Central Nacelle and Centerboard on CRUISING Cat

I posted this in the general design category rather than this multihull one.

An older idea of mine that I think has lots of merit, particularly for a world wide crusing cat. 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 compatible with the industrial belts I site.
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"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.

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 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 http://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 whatever."

couple of older illustrations to suggest the idea,..





RunningTideYachts.com

 

Appears that link to 'Earthling' is dead. Perhaps this link would help

Earthling catamaran's stern | Boat Design Net https://www.boatdesign.net/gallery/earthling-catamarans-stern.21550/

 

Hi Brian, I like the idea of two CBds, but in a different way. Possibly asymmetrical LH and RH may not be as hydrodynamically efficient as a pure NACA form, and swapping CBds on each tack could induce wear at a greater rate. I would propose a CB for deep water and high speeds, with weight down low; and still using your same systems, and a shorter depth and longer chord lower speed NACA form for shallow waters and exploring eg. creeks and narrow / shallow waterways, going at slower speeds. A bit like a two bladed pocket knife with a long blade for heavy work, and a much smaller one for intricate use. Both can kick up. A long CB blade when lifted up for shallow work moves the effort rearward to not optimal, the shape through the water flow isn't optimal when at an angle, and its bottom could be a point of drag. Whereas a short shallow work CB would be more optimal through shallower waters, until it is kicked up for beaching. I have mused about a longer and shorter system myself, for optimal full depth, and useful extreme shallow work.

 

Asymmetrical vs Symmetrical

Lets see if i can use this excerpt & diagram to help explain,..

Remember the 'lift' we are trying to maximize via our board(s) is to offset the sideways push on our boat by the sailing rig. In that second illustration the symmetrical board must be sailed at a higher of angle of attack to generate its lift,...and that angle of attack means it must be sailed higher into the wind which can be challenging for some of our multihulls, particularly cruising ones.

If I were talking asymmetrical daggerboards or centerboards in their individual trunks I might agree with you. but I am placing the FLAT side to my boards against the FLAT side to that central 'nacelle fin',...and utilizing a very BIG bearing. The combination of that big bearing and slippery faces on the fin will make things work with very little friction while offering a nice big area for sideway loads that those boards exert nacelle fin.

I also feel strongly that having that centerboard arrangement 'balanced' right under the CE of the sailplan will be a significant factor.

 

I recall years ago when a designer Bruce King added asymmetrical boards to his SORC design,...very successfully, but the rating guys didn't like it, if I remember correctly. (Many years ago in a racing series known as SORC there was a twin dagger board boat that performed VERY well. It had aysemmetrical boards. It got 'rated' out of existance because it performed so well.)

gybing center boards https://www.boatdesign.net/threads/gybing-center-boards.43772/

one of the postings on that subject thread,....

another posting of mine,..

 

It could work if they are very short and stiff, with tight precision bearings. The leeward board will want to deflect into the centre, and into the rotational path of the other side, in operation, and then on turning the hull, the induced twists could still cause interference if you wait to swap over on a turn. The leading and trailing edges may need compromise to avoid acting like scissors against each other.
Not knocking it, give it a go; maybe on a model sailing boat first, just in case.

 

Actually its the windward board that would be deployed when tacking to windward.

That dwg suggest a rather thin plate they would be mounted to. In reality I suggest it would be a bit thicker to get a bigger flange surface of hi-modulus fibers on its bottom edge. Thus no scissors effect.

And remember this is an idea to be used on CRUISING boats. The accessibility for maintenance (even in remote locations) would be a real plus over any other 'board' designs.

 

Hello hello
Can I ask some advice or should I start a new thread … that is the question

I've been building my 60 foot catamaran for a few too many years and nearly there

Now I'm worried about how to make the dagger boards go up ad down easily - they are hollow and made of carbon and epoxy - and they float

Thinking V rollers in the low end of the case or maybe UHMWPE V blocks

Then there is the question of barnacles

Any thoughts greatly appreciated

 

Michael. Daggerboards go up and down fairly easily when they are not under lateral load when you are sailing, on most cats I have been on as you tack the boards will move easily. When hard reaching have a line leading to a winch or point up upwind. Your real issues are what part breaks if you hit ground, the board or the back of the daggerboard case. The next issue is how do you clean the case out and keep the board clean. Being able to lift the board completely out of the case helps. Finally it is a lot better to have a broad back edge of the daggerboard at the point it intersects with the bottom of the case when the board is fully down. Why? Fine back edges of boards chip, do hairline cracks and try and cut into your case. Have fun from a person who has broken at least 6 daggerboards on large cats.

 

thank you Old Multi - glad to hear the daggerboards should come up and down without too much trouble
I'm getting close to launching and fretting about the slots in the keelson because the shipwrights who are helping me (because I'm the oldest multi in the village) warn that dagger boards are always jamming
As for breaking them - well yes, they are very vulnerable - they extend 2400 below the keel
The cases are framed in spotted gum glassed over with double bias and epoxy and the aft end of the case is bonded to a substantial bulkhead - this T shaped structure is much stronger than the boards
So it's most likely the carbon boards will break off - hard to say where - we strengthened the laminate where the cantilever emerges from the keelson - and the boat will not crack open

 

I have a 9.5 meter open deck catamaran of my own design with single pivoting kick up centerboard under the mast support beam. I really like it, everything is accessible, no underwater parts. I also have transom mounted kick up rudders. There is nothing that needs access or maintenance under waterline just smooth hull surface. A surface piercing foil is less efficient so it costs a little bit upwind performance, but for a boat that's not intended to race it is acceptable tradeoff. I still get better upwind performance than minikeel boat and with board and rudders lifted draft is only 30 cm.

I once accidently hit a barely submerged pile of rocks when sailing at maybe 7 - 8 knots, hulls lifted significantly upwards and slid over, boards kicked up and once in deeper water again rudders were pulled back in by bungee cords and I continued on my way. Also I have hit a huge multi ton log with similar results. There were some scrapes and minor gouges in fiberglass and nothing more, easy to fix later. A similar accident in a regular production boat would likely cause minikeels, saildrives and rudders broken off possibly leading to flooded hulls, maybe even sinking.

Single pivoting centerboard definitely have some significant advantages for cruising boat. Cruisers try to avoid sailing to windward if possible in the first place, waiting a bit for favorable downwind or at least less hard on the wind sail is perfectly acceptable meaning there is less need for board to be deployed at all. It is not as important or used as often as it would be on racing boat. Easy maintenance, accessibility, repairability and very shallow draft is more important to a cruiser than bleeding edge upwind performance

 

Start a new subject thread for that question. It does NOT deal with kick-up centerboards.

 

Happy for your choice. !!
I had a hard time selling the idea when I first suggested it.