calculating displacement

Discussion in 'Boat Design' started by 8knots, Mar 17, 2002.

  1. 8knots
    Joined: Feb 2002
    Posts: 266
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    Location: Wasilla Alaska

    8knots A little on the slow side

    For those of you with a firmer grip on the math aspect of design this will seem a silly question. When calculating displacement, when do you add the weight of crew, diesel, water and all the rest of the weight she must carry? I figured all the areas of my sections with the (DWL) where I would like her to float ready to go to sea. Therefore all my calculations are based on that DWL. At approximately 1750 lbs in immersion, by the time I add 400 gallons of diesel (2880 lbs) 200 gal fresh water (1660 lbs) 2000lbs of engine and gear etc. she will trim out several inches lower. When starting out a new design does one account for this added trim by experience when he/she sets about establishing the design waterline?
    The hull I am working on is the 40’ trawler I have posted before. Her particulars are as follows

    LOA 40’2”
    LWL 38.25’
    BEAM 12’4”
    WL BEAM 12’0”
    DRAFT 4’6”
    B/L 3.3
    M/C .68
    D/L 129….????
    Midship section 29.52 sq ft X LWL 38.25 =1129.4 cu ft X 64 =72,282 lbs X (PC.54%) =39,032 lbs DISPLACEMENT????
    Just for kicks I was reading from one of Gerr’s books and added 15% to that and came up with 44,850 lbs with a D/L of 148 “light for what the books say”(coastal cruiser)
    Anyway I will keep at it if any body has any input I would greatly appreciate it! 8Kts:confused:
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  2. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer


    She is a good looking boat, now on to the really interesting stuff.

    You don't state the basis for your weights so I will just run over the basic procedure. The baffling part of yacht design is that you need to know the weight of your boat before it's designed. Of course you don't, so you take a first guess to work from. Then you draw the boat out, add up all the weights and their centers, then change the hull lines to match displacement and CB. While doing this you must of course keep performance and seakeeping firmly in mind.

    The first thing to do is look at a few comparable designs, published weights are not accurate but they are a place to start. Voyaging Under Power is a great reference for your type of boat. Think in terms of comparable size and construction material. A steel boat will obviously be heavier for it's size than a glass version. So you have a guessed displacement, then you start to draw out the boat. At the same time you need to start a weight study. The best way to keep track of this is in a Lotus or Excel spreadsheet, because all these numbers will change as you go along. Start by adding up your hull structure weights, then deck, cabin, interior, machinery, plumbing, electrical, hardware, and paint. Then add a 5-10% margin to all of this. Then add the ballast.

    You now have "Light Ship" weight, also known as shipping weight, and this is the number most often published concerning powerboats. At any rate, now add up all the payload weights; crew, stores, fuel, water, sewage, and owner's items. Add a margin to this. Take half that payload weight, add it to the light ship weight and you have "Half-Load Displacement". This is a good number to use for your DWL (design waterline). And design waterline is all it is, the boat will never float there as loading will always be something different than what you calculated. That's why we only take half of the payload weight, it represents the midpoint in a range of floatation. Of course with computer programs we can now look at a hull at all stages of loading and trim.

    So that's it, simple.

    Good luck, Tad
    1 person likes this.
  3. 8knots
    Joined: Feb 2002
    Posts: 266
    Likes: 12, Points: 0, Legacy Rep: 352
    Location: Wasilla Alaska

    8knots A little on the slow side


    Thank you very much for the help!! Considering you do this for a living, I think it is very commendable on your part to help the novice like myself! Thanks again 8Kts
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