Steve and Dave Clarks UFO

Dave Clark isn't referring to the drag penalty of a flap on a foil in a theoretical way in which the flap is only responding to changes in heave, that is not the problem.

The problem he's referring to is that, at a nominal given heave position, the wand dances up and down over every ripple on the water surface, moving the flap up and down continuously, so that whilst the average lift is correct to hold the required heave position, the momentary lift is fluctuating at high frequency. This increases the drag significantly above that which would be calculated to generate an average lift with a steady flap.

 

Frequency response can be managed. I am surprised there is no added damper (that I have seen, or is there?). Another possibility would be a second aft wand that could give averaging and pitch info.

 

That's not how I heard it in the video; but never mind, I'm interested in the problem you describe.

Earlier this year, I replaced my previous Mach2 with the newer Mach2.3 and immediately started being aware this (I think). The symptom is an easily-audible, high-frequency fore & aft motion of the control rod going forward from the top of the forward vertical foil. It seems to happen at about 15 knots or so, but not at higher or lower speeds. I was never aware of it happening with my previous boat.

I've tried tightening or loosening the bungee that goes to the "maystick" at the top of the wand, but I'm not sure there was an effect. (I'm enough of a novice at Moth foiling that this is usually the least of my worries).

What advice do you have for dealing with this?

 

planing wand /paddle

Doug, could the problem be that the "paddle" on the end of the wand is too big or too small? Bradfield talked about this because when I first launched the F3 RC foiler it made a noise in light wind that was audible for 100' but it had round wands. When the lower back end was squared off the noise stopped. He had a problem similar to what you described with a square wand on the Rave-he re-did the wand so that it hit the water at more of an angle with a cut flat end that he described as a "planing wand" and his problem ceased.

 

Ufo

UFO at the dock-most impressive: https://www.facebook.com/SailingShot/videos/1757130207874812/

 

The problem you describe may be another issue - high frequency vibration in the control rods/arms, etc. due to their own natural frequencies being excited. This isn't what I'm describing, but you can solve these issues by moving the natural frequencies of the components into bands that are not being excited (change their stiffness).

However, if you have the problem that I am describing, where the wand, and hence the flap, is following the water surface profile all the time, then you can tune the system to eliminate excitation for the higher frequencies that you aren't interested in, from a heave control perspective.

The best way to do this is to use the wand motion as an input signal, process the signal to identify the critical components, and only pass these on to the flap. Not necessarily practical (or class-legal) in a moth, but the lowest drag option. If you use this, then there is no need to use a wand at all, there are other surface sensing systems that have no drag associated with them.

You can also damp the wand motion so that it only responds at lower frequencies, but this then means the wand is penetrating the water surface, which has a drag component but also, more critically, slows its response to real heave changes.

The best compromise in the moth would probably be a mechanical version of the first option, where the signal filtering is done by damping components in the pushrod mechanism or similar. This is not a straightforward problem to solve mechanically, as the power source for flap movement is the wand, and you can easily end up with the second, less optimal option.

 

Good suggestion Doug. A friend of mine just recently changed the shape of his Mach2's wand. I need to ask him more about it.

 

That's very interesting...

This might be a stupid question, but here goes anyway:

Rather than working to make all the control-rod attachments tight, why not just allow some slop to remain. If, for example there was 2mm slop, then the control rod would have to move more than that before it had any effect on the flap.

 

Not a stupid question at all, but what you are describing is an amplitude filter, not a frequency filter. Of course there is a relationship between amplitude and frequency, but it is not linear and by filtering amplitude you are compromising the response time of the system. You would need to increase the gearing to compensate for the lost amplitude, and you would need to be careful that the system doesn't become unstable in heave at certain frequencies due to the phase lag you would be creating.

 

Ufo

Dave Clark from DA:

So many good questions and surmises in this post. Let me try to get through them all. The mast breaks down into three parts in about 15 seconds. The spreaders come off the front of the wishboom allowing it to collapse. The t-foils collapse to flat units as well. All told, the parts can be swallowed into the negative between the two hulls, and in fact the foam insert for this 'boat/ski rack' configuration is in the cards. At a club, you could have small bins for it all, or if your club loves the UFO a LOT, you could store the rigs intact and the foils intact like windsurf gear in some sort of tent or small shed. Did that over the winter testing period. Luxurious.

With a bit of rudder down and the the mainfoil fully withdrawn above the waterline, you have a fully navigating sailboat. It tacks, gybes, goes upwind and downwind. That's another upside of the catamaran configuration. The hulls generate sufficient sideforce to get the boat upwind in lieu of the mainfoil. I'm not sure if it's capable of dodging bullets or anything in this configuration, but the kickemuit isn't without traffic or moorings and I've had no drama. The other thing that this minimum foil configuration is good for is full blown drifter conditions. When foiling is 100% out of the question, pulling the picket fence out of the water is a good play. As a buddy of mine who sailed one of our prototypes out to an island a mile out in these conditions said "it drifts really well". There's also a "shallows foiler" mode we played with two prototypes ago With the mainfoil and rudder 50% down and all systems operational. It reduces the strut drag for takeoff at some points of sail and limits your depth commitment. That was fun.

DRC

 

How would you characterize the filtering that happens when a wand skips along the surface, like in the following video?

https://www.youtube.com/watch?v=6kcApo7ucjM

 

I would suggest that what you want is much simpler. The control you need is slow (response to accelerations of the boat) and what you don't want is higher frequency (the wave shape at 20mph). This is classic suspension; A damper between the flap and the boat, and a spring between the flap and the wand. Then minimize the inertia of the wand. This gives a first order filter and the wand is free to travel up and down the waves -not adding drag plowing and skipping as you mentioned.

Doug H., I would call the wand not following the wave "aliasing". That term should lead you to appropriate control theory, Fourier transform...

And about the audible vibration, What Doug L. said would be the first thing and easiest to fix, the other possibility is that the origin of the vibration is the foil flap trailing edge.

 

In that particular video he is flying too high. The wand is hanging vertical completely out of travel and if you look at the main foil there are lines -I presume optimal fly height -and he is above them all. The lee tip is about breaching.

Interesting, the height control at that moment must be from surface proximity and area reduction from the foil breaching -enough as long as he is hanging from the sail. A couple of the UFO pics on facebook link Doug L. posted showed the same condition and there is one of the boat turtled -the first time I have seen the foils.

 

Using a spring and damper has the same issue, namely that the response time of the system is increased, which can lead to other stability issues. You don't want the system to operate for high frequency input, even of relatively large amplitude, but you want it to function rapidly for low frequency input, even for much smaller amplitude.

Tuning spring and damper values would also be an interesting exercise because, unlike in suspension which seeks to minimise energy transfer, you need the wand force to operate the flap. It may be possible to set up a system that gives benefits for certain frequencies, and I would be interested to see some experimental results for such a system as it may be good enough, even if not ideal.

 

I'm not sure I would characterise this as filtering, as the response of the wand is more due to the characteristics of the wand/flap (low rotational stiffness relative to its inertia) than anything to do with the water surface profile, so the frequency of the wand response is simply its own natural frequency, and the amplitude of the wand response is just a function of the excitation energy from the water impact and the rotational stiffness/inertia characteristics.

The consequence of the skipping is that the mean wand position is higher than the mean water level, causing the flap mean position to be giving more lift than is required, leading to the boat heaving higher than is require, which in turn reduces the rotational stiffness of the wand, etc., etc. This is an unstable system and would lead to ever increasing heave until other factors come into play, such as surface proximity for the T-foil reducing lift.

This can lead to excessively high, but reasonably stable, foiling if the water surface is smooth, as in the video, but can be highly unstable in any rough water as the foil can come out of the water or ventilate, leading to a crash.