Foiling radio control trimarans

Exactly that. If you only want the wand to deal with speed changes you can put it anywhere, since speed change should be pure heave in a properly setup system (that is, the stabiliser should be dealing with any pitch component of increasing speed).

The key benefit of the wand well forward in waves is that the signal to change heave occurs in advance, so the response can be slower, so the overshoot from the ideal position is less, so control is much better. This helps avoid the main foil leaving the water and then the boat crashing down. The additional benefit with the slower response is that, in cyclic waves, the amplitude of movement is reduced because before the hull has risen to the signalled height, a new signal to lower is received. Hence heave amplitude is reduced in a situation where you don't actually want much, if any. The tertiary benefit to the slower rate and reduced amplitude changes is reduced drag, so a faster boat.

 

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Makes sense for a Moth-not sure it would for a wide multihull. Even when the wand is suspended on a bow sprit the point of contact with the water is not very far in front of the foil --I don't think the wand in that position -in speeds over about 10 knots-anticipates incoming waves at all. At any rate, it seems to work well- though based on past history- I don't think the midship wand was rigorously tested.
Bradfield felt that having the point of contact of the wand with the water at or near the longtudinal CG of the boat was important .
Thanks for the post.

 

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Not sure if you meant Bradfield instead of Bradbury? On some Bradfield foilers the rudder foil was adjustable for racing but for normal sailing it was not necessary.
The rudder foil on the F3 did not need adjustment regardless of the conditions. The mainfoil loading on the F3 was 80% , 20% on the rudder foil-zero pitch issues. Foil loading(in lbs per sq.in.F3 or lb per sq.ft on the bigger boats) on the rudder foil was much less than the foil loading on the main foils. The F3 tacked and gybed on foils with a skillful helm . I tacked on foils once in 3 years, but a friend in Minnesota made repeated tacks w/o the main hull touching down. Normally, I could tack with the main hull touching for about one second depending on conditions. Gybing on foils was easy with that boat.
Bradfield never tried a forward wand to the best of my knowledge-there were just too many advantages to his midship position on the Rave, Ospey and SKAT.
Osprey Wand System, 1 & 2 :






Rave wand system:

 

Why would it not make sense for a wide multi? On a wide multi the roll is just local heave on each side, and the sum of both is net heave in the middle, so the mechanism for pitch is identical, just split between two front foils? In respect of distance, remember that the distance is relative to the distance between the foils and the stabiliser, also a small distance, so even just moving the wand out onto a gantry makes a difference on the moth.

I will need to check my sources better, as I had thought Bradfield had considered different wand positions but rejected them due to pitch coupling based on one of your posts many moons ago. Are you saying he never tried any other position? If so, on what basis do you claim this is the best position? I've searched for what resources are online and whilst he was apparently pretty comprehensive in his testing over the years, he doesn't seem to have published much.

 

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As I said earlier: as far as I know he didn't experiment with bow mounted wands. Others did in the 70's. Bradfield's logic to me was to have the wand contact the water near the longitudinal CG. Bradfields system is extremely simple and seems to work well. The designer of the Whisper cat also thinks a midship wand is best. But the designer of the S9 cat uses bow mounted wands and thinks they're best.
I don't necessarily claim that midship is best, just that it is much simpler. Bradfield thought it was the best position. The bit about the forward wand allowing the flap to respond to the same wave that moved the wand doesn't hold up at speeds over 10 knots in several boats I've looked at. 10 knots is 11.5mph which is 16.8' per second. I don't know of a single forward wand equipped boat-not a Moth-not an S9- that has the tip of the wand 16' ahead of the foil it is controlling. At 10 knots the wave that hits the wand is past the foil when the flap moves. So that is a non-advantage but still touted as an advantage by many people.
It also seems to me that, theoretically, its possible for the boat to pitch causing the forward wand to move. If it did, that wouldn't be good. Apparently it works out ok on the Moth and S9 .
But it still concerns me and is something specifically mentioned by the Whisper designer as a reason for his midship wands. From what I've seen I'm not real sure it makes much difference where the wand is. If that is true then having the simplest system seems like the way to go.
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On wide multies dual independent wands do something never done by the wand system on a Moth: their differential movement can allow the foils to provide most of the RM for the boat. That is a key to the F'3's performance and I have a gut feeling that moving the wands forward would possibly introduce some odd factors into the system especially since we know that the most popular reason given for moving them forward is wrong at most foiler speeds. But I'm open and think it would be a great experiment on two identical boats.

 

Your calc above assumes that the wand takes a full second to move the flap. Why do you assume this value?

If this was correct, then the flap would be moving when the wave has already passed the foil when the wand is at the foil, which would be even worse, and is also clearly not what happens, even at 30kts. Response time is in 1/100ths of seconds, not 1 second, and if you look at a wand moving on a moth on a rippled surface (300mm wavelength) it is easily able to follow the surface at 50hz. Of course the amplitude is low in such small ripples, but it illustrates the response time of the wand.

There seems to be a lot of designers with opinions on this issue, but none seem to have actually done any experiments to test their hypotheses and quantify the effects. The one exception to this is the moth class, and their results appear to confirm the theory, which shows that the required response force asymptotes to infinity as the wand is brought back to the foil since the response time reduces to zero. This is not possible in practice, so the wand at the foil can never be as efficient as moving the wand ahead of the foil, which then gives a finite response time, so a lower lift requirement, hence less required flap movement, and hence less drag.

What is your justification for "since we know that the most popular reason given for moving them forward is wrong at most foiler speeds". How do you know this, or was this based on your 1 second response time assumption?

 

Just spent some time watching foiling moths in action. Video from on-board and external shots. Seems that Moths are not designed to "respond" to waves.
At 30 knots the wand is not following individual waves at all.
See the UAE championship video below to see the effect of even moderate waves.

 

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The wand hits a wave with the boat moving 10 knots and one second later that wave is 16' behind the wand and that's assuming no speed to the wave. So if the wand tip is 3' from the foil , the wave would take .18 seconds to reach the foil. At 20 knots it would take the stationary wave .o9 secs to reach the foil and probably much less than that if the wave speed was known. But I think I made a mistake with my example and I think you may have made the best argument I've yet heard about moving the wand forward-worth some study. Thanks.
PS if I remember correctly there is a lag time from when the wand moves to when the boat responds to the flap movement.

 

The boats would shake themselves to pieces if every wand movement triggered an immediate boat movement.
I thought the whole idea of foils was to "isolate" the boat from water friction and wave effects.

 

Never say never. Dual wands with independent flaps to produce RM have been tried on Moths, at least two that I know of. The fact that current Moths use a single wand, foil and flap tells you which system works better (for Moths). What current wand equiped multihull uses downforce from foils for "most of the RM"?

 

The situation is way more complex than being presented here. Waves aren't singular changes in height like a step, the front and back are roughly ramps up and down. The wand starts to respond at the bottom of the trough and keeps pushing the boat up until it reaches the top, then it starts to bring the boat back down, before the main foil has got to the top.

Waves are steeper at the front than the back, and the boat hits them at an angle of about 45° (±20° given that waves often aren't aligned with the breeze). Also, it's very different going up and down wind—waves are rarely an issue going upwind, they can be a huge issue going downwind.

So everything should be taken as a generalisation or approximation, there is no absolute position. I can confidently say putting the wand on the end of a bowsprit "works" for Moths because of the people who have adopted it. They aren't mugs and wouldn't do it if they didn't think it's better than bow or centre mounted wands. I personally didn't think it would make much difference, but I willingly accept their opinion as having far more weight than mine. The extra distance isn't just about more response time, it also makes the wand more sensitive to pitch changes, which is a good thing. Negotiating waves is far more (as in almost exclusively) about sailor skill than the wand raising and lowering the boat over each wave.

Moths also adjust the wand length, pushrod length, gearing, offset and reflex tension depending on conditions (sometimes all of them are adjusted between up and downwind) so there is an awful lot more going on than just one factor.

Finally, your comment that "…Moths are not designed to "respond" to waves" is pretty close to the mark.

 

What I see in the videos is not what you describe. How often do Moths go foiling if the wave height exceeds hull stand-off height? Never?
So, a wave passes under the Moth without touching the hull.
What impact does the wave have on the boat? There is some increase in resistance of the main strut as it must push through a greater depth of water, but then less resistance in the trough.
There maybe some height change if the foil is close to the surface. The foil becomes more inefficient the closer it gets to the surface. So there might be some increase in efficiency, more lift, as the wave crest passes, and less lift at the trough. But the time periods (peak and trough) are so short at speed that effectively they cancel each other. If foils are fixed, boat height is determined by displacement and speed?
The Moth is a monohull so placing the wand mount as close as possible to the centerline of the hull, at the bow, to minimize response asymmetry seems logical.

 

This is the dichotomy of foil control. You don't want the boat to respond to waves below a certain period, but you do want it to respond to waves above this period, and to other heave variations (e.g due to speed changes or heel).

First thing to bear in mind is that the above discussion is talking about wand and flap response, not boat response. Inertia will always mean that there is a lag between flap movement and boat movement as it has to accelerate vertically. This fact, along with changing wand response, is the primary method used with wands to prevent the boat responding to low period oscillations. Moving the wand forward helps with this, as noted above, as it allows a reduced response rate. Your UAE video is the perfect example of what happens when the wave period increases into the region that the wand/flap parameters are unable to dampen. Going upwind the period is low, but as soon as they head downwind the period increases dramatically. Suddenly the system goes unstable, with resulting crashes. This is why wand gantries were introduced, to push the critical wave period to a higher value.

However, even if the boat is not responding, if the flap is moving it is creating drag, which is why the latest moths are damping the wand to reduce flap movement at low periods, hence you will see the wands skipping in some cases, particularly when the amplitude increases. This is a dangerous game, however, as the response time is reduce, and hence why the wand needs to move even further forward to increase it again.

A better system is to measure heave amplitude and process the results to eliminate the periods and amplitudes you don't want to respond to, then just send the processed signal to the flap or other foil adjustment system. Not easy to do mechanically, and banned in most racing classes if done by other means (only the wind and water can move the surfaces) but in open development this is the way forward.

 

Believe that UAE video shows what happens when you run out of water. The boats crest the wave top at speed, fall into the trough, and that is all she wrote.

 

Not often as it's very difficult (as you've observed), but I've sailed in 2m swell and 25kn breeze. It was tough, and sometimes going upwind you have to bear away to avoid breaking crests. Getting hit in the kidneys by a body of fast moving water really hurts. Downwind is plain scary, you try to go slow so as to not overtake the waves too fast, but that's extremely difficult. You have to steer the boat to keep it in the water and not leaping out the front of a wave. It's 90% technique and 10% wand (and I really, really struggled).

Upwind in most conditions (say up to 20kn), almost none. There is a bump from the wand rising and falling, the boat rises and falls maybe 50mm. Downwind over about 13kn is totally different. If you go at the wrong speed the foil comes out of the front of the wave and you swim. Or you bury the bow into the back of a wave because the flap can't lift the boat fast enough (and swim). I can't fully describe the technique, but it requires steering to put the boat in the right place at the right time, avoiding the really steep bits, taking advantage of the flat bits, knowing how fast the boat can respond to wand input and not exceeding that capability. There is a lot of helm and sheet input. Simple up to 15kn, increasingly harder after that. Moths really need constant active input and sailing to the conditions in the immediate vicinity of the boat.

No, by the wand setting. Also, rudder lift is adjusted to vary pitch: more bow down the faster you go to keep ride height level and use minimal flap.

Yes, but not too close as it can induce ventilation of the main strut.

In all of the above, the wand length, gearing and ride height are adjusted for the best settings for conditions, usually differently up and down wind and day to day. It's not a matter of one size fits all, that just doesn't work in varying conditions.[/QUOTE]