# Foiler Design

Discussion in 'Sailboats' started by tspeer, Nov 12, 2003.

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Here is a thought on the canard arrangement to give greater stiffness of heave response that relies on the fact that when foiling, the boat will be going significantly faster than in non-foiling mode.

How about pivoting the canard at a point some way behind the centre of lift, and then controlling the angle of attack with a pre-loaded spring?

By pivoting the foil behind the centre of lift, it will try to increase its angle of attack as lift is generated. By using a spring, the foil could be held at its lowest, design angle of attack under non-foiling conditions when the foil is submerged.

After take-off, the boat should accelerate due to the reduced drag. So if the foil submerges later, it will generate much larger lift which would overcome the spring load, allowing the foil to increase its angle of attack, further increasing the lift until it returns to the surface again.

I have ignored the fact that the angle of attack would be reduced if we were nose diving though - damn!

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### tspeerSenior Member

The 140x14 cm foil supporting 120 kg has a loading of 612 kg per square meter. At 8 kt, the lift coefficient is 0.69; at 16 kt, 0.17; at 24 kt, 0.08. Aspect ratio is 10, and assuming a span efficiency of 0.9, the induced drag coefficients are 0.0169, 0.0010, and 0.0002.

A 14 cm chord has a Reynolds number of about 75,000 per kt. So the Reynolds numbers of interest are 600,000, 1,200,000, and 1,800,000. A 14% thick foil like the NACA 64 014 has profile drag coefficients at these conditions of 0.0131, 0.0064, and 0.0057. According to XFOIL (Ncrit = 3).

From Hoerner, A T intersection has a drag coefficient based on thickness of 17*(t/c)^2 - 0.05, resulting in a drag coefficient based on foil area of 0.0003.

So the total drag of the foil is [8 kt, Re 600,000, CL=0.69, CD=0.0303] 5.3 kg; [16 kt, Re 1,200,000, CL=0.17, CD=0.0077] 5.4 kg; [24 kt, Re=1,800,000, CL=0.08, CD=0.0062] 9.7 kg.

This doesn't include the drag of the strut, which will add profile drag and its own induced drag. And, by changing the lift distribution on the lifting foil (due to side load on the strut), it can increase the induced drag substantially. It would take something like a vortex lattice or panel code to estimate these 3D interference effects.

By "width" I'm not sure if you are talking span or chord. I'll assume you are talking about the span, and simply cutting the foil in half while using the same molds, etc. The induced drag goes up by a factor of 4 because you've halved the span. Presumably there's also a second strut. So the total drag would be 8.3 kg, 6.3 kg, and 10.5 kg.

Cutting the thickness to 8% results in a nasty leading edge stall at Re=600,000, alpha = 5.7 degrees, Cl = 0.61. The profile drag coefficients for the other conditions are 0.0096 and 0.0088. The reason for the drag increase appears to be the formation of a leading edge suction peak that didn't exist for the thicker section, although Cl=0.08 is pretty much the design condition for the 8% section.

Moving the max thickness forward by switching to a NACA 63 008 softens the leading edge suction peak, solves the stall problem and results in profile drag coefficients of 0.0130, 0.0068, and 0.0053.

It's not clear whether you're preserving the span here or not. If you keep the chord the same, the profile drag will vary in proportion to the area (+20%) and the induced drag will go down by 30% in the first case and up by 40% in the second case.

That's the key question, isn't it! A fully submerged foil has a U-shaped drag curve with speed. At low speeds the induced drag predominates. At high speed, the parasite drag is paramount. For the classic parabolic drag polar, the best lift/drag ratio occurs when the induced drag equals the total parasite drag (including foil, strut, rudder, etc.). You could size the foil to place this condition at takeoff. That would result in the minimum takeoff speed, but excessive area at high speed.

Another strategy would be to minimize the maximum drag, so both the lowest and highest speeds have the same drag, and the minimum occurs in the middle of the speed range. This would result in rapid acceleration once you got flying, but may take more wind to get foilborne.

A surface piercing foil can be designed to have esentially constant drag over the whole speed range. At any given speed, a fully submerged foil can be designed that has lower minimum drag. But if the design speed range is wide enough, the maximum drag of the fully submerged foil can exceed that of the surface piercing foil at both high and low speed.

So there's no single best foil solution, any more than there's a single best rig or hull shape.

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### WardiSenior Member

Tom, Many thanks for your response,
I am still digesting the implications....

Firstly though I note that the total drag is actually quite low in Kg, even at 8kts, when compared with displacement sailing. It would therefore seem reasonable to go for an arrangement with perhaps a large foil area, in order to get up early and be able to foil while sailing upwind etc.

Also, increasing the total foil span and placing the area in a single foil seems quite beneficial, so I will continue along that path.

Rather than being forced into the compromise you describe with submerged foils meeting limitations at both the slow speed and high speed ends of the spectrum, I am hoping to run the front canard submerged and lifting for low speed performance, and to have it pop out and run clear of the water at higher speeds, thereby reducing the total immersed area by some 20-30% when going fast. This is a hybrid arrangement, which hopefully provides a solution between the two options you mentioned.
Any thoughts on this?

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### Dick AllanNew Member

I have a Hobie cat 14 . What do I need to do to put foils on it? I assume its fast enough to utilize them.

5. ### Doug LordGuest

H14 foiler?

I don't think a Hobie 14 is a good candidate as a foiler but there is a guy by the name of Dave Carlson who has put foils on many cats; I can't find his e-mail yet but when I do I'll post it.
UPDATE: Go to:
http://wingo.com/dakh/beachcats.html

Last edited by a moderator: Mar 8, 2004
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### Dick AllanNew Member

So do you think a Prindle or Nacra would be better? or add a jib?

7. ### Doug LordGuest

Cat Foiler

My concern stems from sailing the 14. Seems like the boat tended to twist too easily(for a foiler); if a foiler is not rigid in twist the twist can cause unwanted changes in foil angles. Dave Carlson is much more experienced than I am in evaluating beach cats for this purpose-contact him...

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### WardiSenior Member

Hello Tom,
Perhaps we could take the calculations of resistance for a Moth foiler a little further.

It is becoming more critical now for us to get the best minimum "take off" speed. I presume this is determined when the power from the rig cannot overcome the drag from the foils.

For the same foil arrangement discussed previously, and assuming the rig is 8.0 sqm with AR =5, what would be the drag of the foils at 4 and 6 kts and what do you consider to be the takeoff speed of the boat and the windspeed required.

In fact it would be good to have a figure for windspeed to achieve each of the boatspeeds 4, 6, 8, 16 & 24 kts we have discussed. I guess we will have to assume the boat is on a reach with sail at say 30 degrees from centreline.

Any comments on optimising this appreciated!

In practice we are finding a windspeed of around 6-7 kts gets us up and going on a reach.

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### tspeerSenior Member

Now you're reaching the crux of the problem. In principle, one can take off at any speed one wishes - just provide enough foil area. But if the foils are too big, it takes a lot of wind to drive all that claptrap through the water up to the (low) takeoff speed, and then if the foils are fully submerged there's excessive drag at high speeds. Conversely, if the foils are very tiny, it still takes a lot of wind to get up to the (high) takeoff speed.

So in between there must be a foil size that results in the minimum wind to take off. This is not necessarily the lowest takeoff speed. Greg Ketterman advises erring on the high side, as his studies have shown there's not much to be gained at taking off at low speeds. A narrow displacement hull is very effcient to well above hull speed, and the drag flying may well be greater than the drag floating.

For the drag figures I quoted, the parasite and interference drags scale as velocity squared. The induced drag scales as 1/velocity squared. Parasite drag scales with area. Induced drag scales as 1/span squared. So you can use these relationships to stretch the numbers in different directions.

I forgot to include surface effects in the induced drag. The induced drag is doubled at the surface, and between deeply submerged and operating at the surface, the drag behaves like a one demiwing of a biplane with identically loaded wings separated at twice the operating depth. See http://www.desktopaero.com/appliedaero/appliedaero.html for the induced drag of the equivalent biplane.

Calculating the wind required for takeoff really requires a velocity prediction program (VPP) that includes all the contributions:
- hull skin friction
- hull wave drag
- strut/rudder profile drag
- strut/rudder induced drag
- strut/rudder lift (must match sideforce from sail)
- hydrofoil drag
- hydrofoil lift (must match weight - buoyancy)
- sail lift
- sail drag
- hull windage
- crew windage

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### astevoJunior Member

it is this scientific complexity that makes me think an empirical approach would be much easier. things like wing profile shape, and crew windage seem very insignificant compared to control problems.

wardy. if you are looking at this sort of optimisation are you thinking that your current configuration is well developed enough to allow for refinement?

im about to have my interpretation on the illet foiler going in the next week. though ive got to build a new tfoil ( adjustable). on the bottom of an old blade. fit the main foil properly and screw on the control stuff. (already made up.) hope to have results next week.

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### National3434Junior Member

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### WardiSenior Member

Actually, this is the first time I have seen this photo posted myself
http://www.moth-sailing.org/pictures/development/ian_ward_scow_foiler_large.jpg

The photo was taken a couple of weeks ago in light winds by Michael Mottl. He took little notice of the old scow until it jumped out of the water in a 7 knot puff and foiled right past his runabout. He followed for a while and took a few photos. Unfortunately I have not had time to post any of the latest developments as I have been travelling for work.

My test bed is the wingless Scow Moth in the photo and I have now trialled perhaps 40 configurations of foils on bow centreboard and rudder, canting, fixed and with various dihedral and anhedral angles. At present it is a Unifoiler with a single main lifting foil on the centreboard and submerged sensor activated canard at the bow. So far this gives the best results in light airs, but I am also trialling a surface running canard.

The major problem with the Moth is the short "wheelbase" which makes it very difficult to control pitching. The complicating factor is the "hysteresis" experienced by the foils. As the boat lifts off, the foils increase angle of attack and it is rather easy to take off...sometimes too easy! The problem is that once it lifts and starts foiling level some distance above the waves, any gust etc will tend to pitch the bow down, which results in a greatly reduced angle of attack and reduced ability to provide sufficient response to lift the bow again. Hence the boat porpoises, you can see this effect in Rohan Veal's videos. In short, it is easy to get up but not so easy to stay there at a constant ride height.

The sensor activated sub-surface foil also suffers from this same "hysteresis" problem. The solution lies in getting the sensor as far forward as possible relative to the lifting foil. One way is to move the sensor forward. I tried this and it works a lot better, but is not practical unless you like to have a spinnaker pole out the front of the boat! The Hobie trifoiler uses this approach with great success, but that solution involves forward facing sensors with water skis on them. They are ungainly and lots of drag in light winds. An alternative is to have lifting foils on the bow and rudder only. I have trialled this previously and it works quite well. Just needs to be built strong! I am now doing this.

I have also been trying to develop a self levelling, surface running canard, but there is a major problem in that the centre of lift moves significantly between submerged foiling and surface planing modes. After some 25 variations already, I think a simple fixed angle foil with tail reflex as used on some single wing aircraft may do the trick. Anyone with any good ideas for a solution for this??

On a few occasions I have trialled a bow sensor with no other lifting foils. The results in a good breeze both upwind and down are remarkable! The sensor foil lifts the bow sufficiently to prevent nose diving and lifts about 1/3 of the boat clear of the water. I can easily activate and alter this ride height with a single control line. It is very much faster both upwind and down, remains stable as the stern is still in the water and as there is no "hysteresis" effect, so the sensor works really well. The surface running canard also produces a similar effect, but as it has to bear quite a load, it is not as efficient as the submerged foil. This is a similar effect to the forward foils used by the French 60' trimarans.

My feeling that this is not as fast as full foiling, but it is certainly a big improvement in all conditions with excellent control. It may prove to be a good half way measure and better than using a surface piercing method. I would encourage Skiff Moth sailors to give this a go, as it is very simple, cheap and overcomes all of the control problems of full foiling! It may in fact be the best solution for strong winds and big waves!!

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### CloudsipperNew Member

Foils in the archives?

Old newcomer to the forum.. Any chance of reviving the asymetric designs of Bernard C Smith (circa 1980) called the Aerohydofoil? The models were impressive , but I guess they never scaled up. His book "The Forty-Knot Sailboat" is also an excellent introduction to some of the issues in wave drag, sail design, etc.

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### WardiSenior Member

It is indeed a nice reference of amazing ideas. Unfortunately many are impractical for round the bouys course racing which is what I am trying to achieve.

15. ### Doug LordGuest

Sailrocket

This machine was inspired by the work of Bernard Smithand is going after th World Sailing Speed record.Most boats of this type can sail only one direction; apparently this thing and sail two directions.
While manouverability and round the buoys racing is out of the question the most important aspect of this boat is worthy of study: it develops no heeling moment! Therefore stability is not a factor in how fast it will go..more:
http://www.yachting-world.com/auto/newsdesk/20030028110712ywnews.html

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