Variable deadrise

Discussion in 'Boat Design' started by Sawdust, Dec 25, 2013.

  1. Sawdust
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    Sawdust CEO & sweeper

    I'm working on a design for my next project, a 16 foot outboard skiff for fishing on the Finger Lakes of central New York. These waters, like NY weather, can change very quickly--you might go out at dawn on glass, and by lunch time be bucking 20 knot winds and 18 inch chop. So my plan is to build something with a flat broad bottom at the transom and 25+ degrees of deadrise near the bow.

    My concern is this: should the keel line be held flat and the chine line be raised (fwd) and lowered (aft) to accomplish the deadrise variation? Or should the chine line be held flat and the keel line be rockered to vary the deadrise? Are there advantages and drawbacks to either approach? My first pass through the numbers gave me a hull weight around 300 pounds, a design waterline of 7 inches and displacement of 830 pounds with a 20 hp motor.

    By the way, I've still not sold my Simmons replica (but certainly enjoyed owning it!!) My YouTube video about it https://www.youtube.com/watch?v=-19Xf72ZQTg has had over 190,000 views from around the world. It may not have sold the boat for me (yet) but it has certainly inspired a few folks to try building for themselves. I've also had over 6,000 page-views at my website http://andersonboatworks.wordpress.com/ and corresponded with enthusiasts in many countries. I'm lucky my wife sticks with her day job to support my habits.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Your are describing a fairly bluff bowed warped bottom. This type of design is plentiful at the usual sources. It's not going to be a very comfortable boat as the seas build, but it will jump on plane quickly and run fairly efficiently at speeds below 30 MPH. I'm assuming a transom deadrise of 7 or less degrees.

    If you want a well suited warped bottom hull shape, designed specifically for the this area, take a look at the sea skiff style of hull used on the old lapstrake Lymans. They used a relatively fine and steep entry to cut trough chop and a modest warping deadrise, to about 5 - 7 degrees at the transom. These boats are some of the sweetest riding warped bottoms around. Of course, these are round bottom shapes, but a body plan will offer ideas at to where to place the chine.

    Personally, I'd prefer to kiss off some bottom end performance and use a monohedren hull, possible with extra wide chine strakes to help her low end abilities. You'll get a better ride when it gets nasty and a higher speed potential, for when you can afford a bigger engine.

    As to chine and keel profile specifics, this gets pretty technical and the difference between a bad one and a good one are difficult to see, unless you've studies hundreds of warped bottoms and understand why each reacts the way they do. You're best advised to look at the Bateau.com and Glen-L.com offerings and bite the bullet.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Pretty light for a 16 foot powerboat, you are building in which material ?
     
  4. PAR
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    PAR Yacht Designer/Builder

    300 doesn't sound overly light honestly, assuming modest beam and simple layout.
     
  5. Sawdust
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    Sawdust CEO & sweeper

    I've got an excellent source for clear ash; I'm planning the bottom in three layers of 1/6th inch each, and the sides in two layers of 1/8 each. Six ounce glass and epoxy outside, and 4 ounce interior. Two thwart seats boxed below and foamed, plus some foam in the bow. The seat boxes will be the only interior framing other than a 1/2 by 2 inch sheer clamp and 1 by 2 inch chine logs.

    I'm considering a higher-tech fiber mesh but have not yet researched.

    Getting back to bottoms: I realize on further reflection that whether the chine line rises or the keel line dips, the resultant shape will be the same except with respect to the sheer. When I first sketched the concept, I did it with the keel dipping forward, and by virtue of an unsteady hand, drew the keel line from the point of maximum depth up to the zero-deadrise transom in an arching curve rather than a straight line. The resulting slight hollow in the midship bottom intrigues me....will it suck in air and reduce drag? Will it suck in air and reduce lift? Will it tend to help the bow break free as speed increases, or prove unstable at planing and make the bow bounce on and off the water? I can't be the first to ponder this...any suggestions of well-tested plans would be appreciated, as I'm not in a position to just build a few to try the options out (yet).
     
  6. michael pierzga
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    michael pierzga Senior Member

    Not sure what you are asking. The bow of a boat deflects waves, the section of the boat just forward of amidships chops thru the wave and the boat planes of the aft flat run.

    The keel should be straight. flat section aft, sharp vee forward of amidships and flare to the bow.

    A classic skiff shape for low power, modest speed


    [​IMG]
     
  7. PAR
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    PAR Yacht Designer/Builder

    Have you any sketches of the skiff you're describing?
     
  8. Sawdust
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    Sawdust CEO & sweeper

    I've scaled off the hull shape from the sketch that Michael sent as a 17 footer with 6 foot beam and crunched it through Freeship; compared to my original shape for a 16 footer, there were very small differences in various hydrostatic coefficients. The lofting corrections will probably have a bigger effect on performance than the differences between the two hulls. Thanks Michael--I'll stick with your sketch.

    Thanks for the Bateau.com suggestion....look like fun projects for weekend warriors, but I enjoy designing as well as building. I realize that this can be inefficient, but just following someone else's step-by-step plans is not my idea of a good time.

    Stu
     
  9. tom28571
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    tom28571 Senior Member

    The aft bottom buttock lines need to be straight on a planing boat. Arcing the keel up will introduce a convex surface aft and that is just what a planing boat does not want. That is rocker aft which is usually detrimental to planing and also tends to stick the bow in the air.

    Whether the keel rises to the transom or the chine goes down to the same transom deadrise does give the same aft bottom shape but here can be quite a lot of difference in performance between them. A rising keel aft introduces less drag at displacement speed and higher trim on plane. Dropping the chine toward the transom makes the stern drag a bit more at displacement speed and tends to hold the bow down at speed. Low speed drag can be a lot or very little for the immersed transom depending on other factors, mainly weight.

    On a heavy weight boat, I might go with the rising keel but for a lightweight planing boat, I choose dropping the chine.
     
  10. Sawdust
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    Sawdust CEO & sweeper

    Thanks for getting back to the original question, Tom, and for your clear response.

    Stu
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I like "variable deadrise" hulls of this kind for lightweight, lower powered, modest speed boats. Forward of a line drawn from the forefoot to the chine at the transom it is just a monohedron, behind that it flattens right out toward the centreline, by using a curved section at the transom than rises from dead flat to whatever the deadrise of the monohedron would be. The curved area is defined by straight line generators drawn parallel to that dividing line, all intersecting the curved bottom at the transom. The result has a little bit of rocker, but seems to work very well in my experience, a sound compromise between stability and ride. An external keel is advisable.
     

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  12. tom28571
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    tom28571 Senior Member

    Do you have lines of this craft? It doesn't match anything I've seen.
     
  13. Wayne Grabow
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    Wayne Grabow Senior Member

    Very clear diagram. It appears you are describing a developable surface bottom. Considering similar requirements, I built a variable deadrise boat reversing your two areas. The area you have labeled "curved" is flat and the area you have labeled "flat" has a slight curve. The boat has performed well. I need to watch the tachometer to note the transition from displacement to planing (smooth transition), and the speed has been better than expected. The bow rises about 10-12" when planing (on an 18' boat); this would be dependent on how much rocker is utilized as well as other dimensions.

    I figured that for efficient planing, you want flat surfaces, and the area most involved in planing is the aft triangle bounded by the forefoot and the two (port & starboard) transom/chine intersections. What I did was to put a short, slightly rockered section in the keel in the forefoot area and then projected that rocker by conic projection to the chine. The apex of the conic projection would be located a short distance on the contra-lateral side of the keel, but you really don't need to locate it; just make the curve at the chine an enlarged copy of the keel curve. You end up with a straight keel aft and the keel and chine being parallel near the stern with reduced deadrise.

    For example, let's say that the keel is rockered 2.5 degrees in a 2' section of the forefoot; and the midships chine is rockered 2.5 degrees in a 6' long section. The keel will be straight from the forefoot to the transom. The chine will be parallel to the keel at the transom (and for several feet forward of the transom as well). The deadrise will decrease aft because the keel rocker is achieved much sooner than the chine rocker.
     
  14. tom28571
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    tom28571 Senior Member

    Wayne,

    What you describe looks like a normal warped plane and what "Mr Efficiency" proposes looks strange to me.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I'll organize a drawing of the shape i'm talking about later, to illustrate it a little better. It certainly isn't anything original to me, but I have noted it seems to work very well both at rest and underway, offshore. There are any number of ways to arrive at a rounded bottom aft from a straight vee forward, not all developable though. I don't think Wayne's boat is warped plane, he speaks of the chine being parallel to the keel line aft. Maybe it looks like this ?
     

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