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Dirtboats and iceboats - Sailing a solid wing with a slotted flap

by John Eisenlohr on 7 Jul 2013
Holy Gale 2013 John Eisenlohr
I’ve been designing sailing and building dirtboats and iceboats powered by solid wings since the early 90’s. I believe in keeping things simple with my wings. I use an external slotted flap. It pivots near the trailing edge of the main wing or as some say first element. We have found positive control is the best and safest way to control our wings at high speeds.

Positive control means that there is zero slop in the control system. This is achieve via chain and sprocket systems about 13 to one ration that drive the main wing angle of attack. Prior to positive control some tried to control there wings with a simple main sheet system much like the AC 45’s and 72’s. With our higher speed ranges that can be disastrous due to the wing back winding in a tack and going into an uncontrollable flutter. The forces can translate through the whole boat causing the all the wheels to alternately go airborne in a violent dance of death. In fact the first wing I ever made pre positive control met a quick demise after depowering (sheeting out a little) entering a jibe. I ended up with a trailer full of fire wood.

The flap is controlled by a spring loaded trigger system that engages into an angles index system. We use zero to thirty degrees of flap deflection depending on conditions. Our wings are very simple compared to even an AC 45 because our flaps don’t twist. Twist may be beneficial to depower but so far no one has ventured in that direction.

I’ve had my dirtboat up to 83 mph in racing conditions and the same boat up to 80 mph on the ice pleasure sailing.

I attended and raced at the annual Holy Gale held at Smith Creek Nevada July 26 – 30 2013. We had wind every day and some good racing. I ended up with a third in the coarse racing and a third in the enduro with my class two wing boat. The boat weighs over 1000 lbs. with me in it. It 28’ long, 22’ wide and the wing in it’s tallest form is 27’ tall. It can start sailing in about a five mph wind and reach speeds in 40 mph range in those conditions depending on the surface. One evening after racing I decided to make a video describing how to sail a slotted flap wing. I thought the video might be timely for others watching the America’s Cup to understand the basic trimming principle of solid wings with slotted flaps.

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