Bieker B6

[TingTong]

Thanks for clarifying Mark

OK - I see where you are going with this. There actually are THREE effects here that you are conflating. The board will see the normal AoA initially and that will change as it lifts out of the water because when angled the AoA changes. But that's not really the issue here.

The way to think about this is with a notion engineer's call a "Force Couple" - which is a fancy way of describing two forces acting in opposite directions at the opposite ends of a seesaw (ie with a pivot in the middle). In the case of a sail and a daggerboard you have

1. A force acting at the Center of Effort (CE) of the sail pushing the sail directly to leeward.
2. A force acting at the Center of Lateral Resistance (CLR) in a direction that in the vertical domain (as opposed to fore-aft) is a vector opposite of the leeforce - and with the blade down, this acts from below the waterline
3. The "righting force" generated by your weight on the trapeze.

#1 and #2 create a Force Couple http://en.wikipedia....uple_(mechanics) that rotates around the hull (slightly below the waterline). this Force Couple rotates the boat mast to leeward and the blade upwards. This is Effect #1

Effect #2 is the "righting force" - which is a second force couple that is the force of gravity acting on the Center of Mass of the trapezed crew being off centerline of the bouyancy of the hull. This Force Couple rotatates the boat Mast To weather and the blade upwards to leeward.

When Effect #1 and Effect #2 balance each other, you have a ZERO effective set of rotational forces acting on the sails and the blades.

In this balance of couples you still have the difference in the MAGNITUDE of the vectors of #1 and #2. on a Moth, it turns out that when heeled to weather, #2 is GREATER than #1 and the Moth climbs to weather. In an I14, the balance is close at speed but #1 is still slightly greater than #2 - so the boat makes leeway.

sorry. i don't get this. how can, what shall henceforth be referred to as forces no 1 and 2, be anything but equal and opposite? (the couple being counteracted by no3)

Because an I-14 has a net movement to leeward. That means that it is being accellerated to leeward, and thus the force vector to leeward must EXCEED the force vector to weather

if a moth had a net force to windward, would it not accelerate in that direction proportionally to the net magnitude/mass.

similarly a standard i14 with it's force to leeward would accelerate to leeward, presumably until relativistic effects limited it at the speed of light?

And it does.

I'll see about drawing a picture with labels - which should help.

And Moths are NOT "apples and oranges" - the fact that moths can generate enough lift force to weather to accelerate to weather does not mean that I-14s boards are not generating lift to weather. It just means that the net balance of forces is different, but the forces involved are the same (if you exclude the vertical elevation lift forces).

but it can't accelerate, not indefinitely - otherwise it would either hit light speed or be travelling in circles. if speed and direction are constant then forces must be in equilibrium and there is, by definition, no acceleration.

maybe i'm being dim again

 

[Amati]

Since more than a few posts in this thread have been about the feel of the board, maybe part of the advantage of the gybing board for those who feel it is that you can exploit the edges of the centerboard's drag bucket more effectively, or at least in a different manner- which might explain why you can load up during lulls more, since you effectively have centerboard aoa at hull centerline aoa zero? And folks are locking boards in downwind situations- like when sideslip isn't as big an issue?

Taking the feel issue a bit farther, it may be that more drag from more aoa is a help to certain individuals, kind of like when swimmers shave for better times in the pool, even though it has been shown experimentally that leaving hair on the body helps order the flow, which leads to a less volatile boundary layer, which is less drag, but damn, hitting the water shaved is a real rush, and made me at least much more sensitive to all sorts of technical aspects of my stroke.

I don't know what sections I14's are using, but is there a split of section selection between gybing vs non gybing boards?

And that (and D.H. maybe) kind of argues for pretty different different hull designs for gybing vs non gybing boards, if exploiting different modes of lift/drag (hull vs board) viz hull side slip is warranted. I think Hans Geis(s?)ler might have something to say about this, at least at the extreme, esp in lumpy water.

( You even might ask what the reynolds number of flow across the chine, hard or soft, of an I14 going upwind in 10K true is.)

You could also swing the rudder assembly to leeward relative to the centerboard, which might clear the flow the rudder sees, putting the centerboard into more effective upwash from the rudder, which would accomplish the same thing as gybing the board, no? Which might automatically adjust circulation, speed, and aoa to the centerboard? What that would do to the T foil influence to the flow off the transome, might be er, interesting, but I14's rarely sail totally flat, so it might be that since the stern wave would be smaller, and more to leeward, when the hull is heeled a smidge, the T could be directly under the flow of the stern, so you could maybe therefore make it maybe a bit smaller, reducing wetted surface even more, which seems to be one of Paul's aims in the B6. Is a swinging rudder assembly allowed in the rules?

h34r:

Paul

edit- you might even introduce decollage (sp?) :lol:

You'd certainly have the benefits of stagger, instead of a tandem array.

 

[Amati]

as far as sections go, compare the l/d of this section:

http://www.worldofkrauss.com/foils/1539

 

[Amati]

to this one

http://www.worldofkrauss.com/foils/1734

Which one would make the better gybing or non gybing board?

oops- should have been this one-

http://www.worldofkrauss.com/foils/287

 

[TingTong]

i believe most boards are eppler 836, both bieker and copies thereof. thinner have been tried but go slower in chop, so the drag bucket is just about right. because, as many have suggested, the hull contributes very little lift, this works well for gybers and non- alike.

I think it's all gone a bit circular here, probably because everyone's arguing the same things from a different perspective. Here is where this wonderful journey has led me…

There are three main factors being discussed. Complexity. Reduced Drag. Increased power.

Barring the complexity (which is largely a personal issue, but may be responsible for some drag) it is clear that there is something to be found here - evidence is Trevor and his Pegasus testing, but plenty of anecdotal contributions too.

Reduced drag. Hollom argued that the hull's contribution to the foil system shouldn't be ignored, but this shouldn't lead to his conclusion that gybing boards are pointless, rather they might be a good way of seeking out the optimum balance.

Increased power. I think this would have been agreed if around a table, lubricated by alcohol. The gybing board rotates the hull relative to everything else. Sails are trimmed out to keep their angle of attack appropriate to the apparent wind, similarly foils remain unaware under the water and maintain course over ground and angle of attack (the hull rotation is their fault after all). By rotating forward relative to the hull, the heeling force is effectively reduced for any given forward/drive force. The maximum righting moment remains roughly constant as this is relative to the centre of buoyancy, which has rotated with the boat. Drive from sails can be increased (trimming in again, increasing camber), the boat can then go faster and / or higher, but there are cumulative benefits. At the very worst you might have to back off on the t-foil a fraction, as there is essentially a slight increase in the pitching forces.

So by my reckoning, both effects will be greatest when the leeway angles are greatest (you do not want the hull to be dragged to windward). However, the additional power is only available where there is enough wind to provide this (already flat wiring), although there is a slight "cheat" in being able to temporarily increase the angle of attack of the sails by sheeting in further in marginal conditions.

In an I14 this might be at 7-8 kts true wind speed at the earliest. Daggerboard lifting starts at maybe 10-12kts, so the effects are still good here, but maybe reducing. Board max up at maybe 16kts, which is where i suspect most boards are locked centreline.

I know there were a million other factors, but in Long Beach we were significantly higher and faster than the Pegasus boats until maybe 12kts and then the same or slower / upside down. I assumed gybing boards were therefore useless as i thought they would be more advantageous in lighter winds. They had a later transition due to flatter sails, but there was a stepwise matching, then overtaking that may just be attributable to the gybing boards.

What have i got wrong?

 

[Amati]

i believe most boards are eppler 836, both bieker and copies thereof. thinner have been tried but go slower in chop, so the drag bucket is just about right. because, as many have suggested, the hull contributes very little lift, this works well for gybers and non- alike.

I think it's all gone a bit circular here, probably because everyone's arguing the same things from a different perspective. Here is where this wonderful journey has led me…

There are three main factors being discussed. Complexity. Reduced Drag. Increased power.

Barring the complexity (which is largely a personal issue, but may be responsible for some drag) it is clear that there is something to be found here - evidence is Trevor and his Pegasus testing, but plenty of anecdotal contributions too.

Reduced drag. Hollom argued that the hull's contribution to the foil system shouldn't be ignored, but this shouldn't lead to his conclusion that gybing boards are pointless, rather they might be a good way of seeking out the optimum balance.

Increased power. I think this would have been agreed if around a table, lubricated by alcohol. The gybing board rotates the hull relative to everything else. Sails are trimmed out to keep their angle of attack appropriate to the apparent wind, similarly foils remain unaware under the water and maintain course over ground and angle of attack (the hull rotation is their fault after all). By rotating forward relative to the hull, the heeling force is effectively reduced for any given forward/drive force. The maximum righting moment remains roughly constant as this is relative to the centre of buoyancy, which has rotated with the boat. Drive from sails can be increased (trimming in again, increasing camber), the boat can then go faster and / or higher, but there are cumulative benefits. At the very worst you might have to back off on the t-foil a fraction, as there is essentially a slight increase in the pitching forces.

So by my reckoning, both effects will be greatest when the leeway angles are greatest (you do not want the hull to be dragged to windward). However, the additional power is only available where there is enough wind to provide this (already flat wiring), although there is a slight "cheat" in being able to temporarily increase the angle of attack of the sails by sheeting in further in marginal conditions.

In an I14 this might be at 7-8 kts true wind speed at the earliest. Daggerboard lifting starts at maybe 10-12kts, so the effects are still good here, but maybe reducing. Board max up at maybe 16kts, which is where i suspect most boards are locked centreline.

I know there were a million other factors, but in Long Beach we were significantly higher and faster than the Pegasus boats until maybe 12kts and then the same or slower / upside down. I assumed gybing boards were therefore useless as i thought they would be more advantageous in lighter winds. They had a later transition due to flatter sails, but there was a stepwise matching, then overtaking that may just be attributable to the gybing boards.

What have i got wrong?

How different was your boat speed through the water (I'm sorry, over the water :lol: ) in say wind speed 7-8K vs 12K?

Paul

BTW Eppler 836-

http://www.worldofkrauss.com/foils/119

edit: If you're interested, since the L/D stuff I have has differing re regimes, the formula Fredo's team used for re of their foils (in freshwater) was:

Velocity (in Knots) * Chord (in feet) * 137,000 = re

 

[TingTong]

it's 3 years since i sailed with a speedo, but i think we were disappointed with 10kts boatspeed in 12kts wind. 8 in 8.

 

[Amati]

it's 3 years since i sailed with a speedo, but i think we were disappointed with 10kts boatspeed in 12kts wind. 8 in 8.

well, that's re of 361,680 for 8K, and 452,100 for 10K, which is beyond the L/D stuff I have. I'll bet that's beyond criticality, but it doesn't hurt to ask.

I'll see if I can find L/D's for higher re's. As you can see, different re's can give different L/D, etc.

I'm assuming a 4" chord- .33 feet.

Still, it seems that if the hull is contributing to leeway resistance, you might be able to exploit that with a different section. Although that Eppler has a huge bucket. Go off the edges though.....

Paul

 

[TingTong]

it's 3 years since i sailed with a speedo, but i think we were disappointed with 10kts boatspeed in 12kts wind. 8 in 8.

well, that's re of 361,680 for 8K, and 452,100 for 10K, which is beyond the L/D stuff I have. I'll bet that's beyond criticality, but it doesn't hurt to ask.

I'll se if I can find L/D's for higher re's. As you can see, different re's can give different L/D, etc.

I'm assuming a 4" chord- .33 feet.

Still, it seems that if the hull is contributing to leeway resistance, you might be able to exploit that with a different section. Although that Eppler has a huge bucket. Go off the edges though.....

Paul

max chord is typically 230mm. 9". but there is taper in to the tip, obviously.

it still remains pretty close to the limits of what most programmes can deal with. pretty tricky to find sections that can cope, as the range nicely crosses a weird threshold that i do not understand

 

[Amati]

So what's the chord at the tip, or say 2/3 the way down? 14's don't exactly hug the water at 12K TW in a chop.

I really dig the flow regimes between 20,000 and 190,000 re.

One of my favorites:

http://www.worldofkrauss.com/foils/1837 :blink:

It still seems that if the hull is contributing to lateral resistance, you migh be able to exploit that with a different section. The Bucket on that Eppler is pretty wide though. Fall off the edge, and There Be Dragons, though....

Glenrothes, anyone?

Paul

 

[TingTong]

So what's the chord at the tip, or say 2/3 the way down? 14's don't exactly hug the water at 12K TW in a chop.

I really dig the flow regimes between 20,000 and 190,000 re.

One of my favorites:

http://www.worldofkr....com/foils/1837 :blink:

It still seems that if the hull is contributing to lateral resistance, you migh be able to exploit that with a different section. The Bucket on that Eppler is pretty wide though. Fall off the edge, and There Be Dragons, though....

Glenrothes, anyone?

Paul

assume a rectangular top 950mm (but 250 is inside the hull), bottom 900 tapers to approx 4", but in a slightly hypereliptical fashion.

tfoil stops most jumping and keeps the bow down, so board rarely comes clear. typically raised by 400mm progressively between 10kts tws and 16-18kts, maybe 600mm if really flat water

 

[Mal Smith]

I've been following this thread for a while with some amusement, and now feel the need to throw my own opinion into the ring, just to confuse things further, for which I apologise in advance.

The benefit of a gybing centreboard is singular. It reduces hull drag by allowing the heading of the hull to be aligned with the direction of travel. That is all it does. The extra loading on the gybing board (due to loss of any contribution to leeway resistance) is small enough that any change in angle of attack on the centreboard is negligable (at least, for flat bottomed wide planing hulls). There will, or should be a small change in the width of the jib slot, but whether that is a good or bad thing is indeterminate. There is no magic changing of force couples.

In order for there to be a benefit from the gybing centreboard:

(a) the reduction in hull drag must be greater than the added drag due to the gybing board system at the hull exit.

( B) assuming (a) is true, the added time gain must be greater than any time loss due to fiddling with, or tripping over the gybing centreboard mecahanism.

Thats all there is too it. No magic. Sorry.

Interestingly, wide flat bottomed planing hulls which normally contibute little to leeway resistance will also benefit least from a gybing centreboard. The added resistance due to leeway is mainly induced drag due to the lift produced by the hull. A gybing centreboard is probably of more benefit on a deep narrow hull, and this is only because a high aspect ratio centreboard is more efficient than a low aspect ratio hull at resisting leeway. If a gybing centreboard is used on a deep, narrow hull that does normally contribute significantly to leeway resisiance, the area of the centreboard may need to increased to compensate for the extra loading. As stated above, I doubt if this is the case with the i14.

 

[Amati]

This where my head explodes, but I suspect DH's head reached critical mass long ago..

If a hull resists leeway, won't it resist uh, weatherway?

Paul

 

[Mal Smith]

This where my head explodes, but I suspect DH's head reached critical mass long ago..

If a hull resists leeway, won't it resist uh, weatherway?

Paul

Yes, it will. You wouldn't want to to be dragging the hull to windward (i.e. the hull heading below the course made good) or you'll just be adding the induced drag back on again, and without any benefit of the hull contributing to leeway resistance. In fact it would be doing the opposite and adding more load to the centreboard. In general, you would attempt to adjust the centreboard gybing angle to minimse the hull leeway angle to close to zero for best results. As stated by others, the optimum gybe angle will vary depending on boat speed etc.

 

[Trevor B]

The benefit of a gybing centreboard is singular. It reduces hull drag by allowing the heading of the hull to be aligned with the direction of travel. That is all it does. The extra loading on the gybing board (due to loss of any contribution to leeway resistance) is small enough that any change in angle of attack on the centreboard is negligable (at least, for flat bottomed wide planing hulls). There will, or should be a small change in the width of the jib slot, but whether that is a good or bad thing is indeterminate. There is no magic changing of force couples.

In order for there to be a benefit from the gybing centreboard:

(a) the reduction in hull drag must be greater than the added drag due to the gybing board system at the hull exit.

( B) assuming (a) is true, the added time gain must be greater than any time loss due to fiddling with, or tripping over the gybing centreboard mecahanism.

Thats all there is too it. No magic. Sorry.

Agreed.

Small reduction in drag from the hull, plus a tiny gain from being able to oversheet in the light. Worth it in my book.

George, my memories of Long Beach are slightly different than yours, as to be expected.

We obviously overcooked the flat-ness of of sails by a mile mainly because we thought it would be windier, duh, and we were very worried about the big Aussie teams.

I do remember the first race that had what we thought would be "normal" LB weather, starting in 8, finishing in 17. Jibing boards with flat sails finished 1,2,3- it could have been the fourth race.

You, and your sister, had great pace in light air without jibers so maybe it was just fluff....hard to know, harder to prove with about a billion variables to try and freeze out of the equation.

 

[SimonN]

Seeing Trevor pop up reminded me of the first time I ever sailed a boat with a gybing board, a 505 I am sure Trevor will remember weel, "Grace" (Benji and Tucker's boat). After I bought the boat, I didn't get a chance to sail it until our first event. We started about a third up from the pin on a big, left hand track against the tide in about 8 knots. Not only did we get to the layline first, but by then we were the most windward boat (and nobody tacked off!). To say I had a big grin on my face was an understatement. Some time later I was able to do some real testing and if you locked the board off so it wouldn't gybe, it was still a little quicker but couldn't climb compared with the other boats.

That generation of Lindsay and Tuttle boats were pretty extraordinary. Even today, you would be hard pressed to get more beautifully made foils, which I am sure was part of the performance gain. In those days, I didn't ask a lot of questions and just accepted we were fast. I so wasted that boatspeed, but it was a thrill and, most relevent to this thread, it got me hooked on gybing boards.

I have followed everything above and can see both sides of the argument. They all make sense so it leaves only one thing, on the water proof. For me, it's conclusive. I am surprised at TingTong's experience with it, but then it seems to me that the problem wasn't gybing boards as such but the particular execution. Maybe he will get a chance to see how a well executed system really works and will then become a convert!

 

[mark1234]

BB: Because an I-14 has a net movement to leeward. That means that it is being accellerated to leeward, and thus the force vector to leeward must EXCEED the force vector to weather

Disagree. There must be an excess to start the movement. For that movement to remain constant in rate, there must be a balance. An acceleration implies an ever increasing rate.

TingTong: if a moth had a net force to windward, would it not accelerate in that direction proportionally to the net magnitude/mass.

similarly a standard i14 with it's force to leeward would accelerate to leeward, presumably until relativistic effects limited it at the speed of light?

Precisely - what actually happens is that it initially accelerates to leeward, which slightly reduces the sideways force from the rig (imperceptibly unless you pull the board out), and increases the lift from the foil, until all settles into an equilibrium (of sorts)

BalticBandit:

And it does.

I'll see about drawing a picture with labels - which should help.

And Moths are NOT "apples and oranges" - the fact that moths can generate enough lift force to weather to accelerate to weather does not mean that I-14s boards are not generating lift to weather. It just means that the net balance of forces is different, but the forces involved are the same (if you exclude the vertical elevation lift forces).

Apples and oranges because a conventional 'vertical' symmetric foil can only generate lift through having an AOA. That AOA is caused by sideslip, vector motion of the water, whatever you care to call it. If you decrease slip, you reduce AOA, and hence reduce lift, restoring slip. Disregarding gybers, for the boat to move to weather, the foil would have to be at a negative AOA, hence generating negative lift, so where on earth does the weather force come from?

If you gybe the foil, then the hull might move to windward, but not relative to the foil chord line.

Compare that to the moth:

Some quick fag packet maths - I weigh 80kg, the boat 40, and I'm sailing at a (conservative) 15deg windward heel - that gives me 32kg of windward force (about 20% of the total lift) from the horizontal, disregarding the vertical.

That is independent of the lateral course of the boat, or AOA of the vertical. The lift force is automagically controlled to that which is necessary via the ride height mechanism, and the more we heel to windward, the more contribution we get (42kg at 20deg, and so on). The vertical component remains at some 120kg via the flap etc, (up to the point the 'horizontal' stalls).

That is why the moth can move to windward - it's not relying on AOA from a 'vertical' symmetric foil. As I mentioned before, the effect can be so pronounced that the vertical part stalls in the negative sense - with enough windward slip angle (AOA on the vertical) to have the flow separate from the leeward side - usually a fraction of a second before I disappear headfirst into the water...

 

[TeamFugu]

Keep in mind that the big reason you lock the blade off when going down wind has more to do with control than lift. Try sailing a skiff down wind when the side force is dynamic from constantly steering to keep the boat on her feet. Which side the pressure comes from changes and when you have the giber changing directions that are often opposite the direction you want to go, it sure increases the swimming lessons and pucker factor.

I had one on my Swift for a while. Though I thought it was working for the most part, I was having issues getting a gasket to work or stay stuck on the hull. Poor design on my part but I'd probably try to do it again with a cassette if I were to build another boat. I can also tell you it was rather interesting to be out on the wire fighting the gusts and the gibing board on a three sail reach. I didn't think a locking mechanism was that important until I gave it a go. Talk about pucker factor.

 

[Chris Maas]

Keep in mind that the big reason you lock the blade off when going down wind has more to do with control than lift. Try sailing a skiff down wind when the side force is dynamic from constantly steering to keep the boat on her feet. Which side the pressure comes from changes and when you have the giber changing directions that are often opposite the direction you want to go, it sure increases the swimming lessons and pucker factor.

I had one on my Swift for a while. Though I thought it was working for the most part, I was having issues getting a gasket to work or stay stuck on the hull. Poor design on my part but I'd probably try to do it again with a cassette if I were to build another boat. I can also tell you it was rather interesting to be out on the wire fighting the gusts and the gibing board on a three sail reach. I didn't think a locking mechanism was that important until I gave it a go. Talk about pucker factor.

Exactly! And the gyber can make windy pre start maneuvers exciting if you find you need to bear away and gybe. I strongly recomend that my fellow IC sailors not install one. Besides, I'm begining to conclude that it only works for those who believe in it. For instance: I am convinced that I can get better height with the gyber on, and yet I don't see any evidence in the previous posts to support that possibility. Admittedly my brain isn't up to challenge of deciphering the more jargony ones.

 

[JimC]

From a "normal boat" next to you, you would be "nose down" on them but tracking the same CMG - and you would have a touch more speed. And in the Lulls you would maintain point when they have to "nose down".

And that is EXACTLY what you see when you sail a non-gyber next to a gyber.

I cannot see extra speed (with my emphasis) at all. If you make the assumption that hull drag doesn't vary much with attitude and (its only a medium sized assumption since we know that induced drag must be tiny in proportion to total induced, wetted surface drag is identical and wave drag must be very very similar) then if the foils are at the optimum angle to the wind in both boats, and the sails are at the optimum angle to the wind in both boats then all the dynamics are essentially the same. So how can the behaviour in the lulls be significantly different? In both boats the power available will decrease by the same amount, the heeling force will change by the same amount etc etc etc. I agree that in the gybing board boat the angle of the centre line is always nose down by the gybe angle of the board, but what I don't see is why that should make a blind bit of difference to anything other than the jib/main relationship. Which goes right back to my original proposition that the jib/main relationship, which we all know to be crucial to the upwind tuning, is the only factor that appears to change enough to have a significant effect.