# longitudinal position of lifting foils

Discussion in 'Multihulls' started by Jetboy, Jun 20, 2012.

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### JetboySenior Member

If you were building a trimaran for example with a main hull foil providing lateral lift and wanted to add foils to provide vertical lift, ie you were not tied to location longitudinally by center of effort relative to sails, where would you put them? Why?

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MPX Foil System

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I'm developing a system that uses two foils on the main hull-one on the daggerboard and one on the rudder that "support" a curved lifting foil on a small ama. The main foil/ daggerboard is placed where it will support 80% of the boats weight( that puts the main foil just forward of the boat CG). The rudder foil supports 20% of the weight at takeoff. That usually works well for the daggerboard position and can result in the rig being slightly forward of "normal". You will need an altitude control system (like a wand) on the main foil. The beauty of this system is that it allows a very wide(beam=1-1.3 X LOA ) trimaran to fly the main hull in very light air( design for 6-8 knots boat speed take off). Then as soon as the main hull flys, the boat speeds up and unloads the main foils. You should have a single curved foil in the ama capable of supporting the entire boats weight because, as the main foils unload, the ama foil begins to load up. At "X" speed the main foils don't support any of the boats weight but are 100% responsible for controlling the pitch of the whole boat and , therefore the ride angle of the curved ama foil AND the angle of heel of the whole boat. Ideally, this should all happen at around 10 degrees angle of heel. The "wand" controls this angle( by controlling the flight height of the main hull) and also provides RM(righting moment) backup in gusts by causing the flap on the main foil to rise, creating a momentary downforce thru the gust. It is a system with a great deal of potential as a full flying foiler or,using the curved lifting foil as a "foil assist" trimaran.
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Note that daggerboard position is controlled by the 80% load point. But for lateral resistance balance it can be tweaked by changing the daggerboard size vs the rudder size, similar to the shared lift concept developed by Bill Roberts. ( http://www.aquarius-sail.com/catamarans/arc21/index.htm ) You probably won't need to do this, but it is possible if you need it.
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This foil position system, I described above, is the same one used to position the main foils on a Rave, and Osprey foiler. Osprey below:

Pictures: Osprey and model of MPX 12' trimaran with foils on daggerboard and rudder(and planing amas)-

click on image-

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