One of the most exiciting developments to come rc sailing is the use of hydrofoils to"fly" boats that have been traditionally “stuck” to the water. Hydrofoils have often been associated with speed under sail and that is true with models but there is a new dimension particularly when foils are applied to multihulls. That dimension is enhanced control virtually eliminating capsize and pitchpole from small multihulls when the boat is set up properly. Using foils that create their own righting moment means that a multi will just get “stiffer” the more the wind blows with very little or no heel and no pitchpole tendency.
New multihulls are being introduced that attempt to make hydrofoils practical for racing regardless of conditions since the foils are retractable. A retractible foiler can sail in light non foiling or transitional foiling conditions with no drag penalty except the weight of the foils. These same foilers can lift off in 5-6mph of wind or so!
Another new development being tried out is hydrofoil monohulls like the experimental microMOTH: one foil on the daggerboard and one foil on the rudder with movable ballast being used to stabilize the boat.This kind of foiler is no where near as easy to sail as a multihull and there is still doubt as to whether it can be controlled at all. It offers tremendous potential speed since the foils are only lifting not also developing righting moment. Speed could approach or surpass non foil equipped multihulls . Another concept worthy of testing is that of using two hydrofoils on a canting keel model; first tests of this idea did not work very well due to the excess weight of the test boat-but it will be tried again and again.
For more information on design considerations for rc hydrofoils including altitude control please check out the multiONE website created by Kris Harig:www.rcmultihulls.com
I’m the first one to say that just because something works in “large” boats doesn’t mean it will translate into r/c race boats. (I also admit to not being an expert on multihulls). But the following from the Septermber, 2003 issue of “Sail” magazine appears interesting.
On pages 14-15 there is an article describing Ellen MacArthur’s new offshore trimaran. In talking about the design the boat is described as having “no foils on the amas”. Ellen’s business partner Mark Tuner says this is because (and this is a direct quote in the article): “The foils actually help capsize the boat,”
I saw that article; there is a lot more to the quote:“the foils actually help capsize the boat”. Any statement like that used to characterize all foils is a mistake. That statement was likely referring to the foils described below. What the big tris have recently been using is a version of foil that is curved so that when it is deployed in the design conditions the lower end is more or less horizontal. It is curved because that allows the trunk to be curved and the foil to be retractable for light air sailing. That type of foil with its high lift area located so close to the waters surface has a nasty tendency to ventilate: if it goes thru a set of waves in such a way that air is sucked down the foil then the foil BANG loses lift instantly loading the lee hull almost double and very likely precipitating a capsize/pitchpole.The Open 60’s that have used these foils sail over half their races on relatively smooth inshore courses). For the purposes of foiling a fully submerged T- foil works best because its high lift foil area is isolated from the foil that is coming thru the hull close to the waters surface. But the t-foil mounted in a high beam to length ratio hull can’t retract for light air so is not a good solution when applied that way. However, a T-foil can be mounted independently of the hull and be easily retracted…
The foils used-so far-on most of the Open 60 tris are basically foil assist designs designed to lift most but not all of the boat; the problem is that because they are curved and usually (initially)fully submerged and without any altitude control they sometimes can lift the whole boat since lift is a function of speed . But a foil like that lifting the whole boat is risky because the upper part of the foil is right at the air /water interface and can ventillate in a split
second.
Foils can be designed to be effective in rather rough conditions but there comes a point with any foiler where the conditons can be too extreme and the boat would do better with the foils retracted. In the case of the 60’foot tris most have these curved foils that are retractable and make good use of them in the many inshore races that they do with speeds over 30 knots. In Ellens case for round the world sailing the curved foil w/o altitude control is apparently more trouble than it’s worth in her technical teams veiw. But I would venture that a retractable fully submerged foil would probably be of great benefit.
In models ,though, the area of a foil for a particular boat scales very well(almost exactly) but what happens to the foil changes dramatically: the model foil has much less load per unit area and because of Reynolds Number effects relating to the viscosity of water when a model foil “breathe’s air” it usually won’t ventillate; the flow seems to reattach instantaneously. The production model foilers on the market now are all fully submerged foil systems using altitude control and as such add not only speed but impressive control to these boats.
Just to keep the record straight. In the Sail magazine article about Ellen MacArthur, the only quote about foils is “The foils actually help capsize the boat,” There is nothing “more” to the quote or in the article other than that simple statement nor is there any reference to “curved” foils.
In a heat of the International Moth class world champs yesterday a Moth on Foils won the heat by more than 4mins.
Not sure how many swims he had around the course:)
cheers
Brett
Just received a response to a letter I wrote to Nigel Irens ,the designer of Ellen McArthur’s new 75’ trimaran, regarding the plans not to use foils. The main reasons are 1)that the foils typically used on the Open 60 tris, as I described elsewhere, can result in longitudinal instability and 2)-they have to be particularly attuned to sponsor concerns about cost and reliability since this new boat will be making long single handed ocean voyages.
Dr. Sam Bradfields new 40’ SCAT trimaran foiler was also designed by Nigel Irens for the specific purpose of exploring the use of fully submerged altitude controlled hydrofoils in the ocean environment.
Irens and Bradfield are exploring the boundaries of hydrofoils on the Scat and I’m sure we’ll hear of more hydrofoil applications on the Open 60 since Ellen is likely to be be starting a new 60 soon…