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IconHydrofoils on an International MothIcon

From: Ian Ward, 30 Cambourne Ave, St.Ives2075, Sydney, Australia (

The International Moth is a Sailing Class which is one of the only remaining truly development dinghy classes. The rules are simple, 11' long, 7'4" max. beam and 86sq.ft sail area. There are no restrictions on design, weight, materials etc. The class is particularly strong in UK, Germany, Australia and Japan.

As far as I know it is the only International sailing class administered by the IYRU which allows Hydrofoils to be used.

I have developed a foiler Moth over the past 9 months and last weekend for the first time was able to beat the current National champion, while reaching/foiling in 12kts of wind. My boat weighs 64lbs fully rigged on the water.

I am looking now at far higher speeds in stronger breezes and note the limitation on standard foils due to cavitation at around 40kts. I note the work on the Trifoiler designed by Ketterman in the US..

I am interested if your members have any ideas to overcome the cavitation barrier. Can you suggest any references or research on supercavitating foils?

The foil support mechanism I have developed will allow me to change to different foil configurations while foilborne at any speed, so I have plenty of scope to experiment.

I would welcome your thoughts, advice or recommendations of who is the best person to contact on this subject.

[Very few people indeed seem to have done any serious work on super-cavitating foils, outside of research into propellor blades, and I certainly cannot cite references off-hand. A number of people have used ogival sections (bottom is flat, top is an arc of a circle, thickness around 8%). The trouble with these (and maybe with super-cavitating foils) is that with their sharp leading edge they stall readily. Super-cavitating foils are also said to have a poor lift/drag ratio at low speeds, which in a sailboat is a nuisance. You may end up having several sets of foils, and select the set you want according to windstrength and likely dash speed. The Hobie foils (production version of Ketterman's boat) are sub-cavitating.

In Australia, people who might be able to help include the Bethwaites and the team who put together Yellow Pages Endeavour. YPE is not a foiler, but she does move fast, and at one stage, was rumoured to have problems with foil behaviour. Does anyone know whether that was cavitation or ventilation, or the cure?]


As far as I know neither the Yellow Pages syndicate nor the Bethwaites have worked on lifting foils. The Bethwaites are pretty well tied up with the 49er at the moment and Frank occasionally takes out his HSP (High Speed Project) 15' trimaran which has conventional foils.

There are another two local people working on foilers in Sydney. Michael Baranski has a modification of Ketterman's arrangement but uses a ladder foil as his sensor instead of a waterski. I have some photos. I haven't seen it working, only under tow.

A local boatbuilder has a Trimaran arrangement with a central A Class cat rig. He also uses Ketterman style sensor arrangement and can pace it with a Tornado cat in 15kts+, even upwind. He is currently constructing a new lightweight one.

My own development is currently aimed specifically to be a Moth. This is by far the most competitive forum for any new developments as it must perform in all conditions and all points of sailing. Neither a Sailboard nor a Ketterman style foiler can beat a Moth around a course in all conditions!!. This means my design will need to have retractable foils.

I am just now starting to construct the prototype of this retracting mechanism. I am also developing an even lighter Moth hull and self supporting rig, aiming for an all up fully rigged sailing weight of 25-30Kg. Sounds tough but is quite achievable!

The interest in supercavitating foils is perhaps a little premature, but I have a very good unique mechanism for foil control which is great in big waves and allows different height settings while sailing, so I could use the standard foils to say 25kts and then deploy the supercavitating foils. I would like to build these into the design. I have a little information from some websites but little else. This is where your members may be able to help. Unfortunately I have no photos of it sailing yet. That will come in the next couple of months.

Any other clues or contact with people interested in such developments would be greatly appreciated.

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