Foiler Design

Manual Control

Fc- a bit more info. Two of the Raves ,that I know about, were modified to use twin joystick control and according to the owners worked well. An International 14 bi-foiler was designed to be flown manually using twist grip tiller extension control of the rudder flap only. My system would control the main foil flap with a variable amount of rudder foil flap mixed in.
The I-14 performed well with the manual system both upwind and down with two very different foil sets. Here is a picture of it:

 

Hi Doug and Simon It looks like I need to put some way of handling C of G shifts in the numerical model as well! - should be no great problem.

I'm not sure, apart from the theoretical challenge, where my interest lies for a practical boat. Having spent about 14 years on RC helicopter tail control I'm ready for a new control system challenge. I'm not much into racing and got chucked out of the local sailing club 25 years ago for sailing an Albacore I'm hoping that by getting a feel for what foils can do I'll see where it takes me. While I think electronics is not a good practical solution for a production boat it would be interesting to use electronics - say wands with shaft encoders and heavy duty robotics RC servos to drive the flaps with a microcontroller in the middle to define the control parameters as a way of being able to play easily and quickly with wand locations, wand response times, control loop gains etc.

 

FC
Electronics have been talked about as a way forward by the Moth sailors. The problem is that it is against most racing rules but in your case, that doesn't matter.

 

Yes, I did wonder about the limitations of racing rules - one of the reasons I'm not into formal competition - such things tend to get in the way of technical development. Pride of place in my workshop is a copy of a letter from the FAI banning my first gyro design from competition .

 

Sorry, I forgot to add that the I14 that Doug posted a picture of might have foiled but it was actually slowere around a course than a standard I14. It had a reputaion for nose diving.

In the same way, amongst the c-class cats that raced recently, there was a foiler. It foiled well, but significantly slower than a conventional "C". And there lies the problem for a lot of foilers - the same boat non foiling is often faster. Even in Moths, we still see some of the slower foilers being beaten by "lowriders", even though the foilers are actually foiling!

 

I talked personally with David Lugg, the owner of the I14 in question and Alan Smith a foil expert that helped David with the boat. They had two sets of foils, one of which was very fast-faster than other 14's but they chose to use the smaller foils in their few races because they were closer to the proposals going around then about the rules. Those new rules subsequently prohibited enough foil area to fly the boat. Both David and Alan felt that IF the rules had not slowed/dicouraged development their system could have been refined in much the same way the Moth system has been. They hold the record for the first two person bi-foiler in history and they did it within months of John Iletts Moth bi-foiler.

 

One of the reasons I'm interested in a scheme where the foils are fully retractable is that I think it's important to be able to operate in displacement mode with only the penalty of carrying the extra weight of the foils and not to suffer any drag penalty from the foils until the conditions are such as to give foiling a distinct advantage. The temptation to run more foil area to reduce the minimum foiling speed is fraught with the danger that you will then be dragging way too much foil area around and spoiling the foiling performance. Since the available lift goes as the square of the foil area you have, all other things being equal, to increase the foil area by 56% to lower the minimum foiling speed by 20%. There has to come a point where that trade-off is not worth it!

 

You clearly grasp the issues well! Surely the question that needs answering is what windstrength a foiler should be designed to take off in. I believe that this varies, depending on the all up sailing weoght of the boat. The heavier it is, the higher the desined take off speed will be, without having to resort to extreme foils. So, you have a double edged sword.

The Moth is so highly developed that it is rare to sail in conditions where you cannot foil. In non foiling conditions, even non foilers aren't a lot of fun. The problem comes about with oats that don't foil until 10 knots, which can be fun in a non foiler. So, the problem is balancing weight, foil size and sail area so as to get the best boat.

Therefore, I really do wonder whether the retractable foils are a little bit of a red herring. For instance, even in "lowride" mode, a foiling Moth feels like sailing a non foil Moth, just a bit slower! As you rightly point out, the penalty is drag but, IMO, that is it.

 

I think the retracting foils have an advantage from the boat handling point of view - beaching a boat with foils lifted well clear has to reduce the possibility of damage. The Rave foil lifting and lowering doesn't seem to add much to the weight of the system. (I guess a less logical reason for looking at it is that I like catamarans and a cat is probably the easiest thing to implement it on)

I think there is another factor at work here - one that strangely also applies to my other passion - model helicopters - namely that in many ways a good small one will beat a good big one. The reason is the much the same as well - weight generally goes up as the cube of the linear dimensions while areas go up as the square of the linear dimensions. So as we scale a foiler the foil loading tends to go up as the linear dimensions. While Reynolds number effects increase the efficiency of aerofoils with increasing size and this helps the bigger one these effects are not enough to offset the increase in area loading.

 

FC

One of the fathers of modern small boat design, Uffa Fox, famously said that the only only place that weight was OK was in a road roller! It does seem to me that weight is the real killer of early foiling but light weight comes at a cost. To get the Moths as light as they are yet strong enough means some pretty exotic construction materials.

 

I think the power to weight ratio is probably more important than weight alone. You have an RS600FF foiler the weighs 2.5 times what a Moth weighs,and is nearly as fast-faster sometimes. The RS is handicapped by its big wide hull and a rig that is less sophisticated than a Moth but is very impressive nonetheless. And the implications for singlehanded monofoiler design of its weight compared to a Moths weight are wide ranging.
www.rs600ff.com WATCH THE VIDEO!

 

arnie

this is my first post. i have read many but not all of the posts on this thread so if this subject has been covered maybe someone could direct me to it.



i notice there has been some discussion about the angle of incidence of the main foil (on a moth) and that some builders have experimented with fully articulated foils rather than flapped foils. it seems to me that this would be the perfect solution, providing minimum drag or maximum lift as required. has this concept been explored thoroughly and discarded or is it considered unworthy of investigation? any thoughts?

thanks, arnie

 

another question.



if we make the analogy between a moth and an aircraft, the main foil resembles the aircraft wing with similar flaps and the rudder foil, the elevator. an aircraft uses the lightly loaded elevator to control attitude, to fly up or down in the simplest terms, while flaps are used to increase lift and drag at low speed.



wouldn't it make more sense to manually control the (main foil) flaps on a moth as this should only be necessary in the transition to flying and let the wand control the rudder foil? surely the inertial force required to lift or lower the whole craft through it's c of g, in anticipation of an approaching wave or trough is greater, therefore slower than simply changing the attitude through the lightly loaded rudder foil? perhaps the wand could be extended on a bowsprit so it is a similar distance in front of the main foil as the rudder foil is behind. this would put the sensor and the control surface in phase so the main foil is in the right position at the right time?



this all seems a bit too obvious so i probably got something wrong. you comments will be appreciated.



arnie

 

Arnie,

From a purely theoretical efficiency view, I think a simple fully flying main foil is not the way to go. A foil must provide high Cl to give a low take off speed yet be as small as possible in area at high speed. The problem of course is for a small area to achieve a high cl requires a high cambered section to both extend cl max and minimise cdf at high angles of attack. However once you are foiling at high speed in high wind, you need the foil to operate at a low cl, low AOA but your high cambered section will now produce high cdf for this cl. A flapped foil tends to do this automatically as the flap adds both camber and AOA at the same time. However the effect is hardly coordinated to give the ideal min cdf over the whole cl range. So what is really needed is a method to properly coordinate both AOA and flap to keep cdf at a minimum all the time.

As an interesting side note, the maths shows that if you decide on an optimum profile and AOA and hold these values fixed, then area must vary with velocity. In this case it should be noted that form drag force is now pretty near a constant across all speeds. That is a very interesting result because it indicates some truly high top end speeds. It is the only theoretical solution to do so. It would seem a tough call to implement a practical solution based on variable area however it has been done. At one time moths were experimenting with surface piercing V foils possibly partly due to this theory. There were 2 problems. 1. The V foil induced a windward roll due to leeway producing differential AOA. 2. The higher induced drag of a surface piercing foil. Interestingly I came across a more modern example in a well known large tri. The main foils are highly tapered, high aspect ratio and inward pointing from the armas. As the ship rises, the broader foil root comes out of the water leaving the submerged area both shorter and narrower. The aspect ratio is maintained and the mean chord reduces with increasing velocity so I imagine Re and cdi to be fairly stable. It must be pretty near to constant hydrodynamic drag across all foiling speeds. While the surface piercing induced drag must hurt at the lower end of the speed range, I am pretty sure the reducing area would more than compensate for this at the upper end. Certainly the ship is fast. So, it’s not a better solution all of the time, just a different one and has both pros and cons as all solutions do but I thought it rather interesting due to its constant drag basis.

Regards,
Ken

 

ok, i can see one problem.



the rudder foil would automatically pull down while in displacement mode increasing drag while trying to attain flying speed. perhaps this issue could be addressed if there was sufficient advantage while in the flying mode?



arnie