PROJECT WINDRIGGER April 2008 Instalment
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BACKGROUND
WINDRIGGER 6800 with the dory-centrehull was first launched June 5 2007, and its first sailing trial was not until Dec 2007 because of trialling and modification of its outboard-motor mounting and twin rudders.
Photos below show the rudders which were modified following the August 2007 Instalment, by extending their depth by 100mm to increase their effectiveness and the outboard motor lowered 60mm to increase immersion of its propeller and thus reduce cavitation.
TRIALS
The sailing trials were performed using Windriggercat 6800 with the dory-centrehull, twin fore-and-aft-kickup rudders, an aluminium centreplate and twin 7sqm. partially-battened sailboard sails - as shown below in the photos.
DECEMBER 2007 TRIAL
Under sail, the catamaran consistently turned-away from the direction of the wind - exhibiting unmanageable Lee-helm.
Also, it yawed under motor-propulsion requiring continual rudder-correction to maintain course.
I guessed these problems were caused by its centre of lateral resistance acting aft of its centre-of-effort - due to the immersed aft-end of the centrehull and relatively large rudder-area relative to its centreplate.
MARCH 2008 TRIALS
During March 8 to 13, I added vortex generators (winglets) to the bows of the hulls as a means to solve the problems.
This approach to solving the lee-helm problem is based on my good experience with vortex generators published in the Project Windrigger July 2001 instalment - and covered in an article by Bernard Rhodes titled Winglets and Vortex Generators published in MULTIHULLS Magazine Nov/Dec 1990.
The vortex generators I added to the catamaran comprises an aluminium 20mm Tee-section extrusion of length 1000mm, fastened to the outboard bow-surface of each hull about 30mm up from their keel-line, extending 100mm from each bow and running 1000mm aft and parallel to the hulls waterline.
March 13 trial result - The motor-propulsion steering/yawing problem was no longer evident.
March 17 trial highlight - The catamaran sailed effectively against the wind for at least 5 kilometres along the Clyde river - with the centreboard fully-retracted.
This trial comprised a 26 kilometre trip from my Nelligen mooring to the Batemans Bay bridge and return, sailed by Mike Hadfield and myself.
The wind conditions were of variable wind-strength and direction, gusting up to an estimated 10 to 12 knots.
Very difficult sailing conditions but typical of sailing on inland waterways.
This trip took about 6 hours and included motor propulsion during calms.
Initially we used the centreplate but ultimately found that the catamaran sailed better without it.
COMMENTS
This trial showed that a catamaran hull fitted with the vortex generators, can have adequate windward performance without a centreplate.
I am very surprised with this result and very happy to discard its centreplate seal-off the centrecase-slot.
The catamaran can now sail in 200 to 300 mm depth of water.
Photo 1 - Windriggercat 6800 rigged for the March 2008 trials
Photo 2 - Note the rudders in the up-position
Photo 3 - Rudders in the down-position and retained by friction produced by the 2 clamps around the tube spanning the width of the after deck.
The photos 4 and 5 show the vortex generators fitted to the outboard bow-surfaces of the catamaran.
They are located above the keel of the hull so they will not impede moving the catamaran sideways when shifting it back into water after beaching.
The drawing is a 3D illustration highlighting the catamaran's full-bodied bow-sections and smooth keel cross-section and which produce practically no useful lateral resistance.
Ultimately I developed a steerable centreboard to produce useful windward performance.
The vortex generators provide a much simpler and robust means of this windward performance.
CONCLUSION
The main conclusion arising from the trials - I added the Dory hull to the catamaran instead of a round cross-section hull on my understanding that it would produce lateral resistance to improve tacking.
In the sailing trial it did do this but because immersion of the Dory hull is greater aft than forward, the catamaran's bow turned to leeward resulting in excessive lee-helm.
So knowing now that a vortex generator can add lateral resistance to a round-section hull, in future I'll use this shape for centrehulls.
Photo 4 - Vortex Generator showing its bow end
Photo 5 - Vortex Generator comprising Tee-section aluminium 20 mm wide by 20mm high and 1000mm long
BRIDGEDECKS
The October 2007 covered construction of the bridgedeck of my 5600mm loa catamaran and sought help with stress analysis.
This attracted some usefull responses.
So I have added a drawing of the bridgedeck of my 6800mm loa catamaran for further discussion on bridgdecks.
DRG 2 - An outline of the construction of the bridgedeck of the WRC 6800 catamaran.
It comprises basically an open-box covered with plywood epoxy-fastened to and over timber planks each 125mm wide and 20mm thick - selected carry bending loads.
During construction, its torsional stiffness was assessed by lifting one corner of the box to see how far it moved up before another corner moved up.
This lead to inserting timber pieces (such as the coloured one) across the corners of the box resulting in achieving acceptable torsional stiffness.
These pieces produced a remarkable improvement in the stiffness.
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