Adjust dwl

Discussion in 'Fiberglass and Composite Boat Building' started by Scuff, Nov 4, 2021.

  1. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    I'm building a 27'9" trimaran. The plans were for cold molded wood strip but supported foam/glass. I opted for foam/glass and didn't reduce the frames to compensate for the difference. I also lengthened by 1'. Both of these were with designers approval. The foam glass increased the beam by about 1.5" per side (3/4 stringer and 3/4 foam). I'm guessing there would be some change to the waterline .. is there a way to make an estimate of the change? He lists 3500 lbs.for lwl. I've tried contacting designer but not getting a response. I'm an amateur builder. Thanks.
     
  2. TANSL
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    TANSL Senior Member

    Have you got a body lines plan?. With it, modifying what is necessary, it could be calculated quite accurately, although it would also be necessary to know the total added weight.
     
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  3. bajansailor
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    bajansailor Marine Surveyor

    Are you able to get a rough idea from the plans of the wetted surface area of the hull at the design displacement?
    If yes, then you could get a rough estimate for the increase of buoyancy by multiplying this surface area by 1.5 / 12 to give the amount of increased buoyancy in cubic feet.
    Once you have a figure for the increased buoyancy (in cubic feet), then multiply this by 64 to get the increase in the displacement (one cubic foot of seawater weighs approx 64 lbs).
    Re the increase in length of one foot overall, a simplified way of allowing for this would be to then multiply the new displacement above by New length / Old length (yes I know that this is not 100% correct, but it should be close enough for Scuff's purposes).
    If the new length is 27'9", and the old length is 26'9", then the multiplying factor would be 1.037.
     
    Last edited: Nov 4, 2021
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  4. fallguy
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    fallguy Senior Member

    Well, if you make a boat bigger than plans; it floats higher for the same weight, but the weight doesn't stay the same! But let's say you get some marginal increase in displacement and the boat is a bit lighter in foam; the lines might theoretically be lower, but how much?

    I'd say about an inch, maybe more if you did all infusion..

    A lot of posters might get upset for my finger in the air here, but unless you can tell us the displacement or the original hull and the vessel weight differences between foam n wood; it is rather difficult to pin it better,
     
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  5. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    That's why I asked. I've only the listed weight provided by the designer. My goal is to keep the weight below 3500 which is what he lists. The ama's weigh around 250 lbs. based on the glass/resin used. I'll know how much the main hull weighs shortly .. guessing 400 lbs. I wanted to add a hard spot on the bow for an eye just above the waterline.
    I'll look for the lines drawing today and add it here.
     
  6. fallguy
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    fallguy Senior Member

    I wouldn't worry about it much. When you bottom paint; you are supposed to go 3" above dwl. Just go 4.

    If you are too high, you'll ballast down anyway I assume.

    It is always nice to be lighter,

    But you can calculate the weight delta. Just make an estimate of the hull only for each method. I wouldn't worry about the size change much because it is in your favor. On my boat, I made the hulls 1.5" narrower each and I calculated a 200# difference per hull or lost displacement. Your boat is a bit shorter, so you'll pickup say 300# displacement for the size change and the figure the weights difference for the hull. If you are, say 300# lighter, then you can adjust things.

    If you have ppi or kg/cm immersion data, you'll adjust the ppi by the displacement change percentage and then determine the effect of the weight against ppi.

    For example, ppi of say 450#/in.
    Displacement increases from x to x+300 or say 3%.
    Adjusted Ppi is say 450•1.03 or 463.5#/in.
    Weight calculated at 300# less(you can do a real calc).
    300/463.5 = 0.65" higher waterline

    Any hydrostatics at all?
     

  7. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    No hydrostatics .. at least I don't think so. I'm in agreement with the 3-4" above wl for bottom paint.
     
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