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Old 20th June 2011, 10:43 PM   #24
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Question Failed engine here 2 , piston broke

Fins on sailing craft

The use of angled foils for powered craft is relatively easy.They provide reducing SA with speed and hence greatv efficiency and high top speeds. This is far from the case with sailing craft. Generallyt for recreationaL purposes (eg Moth foiling;windsurfers) a T foil is utillised which gives better efficiency (lift/drag) to a planinbg hull but does not reduce its SA with speed (as angled foils would) Hence no better top speed than a "normal" windsurfer.

If angled foils are tried as used let say on a hydro foil (ie foils angled outwards) the down wind foil will ,rather than give lift , pull the hull under with negative lift.(because of leeway/slip angle) If foils are angled inwards (not ideal;as craft rises its doing so an a reducing width base ) the windward foil will now give negative lift !! This can (and is) utilised in foiling cats/trimarans; where the windward foil rather than lifting craft is used to counteract leaning forces. (As in Hydroptere.Windward foil holds hull down; leeward foil gives lift and a T foil on thge rudder gives added lift to balance craft fore/aft)
I beliueve when inward angled foils are utilised with the windward one opposing heeling forces and the downwind giving lift they are known as Bruce foils.

It can be seen from this that when a foil (our skeg) on a windsurfer is angled downwind it will under its own forces try and return to vertica(Negative lift)l. As it angles upwind (railing) it will satrt producing true lift against gravity.
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