PROJECT WINDRIGGER - Dec 2004 installment

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In the last webpage I reported a successful trial of a aluminium tacking-centreplate mounted on Windriggercat MK 2.

This is shown in following Photos 1 & 2.

Photo 1 - tacking centreplate on Windriggercat MK 2

Photo 2 - aluminium tacking centreplate and steerable centreplate

Referring to Photo 2, note the semi-circular wood-sections mounted on adjacent sides of the plate. These act as fulcrums allowing the plate to pivot in the centrecase slot providing 6 degrees angle of attack and a lateral resistance force opposing the sail leeway force. The plate changes this angle of attack automatically when changing tacks. This arrangement made an exceptional improvement to the windward performance of the cat.

Consequently I decided to explore this further resulting in the steerable centreplate (yellow fin) shown in Photo 2. This is mounted on my WRTri shown in Photo 3 and trialled on 15 December on the tri shown in Photo 4.

Photo 3 - steerable centreplate mounted on WRTri

Photo 4 - WRTRi setup for trialling the steerable centreplate

During the trial the centreplate changed its angle of attack automatically on tacking. Its angle of attack is set by movement of the tiller arm limited by the vertical screws mounted on the 25mm square tube shown in photos 2 and 3. For this trial I added a tiller extension so I could trial steering with the centreplate - which performed the same as steering with the Tri's rudder.

But of greater importance, windward performance of the Tri canbe controlled in the following manner. For example the Tri is turned to windward with a 6 degree rotation of centreboard and the turn is stopped by rotating the rudder to maintain a fixed heading - resulting in centreplate and rudder having angle of attack both of which provide lift in opposition to the leeward sailforce.

I used a similar arrangement reported in the webpage Oct. 1997, on the MK I proa, sailing with a balanced sailrig because I had learnt that an outcome of a balanced sailrig is no force-couple to cause the hull to adopt an angle of attack producing lift to windward.

Whereas monohulls (and tris) have a very unbalanced sailrig due to heeling torque causing the hull to move crab-wise and generate much more lift to windward than catamarans So at last I have the means to provide real windward performance for my ply/dory catamaran for which I am currently producing detailed plans to compliment the building method described March 2003 installment.

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