Hull Design

Discussion in 'Boat Design' started by RichardBoatBuild, Apr 11, 2021.

  1. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re your stern side elevations shown above, A would have the least resistance, all else being equal - in fact you ideally want the waterline to be at the chine intersection of the transom and the hull bottom sloping up.
    But that will probably call for lots of iterations of buoyancy calculations and weight estimates, in order to achieve this.

    But very few designs start with a clean sheet of paper (or a blank computer screen) - most have evolved or been adapted to suit from other designs.
    Hence there is no shame in starting with an existing proven design that has been proven to work, and tweaking it slightly to suit your particular Statement of Requirements as to what you want the boat to be capable of.
  2. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Hello Richard! What's your statement of requirements? Like will you be 99% docked to one location or travel rivers? What country / locations? What do you need the boat to do? How long and wide can it be? Trailerable? How many people living on board?

    I give you my thoughts because I want something similar, but I have very little experience boating and none building boats. I'm a newbie myself.

    I'd look into canal houseboats and search for building plans for those. From what I understand the double ended hull shape is the most efficient for slow displacement speeds. Meaning you need little power to go slow. Most sailboats also have this shape at the waterline even if they have a transom. Basically at slow displacement speeds the water rushes back in at the stern and pushes the boat forward, so gives back some of the energy spend to part the water in the front. But it adds length to the boat.

    A houseboat on a river can have a flat bottom and doesn't need flare (angled sides). I'm not sure about this either but I think flare helps against loud wave slapping and splash.

    A box shape where the water has to flow under the hull can be good for planing but you'd need tons of power. I'm not sure if any houseboats actually do plane and if not why you see that hull shape so often.
    If you only ever get towed once or twice while having the boat it doesn't matter and a box shape is easier to build.

    There are also pontoon boats but they are not more stable than a flat bottom boat of the same width because they sit higher up so shift the center of gravity up. But if you do build a pontoon boat wider than it becomes more stable and more efficient. But for a river boat I think that is not that practical. And it's harder to build.

    You mention solar, keep in mind that affordable glass solar panels add some weight to the top, making the boat less stable. About 12kg per square meter. The flexible ones are lighter but more expensive and don't seem to last as long.

    Here are two designs with plans that might be interesting but maybe more involved than a simple houseboat. Also I'm not sure about the stern hull shape of these either. There are lots of other designs too.
    TC35 Michael Storer 38 11.70m 2.13m 1275kg 4300kg
    Mark V39 by Mark Van Abbema 39 11.81m 2.75m 0.92m 4.32kg
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  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Also on Duckworks is this free plan for a small houseboat which is a similar concept to Richard's proposal.
    Lisa B Good - Free Plans
    It would be worthwhile downloading the free plans simply for all the useful advice that you will find within.

    At the other end of the spectrum, and also on Duckworks, is this Eco design from Bernd Kohler - above the deck level she is of similar concept to Richard's proposal, and she is only 6.2 m. long (about 21'), but she is a real 'quart in a pint pot' - with very efficient (compared to a rectangular box hull) catamaran hulls giving her all the buoyancy that she needs.
    ECO 62 Houseboat Plans
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  4. TANSL
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    TANSL Senior Member

    Totally agree.
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  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member


    I think there are a few things you need to consider before you even start doing the designing. I'll put them I lettered categories.

    A.) Use of boat.
    1.) for weekending and maybe one or two weeks of living aboard at a time?
    2.) for living aboard two weeks to two months?
    3.) living aboard an entire season to year round?

    B.) Performance of boat.
    1.) how many people living on board at one time?
    2.) how often will the boat be moving? Every once in a while? Constantly while occupied?
    3.) what's the desired speed? Displacement speed (5 to 7 kts)?
    Semi-displacment speed (7 to say 9 kts)? Planing speed (10 kts+)? With each higher speed category comes a much higher hp requirement. Displacement speed requires about 5 hp per ton. Multiply that by about 4/3rds to calculate the engine hp rating you will need.

    C.) Practical considerations.
    1.) how are you going to get the boat into and out of the water? Marina travel lift? Personal boat trailer? If you choose the latter, you must restrict the Beam of the boat to 8.5 ft (2.59 m) to be allowed to trailer it without special permits.

    Once you have answered the above questions, you will be in a better position to calculate the displacement you will need.

    For any performance level above displacement speed, the engine and its fuel will take up an ever greater portion of the total displacement. This is why we don't see planing freighters (or even semi-planing ones).

    It's probably best to add up all the people who are usually going to be aboard, then multiply that by how much water, food, and personal gear they are likely to bring along. Then add the weight of any heavy items beyond that that are to be carried aboard, such as movable furniture, and perhaps the weight of a portable power generator and its fuel.

    Once this is all totaled up, you can multiply this by about 2.5 to get the first guess on what the total displacement is likely to be.

    Then you can calculate the hp rating you need, then find out how much the engine is likely to weigh. Next, you calculate how much fuel you're going to need. As a general rule, multiply the hp requirement by the number of hrs it will take to get to the next refueling stop, then divide that by 12.00 to get the number of gallons you will need. You might want to multiply that product by 1.15 to give you a reasonable fuel reserve.

    Now you will know the likely displacement of the boat, as well as how much engine and fuel it will likely need.

    You will likely end up with a much lighter boat than you have drawn.
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