Alternative Centerboards & Sail Rigs Aboard Big Cat
BE: I've posted here a copy of a letter I recently wrote to Mr. Rodwell as it touches on some thoughts about centerboards and alternative sailing rigs that might be interesting to some multihull sailors on this forum. For reference B Rodwell had originally posted a message under the subject heading "sock it to me"
One item of your thought processes caught my attention in particular; your desire for a single centerboard, and real shallow draft capabilities. I'll certainly second that motion, that shallow draft idea. One of the greatest attributes of multihulls is their capability to really go exploring ALL the water areas including those tributaries, lagoons, reefs, etc. That's why I had kick-up CB's in each hull of my design.
BUT, what you may not have noticed was
my alternative to the CB's in each hull. Look at the attached drawing, (or the very bottom profile drwg that denotes
"asymmetrical CB's, nacelle mounted". First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam). Now
on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize (possibly 1-foot) diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against. Only one board at a time would be lowered. In fact the two could be linked together such that the act of lifting one automatically lowers (& powers) the other down. And they both could be rigged to 'kick up' upon hitting any solid object and/or for shallow cruising. The control lines (cables) could be routed right up to the cabin top and back to the cockpit.
There are several advantages to an
asymmetrical shaped centerboard. First, it requires less total board area to develop a leeway reducing force....so the board size is reduced. Secondly, since it is asymmetrical, it does not require an angle of attack (does not require the boat itself to be sailed at a skewed angle) to develop the 'board's lift' (leeway reducing force). This actually
may result in the vessel making less leeway.
Plus the drag forces associated with the CB lift forces are on the centerline of the vessel, rather than offin one hull that produces turning moments about the center of the vessel.
This centerline mounting may also improve the tacking capabilities of the vessel as it allows the 'clean' hulls to slip a little while pivoting about the central board.
The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves.
And how about
the maintenance factor, particularly in remote cruising areas. No need to haul-out the vessel to repair kick-up CB problems, or even bottom painting problems. Everything, including the cables, bearings, and boards is all above the load waterline. The initial building cost should be less
by eliminating the trunks in two hulls, and the watertight integrity is much
better. The twin boards might have to be made a little bit longer as they operate with a 'free-surface' end, but then they are asymmetric so they can be correspondingly shorter. I would further suggest that surplus helicopter blades are prime candidate sources for both CB blades and rudder blades....high tech, extremely strong carbon fiber fabrications that have a
prescribed limited life span aboard aircraft, but are perfectly happy for our use.
For rudder designs I would give a cassette system such as the Vara rudder at <
www.socasailboats.com/> a close look in lieu of a kick-up system. Or maybe even Tony Smith's Gemini cat system. Kick-up rudder systems can get pretty complicated, plus they usually don't steer the boat very well at all when they are kicked up in shallow water. (LOTS of weather helm).
If I were looking to use my auxiliary engine in a strictly aux manner, rather than in a motor/sailing demand, I would seriously consider a single engine installation. This engine would be conveniently mounted in an enclosure on the cockpit deck and would belt drive a steerable out-drive leg that would be incorporated into the rear portion of the central nacelle structure. Maybe this rear nacelle might appear as on "Earthling's pod" (attached photo and/or <
www.earthling.co.nz/boating.htm>). This saves the cost and weight of the second engine, trans, shafting, prop, etc, and opens up the rears of the hulls
for a nice master bath, or whatever.
And a final little note for now, I think you would find that most all
textbooks, etc,
would implicate the traditional mainsail as the primary leeway inducer. I believe my mainsail-less rig will thus require less leeway combating forces from my single CB arrangement.
