoil's MDES mode to smooth through the pressure distribution and create at new foil that is fair.  The difference between the original shape and the new shape can often be too small to see with the eye, but it makes a definite difference in the pressure distribution.  If needed, I will use the GDES mode to set the thickness and camber if those are definitely known for the section.

When I want to take a modified profile back to CAD, Xfoil has a top level RSAVE command (which is undocumented, btw) that will write out a command file that can be executed in Rhino to create the shape.  Or else I will strip the name off the Xfoil coordinates file, import it into CAD as a points file, and pass a spline curve through the points.

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8 minutes ago, Basiliscus said:

I don't use XFLR5, so I can't help you there.  But here's how I generate coordinates for Xfoil.  It's perhaps an overly manual process, but it's not so onerous that I've been driven to automate it more fully.

My CAD program is Rhino.

Rhino at USD 995 is way way beyond my budget, but I'll scout around for an alternative CAD app that can maybe do the same.  Thanks for outlining the procedure.  :)

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Wouldn't  the best way to reduce drag and increase lift low down thus allowing smaller foils etc be to allow the foils to vary in angle.

Quite hard to do from a strength perspective but it would allow smaller foils quite aggressive AOA at lift off maybe 15deg then reducing to near zero for the speed runs thus raking boat and mast back for lots of lift and very clean drag profile at speed also.

Flaps for trim and control in the run but small clean foils.

The discussion about super critical above 55knts is fine but for these boats to get around the track 5 or 10 knts faster its not higher top speeds you need its higher average speeds and they are closer to half the cavitation speed than not.  

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3 hours ago, MaxHugen said:

Rhino at USD 995 is way way beyond my budget, but I'll scout around for an alternative CAD app that can maybe do the same.  Thanks for outlining the procedure.  :)

The program that Rhino was developed from (I think) was free and available last time I looked. Buggered if I can remember the name of it though, haven't had it on the last few laptops.

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