Advice on fitting a narrow "under hull" into existing one

Discussion in 'Boat Design' started by Tommifin, Oct 26, 2014.

  1. Tommifin
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    Tommifin Junior Member

    Hi fellow boat enthusiasts!

    I've been designing a new boat in my head and on computer but now I bought a 6,5m sailboat hull, GRB ready for a couple of hundred to base my project on.

    I'm planning to make this a trimaran style by adding a ultra narrow extra section to the underside tha will just about float the boat, and then add outriggers to balance it.

    Problem is what is the best way to get the dimensions accurate enough to make the new underside separately (the boats going to spend the winter under a tarp in finnish winter) and fit it in the spring.

    I will try to make it by stich and glue laminating panels onto glass table and cutting to size, then reinforce after assembled.

    I'm a bit sceptic on opening the bottom of the sailboat hull, so if possible to laminate it on top of the old but might be hard.?.

    How to transfer dimensions from the old boat?

    Any suggestions how to go forward?

    EDIT: this will be going by motor, not sail
     
  2. gonzo
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    gonzo Senior Member

    The volume of the added section will have to be equal to the submerged volume now, plus the equivalent to the displacement of the added materials. In other words, you are making it heavier and increasing resistance.
     
  3. Tommifin
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    Tommifin Junior Member

    Thanks
    I will be adding some weight, but goal is to get some more cruising speed way over the hullspeed. The length/width ratio around 12:1 on the center hull. this would also act as wave piercing hull on lower waves.
     
  4. gonzo
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    gonzo Senior Member

    You would have to add a hull that is probably three times longer than the existing to get that ratio. You will need more power for the same speed, since you are increasing the displacement and the wetted area.
     
  5. Tommifin
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    Tommifin Junior Member

  6. gonzo
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    gonzo Senior Member

    Real budget project and high speed don't go well together. If you are on a budget, put an engine on the boat and go out in the water. What you are proposing will make a heavier boat that will require more power for the same speed as the original hull. If you are only going to motor, the keel can be removed to minimize resistance and draft. Some ballast may be needed, depending on what the hull design's stability is, but can be installed internally.
     
  7. rwatson
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    rwatson Senior Member

    hmmmm .... tricky, I notice they didn't get over 5 knots in the demonstration, and there was a lot of lean in the cornering.

    I am wondering if the have sorted out the power handling totally ?

    Gonzo has raised some good points.

    I think it would actually be cheaper and easier to build a whole new hull than modify an existing one. Getting the engineering to work on a hull that has been designed to just be a simple sailing boat, would be very fraught.

    You could use your existing hull as a mould, and put mdf 'addons' on top to make a male mold you could fiberglass. Later, you could pull the temporary bits off and still have the original hull.

    The big question is what will you gain for all the effort and time and money ? If you just want local notoriety, it will work, but as far as ease of launching, fuel savings, stability, etc - there are absolutely no guarantees.

    Now lets talk about insurance ... :(
     
  8. messabout
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    messabout Senior Member

    Forget the monumental changes to the present boat. Just build a new boat with the design characteristics that you deem appropriate. Chances are that a new build will be less costly and more efficient, Making a silk purse from a sows ear has historically been difficult, sometimes impossible.
     
  9. Tommifin
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    Tommifin Junior Member

    Thank you, all good advice.
    I think I will still try to play with this a little. Still relatively cheep to do some pontoons and already have the comfort of having the main hull with deck, seats and shelter :)
    So I'll consider this a test rig to learn the hard way :)
    *** first into a tree as they say.
    I'll update when I get something conrete done.
    The grainger did do 10Kn with 2x4kw torqueedos so maybe a 30HP old evinrude or something could be a good choice with 4hp 4 stroke for comfortable short distances
     
  10. rwatson
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    rwatson Senior Member

    If you are unsure of the way to measure the appendages, then I am guessing you are not an expert in glass layup engineering either.

    For a start, have you calculated the weight table, including motor, crew and current hull to ensure proper trim and bouyancy.

    You're a braver man than I am. To get the buoyancy and stability right, with a 30 hp outboard, all hanging off a lightweight yacht hull - the chances of wasting your time and money seem high.
     
  11. TANSL
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    TANSL Senior Member

    Tommifin, after everything you've read, my opinion is that you need to keep going deeper into your idea. There may be better things to do that what you want but you must try to do what you want and not what others, less creative and without your motivations, would do.
    It may be foolish or very expensive but, at the moment, nobody knows. If you have a picture or more information about your idea, maybe someone with a reasoned response, could tell you how far you can go with your plans.
     
  12. Manfred.pech
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    Manfred.pech Senior Member

    [​IMG]

    This Trimaran "fingermullet" was designed and built for a long thin "hull under the hull" and it seemed to work sufficiently.
     
  13. rwatson
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    rwatson Senior Member

    sufficiently ?

    All that time and effort - at least it floated I guess. My bet is that he found that the 'easy' original flat bottom design pounded too much, so he had to 'tack on' some kind of wave breaker.

    If you look closely, you can see some construction lines where it looks like he had to add another 6 inches to the original hull, was this a buoyancy problem ?

    Its easy to get caught up in a "maybe this will fix it' cycle and rebuild over and over again, and still get results that are inferior to a readily available plan. I plead guilty to that myself :(

    If you have nothing better to do, and plenty of money for materials, go for it, but otherwise you need to think about it properly.
     
  14. SamSam
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    SamSam Senior Member

    There are various ways to do it. You can make the new shape/hull you want to add on, but make it extra tall. Then in the Spring, you can position it in place and use a compass to scribe the old hull shape onto the new hull. The first scribing will be apporoximate, as you cut the extra parts away and get closer to the final shape, the scribing will become more accurate. If you do 2 or 3 scribings you will get close enough to be able to tab/laminate the new hull onto the old one.

    I wouldn't cut the old hull, at least not until the new one was attached pretty solidly.
     
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  15. bruceb
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    bruceb Senior Member

    Copy something

    You might look at some of Bolger's "Micro" ply designs to get an idea of size for your center "hull". The center hull doesn't have to completely support the main hull for it to work, it is easy to calculate the displacement of a "box" and I think you could make an open top ply "box" and scribe mark and cut and fit as necessary, then epoxy/glass it all together. Crude, but probably more functional than might be expected, particularly if your sailboat hull has a flat run aft.
    Put some positive floatation in it! ;)
    B
     
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