how moths work?
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=13742
Printed Date: 28 Jun 25 at 2:59am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: how moths work?
Posted By: NickA
Subject: how moths work?
Date Posted: 02 Feb 21 at 9:42pm
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Ok, so I get the basics of foiling moths; that a wand is pushed back when submerged and the boat is moving, which through some linkages operates a flap on the main foil. Boat goes up, wand is less submerged and goes forward again. Helm then twists the tiller extension changing the rudder foil attack to keep the boat level. Yes?
Also, that you can change the "gain" of the system by moving bits of the linkage so that the ratio of wand deflection to foil angle can be adjusted, also that the foil flap control is non-linear so you can arrange things to create lots of flap movement at "lift off" but reduce the sensitivity once at ride/flight height. BUT... watching moth videos, I can see that in short chop the wand is flipping back and forth, so is the foil flap wapping up and down too; isn't that quite draggy? Sure, you want the boat to follow a fixed height above slow swell, but there's no point trying to "track" chop and you don't want the flaps moving about unless you want to alter the ride height. Seems like you'd need some damping ( low pass filtering) between the wand and foil flap (eg a spring with an oil damper in parallel )..but I don't see any. What's stopping the flap from flapping up and down as the wand flips back and forth? Anything? ..and isn't wand deflection (hence flap deflection) going to increase with speed, creating more lift at higher speeds, when surely you need less? Or do they just fly higher at higher speeds? ------------- Javelin 558 Contender 2574 |
Replies:
Posted By: Daniel Holman
Date Posted: 02 Feb 21 at 10:24pm
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All good questions. My limited understanding is that the rudder angle is set and forget by and large - its easier to move ones arse 3" either way. I also think that when the boat is high, the wand can be flopping around vertically whilst not really affecting the flap much, I think due to the cam profile at that angle as it actuates the flap being quite circular. And you can adjust the point at which that happens to some extent with ride height. The flexibility of the wand can provide some compliance (spring rather than damper). The wand length is often adjustable too. So gain as you describe it handled by the distance between wand end, length of wand and cam profile. There is some compliance due to wand stiffness / length relationship. Not aware that there is any damping per se. My limited experience of sailing a posh exocet is that once up and sailing, the wand looks as if its skipping and hopping like a frog in a sock but you don't feel that is really affecting the flap at that sort of frequency, its really serene or at least it is on flattish water. Iguess it shouldn't be responding much to stuff below a certian height, you'd want it to follow bigger wave elevations for obvious reasons. Its bloody clever, I think lots of clever people have put a lot of effort into getting them how they are nowadays.
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Posted By: NickA
Date Posted: 03 Feb 21 at 6:21pm
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"Its bloody clever, I think lots of clever people have put a lot of effort into getting them how they are nowadays."
Too right. Video of an "old" Exocet ( from way back in 2019 ) has control lines for ride height, wand return force and linkage ratio ( gain in my control engineer parlance). If they needed low pass filtering in the control, I'm sure it would be there. ------------- Javelin 558 Contender 2574 |
Posted By: aardvark_issues
Date Posted: 04 Feb 21 at 1:45pm
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A few people have messed around with some sort of damping. I've had an inline damper robbed from an RC car to have a controlled amount of slop - other people have just straight up introduced slop in the control systems. Long story short, it's faster to have more control and ride closer to the height limits than it is to try and engineer out the bouncing you're seeing in the wand. ------------- http://www.aardvarkracing.co.uk" rel="nofollow - Home of Rocket Racing |
Posted By: dohertpk
Date Posted: 04 Feb 21 at 9:51pm
Posted By: dohertpk
Date Posted: 04 Feb 21 at 9:52pm
| Interesting discussion. It's precisely this level of complexity that makes me think I'll never have the kind of mind needed to own one of these boats! |
Posted By: Rupert
Date Posted: 04 Feb 21 at 10:12pm
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I thought the answer was "magic". And if magic is simply science beyond a person's comprehension, then I'd be right. ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Sam.Spoons
Date Posted: 04 Feb 21 at 11:02pm
Sailing anywhere other than directly before the wind is magic to most people  ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: Riv
Date Posted: 05 Feb 21 at 2:14pm
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Remember double de clutching cars without synchromesh?
Now we have all sorts of flawless auto boxes. Anyone can drive them. Same will be true of Moths on the next few years. All the difficulty of the control systems will be designed out and almost anyone who wants to practice the basics will be able to fly easily. ------------- Mistral Div II prototype board, Original Windsurfer, Hornet built'74. |
Posted By: Sussex Lad
Date Posted: 05 Feb 21 at 2:26pm
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[TUBE]https://www.youtube.com/watch?v=J1ho8di4ywQ[/TUBE] |
Posted By: Daniel Holman
Date Posted: 06 Feb 21 at 5:38pm
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End of the day I don't think they have damping because its not needed. The object of the wand system is to manage the flap as the boat transitions up onto the foil (max flap on to flap pretty neutral) and from there, control the ride height of the boat over larger amplitude / larger wavelength waves. little wavelets and chop won't cause that many issues, and on these modern moths (exocet. mach 2, aardvark rocket) even though the wand is moving a fair bit you just don't see uncontrolled heave and pitch response decay, or feel anything when sailing, which is what you would need a damper for, if you use the car suspension dynamics analogy. I sailed a newish exocet year before last, and in a straight line it looks after you, flight seems very stable with no intervention from the helm and I think this is one of the biggest improvements over moths of old. The boat largely foils itself.Apart from in tacks and gybes but no con trol system will do that on the moth foil config with 70% of the toal mass going from one side of the boat to the other!
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Posted By: laser193713
Date Posted: 09 Feb 21 at 1:34pm
I'm not sure that's going to be the case exactly. The development route will focus on performance for the sailors who can already sail them. I doubt the focus of development will be make it achievable for everyone. By all means it may be that other developments make the boat easier to sail, but I don't think "fly easily" will ever really apply... Having sailed a few moths/foilers, from old early foiling moths, a 600ff, a laser with the foiling kit, a bladerider and a mach 2, there has definitely been a progression made towards easy foiling, but that wasn't exactly hard from the early days. The 600ff was almost impossible to sail because to get windward heel your feet moved away from the wing, and there was just too much sail area to try and depower very rapidly as you started foiling. The laser foiling kit is quite frankly pointless, you can't really VMG foil and it isn't very stable because you are running at such high angles of attack to try to pick the weight up that the foil stalls too easily. The moths have certainly got easier and the foil systems more complex, but with a simpler user experience. Mass market foiling is still never going to really be attainable purely because of the speed, and the deceleration when it goes wrong. Injury risk is a big factor for the general public, the same reason you see people on big downhill bikes but they don't ever go down big stuff. The bikes are far more capable, but the speeds and risks involved with actually doing what the bike is capable of is more than Joe Public will put themselves through, the potential to not make it into work on the Monday morning and the mortgage payments that will suffer as a result prevent that. In short, you could make a fully autonomous foiling boat, but you still might not convince many people to sail it, or to sail it hard anyway...
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Posted By: NickA
Date Posted: 10 Feb 21 at 9:49pm
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It's all in the cam design! Once you're up and flying the wand doesn't much affect the flap. It's a strange system from a control engineering POV relying on a carefully managed non linearity between the wand (height sensor) and flap position. But I'll bet the flaps are still moving when you hit a wave that knocks the wand back far enough.
If you were doing it by electronics and software you'd want to detect average height above waves ( which would need low pass filtering of the wand deflection) then a PID controller to operate the flaps to maintain that height. Seems the moth designers have made a mechanical analogue computer that pretty much does all that. An AC75 does the same with digital computers .. I guess. So yes you could probably make a self stabilising moth that worked out all the settings for you ( more so than the current system does ). Would that spoil the fun? Presumably there will be two foiling streams, the performance oriented hard to sail cutting edge ( moths) and the easy foiling for the masses ( the UFO and TFD etc ), which will probably end up with electronics. Interesting to see if a software controlled boat could be quicker than a mechanical one ( answer= yes judging by AC75s) ... and where that would leave the moths. ------------- Javelin 558 Contender 2574 |