freeboard, how much is enough?

Discussion in 'Sailboats' started by dionysis, Feb 6, 2003.

  1. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    I thought so...

    Thanks Michael,

    your point about adequate freeboard when having sustained damage is decisive. I also take your point about designing for commercial use. I am aware of the AMC and am in the process of negotating borrowing rights.

    Thanks again, dionysis.
  2. Guest

    Guest Guest


    Light boats don't have much depth of hull, so in an effort to provide 6' plus headroom, light boats usually end up with lot of freeboard, and a good bit of coachroof, too. If you design for headroom, chances of designing a boat without enough freeboard is just about nil.

    Tim Dunn
    near Seattle, USA
  3. dougfrolich
    Joined: Nov 2002
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    Location: San Francisco

    dougfrolich Senior Member


    Check out SNAME Transactions, Vol.98, 1990, pp.187-237. For a paper titled Sailing Yacht Design- A new appreciation of a fine art. Basically it compares many commonly concidered design ratios of "traditional" vs "Modern" sailing yachts.
    I like sailing small ULDBs in the ocean, and have found freeboard forward and bow shape are of criticle importance for a well behaved design. Attached is one of my favorite small ULDBs with the correct freeboard,volume,shape combinations.


    Attached Files:

  4. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    ì like to learn what the numbers in GZ mean and how to make a basic horizontal lever arm (GZ) drawing like Nemo made, i asume it takes the sums from CG and CB making a GZ? maybe its rite there in the ani-gif in size? happen to know also how to convert pound and feet torque to metric?

    thanks ahead, yipster
  5. Guest

    Guest Guest

    The generation of GZ curves for the three hulls, with different freeboards, is the beginning to identify the problem.

    The hull form can include the deckhouse with weather-seal windows and weather seal doors. Once the deckhouse appendage is inserted to the model, the GZ curve will rise after the deck edge immersion. This will enable some yachts with lower freeboard to meet the standard requirements.

    Refering to the applicable rules. I believe UK have some yacht rules, use those. In the states USCG rule on power to carry sail can be used to evaluate the safety of wind sail vessels.

    Work out the sail moment effect and impose over the GZ curve. Draw either a common turnng lever or use cosine or cosine(sq) function to change the GZ baseline. The energy in the GZ curve must be ample to support the wind force applications. Apply gust factors too.

    I agree to stay on high freeboard, but make sure you have the draft at least half the depth over the bilge, so that the bilge will not emersed in the large angle assessment.

    Make sure that the GZ curves are for free to trim status for each case in rising heel angles.

  6. dweezle99

    dweezle99 Guest

    freeboard is optional

    First figure out what the boat is used for. width/beam can affect stability without loosing speed, however if you add 1.25% to the lwl you can increas sail area by 30 to 40%. Wider is more stable. Alonger water line is faster and freeboard is a matter of style and use . (to a point) Im a custom builder and desighn a nice 30' hard chine hull. Cabin space is comfortable for sitting and makes a nice weekender. I have a beam of 8'11" and a freeboard hieght of only 1'8" at the lowest point. On a good day with moderate winds this boat made 21 knots and sails almost upright even in headwinds. Alot of people get lost in designing boats and forget that style is just as important as all that complicated stuff about down wash cg's ect. If youre going to heel a boat more than 60deg. than youre not really sailing anyway for rudder compensation. Are you ocean going or just playing in the bay. Thats a question you have to ask yourself. However if your looking at cruising I would add a little waterline definately some freeboard and see what beam grabs my imagination. Sail magazine has a very good two part article on these calculations and where to research them in the Sept/Oct issues.
  7. water addict
    Joined: Jun 2004
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    water addict Naval Architect

    How much freeboard is enough?
    3 feet.
    Seriously- your question is a bit nebulous. If you are asking, you should know the answer is variable depending on the situation.
  8. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    Consider this 38-foot class, one of the fastest monohulls in the world and for many years the fastest vehicle on earth, on land or sea. It's been raced continously since 1896. Does it have sufficient freeboard?


    The success of this class, and it's smaller sister classes, imply the answer can only be, "Yes."

    Considering, of course, the waters on which they sail and and the objectives of their sailors. You can't even begin to ask the question about freeboard until you establish these factors.

  9. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    why, yes

    I originally posted the question because when I was drawing boats for offshore use I always ended up with low freeboard. (it looked good) I though this was great since I could reach up, if I was in the water and be able to touch the deck at the lowest point of sheer. I would then check to see where the waterline was when the boat was heeled at 30 degrees, and it would be ok,( a little water on deck) , but when I looked at recommended heights, I always got around 10% lwl or more, which seemed to me to look too high.

    I was looking for other's ideas on what needs to be taken into account from a seaworthyness perspective, when deciding on freeboard height.
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