Picnic Cat, & proof of concept Aftmast Catamaran Design

[brian eiland]

Picnic Cat, & proof of concept Aftmast Catamaran Design

With this subject thread I am proposing a relatively inexpensive vessel that will act a a proof of concept for my aftmast rig idea, AND end up being a very nice picnic cat for the rest of its life.

I chose the 'picnic cat' basis for 2 reasons. The Thailand one I referenced long ago was built of plywood,and its shapes were very efficient at producing a nice looking vessel with relatively flat panels of plywood,...

Thread 'Weekender/Picnic Vessel, Power and/or Sailing Catamaran'

Jul 16, 2010

Looking thru some photos recently and this one caught my attention again. Just couldn't let these photos languish on my computer. These were taken in Thailand.

I titled it weekender/picnic boat, but really she is large enough for a nice long cruise.

• brian eiland
• Replies: 74
• Forum: Boat Design

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Now forget those twin outboards in the sterns for the moment, as we are going to employ a single motor power in the central nacelle.


In fact that central nacelle is going to be all important to our new prototype. It not only is going to provide a longitudinal backbone to the cat as I have made references to in the past, but it is also going to carry the twin asymmetrical centerboard arrangement I have suggested before, It is also going to house the single propulsion motor/prop like the Stiletto 30 did, AND a simplified single rudder design,..

Weekender/Picnic Vessel, Power and/or Sailing Catamaran https://www.boatdesign.net/threads/weekender-picnic-vessel-power-and-or-sailing-catamaran.33751/page-3#post-872786

Now imagine this Kelsall cat in 2 versions,..one open center deck as shown, or covered with a cabin structure similar to the picnic cat I posted above. The center 'deck plate' between the hulls would be fashioned in a thicker sandwich manner at the base of the aftmast to provide for the extra compression loads by the mast. And those compression loads would be much lower due to the rake of the mast being only 6 degrees,...or zero degrees, and the continuous-line backstays being rigged out to more extended sterns than shown.

I imagine this to be a very economical vessel to build, and one that should be a delight to the owner,..experimentor.

Sorry can't provide any more sketches at the moment as I recovering from a broken wrist.

 

[brian eiland]

A wee bit more on that central nacelle,..

Thread 'Central Nacelle on Crusing Catamaran (lots of pluses)'

Jun 11, 2024

This is an old idea of mine that i think still has lots of merits

I'll come back to a few of your other questions a bit later, but meanwhile I'll post this portion of a previous thread I tried starting with no results. It considers a centerboard mounting that is above the waterline (easy maintanence and repairability without hauling), and a single 'cockpit mounted' engine with steerable outdrive....and the relatively small HP you are considering would be easily compatable with the industrial belts I site.
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"One item of your thought processes caught my...

• brian eiland
• asymmetrical cb's central center board central nacelle wave splitter
• Replies: 8
• Forum: Boat Design

 

[brian eiland]

Single Central Kick-Up Centerboard

"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 off in 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. And the central mounting results in less resistant forces to turning than presented by individual hull mounted boards with their extra lever arm induced forces.

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.

 

[brian eiland]

Bridgedeck Centerboards

This is a VERY long subject thread, but with LOTs of interesting postings in it,..
You might have to devote an afternoon to reading thru it,...maybe just visit some of the later postings.

V

Thread 'Bridgedeck centreboard why don't they work?'

Jan 10, 2017

I was wondering why that single bridgedeck centerboard are not more use on cruising catamaran?

I have try to find the answer and they is no clear answer. On Sir Woods website I have read that bridgedeck centerboard don't really work and that the Prouts brother have tried them with no success.

I can't find a definite answer to the problem of the single bridgedeck centerboard. Is a structure problem? A cavitation problem? Can someone with experience tell me what is wrong with this type of setup?

Thank you for the answer to this issue.

• valery gaulin
• Replies: 550
• Forum: Multihulls