Foil performance discussion

The boat at https://vimeo.com/127926604 is a 7.5m/25' proa. It has a fixed foil under the 4m windward hull. 300mm chord, 12% ogival section with a flat high pressure side and a rounded tip. The kite is attached to the other hull, so the foil is lifting the crew, hull, half the beams and itself. About 180 kgs. The surface of the foil is peel ply/40 grit sandpaper, ie pretty rough by most foil standards. The edges are sharp, except for a few dings. Angle to the water surface is about 50 degrees at rest, 40 when flying. Lift off speed is about 10-12 knots.

The foil was built for another purpose and used for this boat because we had it. Due to it's crudeness, I thought it would fail, was pleasantly surprised at how well it works.

The kite/proa/foil arrangement has been successful enough that we are thinking about scaling it up to a 15m and sailing it offshore.

Questions:
1) How should we scale the foil up for the bigger boat. Kite sizes will be 50% larger, weight to be lifted will vary between 140 and 300 kgs, depending on where the crew sit. Lift off speed about the same.
2) the foil ventilates for half it's chord and almost it's entire length a few times in the video. I thought that this would cause loss of lift and/or large drag. Doesn't seem to. I also thought it would be hard to get the flow to reattach. Doesn't seem to be.
3) The faster the boat, the more foil comes out. So the tip shape gets more important. At present it is bog rounded with a grinder. What is the best shape?
4) Downwind, the kite loops lift the bows up to a metre out of the water and speed increases, a lot. No idea how much, but let's say 20 knots. However, it does not foil, even with my 80 kgs moved onto the long hull.
5)What would the optimum foil size, shape and section be for the boat in the video, and how much improvement would each aspect make?m Ditto for the 15m.

Explanaiions, suggestions, improvements, etc appreciated.

thanks.

rob

 

Very interesting ...

I have some tentative thoughts about some of your questions, but before sticking my neck out too far, I'd like to clarify one thing :

Is the primary purpose of the foil to resist the sideforce from the kite (like a daggerboard), with the vertical lift being sort of a bonus. Or do the hulls and/or other appendages provide enough lateral resistance that the foil is mainly there for its vertical lift ?

 

Foil

Rob, I can't quite make out how the foil is mounted to the hull-is it straight coming off the hull at an angle? Could you do a quickie drawing of the hull and foil viewed from in front or behind?

 

Both. There are no other appendages (steering is by moving the kite line attachment). The hulls are semi circular cross section and draw comparatively little. We go upwind without the foil a little better than a well sailed twin tip kite board.

Doug,
The foil is attached to a tube between the beams and bears against the hull. Pretty ugly, but I had so little faith that it would work that I did not bother with elegance. I have added a fairing piece where it connects to the hull to spread the load (first test, the foil holed the 5mm foam with 300 carbon each side hull) and fill the gap between the round foil and the hull.

 

Rob : I'm not going to pretend that I can answer all of your questions, but I do have some thoughts about the foil on your proa that might be helpful.

In my earlier post in this thread, I asked about what you considered the primary role of the foil to be. My own view of this is that it is primarily an augmentation to the lateral resistance, with a bonus that it can be used to lift your windward hull sometimes. I emphasize the word "sometimes" because a boat that has only a single inclined planar foil needs more than just boatspeed to get the foil to lift. Unlike most other foil systems, it
depends heavily on the sideforce, because the force on the foil (excluding drag) is essentially entirely in the direction perpendicular to its surface. This is not true for systems with 2 or more foils or with foils having fins or other nonplanar shapes.

To explain what I mean, let's consider the balance of forces. If we can assume that the hulls contribute very little to the sideforce, then these force balances are especially simple. The sideforce on the foil is simply the same as the sideforce generated by the kite (in the opposite direction), and the vertical force is that sideforce divided by the tangent of the foil's dihedral angle. (Assuming the foil is large enough to generate the force required). If you try to increase the vertical force by increasing the foil's incidence (in pitch), the boat will simply slide sideways & adjust its leeway angle until the force balance is reestablished.

In your case, the vertical & horizontal force components would be about equal, since the dihedral angle is close to 45 degrees. So, if you can estimate (or measure) the forces on your kite, you can easily determine whether your foil will be able to provide the sort of vertical force you need.

Obviously, in many cases, it won't be able to. In particular, the foil will never lift when you are going downwind, because there is little or no sideforce. This explains the situation in question #4 of your original post.

Including the hulls' contribution to the sideforce will probably make the situation worse, since it reduces the sideforce that the foil must develop. However, since your foil has a large amount of camber, you may encounter some situations where the hulls & the foil are lifting in opposite directions (ie - negative leeway angle), & then the sideforce on the foil would be larger than the kite's. That's a weird situation, though so I won't discuss that more now.

It was interesting to watch all the ventilation & reattachment in the video, especially how readily the flow seemed to recover. Supposedly, ventilation will not occur unless there is flow separation present & the sharp leading edge is certainly conducive to separation if the incidence is outside a very narrow range. It might help to diagnose the situation if you could use the force estimates (determined as above) to calculate the force coefficients that the foil is operating at.

Determining the coefficients should also be of some help in trying to optimize the foil on your current boat & deciding how to scale the foil for your larger boat.

All-in-all, this is a very interesting project & I wish you all the best in it!

 

Thanks. Very helpful.
Given a choice of foiling or preventing leeway, I would certainly chose the latter, so your view is correct.
It is difficult to measure the kite loads due to the complexity of the strings set up, but I will endeavour to do so and let you know. Meanwhile, lets assume 100 kgs when the boat is going upwind in 20 knots with a 17 sqm kite (projected area about half this). The lines are about 45 degrees above horizontal and about 15 degrees forward of the beam. How do we get the coefficients and how do I relate them to the foiling capability and foil size?

When I used the boards as rudders (there original purpose) the separation/ventilation at the leading edge was very noticable and different to that in the foiling video where the ventilation appears to occur from the centre of the foil, not the leading edge. I figured this was due to the board being too thick for it's chord, rather than a sharp le problem. Comments?

Re measuring kite loads. Are there any cheap, in line load cells that record the loads? Ideally waterproof. Alternative is to use spring scales, but these don't record.

Thanks again,

rob

 

Rob : Here's a quick look at some numbers :

If the kite is pulling on the boat with a force of 100 kg at 45Deg above horizontal, then projecting onto a horizontal plane gives 100 * Cos(45Deg) = 70.7 kg. If that force is angled 15Deg. forward of the beam, then the side component is 70.7 * cos(15Deg.) = 68.3 kg, & assuming a foil at 45Deg, the vertical force component is also 68.3 kg.

This is a far cry from the 180 kg you would like it to lift, but maybe the assumed force of 100 kg is too low. Would you know, for instance if the force could be twice that much ? Or did you just pick 100 kg as a nice round number to illustrate the calculations?

In any case, given the force components on the foil, the total force is just the square-root of the sum of the squares of the components. (Fn=96.6) And the normal force coefficient (Cn) is just the total force (Fn) divided by the product of the dynamic pressure (q) and the planform area (A). (Cn = Fn / (q * A))

To go further, I have to make up some numbers that you didn't tell me. To get the dynamic pressure (q = 1/2 * waterdensity * boatspeed^2), we need the boatspeed. In 20 knots windspeed & sailing upwind, a boatspeed of 15 knots is a possibility. Assuming that, gives q~ 3114.5 kg/m^2. To get the planform area, we need the span, but given a chord of 300mm, I'm going to guess a span of ~1m. That gives an area of ~0.3m^2.

Putting those numbers together gives Cn= 96.6 / (3114.5 * 0.3) ~ 0.10

This is the value assuming that the entire foil is underwater. If it is only 50% in the water, then the value grows to ~0.2. But this is the total force coefficient & the largest local values will be somewhat higher. If they are as much as 50% higher, then the value of Cn would still only be ~0.3, which is a small enough value that you wouldn't normally expect to see very much flow separation.

Since this is all based on a lot of very tentative numbers, I'll leave it for later before speculating about the reasons for the behavior of the foil.

 

Proa foil thoughts

Rob,

Doug has covered the ground well, but I have a few additional points to consider.

The configuration has some issues with heave stability. Consider the J foil. Downwind, its configured like a V foil. V foils are heave stable through reduced lifting surface and leeway stable if the opposing V side forces are in balance with the rig side force. Upwind, more like an L with an uptip. In the upwind mode, lift results in leeway as vertical foil area and lateral resistance decreases. This leeway decreases the AoA of the uptip resulting in reduced lift. The result is leeway modulated heave stability. Premised on Doug's observation that the side force is proportional to the lift from a single inclined plane, your foil will be subjected to increasing AoA and increasing lift coefficients as the windward hull flies. It’s worth noting that as the craft heels the foil incline becomes shallower. This results in the lift component increasing relative to the side force resisting component. So, for a given side force, the resulting lift reduces the foil’s lateral resistance. The combination of these effects, without further consideration, would appear highly dynamically unstable.

There is however a modulating effect from the trailing edge ventilation. When used as a rudder the leading edge would cavitate at high AoA. In foil mode, the AoA will be much smaller, so cavitation will be less significant, at least at moderate speeds. The significant difference wrt ventilation relates to the reduced immersion with length and perhaps to the wash down toward the tip of the blade due to leeway. The low pressure downwash resulting from the lift generated by the foil at the surface is sucking air. This effect is markedly lower with a vertical foil. This effect becomes stronger with reduced aspect ratio as immersion reduces: the pressure gradient along the foil compresses.

Hydroptere overcomes the ventilation (presumably because it seems to work) through: fences positioned along the foil, a tapered foil that preserves AR, and vertical tip that at least partially decouples the lift leeway relationship. Plus of course the windward foil is typically partially immersed, resulting in some cancellation of the leeway resisting force.

Like you, I’m considering a lifting foil under the windward hull for my proa. Amongst configurations I’m considering is a T foil. Like the Veal heeled moth and the Sunnucks M20 Vampire. This foil could be angled slightly to leeward. This creates a partial V, generating greater stability. A T foil is probably less of an engineering challenge than curved foils. Angled to the lee with a fairly short stem, the lifting element could be fully immersed until the hull had comfortably cleared the water. The lift will be modulated through surface piercing ventilation of the leeward tip. The stem would provide a ventilation fence at higher speeds.

David

 

Foil performance

The reason flow reattaches quickly after ventilation should be evident from the discussion in my previous post. With ventilation induced lift collapse, surface area, AR, angle of immersion and immersed length increase, so leeway, AoA, the lift coefficient and pressure gradient along the foil decrease.

 

If the system consists of the foil alone & given the inability to even balance the forces in that mode, then it's not surprising that that there might also be a dynamic instability.

Yes, ventilation is one of the ways that the foil loses lift as it reaches the surface, but what's surprising to me is how little it seems to be affecting things. That raises a number of questions :

1-Is the foil providing all the heeling moment, or is the positioning of the kite control lines also somehow a factor? The results of the example I posted yesterday make me doubt how much lift the foil is actually producing.

2-What is the nature of the flow on the foil when it is not ventilating ? Maybe there is always significant upper-surface separation, but it's only visible once the ventilation occurs. That might mean that most of its lift is due to higher pressure on the lower surface & it doesn't lose much as it transitions into planing mode.

There are obviously more questions about what's going on & how to proceed. Too many for me to think about all at once. I foresee this thread becoming even more interesting ...


I do have one simple suggestion for Rob , though, : Try canting the foil forward 5 - 10 Degrees, if possible. This will promote spanwise flow up the foil & at least partially inhibit the ventilation from traveling downward.

 

Didn't mean to post here

 

The dynamic stability issues I raised are certainly far less significant for Rob's application, being partial as compared to full foiling. There's an aspect of the dynamic problem that that I didn't consider yesterday. That is the effect on attack angle of vertical velocity component. With dynamic instability, the vertical foil accelerations are significant, relative to both the gravitational and water surface inertial planes. Consequently, with upward velocity, the attack angle is reduced; and downward velocity, the attack angle is increased.

From this I conjecture: that as the the foil decelerates upward, the attack angle increases, increasing the lift coefficient and ventilation. The resulting ventilation, reduced lift area and reduced AR eventually dominate. The upward deceleration becomes a downward trajectory. The foil decelerates downward, reversing the described effects. This speculation hints at acceleration maximas at the extremes of travel, but this is not important. What is important is the the relative vertical components of velocity.

I'd be interested to hear from Rob as to whether the windward hull had any tendency to fly without the foil. I personally doubt it. Another way of looking at the forces is to consider the rotation around the centre of mass produced by the upward kite pull. My view (without running any numbers) is that the resulting rotation around the centre of mass will tend to lift the lee hull rather than the windward. This would depend on the flying height of the kite, which has been fairly low compared to other kite flying examples I've seen.

Taking this train of thought to the extreme, Sail Rocket is using this lift to fly the lee hull. The force generated by the rig then requires an equal and opposite reaction from the windward foil. It would be a logical extension (excuse the pun) to add a 90 degree downward dog leg that provides a down force. Heave stability subject to at least partially solving ventilation. I'm aware of Rob's objections to Bruce foils, but it would be fun on a performance craft.

 

On the contrary, a deceleration of the upward motion only means that the angle of attack isn't being reduced as fast. As long as there is still any upward motion, the angle & the CL would both still be reduced.

Furthermore, no mention has been made yet of the reduction in lift that occurs simply by the proximity of the free surface, even without any vertical motion or acceleration. This surface effect is powerful enough that it has been used as the sole heave control mechanism for heavy Russian ferryboats (with fully-submerged horizontal foils) operating in smooth waters in about the 1950's.

 

100 was pretty arbitrary, based on kite boarding guesses. ie, a kite will lift 100 kgs in a jump. But this is higher loads than when the kite is only propelling the board and rider. (I think). I will mount my trusty fish scales and see if I can get any more accuracy.

Estimates are:
Boatspeed at lift off is ~10 knots, 12-15 upwind.
Span in the water is about 1m, and about ~75% of that is out of the water in the video.
I have refitted the foil a little more securely and got my gps working. Took it for a motor yesterday and with me on the lee hull, and turning quite tight circles with the foil on the inside of the curve, it foiled at 9 knots.

David,
It does not appear to be particularly unstable, which was a big surprise to me. Once it is up, it stays there. Even with the massive ventilation in the video, the drag did not change appreciably (steering with the kite force shows drag differences in the little hull very quickly), and the flying height was steady, compared to most foils that lose lift.

Doug questions
1) The kite is attached to the lee hull, about 800mm off the water. At low flying angles, it might be lifting the little hull, but we rarely fly the kite at low angles, so suspect not. Without the foil, the hull sits low, is wet and draggy.
2) No idea of the flow, but this is a possibility. How do we determine the nature of the flow on the upper surface? I will put some tell tales on it (not sure how, any suggestions?) and see what happens.

Canting the foil forwards is a good idea, but not possible with the current set up.

Doug,
Bruce foils are cool, but too much wetted surface. I did wonder if the kite boat foil was acting partly as a BF. ie, the sideways component of the leeway was causing the foil to lift.

Please keep the theories and things to try coming. We are going out again on Tuesday, with gps and cameras. Will let you know the results.

rob

 

Rob:

Just by coincidence, the foils on my trimaran have almost the same horizontally projected area as the foil on your proa. (~0.2 m^2, at takeoff ). They have to lift ~160kg (including my weight). I find that, depending on how I set my foils, I am able to take off at speeds as low as 10 knots (CL~0.6), but then the top speed only reaches the mid-teens. If I delay the takeoff to ~13 knots (CL~0.35), then I can reach top speeds in the mid-twenties. Of course, my foils are different than yours in several ways, but these general numbers might be a useful comparison for you.

This reinforces my suspicion that there is a lot of separated flow on the upper surface, even when you can't see it. I wish I had some good advise on how to visualize it, but my only experiences with telltales underwater have been unintentional

Since you'll have both gps & cameras, I would highly recommend getting some software like Dashware ( http://www.dashware.net/ ), that will let you see the gps data on your videos. I've been using it lately for both my trimaran & my Moth & it's really a big help in figuring out what's going on sometimes.

The software is easy to download & install, but you don't have to do that before the next time sailing. The only tricky part of using it is in synchronizing the video & the data, and for that a little thought ahead of time can be a big help. It's possible to do the synchronizing by identifying moments on the video where the motion starts, or changes direction abruptly, but the best I've found so far is to simply call up the gps time on a phone [U]http://www.timeanddate.com/[/U] and make sure you record a few moments of that at some point in each clip of your video (as long as you can do that without ruining your phone, of course.) If your gps is capable of displaying the time, maybe you can just photograph that, but mine isn't.

Hope you have good luck on your planned sail tomorrow.