Split trimaran?

Discussion in 'Boat Design' started by pbmaise, Sep 15, 2019.

  1. pbmaise
    Joined: Jul 2010
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    Location: Cebu the Philippines

    pbmaise Senior Member

    So I have sold my 65ft by 40ft trimaran and have been thinking of going larger. A few problems with my previous vessel come to mind.

    #1 I couldn't beach it to paint.
    #2 I worried about the single engine.
    #3 None of the local yards I wanted to use could haul it out.

    I am thinking along the lines of motorsailing and commercial dive boat operations. This means maximizing width for stability and accommodation.

    I am thinking about using a sail plan that is geared mainly for running and decoration. Sail area will be unbalanced and tend to push the bow to the lee when beam reaching.

    Picture now a trimaran as follows:

    LOA 70-72ft and width 50-55ft

    The center hull is split down the middle and the two halves of the vessel bolt together with a rubber seal between two flat surfaces. The wall interface would extend below the vessel to form a fixed continuous daggar board that would support the vessel when beached for an update to the bottom paint.

    The same wall would extend continuously upward till it was above the waterline. This would allow the two halves of the vessel to be unbolted in calm water and travel lifted separately. Initially I am thinking of keeping all bolts above the waterline too.

    Target speed under engine is 8 knots and I am thinking along the lines of two 150-200 hp diesels mounted in the two ama. I will want to be able to pull the shaft when beached and replace the cutlass bearing.

    Trips are under 200nm return and I can accept not departing if either engine is not working. Further I can accept limping or towing should an engine fail during a trip.

    I am thinking of going rudderless and control engine throttle with an autopilot to provide steerage. While under sail one engine would be run to balance sail forces and power electric loads.

    It would took quite more text to explain the sail plan I am thinking of. There would be four rather short spars and no booms.

    Has anyone designed a split vessel in such a manner?

    Is rudderless practical?

    300 hp engines and
  2. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Your concept certainly contains some “out of the box” thinking!
    It’s unusual configuration might make it very hard to insure, especially for carrying paying passengers.
    I’d say that the rudderless configuration would be horrifying to sail, and not very good under power, though I do like the idea of using an autopilot to control the thrust.
    Why not use a viable sail plan? It wouldn’t be much more difficult, and could save your *** someday.
    Since you’re planning some type of keel on the center hull, hanging a rudder there shouldn’t be a problem, even if slightly off center to remain on one side when the hull is split.
    Cutlass bearings can be easily changed without pulling the shafts, but with correct installation and alignment, the bearings should last many years without changing.
  3. pbmaise
    Joined: Jul 2010
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    Location: Cebu the Philippines

    pbmaise Senior Member

    Thanks for the suggestions. Yes if I place one central rudder behind a protective keel it would likely be fine even if offset slightly from the center line. My previous rudder was 2.2 meter tall and one of main reasons beaching was out of the question. That and what is the point of beaching if most of the hull in the previous design would be resting on the ground and hence unpaintable.

    The reason for not going with a full rig is partly owing to the fact that during the dive season winds are next to nill. It is also owing to cost.

    Guests for a dive trip are concerned first with the diving, second creature comforts and meals, and sailing is not a priority. One competitor with an actual sailboat never puts his sails up unless taking a photo shoot for advertising their trips
  4. CocoonCruisers
    Joined: Dec 2015
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    Location: Marseille & BuenosAires

    CocoonCruisers Junior Member

    Love these radical threads, especially when there is an actual use case. The watertightness challenge for your center joint looks scary though.

    (I suppose you could get away with half hulls that are actually closed. I guess your main hull is mostly for toilets, kitchen, tanks and engines in that usage scenario, so perhaps splitting the livingspace isn't such a big deal in your case. You could even have protrusions from one side into the other.)

    Perhaps you could consider hinged amas that can be lowered a few feet. Beach, and ballast one ama to raise and paint the other, lower both to jack up the main hull for painting and any repairs on the propulsion. Depending where you are, it might even be cool to take the clients to a beachside cocktail, giving the seasick ones an hour of relief on a stable platform, and letting the more adventurous ones wade over safely. Unbolt one ama before hauling out. A hinged ama design would probably look less scary to insurers because there already are quite a few folding trimarans. You'd set that up with substantial locking mechanisms for underway, and even if one of them fails the you'd just end up with a part of the deck inclined, or closed to the public because it is wobbling about 15 degrees or so; Who cares, boats roll, sailboats heel and everybody is comfortable with that normally. And you'd still have enough stability to motor home safely.

    Engines in the amas sound suboptimal in terms of weight centering (resulting in gentler motion - guess that matters for day charter), because of prop ventilation in waves, and also because of the inflation in ama volume that this would generate (for engine width and access, and to handle the engine's weight/inertia). For efficiency too, you'd better concentrate the weight in the main hull. You're targeting powerful engines with large props for 'displacement' speeds. Then spacing them just as much as hull width allows should already give enough leverage to manoeuver like a motoryacht, or to offset rig unbalance without giving much rudder. And then you could really rely on either engine/driveline to get home, if one fails.

    Rudder: as long as you plan to always have an engine running, motoryacht-style arrangements with small protected (beachable) rudders behind the props should do. Else, or in case you need some kind of centreboard, have you seen the kick-up rudders on the imoca's , mini 6.50 and class 40 ?
    Last edited: Sep 16, 2019
  5. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    How will you steer without a rudder if you have an engine failure? What’s the issue with having one? They’re not very complicated, certainly a lot less complicated than an autopilot that would be controlling two engines.

    On a boat that size you could have twin engines side-by-side in the center hull.

    Rather than going through all of the headaches of splitting and sealing the center hull, consider hinging one of the amas. By doing that you could tilt it up to paint under it, then roll the boat to paint under the other. Configure the hinge so it can be completely separated for haulout - put a temporary float under the hinge on the single ama side.

    If you’re set on the split hull, why not two solid bulkheads that mate together. Then sealing between them isn’t an issue. For the short trips that you’re doing, could you live with two long, narrow cabins? Or a minimum number of penetrations between them, then the only thing requiring sealing would be around the perimeter of the penetration. I think you want bolts near the bottom of the dagger board for maximum structural strength, recessed to minimize drag.

  6. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    There are simpler ways to achieve what you want, namely beam reduction and beachability.
    First the beam reduction. Here I see two simple posibilities, demountable beams and telescoping beams. Both rely on staggering the beams fore and aft. Either put them in a box and make them slide in and out (search telescoping trimaran for pictures) or bolt them to the main hull. If bolted, when dismantling you just unbolt the beam ends from the main hull and drop them on inflated tractor innertubes lashed underneath. Use a dinghy to maneuver the resulting raft. Both options require more structure in the main hull then continuous integral beams but are much simpler to make then splitting the main hull. All fasteners are above deck.

    For beaching all you need to elevate the boat off the bottom are folding legs under the main beams. Put a hinge on the main hull side so that the legs fold up sideways under the beams. On the outboard end of the leg another hinge for a strut that gets braced on a socket underneath the beam, ama side. When beaching you drop the main leg to vertical and move the strut to the socket creating a triangle with the main beam. Triangulate further diagonally between main legs bottom and the other main beam to create a pyramide. Under the legs you need some plywood pads to spread the weight of the boat on the soft bottom so they don't get driven into the sand. If you want you can put wheels on the end and drag the whole boat up the beach with a winch or a tractor.
    When not in use you simply fold the struts under the beams and trampoline and fix it there by lashing or dedicated sockets. If you want to it can be designed to diagonally brace the beams when folded.
    The rudder you make pivoting or retracting, standard stuff.

    I would put the engines on wings on the main hull with retractable driveshafts. Something like a longtail fixed in the middle of the boat that can only pivot up and down would do. Retractable nacelles like some multis use for outboards can also be done with diesels and I/O drives or saildrives.

    If you sail only downwind just put a folding mast on the boat and carry some spinnakers.
    CocoonCruisers likes this.
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