Centre board box hieght

Discussion in 'Boat Design' started by lindum, Jul 2, 2014.

  1. lindum
    Joined: Jul 2014
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    lindum New Member

    I'm saving hard at present to build a 20' pocket cruiser early next year and want to get several things sorted out before I order the ply and other wood.

    How do you calculate the depth of the center board box when constructing the hull. Personally I think you would have to know the weight of the boat before you could build the box. (Just my thought).

    What type of wood will I need for the stem.

    Thanks for your help.

  2. viking north
    Joined: Dec 2010
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    viking north VINLAND

    Michael, take an old builders advise, save yourself a lot headaches and money and track down/purchase a set of plans which will supply you with all the necessary info.
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    All you need to know is the size of the board. The box is big enough to house the board.
  4. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    I assume you are concerned about making sure the top of the box being above the waterline. Are you designing the boat yourself? If so you should be able to determine the highest the waterline can be, including when heeled. If you're not able to do so then you need to do some more studying or switch to using a set of plans.

    The top of the box itself does not need to be above the waterline, as long as there is no opening below the waterline. The box can be sealed except for a tube through which the centerboard pennant runs to a sufficient height. Keep in mind that if the boat is intended to float when swamped that any openings in the top of the centerboard need to be above the swamped waterline, or able to be closed in order to bail the boat when swamped.
  5. alan white
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    alan white Senior Member

    You may want to have the top of the trunk well above your waterline due to openings for the pennant and possibly downhaul being at the top of the trunk. In that case, first determine your waterline based on a heavy condition (boat somewhat overloaded). 4"-6" above that should be adequate.
    Obviously, determining your waterline is part of the design process and you should know with a fair degree of certainty where that will be whether you are the designer or not.
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Lindum, welcome to the forum.

    No insult intended, but if this is the type of question you need help with, you are in no position to consider self designing much more then a very small dayboat, that should be considered disposable.

    I designed my first boat when I was 13 and I patterned it after other 8' prams I'd seen at the local docks. It wasn't quite as good at the ones that had been "designed" by a professional, but I worked for me. I fished it for years, until the bottom literally wore out from dragging it up on the beach.

    If thinking about a 20' cruiser, there's enough investment in sails, materials and hardware, to force you to consider a lot more understanding about the realities of the various aspects of design work. If it's a sloop, say with 180 to 200 sq. ft. of sail, you're looking at $1,500 as a minimum just for a 100% jib and mainsail. Getting a boat to balance over its appendages, no matter how high the centerboard case is, would seem like a primary consideration.

    Lastly and FWIW, I've designed boats where the case and the board where entirely under the LWL, so the height of the case wasn't a consideration. In fact, this was the reason for it, to remove the case completely from the boat's interior.

    Maybe you can post some sketches or the GA, lines, general dimensions and hydro information, so we can have a look at what you're thinking.
  7. lindum
    Joined: Jul 2014
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    lindum New Member

    Thanks for the replies lads. It was just a nagging thought at the back of my mind and I wanted to get an answer for it.

    I wouldn't dream of building a boat without a plan. But, as you say Par, the cost is somewhat overwhelming.

    I'll continue to save and look round for a suitable set of plans.

    Thanks again for you replies.
  8. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    There is a thread on Sub 20' micro cruisres somehwere on this Forum. That should throw up a few ideas. Note, quite a few are wood.

    There are also a lot of older lines plans around which also give some idea of what has been done. So there are different shapes for different coastlines all over the world. Such as shallow draft stuff, for the East coast of the UK and deeper draft better windward going West country craft.
  9. CloudDiver
    Joined: Jun 2014
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    CloudDiver Senior Member

    Since you are in the UK, check out Keith Callaghan's designs. Besides some fantastic racing dinghies (Merlin Rockets) he specializes in trailerable Pocket Cruisers. http://www.bluelightning.co.uk/


  10. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Thanks ClouDiver. I'd second looking at Keith's designs. My father had one of his Nat 12's a Havoc design and that was pretty good, best with a light weight crew!. Have crewed in one of the older Merlin designs too. Quite well behaved boats and the lighter you can keep them, the better they perform IMHO. Partly due to the U section, which gives better lift onto the plane and better initial stability, even on a keel/ballasted boat. The rocker profile then becomes more paramount, but he has been wise in his selection and choices, especially with a lot of experience.

    Blue Lightning is shown on video against the tide in a typical East Coast (UK) moderately flat lump of a sea which is quite awkward, at least to those of us that know it. To my eyes, she is performing very well indeed, maybe better than some 24' stuff (including Sportsboats) I have sailed in the past.

    Well worth a thorough investigation. Try giving him a contact, maybe you can get a trial sail?
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