Getting started in Design & Construction

Discussion in 'Boat Design' started by BlueWaterMD, Aug 26, 2009.

  1. BlueWaterMD
    Joined: Sep 2007
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    Location: NJ

    BlueWaterMD Junior Member

    I have been wanting to build my own boat for some time now. I have spent countless hours looking for the right plan, but can't find what I am looking for. I have found some designs that are close, but not the exact boat I want.

    Is it feasable to design a small, simple skiff by myself for plywood construction? I have a degree in engineering, but no experience with designing boats. I have lived and worked around boats all my life, so I do understand alot about them.

    Where do I get started? Where can I find a basic overview of going from sketch of the boat, to the actual design, to transferring the design to plywood pannels for construction? Do I need CAD software or can I design by hand as this will be a very simple skiff?

    My Boat:
    The boat is a flats skiff to be used for fishing the shallow water flats of south florida. It will be most likely be constructed in plywood with a fiberglass skin. Weight is important. Skiff will be used in calm, shallow waters only. Relatively low HP
    LOA 17'
    Beam 6'
    Draft 6" loaded with engine, gear, and 2 anglers (This is inportant)
    I want a boat with a slight V foreward progressing to a deadrise aft of only a few degrees (if not totally flat). I would like a tiny bit of flare in the bow.

    I really would like to replicate a Mitzi Skiff - www.mitziskiffs.com
    The closest thing I have found is the Bateau FS17. I don't like the way the hard chine is exposed. On the Mitzi, the chine is submerged when the boat is stationary. It makes for a quieter boat while fishing, and is more asthetically pleasing to me.

    Note there are several other plans out there for flats boats. They are heavier boats that require more horespower. I am really looking to build a light skiff.


    I understand that my questions are very broad, and incredibly basic. I appreciate your patience and assistance in this matter. Thank You.
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Flats boat

    You seem to have your mind made up about the way you want your boat to look. Pardon me if I suggest that preconcieved appearance or design is mistake number one. A Florida flats boat may have little in common with Jersey sea skiffs or other boats in your area.

    You said low horsepower. That is a relative term. Be more specific. Florida bass boat dudes, and some flats fishermen, think that 70 Hp is shamefully inadequate. For them, 200 HP is about right. It comes down to how fast you feel that you are compelled to go. You are describing a rig that will displace less than 1000 pounds in full operating mode including two person crew. thirty Hp is probably enough to make sufficient speed.

    Six inch draft is no problem for the boat at rest. It will not need that much draft if you keep the bottom flat or nearly so. If you are concerned with sound ( like maybe bonefishing) then the chines should be immersed and so should the fore foot. You will not be able to use much if any deadrise if you are to keep the chines immersed. Let us say that the bottom wll be 60 inches wide. That will give your sides plenty of flare to get to the six foot beam you prescribe. A thousand pounds is on the high side of an estimate but things have a habit of getting heavier than we intended. You will need 15.6 cubic feet of displacement to float that load. If you must have flare at the bow then the stem is going to slope. Lets say that the at rest waterline length is 190". 15.6 cubic feet = 26957 cubic inches. divide by 190....you will need about 142 square inches of average section. The prismatic coefficient of such a boat might be something like 0.65 to 0.70. Divide 142 by ...lets err on the conservative side....0.65 you get a theoretical center section of 142/0.65 = 218.. The bottom is 60 inches wide....218/60 = 3.6 inches of draft. These are rough figures but they are in the ball park.

    So you want immersed chines and you draw 3.6 inches. Dead rise must be nearly non existant. If you must have deadrise then you will need round chines to help keep the noise down. Round chines and plywood are a possibility but construction becomes a bit bitchy.

    Now you have a 60 inch bottom span and that is where the trouble begins. A 60" expanse of plywood will need to be pretty thick, or it will need plenty of interior support structure, if it is not to pant dangerously. As an engineer you will be able to connect with that reality. Common static loads of 10 pounds per square foot more or less will not be too hard to deal with. Never mind static, dynamic loading can get to orders of ten higher than static. Depends on how hard you drive the boat into that nasty chop. Some times you are forced to run hard in order to seek shelter in time to survive. OK there goes the hoped for light weight. There are ways to reconcile all this of course but it becomes either expensive or laborious or both.

    I have not asked you whether you must be able to power in that 6" of water. If the answer is yes, then you are looking at something on the order of Rescue Minor. That is a unique design usually attributed to the Atkin Brothers design firm. RM is a hull with a cavity in the bottom (not a tunnel hull) that houses the prop and its shaft. It'll run in six inches no problem. It is not fast, maybe 20 MPH tops but it is one helluva boat for such an application. Another possibility is jet drive of course.
     
  3. Fanie
    Joined: Oct 2007
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    Location: Colonial "Sick Africa"

    Fanie Fanie

    Hi BlueWaterMD,

    There was a time I considered long and hard building something like the skiff's they use for shallow water fishing. In the end I didn't, although I do like the idea. I fish fresh myself too, and quite a bit in the shallows.

    If I have a choice I would build myself a displacement catamaran that is wide and foldable. I know it sounds way off, but the more I fiddle with displacement hulls the more I like them. I also like the space and safety aspect of the wider boat, softer ride in chop, although they are not as fast as planing hulls if displacement only. They can however be made to plane if you have the right hull shape and big enough motors, doesn't need to be 200's either, even 40's may do it depending again on some design cryteria.

    Consider as many options as you can, each will bring different advantages, choosing exactly the right thing is difficult, there is no such thing as a perfect boat.
     
  4. foxy
    Joined: Aug 2009
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    Location: Florida

    foxy Junior Member

    I'd say go for designing your own skiff, its just not that hard and you will learn a lot. For books, I'd suggest "The Gougeon Brothers on Boat Construction". Skene's will give you all the formulas although Simpson's rule (for displacement) is more complex than you need. You can easily figure displacement for this type of project by adding the areas of the stations and multiplying by the station spacing.

    You don't need CAD. I worked for twenty years on a drawing board and another twenty on "the box". However CAD programs can do a lot for you if you have the inclination to learn them. The Wolfson Unit has a freeware lines program called ShipShape <http://www.wumtia.soton.ac.uk/software.html> which you can try and several of the software companies provide demo versions. Good luck with your project.
     
  5. Wayne Grabow
    Joined: Aug 2003
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    Location: Colorado

    Wayne Grabow Senior Member

    A degree in engineering should be enough background for you to design a simple skiff. Messabout has introduced you to the sort of calculations you will need to use. The design process is an iterative sequence. Start with estimated length & width and displacement. Then you can compute draft and reconsider your initial estimated figures. Once you have designed the underwater part of the boat, proceed to the above-water portion. In addition to considerations of what type of water it will be used in, consider the building technique, building space, building materials, propulsion & controls, storage, transport, launching facilities, esthetics, possible canopy, and alternate uses. Its great fun. During building always consider how each step will influence subsequent steps.

    If you are set on plywood, then you will need to use a developable design technique. For a simple first skiff, I would suggest using constant 2-D cross-sectional slopes. Prettier shapes can be created with 3-D, but the design gets more complicated. CAD is not necessary; drafting will work.

    Best of luck whatever you decide. It is a real commitment and needs to be a labor of love.
     

  6. adamadam
    Joined: Sep 2009
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    Location: Australia

    adamadam Junior Member

    There are a few people who sell plans for flats boats. www.bateau.com have one in 15 (with a tunnel), 16, and 18 feet. Glen L has an old Ken Hankinson plan called the flats flyer too. They all look pretty nice, and are plywood and glass construction, except for the 15 foot version, which you can also get in a foam sandwich version. Here are the links to some.

    Hankinson flats flyer

    Bateau's Phantom 16

    You could base yours off these, or even get a set of plans for the construction and plan tips. The Mitzi skiffs look like they don't lend themselves well to plywood stitch and glue method, and seem a bit curvy.

    Hope any of that helps

    Adam
     
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