Photo 1 - Note the Dory-centrehull and aluminium centreplate.
During the last 5 months I have been working mainly on providing comfortable cabin accommodation onboard my 6800 loa catamaran. This involved constructing and adding a centrehull to the bridgedeck of the cat.
Instead of using the circular cross-section hull shown in the Dec 2006 installment, I designed and constructed a Dory-shape hull with a forward-kickup centreplate and a transon for mounting an outboard motor. The main reason for selecting the Dory shape is to minimse its buoyancy and provide the catamaran with some inherent lateral resistance at low hull speed.
This construction evolved into a lot more work than initially expected, so it was a real relief to launch the catamaran on 5 June and see it float to its design waterline and trim.
Its now winter so results from trialling the catamaran under sail could be months away.
The following includes some photos of it on-land and during trial of the rudder system under motor power.
Photo 1 - Note the Dory-centrehull and aluminium centreplate.
Photo 2 Construction of the Dory-centrehull and twin kickup rudders.
Photo 3 - Centreplate arranged to kickup and lift on grounding.
Photo 4 - My mate John sitting in the cabin with his feet resting comfortably on the centrehull floor.
Photo 5 - Rudders retracted aft.
Photo 6 - The rudders are retained in this operating position via the friction produced by the clamps fastening the 50mm aluminium tube to the centrehull deck.
Photo 7 - Rudders in the forward retracting position - to prevent rudder damage following grounding by being driven astern.
Photo 8 - Note the cabin shape and windows and foredeck of the centrehull.
Ultimately I intend to add a cockpit over the aft end of the centrehull and enclose the aft end of the cabin for sleep- onboard accommodation.
I see the appearance of the cabin as asthetically acceptable although it could be improved by changing the shape of its aft side-window to match that of the fore window.
The area of the fore-deck of the centrehull needs to be increased to provide more footing-space for boat-handling such as picking up moorings.
I do not think the immersed centrehull will be a problem in a seaway because the space between the catamaran hulls and its bridgedeck will not impede the passage of water flow around the three hulls.
My next task is to test this feature in a rough water.
My unpublished work covers many hours of design and construction activity covering for example design of Dory hulls using coordinate geometry, design strategies for construction of the bridgedecks for my catamarans, experiences arising out of my use of epoxy/non-woven fibreglass laminates and their resistance to osmosis, use of plywood in hull and cabin construction, problems with assessing sailing performance and my experiences covering relative merits of proas, trimarans and catamarans.
I intend to publish details of these subjects on this website to attract comments from others - to help improve on the technical quality of mulihull design and construction.