MultiFoiler Design: altitude/RM Control

Rave multifoilers use an altitude control system that relies on two independent trailing wands-one on each side of the boat controlling the main foils.. Not only do they control altitude but because they are separate they also control Righting Moment.
I was thinking that perhaps, one wand could be used to control only altitude on BOTH foils while shroud tension would be mixed in to both foils to create/control Righting Moment eliminating the need for one wand.
I know that the original monofoiler "Monitor" used shroud tension to add RM but it was a surface piercing foiler. I'm wondering if anyone has thoughts on this concept and/or has done experiments along this line with fully submerged foils?

 

Sounds difficult

As you say, this was done on the Monitor with a ladder foil arrangement. It does however seem a very indirect method of controling the roll (righting moment) and frought with technical difficulty. The trailing arm wands or forward planing sensors create very little drag and are directly in contact with the surface of the water which you are trying to stay relative to. I am having enough difficulties trying to get my A Class cat foiling with two sensors let alone trying to work out the dynamic relationship between shroud forces and required angle of incidence.

Just my thoughts

PS. Have you seen that the Airbus C class boys are having a look at getting their machine foiling.

http://www.team-invictus.co.uk/Team-Invictus/Invictus2/invictus-2.htm

 

Foil RM

Appreciate your comments. I'm not convinced it would be an improvement either.Roll in any kind of small waves might be ignored by the system-I'm not sure.
For your A-class: Dr. Bradfield set the Rave up with symetrical foils with the forward foils at +2.5° and the rudder foil at 0°. They are set up for the two main foils to carry 80% of the load.
Have you seen the Aussie A class site? It has a forum with 6 pages of hydrofoil discussion...

 

Aussie A Class Site

I haven't seen it in the Aussie site but have seem the discussion on the Dutch A Class site.

http://p078.ezboard.com/faclasscatamaranforumfrm2.showMessage?topicID=67.topic

Is this the one you mean Doug?

 

Aussie A site

Thats the same one D-but what a tragedy! Seems like the last few pages of posts were lost and can't be recovered! Wow- the loss is huge- Rohan Veal Moth foiler Champ (and A class owner) describing racing(and beating) a fleet of A class cats with his foiler Moth; Steve Clark suggesting that partial lift banana boards might be a better solution than full flying foilers in the A class and much more. Wonder what could have happened?

 

It's a thought but I havent tried any practical experiments since the hydrofoil experiments I did some years ago aimed to laterally stabilise a small hydrofoil boat solely by moving crew weight and adjusting sheeting angle. I dought that you would make a successful lateral stabilisation system purely relying on a control input from a shroud tension or other heeling moment sensor. The reason is that the function of such a system is to keep the boat more or less level and you will have a hard job builiding a system which counters heeling moment so accurately that the boat never heels too far - small errors will tend to accumulate. In other words, you can pretty accurately match the heeling moment with a righting moment, but still get the angle of heel wrong. But maybe you could consider a system where there is a control input from a heeling moment sensor (eg a shroud tension sensor) combined with an input from an angle of heel sensor (eg wands sensing the water surface). Maybe such a system might be less affected by wave action than one relying solely on surface sensing wands. You could also consider inputs from gyros etc. but I tend to think that a true sailing boat should not have electrically powered systems unless you are prepared to generate the electricity from the wind, which may of course be possible but is more complicated and adds drag.

John

 

New book on Monitor

There is a new book out called Monitor Hydrofoil Sailboat - Design in review by Neil Lien. Neil apparently worked very closely with Gordon Barker and was one of the 'test pilots'.

 

Review

Here is the review from the International Hydrofoil Society Website.

I'm not sure how to obtain a copy but I haven't tried very hard. If anybody finds out, can they post the details here?

Lien, Neil C., MONITOR Hydrofoil Sailboat, Design in Review, soft-cover, self-published 2004. The book details hydrofoil design developments of the Baker Manufacturing Company, with a focus on the MONITOR, developed with US Navy backing, one of the earliest successful sailing hydrofoil designs. The book also covers the earlier developmental towed hydrofoil boat, hydrofoil runabout, and smaller scale 16-foot hydrofoil sailboat built by the company. The 61 page soft cover book includes 19 drawings and sketches of various elements of the MONITOR design covering the foils, hull, sail and control arrangements. Also contained in the book are 25 photos of MONITOR and other Baker hydrofoil craft. The back cover features a screen shot from a simulation by Hanno Smits. Intro: “When sailing ‘E’ Scows on Lake Mendota, Gordon Baker conceived the idea of putting hydrofoils on a sailboat to drastically increase speed by lifting the hull above the water. When he returned in 1938 to Baker Manufacturing Company in Evansville, WI after a stint of 10 years at Westinghouse Research Laboratories, he instigated a program for his engineering department to research and design such a boat.” MONITOR was first sailed on 24 Aug 55 on Lake Mendota, Madison, WI, and a pace boat clocked her at 25 knots. In October of the following year she was paced at 30.4 knots. It was reported that MONITOR attained speed to true wind speed ratios of just over 2.0, and at times unofficial boat speed measurements close to 40 knots were observed. U.S. Navy backing of MONITOR was motivated by its objective to learn more about the foil structural characteristics and construction methods used by Baker. After Neil re-discovered the craft in a deteriorated condition when he returned to Baker Manufacturing Co. in 1974, he and others in the company restored the craft. Today, MONITOR is on display at the Mariner’s Museum “Small Boat Exhibits” outbuilding in Newport News, VA.

 

RM from shroud tension

Seems like if you could get an RM input from shroud tension and couple(mix) that with a single wand input for altitude that getting rid of the other wand is a beneficial thing in terms of reducing drag.
What concerns me is that with such a system if the crew got up on one side from a center control position or some input to the boats motion was received that was similar to that then the boat might roll w/o control to some degree. Any input along these lines that was momentary would seem like it would be damped out by the rig but I'm not 100% positive.

 

If the system is intended to control the angle of heel then I still think it needs an input which is a measure of angle of heel. If surface sensing wands are used then that requires two of them. Of course it would be interesting if you can demonstrate otherwise by a practical experiment.

I agree that sails provide powerful roll damping, one thing that rather surprised me from my own experiments was that controlling the angle of heel was much easier than controlling pitch, partly I think because of the damping from the rig.

 

Controlling altitude with one wand is certainly feasible, but using shroud tension to control roll needs consideration of trim vs stability.

With a conventional sailboat, the righting moment curve from the hull provides a correspondence between the steady-state heeling moment from the rig - indicated by shroud tension - and the heel angle. High stability is used to generate the trim restoring moment by allowing a steady-state error in the roll angle in response to the disturbance from the sail rig.

However, a fully submerged hydrofoil has little change in lift with depth and therefore little change in rolliing moment with heel angle. The equivalent would be a righting moment curve that was extremely flat. The stability (change in righting moment per change in heel angle) is very low. So you don't have the same correspondence between heel and shroud tension. Shroud tension tells you almost nothing about the roll angle itself.

A change in shroud tension from the steady state value is probably more closely allied with roll acceleration than it is with roll angle. So shroud tension could be used to provide lead in the control system. If you see the tension going up, you know the boat will be rolling in the near future, so you can start to apply the opposing control. The lead-time provided by the change in tension helps to compensate for lag in the control system. But using tension in this way needs to account for the very large trim value of the tension - it needs to be subtracted out from the instantaneous tension to see the change.

The other issue is trim. A sailboat has a very large steady-state heeling moment from the rig. Shroud tension could be used to actuate the controls that provide trim rolling moment. But this moment will be essentially independent of roll angle.

So there needs to be some other system to sense heel and regulate the boat in the roll axis. This feedback will mimic the righting moment curve of a conventional boat and probably be more like a multihull in that regard. The more the boat heels, the more righting moment is commanded by the roll feedback. A sophisticated roll axis control system can also include integral compensation, so there's no change in the steady-state roll angle as the load on the rig changes.

 

Monitor's controls

There is no reason why the Monitor's system of controls couldn't work with non-surface piercing foils. In my opinion (and I speak after having studied the patent, met with the associate of the inventor and seen film footage) the Baker (Monitor) system remains by far the most performing mechanical control system for sail-powered hydrofoil. Its superiority upon the surface sensor types, all derived from Christopher Hook 'surface sensors of the late 1940s, is as followed: the Hook (surface sensor) system feels a trim imbalance and repairs it; the Baker system prevents it from building; by doing so, it maintains the main foils within the range of the advantageous lift/drag "bucket", and minimizes drag.
Schematically, it works this way. (a) The boat has two main foils which carry most of the weight, the third foil being from trimming and direction. (b) The rig pivots freely fore and aft (one sees it balancing at the moorings). Forestay and backstay come back underdeck on a pivoting frame. To this pivoting frame is also attached a push-pull tube to command the angle of incidence of the trimming foil, and a fairly substantial spring whose tension can be adapted by the crew to wind and seaway. Now, when the lift produced by the wind in the rig increases, it creates a forward capsize moment that, if applied directly to the hull, would change the trim, diminish the angle of incidence of the forward foils, diminish their lift, and eventually cause a crash-dive. On Monitor, the increase lift simply pivots the rig forward; by doing so, and through the tension of the longitudinal stays, it moves the pivoting frame, which reduces the angle of incidence of the aft (trimming) foil, the boat tips up, the incidence of the forward foils increases, and so does the lift, then -and only then!- the spring stops the pivoting frame and connects the rig with the boat. It seems slow -but it happened very quickly. Monitor didn't change longitudinal trim at all -none!- she was longitudinally as stable as a dreadnought. The advantage of the system is that it works whether the rig-produced lift increases or decreases, or whether the drag of the hull increases or decreases. This is why the Monitor reached such astonishing speeds in 1955, despite her dreadful sails.
The problem of sailing hydrofoils is how to reduce drag (answer: Baker system) and how to reduce brutal stresses induces by brutal changes of foil lift. France "L'Hydroptère" spends most of her time repairing breakage, because her architecture is so inefficient. In the early 80s they were quite a few foil-stabilized offshore multihulls. This was abandoned because if one wanted to avoid breakages due to sudden stresses, one had to boost the structure so much that the boat became too heavy. Today 60' all have dagger-like foils, which break before breaking the hull.

 

Roll Input/Control

Daniel, that was very interesting! How did the shroud tension system work in roll? It seems to me that the surface piercing/ladder aspect of the Monitors athwhartship forward foils probably played a big role (no pun intended) in keeping any heeling/roll moment under control. Adapting a shroud tension system to fully submerged foils won't have that natural stabilising tendency so it seems to me the question is how to make up for it? Any thoughts?
I read that the Monitor system had a facility for manual input in (at least) roll-was that used in flying the boat?
Also, it seems to me that fully submerged foils will require at least one wand to set altitude;any thoughts would be appreciated.

 

Doug, The Baker (Monitor) system is only in use longitudinally. To allow the rig to pivot freely, the shrouds MUST be in the transverse plane of the mast.
Lateral stability is on paper very simple: line of lift goes thru centre of effort (sail area). In practice it is not that simple, because to create lateral lift some amount of drift is needed. Except if one builds toe-in angle on the side foils of the ladder. Now, don't forget that Baker came from Evanston, not far from where Buddy Melges built his scows, and that scows hulls don't drift because there is toe-in in their centreboards.
On monitor, if I well remember, you could modify the alpha of each ladder; it was specially necessary because the sails were I believe second hand, not well adapted and anyway they would have looked gross to a potato sack.So they'd trim laterally, obviously increasing the alpha on the lee foil.
Re: T-foils. It really depends of the size of the foil, but getting close to the surface would reduce lift, and the boat would sink; with a Baker control system, if the boat sinks its resistance increases, which will automatically increases the alpha of the main foils until they are high enough to decreases the resistance again and reestablish the equilibrium.
Re: T-foils vs ladder. ladder foils are surface piercing and drag inducing -BUT they are structurally very sound, and very light. If you just compare drag, they are loosers, of course -but if you compare drag+weight, it's not sure at all: cantilever foils can be really heavy, unless they are very hitech and expensive. As soon as one goes beyond the A-class size of crafts, structural weight becomes a real problem.
Re: foils in general. low drag foils aren't any good beyond 40-45kts, a speed reached regularly for short period by maxi cats like Orange. At hi speed as soon as they are loaded they cavitate. So one has to have very lightly loaded foils, the reason why they were triplane foils on Yellow page.
Please note that above 20 kts, surface effect is as active as foils, with a tenth of the drag.