Foiler Windward Heel: Upwind Best of All??

The foiler Moth revolutionized foiling by sailing a monohull on just two foils for the first time in history. Since the first bi-foil Moth sailed a technique was developed by Rohan Veal which may be as significant as foiling on just two foils: windward heel, particularly upwind. Rohan and others who have done it say that it makes an incredible difference in the upwind VMG of the Moth-which in comparison to most other small boats is simply outstanding.
I'm curious if there are not more things at work here than may be understood and am looking forward to any theories on the subject.
In discussions of gybing boards or of the twin rotating foils on CBTF boats it has been pointed out that the foils sail at the same angle of attack regardless: what happens with a gybing board(or CBTF boat) is that the BOAT rotates with the practical effect of physicaly pointing LOWER than a boat with a fixed foil. But, assuming that the boats are both going the same direction(angle) the boat with the gybing board(or rotated foil) can go faster because: 1) it can ease the sails slightly and 2) the hull is going thru the water straight-not at an angle like conventional boats. At least thats how I understand it. Many people,including me, before being enlightened on this fact by Tom Speer had visions of a CBTF or gybing board boat "lifting " to weather(which a CBTF boat can actually do but for only short periods).
Where all this gets a little murky is in the case of the foiler Moth-heeling to weather upwind.
Does the same case hold as previously described or is it possible using this method, with the boat heeled at a certain angle to weather, that the boat could actually make a positive angle to windward?(movement toward the wind at 90° to its course heading-more or less)
Does the rig over to weather unload the foils even more?


===============
Here are some comments on part of this subject made by Tom Speer in 2005 from post 377, p26 of "Foiler Design":

My question is: Because of the orientation of the rig and the foils can the lift to weather be greater than that necessary to match the side force?

 

since the strut is presumably not lifting much, there is less induced drag from the config since the main foil is not disturbed, I think I read.
Is he going to fall down now?

edit: i thing the foils are very efficient in providing vertical lift, so i am sceptical that sacrificing horiz lift from the sail to help them is beneficial.

edit: it would be funny if you could turn the daggerboard to leeward so that the boat popinted straight into the wind.

are the fabric wings less draggy than netting?

 

AIUI a surface piercing foil with side load on has much more drag than a fully immeresd one, plus problems with ventilation, variable side force etc. By taking the lateral load off the daggerboard all those issues go away.

In addition windward heel upwind is fashionable in quite a lot of boats at the moment. There are theories that it ups the rig effiency.

 

I can see a lot of reasons why windward heel might be beneficial:
- there is an upward component of the sail's lift that helps unload the foil
- the tip of the foil is lowered, increasing depth and the span of the side-force producing area
- the pilot's body is lowered below deck level, reducing windage
- the spanwise lift distribution on the foil and strut is altered, reducing induced drag due to either side force, vertical force, or both.

There are undoubtedly disadvantages, too. You'd have to do a pretty thorough drag breakdown to see which effects are significant and which are in the noise.

 

righting moment

in short i think its possible to point the boat straight and use the foil to get lateral lift using heel. its just difficult to get the boat to heel that far without hitting the windward wing on the water surface and tripping over. there is power to spare in anything over 10 - 12 kts, (so you heel a long way and probably stay upright) remembering that windward heel also powers up the boat by moving the hull Centre of gravity to windward.

question: does generating lift from other places and unloading the foil reduce drag? wont it just lift the boat higher? do you have to slide forward and decrease angle of attack to compensate? (as flap angle is purely controlled by ride height, and is set up for a specific weight)

also it just occurred to me, the vertical component of lift that is lost from the foil when its on a tilt, is probably (mostly) taken up by the tilted sail. the horizontal lift component gained by the foil acts to power up the sail through apparent wind, and so the horizontal lift of the sail stays the same. so your getting a free vector, pointing horizonatlly to windward... so i guess at some point it must work, if you can power up the sail so that at a given tilt it can compensate for the loss of lift from the foil.

may need to raise the wings too, so they dont hit the water so early. i imagine its marginally harder to heel a bladerider to windward compared to a prowler.

 

looking at the different L/D ratios of fully submerged asymmetric compared to symmetric surface piercing foils, it makes perfect sense to use the lifting foil to provide the necessary sideways force.

boogie

 

windward heel

My question still remains: can this technique allow the hydrofoil to efficiently produce MORE than the required sideways force?

 

definitely its just a matter of balancing the vectors. you may need more than 20 or so degrees of windward heel is my guess. the boat needs to be designed so that it can be sailed at that angle. or you could cant the foil.

 

canting the foil upwind may be an alternative to using a flap to kill upward lift. hey it could even hinge at the T joint, so the weakest part doenst have the hinge, as with a canting keel. if that was automatic it would be a seriously weird sensation! get a gust, then get some random uncontrollable sideways action!

 

Ask a world class sailor why he does something the way he does, he'll say because it goes fast when I do that.

in straight line sailing (equilibrium), the foils in the water won't produce any more side force than that generated by the rig in the opposing direction - ever. Unless of course we are proceeding past warp 1.

 

To Doug Lord, I think yes.
Like a proa with excess asymmetry = windward elevator, but less boatspeed --well documented I think.
* * *
As for windward heel, seems clear, no endplate but better flow...

 

But how much lift does the inclined rig really develop? How much is the sailor steering the boat to windward?

Paul

 

Assume the rig is heeled and sheeted to maintain the same angle of attack. This is equivalent to rotating about the apparent wind velocity vector. The vertical component will be lift*sin(heel) and the loss of horizontal compnent will be lift*(1-cos(heel)). The drag will be unchanged (to a first approximation).

Ten degrees of cant will give you 17% of the rig's lift vertically at a cost of 2% of the horizontal driving component. 15 degrees of cant will give you 25% vertically at a cost of 3.4% of the horizontal drive.

Of course, the same ratios apply in the opposite direction, too. If you heel 15 degrees, you've added 25% of the lift of the rig to your displacment - hence the admonition to sail'er flat!

 

I think he could fall down now. And judging by his non attention to the state of his foils, it could be a photo op? (Just bring a camara to the Gorge, and watch what happens out on the river!) Or maybe a good Moth Foiler can scallop to windward? Go to the point of a crash, but then fall to windward, while accelerating and turning back to leeward, and using the speed, apparent wind and with any luck a gust tongue, to accelerate the rig back to the ambient course? I used to do this on my Div. I and II Windsurfers (tm!). At times it would give you a 1-3 foot advantage. This pic could be a mid point of slow motion Moth style pumping, kind of like WWII Heinkel pilots who used to bang their bombers into what they called a step climb- build speed laterally, then bang the wings into a short violent climb (repeat as needed). It would explain why he's looking forward. And the set of his sail- if I see it right, there is a small luff going on. Starting the fall to leeward? Then a smooth fast pull on the mainsheet, while falling off. It would also change the loading (and drag) of the foils.


Paul

 

Definitely a photo op. Thierry has been to visit Rohan several times purely to take photos - like that one. There would be a crash very soon afterwards.

No not really. The best speed normally comes when the boat is trimmed consistently, not bouncing or steering up and down.