Hull speed/drag...theory vs practice?

Discussion in 'Boat Design' started by Bill Fish 6, Dec 2, 2004.

  1. Bill Fish 6
    Joined: Nov 2004
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: pensacola, florida

    Bill Fish 6 New Member

    Hi all.

    Lets say you want to design/build a relatively narrow hulled boat, 18 to 24 foot long, 2000 lbs all loaded up (give or take).

    And lets say your trying to design it to minimize the amount of horsepower needed to make it go 10 to 12 knots (roughly twice "hull speed").

    And lets say you want to build it with only handful of developed surfaces (ie surfaces curved in only one dimension).

    And you dont really care about how things are laid out, its beam or draft, or stability or anything like that. Your overiding goal to carry a given payload (1000 lbs give or take) at said speed with minimum power. Everything else is secondary.

    Now, the questions. How accurate are the cheaper/easier hydro/boat design codes going to be in predicting the power requirements? 10%, 20%, 50%!?

    And even if the codes are off a good bit, are they going to be helpful in finding/developing a design that represents a "local minimum"? Or in other words, even if the predicted power requirements are off, you still end up with a design that has about the lowest power requirement given the constraints you've placed on it.

    Of course I said "long narrow" hull, but might a shorter, fatter one end up being the optimum?

    And probably the question I am really most interested in is this.

    I go to design and build said boat (with hopefully some guidance from the "experts"). I build the boat with a handful of developed surfaces and end up with decently smooth surfaces.

    Now a well funded and very experienced bunch of boat "gurus" go to design the same boat. But their boat must be the same length, same weight, and have the same decently smooth surface. BUT they are allowed to make their hull ANY shape they think will minimize the power requirements.

    How much better is the "guru" boat likely to be? 10%, 20% 50%? And how would the answers change if our design goal was more like 1.5 hull speed? Or even just hull speed?

    And

    Once you get a decently shaped hull (but not guru level), are you better off putting your energy in trying to get the boat structure lighter while still maintain sufficient strength? ie you've designed the strength in using engineering principles, rather than just brute forcing the strength by using thick plywood for the hull for example).

    Thanks for any input!

    take care

    Blll
     
  2. fcfc

    fcfc Guest

    There are 3 problems in your problem.
    1) getting the hull with the minimum drag
    2) getting the propulsion system with the best efficiency.
    3) Merging both above...

    Another point, a professionnal designer is liable. So he migth not go too far off regulations. Everything else will NOT be secondary, contrary to what you asked for.

    From larson & eliason, principle of yatch design,
    speed is dependend of LWL.
    drag is dependent of displacement.
    All other parameters have a total influence of less than 20% for normal (not extreme) monohull sailboats.

    The price is probably not a secondary problem, too.
    The lightest boat is unfortunately most of the time the most expensive one. Vacuum bagged and cured carbon prepegs give some ligth boats. Can be used to build developped surfaces too although I do think you plan to use such materials.
     
  3. fcfc

    fcfc Guest

    given 18-24 ft, 10-12 kts, 1000 lbs loading, you can have very different boats.

    a 18 ft optimized for 12 kts will be planning. If you put 1010 lbs inside, or a 2 inches chopp, performances will be destroyed. Trim may be also important. But given an adequate (a bit overpowered for 12 kts) engine, the usable speed range may be 12-20 kts.

    a 24 ft optimized for 10 kts will be in (semi)displacement mode. Do not hope to do 10.1 kts under any conditions. The usable range will be 4-10 kts.

    In both case, the lighter, the better.

    Also you may not rate the same a gasoline outboard and a diesel inboard.
     
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