Lengthen Hulls to Increase Buoyancy/Load

Discussion in 'Boat Design' started by abosely, Apr 28, 2015.

  1. abosely
    Joined: Mar 2015
    Posts: 190
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    Location: Big Island Hawaii

    abosely Senior Member

    I have decided to build a Wharram Maui Cat so I have something to play with while building Tanenui.

    It's a Classic design built with backbone keel, bulkheads, stringers & 6mm ply, not Stitch & Tape.

    Basic specs:
    LOA 16' 9"
    WLL 13' 7"
    Beam OA 8", widened to 9', the max trailerable width here on Big Island.
    Hulls beam 2'
    Weight 400 lbs
    Load 450 lbs
    Draft 9"
    Sail Area 100 sqf

    It's a strong design, probably overbuilt by today's standards. But I like that.

    I would like to increase payload to approx 900 lbs by stretching Hulls 2' to 3' and increasing freeboard by 4".

    So my questions are...

    Is this realistic to accomplish?

    Since I have no Marine design knowledge, is there someone who would calculate this for me? Or is this something I need to pay someone to figure for me?

    Hopefully this is reasonably straight forward to figure and something that you all can help me with. :)

    I'll do a thread and document the build since the Maui Catamaran is not a very common boat now days. I know Wharram has a stitch & tape Cat similar to the Maui. I think of the Maui as more of a sailing Land Rover! :)

    I'm not looking for high performance sailing, it's more of a rugged work boat used for snorkeling, freediving, spearfishing, fishing off and exploring, camp sailing being beached around the Big Island.

    Cheers, Allen
  2. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect


    You can simply increase the frame spacing pro-rata to get the Lwl you wish. This way all the frames/section remain fair.

    But you would need to post the Lines fro anyone to calculate if the increase in length provides you with the buoyancy you are after.

    The scantlings may increase a tad too. So before you go down this route you need to double check your weights (adding in for the extra length such as structure weight) to ensure that she will float on the draft you wish and thus if the lengthening of the Lwl will provide you with that buoyancy to offset the increase in weight. Or she may even float too high....you just need to double check..
  3. abosely
    Joined: Mar 2015
    Posts: 190
    Likes: 2, Points: 18, Legacy Rep: 10
    Location: Big Island Hawaii

    abosely Senior Member

    Ok thanks. I was guessing to increase bulkhead spacing evenly to get proportional length increase.

    I'll post up the the dimensions when plans get here.

    So, this should be feasible, subject to checking starting dimensions?

    In reference to scantlings possibly increasing. That refers to that it's a V hull, so when increasing freeboard (bottom of keel to top of hull/deck) and keeping angle or taper of hull the same it will gain some width across the top of the hulls deck also, correct? Making sure I'm understanding terms correctly.

    If so then that is a positive, as the deck being slightly wider is a positive.

    Cheers, Allen

  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    You wish to double the payload by adding 2 or 3 feet of length. I do not think that is going to be enough of a mod to satisfy the extra load. Also with the additional weight, 100 sq foot sail is going to be at a disadvantage, You will need to add more sail area to get equivalent performance from the boat if at all. The hulls have been designed to operate within a specific range of displacements. A massive increase in load will negate the calculations and work that the designer applied

    If you want a modest performing boat that is far more practical for fishing, snorkeling, and exploring, then a monohull will be a better choice. The mono will have more comfort, ability to conveniently store the beer cooler, fish box, and miscellaneous gear. The mono will also be more forgiving about weight variations. Could be easier to build too.
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