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Long-board Kite Hydrofoil Tests

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lightandfrost
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Long-board Kite Hydrofoil Tests

Postby lightandfrost » Sun Jul 22, 2018 6:32 am

Light and Frost has installed a 70cm curved mast on a long board then took the board out with a kite and tuned the fuselage angle on the mast. The wing/fuselage/stabilizer was previously tested on a short board, so functionality of those components was not an issue here.

For those interested in such projects here are some details

The mast was installed just forward of the thruster fins. The board was opened and two channels one on either side of the stringer were routed out in the foam. Two lengths of wood 30cm in length wrapped in three layers of glass were installed one on each side of the stringer so the mast bolts would go thru the width-center of the wood. The board was then resealed, a fill coat of micro-ballons and epoxy applied then sanded flat and two layers of glass and a seal coat was applied on the bottom over the mast area (the yellow area on the board). Long bolts were installed which go thru the board. The bolt holes were sealed with epoxy and silica powder to prevent water intrusion into the interior of the board. How that was done is not covered here. Strengthening the area around the mast is structurally important. We have seen many many examples of production hydrofoil board structural failure around the mast area. In ALL cases the designer relied on board thickness and carbon fiber to provide the strength as no stringers were found. However carbon fiber is high susceptible to impact resonance failure were the material slowly crystallizes then fails. A few pieces of wood around the stress areas easily solves these problems.

To tune the fuselage angle on the mast, which controls the center of gravity on the board, the board was taken out and water tested with a kite. A long board rider was consulted on normal foot positions when riding as we wanted to find out if foot position for foiling on kites during tuning match foot positions when riding a normal long board with no foil installed. We found that the required fuselage angle on the mast was exactly parallel to the main fin box and the foot positions when riding with a kite exactly matched the foot position when normally riding a long board. One very loose inline foot strap 21cm in length between bolts was installed where the aft foot strap bolt is 33cm from the front board mast bolts.

Hydrofoils when moving thru the water have essentially four states, 1) Board bottom on the water, 2) board light on the water, 3) board out of water on the hydrofoil where the board is perpendicular to the water and 4) board on hydrofoil angled to one side with the kite lower in the power zone. The board functioned fine in all four modes. The rear foot position is over the mast bolts. Putting a pair of angled front foot straps is being considered but that is for further tests.

The board turned out to be very easy to ride and very forgiving.The board can be ridden in extremely light wind. In some cases the wind dropped below ram-air kite flying speed. The rider had to pump the back stalling kite airborne by pulling in the center-lines above the bar to re-fly the kite while standing on the board. Hydrofoil front wing stall simply result in the nose intersecting the water at a gentle impact angle wherein the board settles on board bottom in state 1 above and continues in forward motion wherein the rider simply powers up the kite and continues the ride. The water surface was 2 foot chop with occasional small but steep breaking wave. The board went out thru the wave on the board fine and on the foil also well as long as the 70cm mast/wing was operated nap-of-the-wave which is a standard requirement when using short masts in rough water.

The board will be taken out into larger waves in the near future.
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