PROJECT WINDRIGGER April 2009 Instalment

Last October I removed my 6800mm loa fibreglass catamaran (WRC6800) from its mooring, detached the hulls and dumped its bridgedeck. This catamaran first launched in January 2002, was a good platform for testing ideas, but has outlived its usefulness. I intend to design a new catamaran incorporating following outcomes of this project - the beam-change/trailering system, biplane sailboard-sailrig, fore-and-aft kickup rudders, the vortex generators and steerable centreboards/leeboards, design a new bridgedeck for camp-onboard accommodation followed by design of new hulls. But at present I am undecided whether to design round-bottom or Dory hulls - for the following reasons:

1. Sailing the round-bottom hulls catamaran was never as exciting as sailing my Dory-hulls plywood catamaran, which in 15-20 knts. wind would suddenly accelerate and throw up lot of spray - like planing when sailing a 505 and or Flying Dutchman. I had never considered Dory's to be planing hulls until recently reading a book (ref. 1) which lists them as planing hulls.
2. When trialing the proa shown in Photo 1, I found its performance under outboard propulsion to be unexciting - sluggish. I designed this hull following the accepted practice of "round-bottom to minimise wetted surface and length to beam ratio of at least greater than 10, to minimise wave-making drag. At speed - it shipped a lot of water over its bow and into the cockpit due to the smooth curvature of this hull which offers no means to deflect the bow-wave water flow off the hull - so its wetted surface must increase with increasing hull speed. Contrary to this, the waterflow from the bottom of planing hulls is deflected sideways from its hull.
3. According to Hullform hydrostatic data, my Dory catamaran has less wetted-surface than the round-bottom catamaran (3.1 and 3.4sqm) for the same displacement.

This trimaran differs from existing trimarans in that it is designed to sail with negligible heeling and to have high resistance to pitch-polling - to maximise on safety. To achieve this its outriggers are located forward of the hull centre-of-buoyancy to provide reserve buoyancy up-forward.

Photo 1

Photo 2 - DORYTRI - 3000mm loa, trailering beam 1700mm, sailing beam 3000mm. The centrecase slot has been sealed off.

Photo 3

Photo 4 - DORYTRI - Note the stern-mounted kickup-rudder.

Photo 5

Photo 6 - DORYTRI - hull bottom shape.

TRIAL RESULTS

1. Under outboard power, the trimaran planes with about 1800mm of its hull forward out of the water - at a speed of 14km/hr measured onboard using a TOM TOM car-navigator.
2. Its windward performance appears acceptable - the leeboards automatically change on tacking.
3. Sails with very little heeling - both outrigger hulls stay in the water - no flying outriggers.
4. No noticeable impacts on its hulls when sailing in the choppy waves of the River Clyde estuary.
5. The tiller of the transom mounted rudder interfered with the aft stubmast as shown in Photo 6. Remedies is shown in photo 7.
6. The outboard mounted as in Photos 2-4, is well-placed for controlling it by a helmsman - but at top speethe outboard leg creates a lot of turbulence and water flow into the cockpit .

Photo 7

DORYTRI RUDDERS

Photo 8 - rudder control arrangement devised to overcome the tiller-interference problem shown in Photo 7

Photo 9

Photo 10 - DORYTRI on its trailer.

FUTURE ACTIVITY

REFERENCE