Depth of hulland freeboard

Discussion in 'Boat Design' started by Mik the stick, Sep 22, 2015.

  1. Mik the stick
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    Mik the stick Senior Member

    If you can do tech drawing (I can't) you draw your ship calculate its weight and you will know where the lwl will be. You could draw a boat do all the calculations and find the boat might be unsafe because it has to much or too little freeboard. My problem is I can't draw. From Gerr's Boat Strength Fish n Squish (40ft) has a depth of hull of 5.91 feet ~15% Draft being less than half of that. Is there any way to calculate what the freeboard or depth of hold should be for the length of a boat.
    Mik
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Much depends on the kind of boat, and particularly where it will be used. The idea that high freeboard "keeps the water out" and therefore makes the boat safer, is far from "holy writ". Many very good boats appear to have an economy of freeboard, but get an advantage from a lower centre of gravity, and lower windage.
     
  3. gonzo
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    gonzo Senior Member

    You can inspect similar boats and determine what the average freeboard is. That will give you a good baseline.
     
  4. PAR
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    PAR Yacht Designer/Builder

    I don't understand the question. There's no ideal freeboard and it's determined by several factors, governed by the requirements of the SOR. Simply put, if you run around the "spiral", the freeboard you get can be fudged if desired, though some justification is necessary.
     
  5. Mik the stick
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    Mik the stick Senior Member

    Thank you gentlemen you have forced me to think. The reason for knowing the freeboard is to calculate the depth of hold which is required to produce a scantling number to use with D. Gerr's "Boat strength" book. From his book "Nature of Boats" (LOA^.5 + BOA)/16 = 1.18 inches for his Boat Stength example boat. Sn for that boat is 2.969 for a plank width of 1.144. Using Plank thickness = 0.74 * SN^.4 SN must equal 3.22 for a plank thickness of 1.18.
    I can either use the SN generated by the first formula and have a margin of safety or reduce it proportionally to equal the second formula.
    Thank you Mik
     
  6. DCockey
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    DCockey Senior Member

    Remember that Gerr's formulas are empirical and not exact or precise.
     
  7. Mik the stick
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    Mik the stick Senior Member

    Imperical yes, but if you are going to design a boat you have to have a method. I have learned more from Dave Gerr's books than from any other source That said I am about to start a thread because I don't agree with everything I have read in Boat Strength on plywood.
    I can understand the methodolegy of Boat Streangth and believe it would produce a safe boat. Understanding boat design for me is a hobby I can't draw but I may yet Post a design on this site:D
    mik
     
  8. Petros
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    Petros Senior Member

    usually if the boat is designed for smaller inland waters, it would have less freeboard. And it if intend for near shore coastal cruising it would need more, but less than a boat designed for deep water cursing in the open ocean.

    But there are large variations in free board within each of those groups.
     

  9. Mr Efficiency
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    Mr Efficiency Senior Member

    It is good to take a little water over the gunwales from time to time, keeps everyone alert.
     
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