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Chris White's MastFoils™ & Flaps

I recently posted an article showing the development of masts and rigs—briefly covering the range from conventional sails with fixed masts as used by the great majority of sailing boats—up to the rigid wing sails now being set on the latest America's Cup AC45 and 72 catamarans. A table of maximum predicted speed relative to wind speed was also posted, just to give some idea of 'relative performance' when associated with the most efficient hulls for the speed in mind. See this link: Mast-Rig Options.

Before adding anything to that list, perhaps it needs a little clarifying. For sure, the rig on its own without a suitable high performance hull, would show far less of a range in performance than indicated, but when coupled with the most efficient hull for the intended rig, then such differences are possible. In fact, the hulls may have as much or even more effect on performance as the different rigs—as much by their weight as anything else, as lighter weight permits slimmer hulls.

Nevertheless, rigs DO make a sizeable difference and if anything can be done to make them easier to handle, then their effectiveness and area can be carried for longer periods of time, so increasing average speeds while fast cruising.

Atlantic 47 Trimaran

One such rig is presently under development and has a number of neat features that are well worth outlining here. This is the new MastFoil™ as conceived by master multihull designer Christopher White.

Chris always has something worthwhile to offer—well based on practical values, with safe but fast cruising at the heart of most of it.

With the argument that the mainsail is the most difficult sail to handle and reef quickly, Chris presents his new MastFoil™ that gets rid of the mainsail entirely!! The required area to drive the boat is then split between two masts and will primarily be provided by two large jibs that can be roller furled. The masts themselves will apparently be straight, round carbon fibre tubes that will be held up with three simple stays running to a masthead fitting. But around each mast will be fitted a rotating wing, with a trim flap pivoting off the rear of what then becomes effectively a rotating wingmast. The flap itself will be small, but it can seemingly be angled up to 50° or more relative to the rotating wing, so giving this small foil, a great variety of possible positions. Only experience and experimentation will determine the propulsive effect of these small foils, but one can imagine many possibilities from winging out a cupped form to go downwind, to aiding the flow off the adjacent jib leech, to adjusting to use them alone to sail off a dock or even propel a boat backwards!

Atlantic 47 Trimaran

Anyway, here's a rough sketch that I throw in just to clarify how I presently see the MastFoil™, but hopefully we'll soon see Chris awarded with the appropriate patents so that detailed drawings will then be available for all to share.
After a smaller prototype was tested, this rig is now being fully developed for the new Atlantic 47 catamaran by Chris White and the first boat is presently undergoing tests in Chile.

Here is a short YouTube of how she goes, and clearly, this boat will not be slow www.youtube.com/watch?v=73n56dxBgYI.

If you want to read what Chris himself says about this rig, here's a link to his personal page on this:


Keel Trim Flaps

Chris has also long defended his use of low-profile keels as the safer solution for a cruising cat, even if he acknowledges that one could lose say 3–4° of pointing ability compared to a deep daggerboard.

But now, Chris is experimenting with yet another trim flap. This time by adding a small one at the rear of the low aspect ratio (LAR) keel, to improve the overall efficiency. Here is what Chris himself writes about this on his website:

“This makes so much sense. It is simpler to construct and maintain [than daggerboards], weighs very little and is not vulnerable to damage. Yet a flap has the ability to dramatically improve the performance of the fin. Let me explain: The typical symmetrical daggerboard creates zero lift unless the hull is sailing a little bit sideways in the water. In order for anything beneficial to result from the increased draft of the daggerboard it must go through the water at some "angle of attack" to the water flow. Normally a good sailboat going to windward will be "making leeway" of 3 to 5 degrees. This means that the boat is turned 3 to 5 degrees to the rush of water coming along the hulls, which is highly inefficient. Unless the hulls are operating at this inefficient angle to the water flow the boards have no angle of attack and cannot provide any lift to combat the sideways push from the sails. So in order to get "lift" from the board you need to accept an offsetting "drag" penalty from the hull.

What makes the most sense is to have an underwater foil that is adjustable so that it can provide lots of lift while the hull tracks nearly straight thru the water. By far the simplest way to do this is to install a flap on the trailing edge of a fixed fin.

I can see all sorts of beneficial uses for fin flaps beyond just sailing to windward. They can be used to balance the boat under different sail trim. They can be used to combat crosswinds in a docking situation. They can be very effective sailing down wind too. Flaps set at negative angles will reduce drag and help "push" the boat to leeward which is just what you want when deep reaching.

There is even the possibility for using the fin flaps as a built in drogue. By setting them in opposite directions at high deflection angles they will be very effective "brakes" and could be handy in conditions where you need to slow down. Each flap will have an independent control and will be adjustable from the helm station.”

This is all very interesting to me personally as, way back in the 50s, I had the same conclusion about the required leeway or angular drift to create board lift. So I came up with specially widened centerboard case that accommodated a board with an articulating cam at the rear edge. This could be turned via a cockpit handle, so that when lowered, the board could be angled about 3° to each side, to provide lift without having to rely on leeway  (today this is called 'a jibing or gybing board').   The problem in my case was that this was mounted in a short monohull that already drifted fairly easily sideways to create the lift angle it needed, so I was never able to prove to myself that the gain was worth the extra drag.    But such a foil on a long slim catamaran would seem to make even more sense, so the results of this one will be interesting to watch.  For the Atlantic 47, I imagine this flap will appear as a small rudder aft of the LAR keel, with an actuator up inside the hull. Probably only a small movement will be required, unless of course Chris also plans to use them as brakes!

Kudos to Chris for these most interesting new developments.

Feb 2013, mjw

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