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Thread: Hull Construction weight as percentage of design WL Displacement

  1. #1
    Join Date
    Sep 2009

    Default Hull Construction weight as percentage of design WL Displacement

    Is there a majic number that expresses the ideal, or near ideal, hull construction weight (With or without rigging)? (Ideal = X% x design displacement)

    I have a set of lines (Proa/Walap), but now must decide how robust (And primitive) the build can be, and still allow provision for more than a weekend sail. I'm at 12:1 Length to Beam with a projected 2600# disp.

  2. #2
    Join Date
    Jan 2010
    Northern Europe

    Default Re: Hull Construction weight as percentage of design WL Displacement

    The only thing i know that comes close is the displacement/length ratio, higher figures given to more robustly built hulls, hence the higher figure and higher displacement. Ideal hull construction weight will be dependent on its material. If you already have a displacement figure of 2600lbs, how did you get to that figure?

  3. #3
    Join Date
    Apr 1999
    Hyannis, MA, USA

    Default Re: Hull Construction weight as percentage of design WL Displacement

    No magic number. Different designs call for different construction scantlings. Also, designed displacement is usually calculated with all the usual ancillary stuff - engines, cabin furnishings, anchor gear, rigging and that - plus tankage half full. If you start from that designed displacement, the larger the displacement, the smaller the porportional impact of adding weight like crew bodies, food, personal gear and all that.

    There's no real magic number for that either, but you can make some approxomate eye-ball calculations from published plans if you figure the surface area of the boat at various waterlines up from the load waterline and then figure how much you push the boat down for a given load. You'll instantly see that some boats suffer greatly from loading more than a couple of inches while others - the LFH Marco Polo hull is a great example - are designed with many of the principles of cargo boats and keeps its general form and sailing trim even if laden for a year's voyage.

    Also, of course, weight placement matters hugely. Many cruisers end up carrying a few drums of diesel on deck or, say on a Bristol Pilot model small cruiser, a dozen 5-gal gerry containers lined up six to a rail. Depending on the size of the boat, this can have noticable impact on the metacentric and besides the boat getting loaded to the point of being a bit loggy, she may also be losing some stability.

    Understanding a boat's design limits coupled with experience solves all this.


  4. #4
    Join Date
    Oct 2001
    Flattop Islands

    Default Re: Hull Construction weight as percentage of design WL Displacement

    This is (guessing) a substantial proa at 2600 pounds. The 40' Newick design Cheers(1968) was 3000 pounds with 16'8" total beam in an "Atlantic" configuration(carrying the ama to leeward). The best guide for scantlings will come from existing vessels. Skin thickness is based on panel size (distance between framing) and speed, more speed and less framing equals a thicker or stiffer skin. Russel Brown knows as much as anyone about proas so either study his work closely or contact him directly.
    cogge ketch Blackfish
    cat ketch Ratty

  5. #5
    Join Date
    Oct 2000
    Padanaram, MA USA

    Default Re: Hull Construction weight as percentage of design WL Displacement

    It'll be an ongoing process until after the launch, but I'd recommend that you develop a weight budget for the boat. The weight of the boat, gear, provisions and people should match the displacement of the design. If they're out of sync, it's better to discover that early.

    The budget can start off with 10-20 items and will eventually be hundreds. Develop a comprehensive list of the unit weight for the boat's materials and components and get careful with takeoffs of areas and lengths. Excel is your friend. If you're worried about center of gravity, and I would be for a proa, the same table can be the key tool in that if you record the positions of components. Calculating moments and he CG position is not hard.

    Be very skeptical of the claimed displacement of other boats. It's one thing for a designer to draw a hull, calculate its displaced volume and declare what the weight needs to be and it's quite a different thing for a long-suffering builder to bring the boat in at that weight. You're only gonna do it if you plan it up front, in reasonable detail.

    Probably the most important step in developing the weight budget is in selecting scantlings for hulls, deck, bulkheads and in your case, akas. I've gotten help in specifying scantlings from the manufacturers of core materials and Dave Gerr's Boat Strength book is a good resource, too. The Gougeon book, too. For more traditional construction, where dimensions and scantlings are pretty well described, it's critical to know the density of the actual material you'll use.
    Building light is full of trade-offs. If you start with a structure that's just strong enough, adding lightness requires adding money and/or adding time. Doing the details with light weight in mind can get incredibly fussy and I've sometimes wondered whether selecting a lighter general scantling scheme at the outset would have saved time in the project as a whole. When you're vacuum bagging a foam-glass cabin seat, you wonder whether using carbon in the hull might have given you the slack in the weight budget to do the seat of plywood. An appraoch to resolve that dilemma is to keep in mind the 'shadow price' (look it up) of a pound. I.e., what's the value of a pound saved or what's the reduction in the value of the boat if it's a pound heavier. For a light multihull, I adopted a value of $50/lb. It'd be higher today.
    Last edited by JimConlin; 02-14-2013 at 10:14 PM.

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