26' power trimaran, trawler style family boat

Discussion in 'Boat Design' started by socalspearit, Oct 3, 2022.

  1. socalspearit
    Joined: Apr 2021
    Posts: 82
    Likes: 22, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    I am far along enough on my other build (very small but somewhat sophisticated strip plank carbon fiber composite) that I am stupid enough to start thinking about the NEXT boat, which is very much on the opposite end of the spectrum.

    I want a cheap and cheerful family boat for southern California, mostly to run from an LA harbor to Catalina Island (about 25kts) and hang out for a few days to a week, anchored in one of the coves or on a mooring, maybe head to a further island time and weather permitting. I have been looking around and seen some interesting designs but understably at that size, most boats have some compromises in order to be easily trailerable. I just want something that will fit my 26' slip, or that can be dry docked for even cheaper at one of our harbors, and I am not interested in an easily trailerable boat.

    What I want out of the boat:
    -Outboard powered
    -Lots of living space, much of it indoors/covered
    -Stable and comfy on anchor (good initial stability)
    -Cheap/efficient to run, doesn't have to be super fast
    -Space for 4-5 adults to sleep comfortably with some options for privacy
    -Decent cooking area
    -Head/shower
    -Diver/watersport friendly
    -Needs to fit in 12' wide slip but does not need to be easily trailered
    -Seakeeping is always a concern but it would not be intended to run 'all-weather' or far offshore where weather changes suddenly from good to very bad with no warning
    -maybe 300 mile range
    -hook and line fishing is not a concern whatsoever me but it would be nice for possible resale if a more serious fisherman could look at it and see himself/herself fishing while the rest of the family is happily occupied

    I do not have a ton of experience with multihulls but I am liking the idea of a semi-displacement trimaran--something that would have characteristics that fall between a mono and cat. I liked the floor plan of the Skoota 24'. This build I would do plywood stringer frame with a layer of glass, and generally try to keep everything above the waterline light and sturdy enough for a family but not a commercial/industrial strength.

    I am away from my computers so this is not much more than a 'back of a napkin' sketch for now but this concept has:
    -seperate/private aft queen berth
    -midships area is covered outdoor galley, shower, head (composting toilet)
    -all tanks/batteries, etc, in the center hull down low
    -spot for tender and little lifting boom
    -twin berths in the pilothouse/fore cabin
    -self bailing

    I'd to rough it out in Fusion 360 to work out basic weight/buoyancy/waterline, but if I drop the center hull deeper it would take on more monohull characteristics, right? If the hulls in general are also finer and deeper, that could also reduce tendency for vertical heaving also? Not sure if I am totally missing something, but why are seagoing trimarans of this size almost never done with this type hull in order to maximize deck space? Would it be a nightmare in certain seas for some reason I'm not understanding? Also--for our waters we have consistent long period swells 2-3 ft on many days, 4-6' is a 'rougher day' but outside of storm conditions not much more wind than 15-20kt gusts in the afternoon, with calm mornings, evenings, and nights. Our waters are also deep so draft is not a big issue.

    PXL_20221003_090849855~2.jpg
     
  2. Alan Cattelliot
    Joined: Jul 2021
    Posts: 262
    Likes: 71, Points: 28
    Location: La Rochelle (Fr)

    Alan Cattelliot Senior Member

    Hi,
    I've studied the hydrodynamics of this boat LEEN 56 - LEEN-TRIMARANS https://leen-trimarans.com/fr/gamme/leen-56/ . A little bigger than yours, but numbers can be extrapolated, i think
    upload_2022-10-3_14-13-44.png

    I drop the center hull deeper it would take on more monohull characteristics, right ? If the hulls in general are also finer and deeper, that could also reduce tendency for vertical heaving also?
    => That's partially true. However, it shall be balanced with the "semi-planning" navigation mode that you are seeking, taking into account, also, that in general, such configuration is not really made for planning. Their advantage is their low drag and their excellent stability. So you have to carefully balanced the hulls depths and the position of the centers of the hull. In the drawing down below, are represented the Center of Floatation (CF) and the Center of Buyoancy (CB) of the central hull and the two floats, put together, for the LEEN56. We explore designs with the center of buyoancy placed in front of the center of floatation, in order to help attaining the semi planning mode.

    upload_2022-10-3_14-18-10.png

    Down below are the final hulls parameters, where CC stands for Central Hull, and FL for Float. Dimensions are in m, areas in m2, Displacement in m3. location are given from the forward perpendicular.

    upload_2022-10-3_14-44-23.png

    To illustrate the trade-off that shall be made between beam, depth and drag, here is the result of drag calculations @ fixed speed, made by CFD computations, for the LEEN70, big brother of the LEEN56. I hope this will help in giving you a direction where to go when drawing your hulls.

    upload_2022-10-3_14-51-17.png
     
  3. socalspearit
    Joined: Apr 2021
    Posts: 82
    Likes: 22, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    Thank you! That is very helpful for understanding the multi-hulls. I did not consider Cb and Cf in the last design but for a well behaved multi-hull I can see how that would be critical.

    The LEEN boats were obviously something I looked at for this design as they were the closest thing I could find to what I was imagining--3 similarly shaped and sized hulls to create a large platform as opposed to center hull with tiny amas. Having the hull detail and numbers will be very helpful for study.
     
  4. socalspearit
    Joined: Apr 2021
    Posts: 82
    Likes: 22, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    By the way, it would seem like the foils on the LEEN hull could help immensely when it comes to finer trim. Because of our kelp forrests such a thing would be absurd out here unless on an enormous scale--but if it could be designed to 'clean' itself by folding the wings down flat against themselves then swiveling up to be longitudinally in line with hull it might work. That would be a lot of moving parts and so outside my scope but anyway a little adjustable foil seems like a way to improve multihull planing.
     

  5. Alan Cattelliot
    Joined: Jul 2021
    Posts: 262
    Likes: 71, Points: 28
    Location: La Rochelle (Fr)

    Alan Cattelliot Senior Member

    Yes that's true. We had issues with earlier hull design with a too excessive trim of the boat. That's why we choose to explore the foil option, together with a trim tab option, as trim is essential for a boat to achieve planning In the end, the simpliest solution has been retained, with a re-centered weight and a lowered maximum speed, keeping a desire distance range in accordance with the choosen engine while having a correct trim angle for the vision at helm. Sadly leaving the foil option aside...

    Still, the conclusion of our study was :
    - at speed below 12tks, a trim tab reduces the boat drag more efficiently than foils.
    - above 12kts, a trim tab does not change the boat drag compared to a configuration without trim tab
    - above 12kts, foils diminuishes the boat drag, and would have allowed a greater speed with the same propulsive force.
    - the use of foils allows to control the planning speed of a multihull


    Please note that the value of 12kts is specific to our earlier design. This value will differ from one configuration to another. In the figures given in the graphic downbelow, the surface of the trim tab and of the foil, as well as their trim variation with speed, have been specifically choosen to overcome the formation of a big bow wave at 12kts.
    upload_2022-10-4_10-32-26.png
    Theoretical power curves without foils (wofoil), with foils (wifoil), with trim tab (witrimtab). Horizontal : Boat speed in kts. Vertical : Propeller power in kW.

    upload_2022-10-4_10-35-46.png
    Theoretical trim curves without foils (wofoil), with foils (wifoil), with trim tab (witrimtab). Horizontal : Boat speed in kts. Vertical : Trim angle in degree.
    The data points wifoil and witrim are superposed, as we deliberatly impose a choosen trim angle.
     
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