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Busting 50 - Non-Cavitating Foils


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#1 Guitar

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Posted 10 October 2013 - 04:34 PM

So rumor has it the 72's may be coming back. What design for the foils will allow them to bust the 50 knot barrier / wind limits?



#2 Basiliscus

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Posted 10 October 2013 - 09:22 PM

50 kt with subcavitating foils is tough.  The foils will need to be thin and can only operate over a narrow range of conditions.  

 

Take the Eppler E817 hydrofoil section, for example.  It's somewhat thinner than the sections used on the AC72s, and it has a wide range of conditions at which it can operate without cavitation.  It's shown in blue in the chart below.  To go much above 40 kt, it needs to be thinner.  The dashed lines show the E817 scaled down to 7% thick and 5% thick.  You'd need it to be to down around 7% to 8% thick if you wanted to operate at 50 kt.  The foil loading will need to be 800 - 1200 lb/ft^2, and at that loading the foils will be cavitating at the leading edge if you try to fly below 25 kt because of the pressure peak that forms there at high lift.  You might be able to use bigger foils, with a loading more like 500 - 600 lb/ft^2, at the risk of some cavitation on the underside of the leading edge at high speed.  But it would get the low end of the foiling regime closer to 20 kt of boat speed without cavitation.

 

If you have a thin foil, it's hard to make it strong enough to carry the load.  That drives you to short spans to reduce the bending moment and to get more chord for more physical thickness.  And the short span means higher induced drag, especially for lower speed situations like marginal foiling winds or foiling upwind.  So you have to sacrifice a lot to get 50 kt.

Cavitation2.gif

 

(In the chart above, the Y axis is the maximum local speed encountered anywhere on the section, divided by the free-stream speed.  The horizontal lines show the maximum speed ratio that can be tolerated at a given boat speed before cavitation bubbles start to form.  If you know the boat speed (horizontal lines) and you know the lift coefficient (X axis), you know the foil loading.  So the curved diagonal lines represent the foil loading that corresponds to the the cavitation thresholds.  The colored lines are the maximum velocity ratios for individual sections.  If the line corresponding to a given section is below the threshold, then cavitation cannot occur at that condition.  The steep increase in local speeds that forms the sides of the cavitation buckets is due to the formation of a leading edge suction peak when the angle of attack gets out of the design range.)



#3 ~Stingray~

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Posted 10 October 2013 - 10:28 PM

Is it too big a challenge to design and build adjustable-thickness foils, with loads that high?

#4 Terry Hollis

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Posted 10 October 2013 - 10:37 PM

50 kt with subcavitating foils is tough.  The foils will need to be thin and can only operate over a narrow range of conditions.  

 

Take the Eppler E817 hydrofoil section, for example.  It's somewhat thinner than the sections used on the AC72s, and it has a wide range of conditions at which it can operate without cavitation.  It's shown in blue in the chart below.  To go much above 40 kt, it needs to be thinner.  The dashed lines show the E817 scaled down to 7% thick and 5% thick.  You'd need it to be to down around 7% to 8% thick if you wanted to operate at 50 kt.  The foil loading will need to be 800 - 1200 lb/ft^2, and at that loading the foils will be cavitating at the leading edge if you try to fly below 25 kt because of the pressure peak that forms there at high lift.  You might be able to use bigger foils, with a loading more like 500 - 600 lb/ft^2, at the risk of some cavitation on the underside of the leading edge at high speed.  But it would get the low end of the foiling regime closer to 20 kt of boat speed without cavitation.

 

If you have a thin foil, it's hard to make it strong enough to carry the load.  That drives you to short spans to reduce the bending moment and to get more chord for more physical thickness.  And the short span means higher induced drag, especially for lower speed situations like marginal foiling winds or foiling upwind.  So you have to sacrifice a lot to get 50 kt.

Cavitation2.gif

 

(In the chart above, the Y axis is the maximum local speed encountered anywhere on the section, divided by the free-stream speed.  The horizontal lines show the maximum speed ratio that can be tolerated at a given boat speed before cavitation bubbles start to form.  If you know the boat speed (horizontal lines) and you know the lift coefficient (X axis), you know the foil loading.  So the curved diagonal lines represent the foil loading that corresponds to the the cavitation thresholds.  The colored lines are the maximum velocity ratios for individual sections.  If the line corresponding to a given section is below the threshold, then cavitation cannot occur at that condition.  The steep increase in local speeds that forms the sides of the cavitation buckets is due to the formation of a leading edge suction peak when the angle of attack gets out of the design range.)

 

Maybe a bi-foil arrangement so that the top foil lifts out of the water at 40 knots leaving the bottom foil to cover the 40-60 knot range .



#5 Bulbhunter

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Posted 10 October 2013 - 10:47 PM

50 kt with subcavitating foils is tough.  The foils will need to be thin and can only operate over a narrow range of conditions.  

 

Take the Eppler E817 hydrofoil section, for example.  It's somewhat thinner than the sections used on the AC72s, and it has a wide range of conditions at which it can operate without cavitation.  It's shown in blue in the chart below.  To go much above 40 kt, it needs to be thinner.  The dashed lines show the E817 scaled down to 7% thick and 5% thick.  You'd need it to be to down around 7% to 8% thick if you wanted to operate at 50 kt.  The foil loading will need to be 800 - 1200 lb/ft^2, and at that loading the foils will be cavitating at the leading edge if you try to fly below 25 kt because of the pressure peak that forms there at high lift.  You might be able to use bigger foils, with a loading more like 500 - 600 lb/ft^2, at the risk of some cavitation on the underside of the leading edge at high speed.  But it would get the low end of the foiling regime closer to 20 kt of boat speed without cavitation.

 

If you have a thin foil, it's hard to make it strong enough to carry the load.  That drives you to short spans to reduce the bending moment and to get more chord for more physical thickness.  And the short span means higher induced drag, especially for lower speed situations like marginal foiling winds or foiling upwind.  So you have to sacrifice a lot to get 50 kt.

Cavitation2.gif

 

(In the chart above, the Y axis is the maximum local speed encountered anywhere on the section, divided by the free-stream speed.  The horizontal lines show the maximum speed ratio that can be tolerated at a given boat speed before cavitation bubbles start to form.  If you know the boat speed (horizontal lines) and you know the lift coefficient (X axis), you know the foil loading.  So the curved diagonal lines represent the foil loading that corresponds to the the cavitation thresholds.  The colored lines are the maximum velocity ratios for individual sections.  If the line corresponding to a given section is below the threshold, then cavitation cannot occur at that condition.  The steep increase in local speeds that forms the sides of the cavitation buckets is due to the formation of a leading edge suction peak when the angle of attack gets out of the design range.)

 

Maybe a bi-foil arrangement so that the top foil lifts out of the water at 40 knots leaving the bottom foil to cover the 40-60 knot range .

Very smart man! LOL

Yes the issue with speed across the foil reaching a max point where the foil shape must change or your speed limit has been maxed out physically due to physics of the foil would suggest either some type of foil shape change capability while on the fly or a rotation to another foil at a certain point ie speed etc.



#6 aldo

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Posted 10 October 2013 - 11:17 PM

Would an Adjustable oblique angle rudder foil be too complex?



#7 floater

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Posted 11 October 2013 - 01:18 AM

If we assume no shape changing or multiple foil packages (hey - were trying to save money here!). Then I guess the game remains about what we saw during the final.

Downwind - max velocities similar between boats.

Upwind - it's game on as to which boat can foil the fastest.

Okay to assume we aren't going to be hitting cavitation limits upwind?

#8 P Flados

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Posted 11 October 2013 - 02:57 AM

Any boat designed for racing around the cans can not give up good off max speed performance.  

 

Good general foil performance does not really allow much in the way of larger surface area or super thin sections.  And do not even think about super-cavitating.

 

I have posted the following at BDN:

  • Fast with the AC 72 was approaching (or may have reached) serious cavitation challenges.
  • If you want anything resembling traditional in shore racing, you need the handling that goes with traditional foil sections that do not deal with cavitation well.
  • As much as I pulled for "faster is better", future AC will probably be better off trying to limit speeds to stay away from all of the agony of trying to overcome / work around cavitation (as Paul Larson how fun this can be).

Speed limiting rules seem weird, but keeping Bmax to say 75% of LOA  would not be so terrible (I think S Clark should get credit for pointing out advantages for reducing width).   



#9 skins

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Posted 11 October 2013 - 03:33 AM

The next gen polymers will be able to absorb (or interact with!) the bubble collapse so that the only challenge will be to design the right shapes to work in two phase flow.

I think the really tech pump guys are all over this...

#10 fireball

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Posted 11 October 2013 - 03:38 AM

Any boat designed for racing around the cans can not give up good off max speed performance.  

 

Good general foil performance does not really allow much in the way of larger surface area or super thin sections.  And do not even think about super-cavitating.

 

I have posted the following at BDN:

  • Fast with the AC 72 was approaching (or may have reached) serious cavitation challenges.
  • If you want anything resembling traditional in shore racing, you need the handling that goes with traditional foil sections that do not deal with cavitation well.
  • As much as I pulled for "faster is better", future AC will probably be better off trying to limit speeds to stay away from all of the agony of trying to overcome / work around cavitation (as Paul Larson how fun this can be).

Speed limiting rules seem weird, but keeping Bmax to say 75% of LOA  would not be so terrible (I think S Clark should get credit for pointing out advantages for reducing width).   

 

They're 22m long and 14m wide now, so they're already well under 75%. Maybe reduce it to 50%?



#11 tls

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Posted 11 October 2013 - 04:13 AM

Good general foil performance does not really allow much in the way of larger surface area or super thin sections.  And do not even think about super-cavitating.

 

Well I don't think there can be much harm in thinking about super-cavitating foils.  The traditional challenge is that super-cavitating foils are really much less efficient at lower speeds (where lower means <40 knots) than normal sections.  I could imagine a future in which super-cavitating foils are used when reaching and possibly running, while a separate board with low drag foils is used upwind. As a practical matter, the current boats are only fast enough to really run into cavitation limits on the lifting foils when reaching.  If they are running greater than 45 knots downwind they are probably running hotter than their polars suggest are optimal (although that might change if the wind limit was 30 knots). 

 

Ultimately I suspect they will use separate upwind and downwind foils if the rules allow it. Not to solve cavitation but because the optimal longitudinal position of the daggerboard changes. 



#12 skins

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Posted 11 October 2013 - 04:37 AM

The next gen polymers will be able to absorb (or interact with!) the bubble collapse so that the only challenge will be to design the right shapes to work in two phase flow.
I think the really tech pump guys are all over this...


Sorry to reply to my own post...

I think bourbonight already commented about harnessing the power of cavitation around here somewhere...it's next level enough.

Imagine what you were thinking in early 2011 compared to the AC34 Match...

Big moves will be made. Polymer science will be huge

#13 GauchoGreg

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Posted 11 October 2013 - 02:40 PM

If we assume no shape changing or multiple foil packages (hey - were trying to save money here!). Then I guess the game remains about what we saw during the final.

Downwind - max velocities similar between boats.

Upwind - it's game on as to which boat can foil the fastest.

Okay to assume we aren't going to be hitting cavitation limits upwind?

 

And if this is the case, it is actually more likely than not that lower-budget teams might be able to compete with the higher budget teams, as all teams will have similar upper speed barriers.  Of course, this assumes that a team does not break through the barrier with some foil innovation . . .



#14 floater

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Posted 11 October 2013 - 04:51 PM


If we assume no shape changing or multiple foil packages (hey - were trying to save money here!). Then I guess the game remains about what we saw during the final.

Downwind - max velocities similar between boats.

Upwind - it's game on as to which boat can foil the fastest.

Okay to assume we aren't going to be hitting cavitation limits upwind?

 
And if this is the case, it is actually more likely than not that lower-budget teams might be able to compete with the higher budget teams, as all teams will have similar upper speed barriers.  Of course, this assumes that a team does not break through the barrier with some foil innovation . . .
I'm still a bit flummoxed that OR and ETNZ boats so different - yet performance so similar.

#15 Bulbhunter

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Posted 11 October 2013 - 05:00 PM

How about the secret sauce pumped through surface membrane on the foil to alter the nature of the water just enough to enable faster flow before you get flow speed that boil's the water across the surface of the foil?



#16 GauchoGreg

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Posted 11 October 2013 - 05:39 PM

 


If we assume no shape changing or multiple foil packages (hey - were trying to save money here!). Then I guess the game remains about what we saw during the final.

Downwind - max velocities similar between boats.

Upwind - it's game on as to which boat can foil the fastest.

Okay to assume we aren't going to be hitting cavitation limits upwind?

 
And if this is the case, it is actually more likely than not that lower-budget teams might be able to compete with the higher budget teams, as all teams will have similar upper speed barriers.  Of course, this assumes that a team does not break through the barrier with some foil innovation . . .
I'm still a bit flummoxed that OR and ETNZ boats so different - yet performance so similar.

 

On several of the 1st leg reaches, and particularly on that last downwind beat in Race 19, I was thinking how amazing it was that two totally separate design houses came up with significantly different boats over the past few years, and we got to see them racing within spitting distance of each other, and going at identical speed . . . and I was thinking how lucky it was for us to be enjoying it, and how I wished it did not have to end . . . no matter who won.  Maybe it really isn't as much about luck, but about plain old physics.  If that is the case, maybe we all get what we want, boats going about as fast as modern design and technology permits, AND great close-up competitive racing.



#17 Basiliscus

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Posted 11 October 2013 - 05:41 PM

The next gen polymers will be able to absorb (or interact with!) the bubble collapse so that the only challenge will be to design the right shapes to work in two phase flow.

I think the really tech pump guys are all over this...

OTUSA were repainting their rudders every night because of cavitation damage..  But it's not the cavitation damage that's the problem - it's the drag.  

 

 

Well I don't think there can be much harm in thinking about super-cavitating foils.  The traditional challenge is that super-cavitating foils are really much less efficient at lower speeds (where lower means <40 knots) than normal sections.  ...

 

The high-speed drag of supercavitating sections makes them a non-starter as well.  Imagine sailing around with the board stalled.  That's basically what a supercavitating section is doing.  The difference is the separated region is filled with vapor instead of recirculating liquid.

 

There's a much simpler solution.  The section just needs to think it's going 40 kt when the boat is going 50 kt.



#18 mad

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Posted 11 October 2013 - 05:49 PM

You could always just ask Paul Larsen and the design/build guys, bet they have an idea or 2!

#19 Xlot

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Posted 11 October 2013 - 06:24 PM

But it's not the cavitation damage that's the problem - it's the drag.  


Where does drag originate?
 

There's a much simpler solution.  The section just needs to think it's going 40 kt when the boat is going 50 kt.


Swept-back foils, I think you were saying. And what about supercritical sections?

#20 Basiliscus

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Posted 11 October 2013 - 07:33 PM

But it's not the cavitation damage that's the problem - it's the drag.  


Where does drag originate?
 

>There's a much simpler solution.  The section just needs to think it's going 40 kt when the boat is going 50 kt.


Swept-back foils, I think you were saying. And what about supercritical sections?

 

 

Yes, sweep is the classic solution when there's a limit to the local velocity.  Simple sweep theory would imply a sweep angle of 37 deg would make a foil behave at 50 kt like it was at 40 kt. Of course, things are never quite that simple!

 

The sections that are being used now are similar to supercritical airfoils.  They are both driven by the same requirement, to stay under a fixed local velocity, which drives them to flat "rooftop" pressure distributions and similar shapes.

 

The board/wing junction is the real problem area.



#21 tls

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Posted 11 October 2013 - 08:10 PM

The high-speed drag of supercavitating sections makes them a non-starter as well.  Imagine sailing around with the board stalled.  That's basically what a supercavitating section is doing.  The difference is the separated region is filled with vapor instead of recirculating liquid.

 

I agree that supercavitating foils are low L/D but they are not lower than normal sections in the velocity*lift coefficient regime in which cavitation occurs. It is also clear that other types of drag (induced and spray) are huge contributors to the overall drag, and so section drag is not always the limiting factor. 

 

At any rate, I think the role that cavitation will play in the next AC is vastly overstated unless they add more reaching legs.  As I understand the cavitation problem on Oracle, it was due to a rudder (not rudder lifting foil) requiring high too high of a Cl when undergoing maneuvers with a partially submerged rudder. They simply did not have enough submerged area to control the boat without going to high AoA, causing both cavitation and ventilation.  This particular cavitation problem might be solved just by allowing more controls for the rudder lifting foils to keep the actual rudder foil submerged. 



#22 P Flados

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Posted 11 October 2013 - 08:11 PM

The only reasonable solution that I see would be the 4 dagger board foils per boat choice.  

 

One set of super-cavitating daggers that would only be used when the "normal" ones reach their limit.  

 

I am just not sure that this would be such a great idea for AC competition at this point.

 

It would definitely devolve into a "the best foils win" kind of thing where more money would be a huge issue.  

 

Oh, and yes a British challenge would seem to have a potential head start if they want to lean on Paul & his team. 



#23 Keith

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Posted 11 October 2013 - 10:11 PM

Paging the Hydroptere guys, paging the Hydroptere guys......



#24 floater

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Posted 12 October 2013 - 01:03 AM


 



If we assume no shape changing or multiple foil packages (hey - were trying to save money here!). Then I guess the game remains about what we saw during the final.

Downwind - max velocities similar between boats.

Upwind - it's game on as to which boat can foil the fastest.

Okay to assume we aren't going to be hitting cavitation limits upwind?

 
And if this is the case, it is actually more likely than not that lower-budget teams might be able to compete with the higher budget teams, as all teams will have similar upper speed barriers.  Of course, this assumes that a team does not break through the barrier with some foil innovation . . .
I'm still a bit flummoxed that OR and ETNZ boats so different - yet performance so similar.
 
On several of the 1st leg reaches, and particularly on that last downwind beat in Race 19, I was thinking how amazing it was that two totally separate design houses came up with significantly different boats over the past few years, and we got to see them racing within spitting distance of each other, and going at identical speed . . . and I was thinking how lucky it was for us to be enjoying it, and how I wished it did not have to end . . . no matter who won.  Maybe it really isn't as much about luck, but about plain old physics.  If that is the case, maybe we all get what we want, boats going about as fast as modern design and technology permits, AND great close-up competitive racing.
agree with that - the physical limitations of the design rule dictated largely similar power from the wing - and drag from the foils.

In that sense, the box rule worked perfectly - both boats within the box ended up with similar performance simply due to the physics dictated by the dimensions and shape of their equipment.

My intuition says - makes perfect sense.

Problem is - I've learned not to trust my intuition when it comes to these beasts!

#25 RMac

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Posted 12 October 2013 - 01:48 AM

Will cavitation really be an issue if the boats stay as fast multis?  It seems like all the solutions to cavitation require a considerable compromise in either low speed (like 25kts) performance or extra gear and weight.

 

I'd imagine that if the designers were to have another go with just OR2 and NZA, they probably wouldn't be focusing too finely on top end speed, and would instead be working hard at upwind speed, improving systems for manouvers, and getting the boats foiling earlier in the wind range so we'd never have to see another code zero again. 

 

The leg that I think cavitation would be the biggest hindrance would the first reaching leg.  If you're hitting speeds on the downwind where cavitation is an issue, why wouldn't you just push the angles deeper to go same or slower speed but closer to mark?



#26 Blackman

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Posted 13 October 2013 - 08:04 PM

First, they have to delete all control limitation, if they want foiling, as OR put many limits to be sure it's not possible to foil within the rule, and missed the goal. if You can have trim tabs, then you can make everything, with big lift at low speed, and adjustement of the tab at high speed...  The game was really complicated by the rule limitating the controls, open the rule, and you'll see miracles!



#27 Bill R

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Posted 14 October 2013 - 08:32 AM

is this 50 knot barrier similar to the sound barrier? Aircraft have all sorts issues getting past the speed of sound, would foils in water have to get round the same sort of problems but at a much lower speed? Is there such thing as a sonic boom for a foiling boat? or are we in unknown territory?

#28 Francis Vaughan

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Posted 14 October 2013 - 09:14 AM

Speed of sound in water is about 1500 m/s, which is well over 4 times that of sound in air. We won't be seeing sonic booms in water whilst sailing anytime soon. The problem with aircraft and the speed of sound is that the air doesn't get the message that you are coming, and doesn't have time to get out of the way.  Cavitation is sort of the opposite thing.  The water has got out of your way so quickly that it has actually pulled a vacuum in the gap between itself and you. Cavitation is because water has inertia.

 

Water running past a whole lot of nothing doesn't behave in the same way as water running past a solid object, and a lot of the expectations about how things will work start to fail. Once the water ceases to flow along exactly the path you want over a foil you are going to be losing lift, and generating more drag, and generally losing out.  Super cavitating foils work out how to harness the flow in the presence of cavitation to still maintain a controlled flow.  This seems to be done by deliberately inducing cavitation with a step in the foil close to the leading edge.   So rather than uncontrolled cavitation you have cavitation in a known place, and you can start to tune the system to gain back what you need.  The down-side is that this step in the foil shape is not exactly fast when you are not cavitating. 

 

There is a lot of knowledge about fluid flow, and people that design turbo-pumps deal with cavitation all the time.  However anything that involves turbulent flow is still much of a black art.  We are not exactly in unknown territory, but the lights are dim and the terrain uneven.

 

I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic.

- Horace Lamb -



#29 Bill R

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Posted 14 October 2013 - 09:36 AM

thanks for that, I like your last sentence about the lights being dim, Obviously there is a lot to learn and experiment with to get the boats going past 50 knots.

#30 The Advocate

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Posted 14 October 2013 - 09:51 AM

It's not the 50 knots that is being focused on that's the issue. Plenty of foils can do that in water.

 

It's about having a foil that will be efficient from 1 knot to over fifty. Fifty is not a barrier, the ability to be efficient at all speeds, is.



#31 Steam Flyer

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Posted 14 October 2013 - 12:19 PM

It's not the 50 knots that is being focused on that's the issue. Plenty of foils can do that in water.

 

It's about having a foil that will be efficient from 1 knot to over fifty. Fifty is not a barrier, the ability to be efficient at all speeds, is.

 

 

Zackly. If the boats were to start sailing at 45 kt it wouldn't be an issue; given todays engineering art. I am sure that there will be foils that are efficient over a tremendous range of speed but right now the closest thing we can do is put on flaps & slats and those were forbidden by the AC rules.

 

Shucks TURBINIA had cavitating props back in 1897 so it's not exactly a cutting edge field. But as FV said it's difficult terrain

 

FB- Doug



#32 Basiliscus

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Posted 14 October 2013 - 03:45 PM

is this 50 knot barrier similar to the sound barrier? Aircraft have all sorts issues getting past the speed of sound, would foils in water have to get round the same sort of problems but at a much lower speed? Is there such thing as a sonic boom for a foiling boat? or are we in unknown territory?

The two barriers are similar in that the flow physics exhibits a change when the local velocity exceeds a threshold value.  The nature of the change is completely different for the two cases.  But the impact on performance is similar in that there's a drag rise.

 

The cure is also similar in that one simply wants to avoid exceeding the threshold.  So that drives the design of the section to be based on pressure distributions with long runs of constant velocity that is just below the threshold.



#33 Lohring Miller

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Posted 14 October 2013 - 07:04 PM

Base vented foils are in between fully wetted and supercavitating foils.  They sacrifice some lift to drag for increased cavitation resistance. 

 

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Attached File  Base vented foil.jpg   23.52K   15 downloads

 

Attached File  Wetted vs Vented foils.jpg   107.84K   9 downloads



#34 The Advocate

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Posted 14 October 2013 - 09:31 PM


 

The cure is also similar in that one simply wants to avoid exceeding the threshold.

Perhaps, but really, where is the fun in that? :P



#35 GauchoGreg

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Posted 24 October 2013 - 05:37 PM

I was just thinking of Banque Populaire V.  It hits speeds above 45knts.  I believe in its transat record, it hit 47-48knts.  Different kind of foils, but it would seem to deal with similar issues.  I know, with the nature of the foils being lift-assist, not a big worry about the boat crashing down, but are its foils getting damaged by cavitation up at those speeds?



#36 bruno

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Posted 24 October 2013 - 05:59 PM

I am just guessing but is not cavitation a function of lift in some way? So if one is not getting a lot of lift then one's propensity for cavitation is reduced even at greater speeds? Is that not why miss bud is a planing body rather than a foiler?

#37 GauchoGreg

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Posted 24 October 2013 - 10:26 PM

I am just guessing but is not cavitation a function of lift in some way? So if one is not getting a lot of lift then one's propensity for cavitation is reduced even at greater speeds? Is that not why miss bud is a planing body rather than a foiler?

 

If you are responding to me regarding BPV, while not full-lift foils, the "C" foils are still providing a ton of lift, just not enough to lift that huge boat out of the water all the way.



#38 bruno

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Posted 25 October 2013 - 02:50 AM

Greg, sorry, yes, but I misspoke, change lift to pressure. I think that may be what seperates fractional foils from 100%, but happy to be wrong here.

#39 bruno

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Posted 25 October 2013 - 03:00 AM

Greg, sorry, yes, but I misspoke, change lift to pressure. I think that may be what seperates fractional foils from 100%, but happy to be wrong here.
http://www.newscient...ml#.UmnePDK9KSM

#40 Basiliscus

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Posted 26 October 2013 - 06:39 PM

I am just guessing but is not cavitation a function of lift in some way? So if one is not getting a lot of lift then one's propensity for cavitation is reduced even at greater speeds? 

See post #2.

 

Is that not why miss bud is a planing body rather than a foiler?

Miss Budweiser's being a planing boat probably has more to do with the H1 Class Rules (http://www.h1unlimit...v1.1_042812.pdf).

"A. DESIGN CONSTRAINTS 

1. Any unlimited-class hydroplane shall be designed to function as a planning hull; 
i.e. a hull supported by a combination of hydrodynamic and aerodynamic forces 
generated at and above the surface of the water by the boat's forward motion. 
(a). The intent of this general specification is to eliminate hydrofoil or 
displacement-type hulls. Any number of planning surfaces is permitted. "


#41 Lohring Miller

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Posted 27 October 2013 - 04:11 PM

Hydrofoils are suited for relatively small boats at moderate speeds in the approximately 20 to 60 knot area.  Their advantages can be better lift to drag in this area.  The faster you go the more power you need and the more you need to reduce contact with water.  Water is 800 times as dense as air so the same object in water has 800 times the drag.  You get a continuum of designs that range from displacement hulls to airplanes.  In between designs range from hydrofoils to planing hulls and wing in ground effect vehicles as speeds increase.

 

Sailboats are relatively small boats that are just starting to operate in the speed range where foils may have an advantage over displacement and planing hulls.  Internal combustion powered hydroplanes have the power to operate well above these speeds.  They are actually slow speed wing in ground effect vehicles.  Even the fastest sailboats are now operating above the area where foils are best.  The Sailrocket is a planing hull that's partly supported by a wing in ground effect just like powered hydroplanes.  Foils in the water have too much drag compared to planing surfaces at these speeds, no matter what the foil design.

 

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#42 nroose

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Posted 27 October 2013 - 05:13 PM

It seems like there might be some significant possibility, and it kind of already happened, that while most sail boat design recently has been focused on maximizing reaching and downwind speeds with minimum detriment on upwind speeds, we could see upwind speed being the focus, since the boats are going so fast on reaches and downwind that gains there are difficult/expensive/impossible.

 

I just googled cavitating foils and got the image below.  Pretty wild!  At first I wasn't sure which direction the flow was supposed to be.  And it got me thinking.  Instead of trim tabs, they need to allow cavitation tabs that open up the trailing edge...  And perhaps a way to make something pointy come out of the leading edge...

 

1340130602025.png



#43 MR.CLEAN

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Posted 27 October 2013 - 05:27 PM

Hydrofoils are suited for relatively small boats at moderate speeds in the approximately 20 to 60 knot area.  Their advantages can be better lift to drag in this area.  The faster you go the more power you need and the more you need to reduce contact with water.  Water is 800 times as dense as air so the same object in water has 800 times the drag.  You get a continuum of designs that range from displacement hulls to airplanes.  In between designs range from hydrofoils to planing hulls and wing in ground effect vehicles as speeds increase.

 

Sailboats are relatively small boats that are just starting to operate in the speed range where foils may have an advantage over displacement and planing hulls.  Internal combustion powered hydroplanes have the power to operate well above these speeds.  They are actually slow speed wing in ground effect vehicles.  Even the fastest sailboats are now operating above the area where foils are best.  The Sailrocket is a planing hull that's partly supported by a wing in ground effect just like powered hydroplanes.  Foils in the water have too much drag compared to planing surfaces at these speeds, no matter what the foil design.

 

Lohring Miller

Planing is faster with unlimited power.  What's the HP/Weight ratio of an AC72?



#44 DA-WOODY

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Posted 27 October 2013 - 05:49 PM

Hydrofoils are suited for relatively small boats at moderate speeds in the approximately 20 to 60 knot area.  Their advantages can be better lift to drag in this area.  The faster you go the more power you need and the more you need to reduce contact with water.  Water is 800 times as dense as air so the same object in water has 800 times the drag.  You get a continuum of designs that range from displacement hulls to airplanes.  In between designs range from hydrofoils to planing hulls and wing in ground effect vehicles as speeds increase.

 

Sailboats are relatively small boats that are just starting to operate in the speed range where foils may have an advantage over displacement and planing hulls.  Internal combustion powered hydroplanes have the power to operate well above these speeds.  They are actually slow speed wing in ground effect vehicles.  Even the fastest sailboats are now operating above the area where foils are best.  The Sailrocket is a planing hull that's partly supported by a wing in ground effect just like powered hydroplanes.  Foils in the water have too much drag compared to planing surfaces at these speeds, no matter what the foil design.

 

Lohring Miller

Planing is faster with unlimited power.  What's the HP/Weight ratio of an AC72?

 

what's a HenWeigh



#45 Lohring Miller

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Posted 28 October 2013 - 03:05 PM

Planing is faster with unlimited power.  What's the HP/Weight ratio of an AC72?

There's lots of power from sails too.  What's fun (as you know) is figuring out how to harness it.  My experiences working with power boats indicate that high speeds in smooth water require as little boat in the water as possible.  In rough water, speed depends on reducing the effects of the waves.  Submarines are best in the lower speed ranges, foils work in the intermediate area, and wings in ground effect are best at high speeds. 

 

It's still tough to beat displacement and planing hulls in the intermediate speed range when lift to drag is the only consideration.  That's why you can go 100 mph with less than 90 hp in a hydroplane in smooth water.  If you add in waves, foils look a lot more promising.  Planing hulls in waves are a lot less fun when you're on board and not in the helicopter.

 

Lohring Miller



#46 bluelaser

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Posted 28 October 2013 - 04:25 PM

I know the structural challenges are significant but the advantages here look like everything someone would want, and the inherent stability issues would be largely mitigated by the scale of the foils compared to the rest of the craft:

 

http://en.wikipedia....ward-swept_wing



#47 Koukel

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Posted 29 October 2013 - 01:56 AM

That's why you can go 100 mph with less than 90 hp in a hydroplane in smooth water.  If you add in waves, foils look a lot more promising.  Planing hulls in waves are a lot less fun when you're on board and not in the helicopter.

 

Lohring Miller

 

Will-a-met-ee.  Light to heavy, variable winds.  Frequent 180 degree wind shifts.  Average power boat skipper is not aware what the phrase, "right of way" indicates and thinks that "tonnage rules" are for when your hydroponics do really well.

 

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