optimal relative transom width for low power planing hull skiff?

Discussion in 'Boat Design' started by BE_, Apr 3, 2022.

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  1. BE_
    Joined: Jul 2021
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    BE_ mr. man

    I am trying to design a small (12'x4') ply-on-frame flat bottom skiff that can plane on somewhere around 6-8 horsepower and go about 10mph. Would it be better to have the transom be nearly as wide as the widest part of the hull, or significantly narrower to achieve this? I keep seeing people say both.
     
  2. bajansailor
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    bajansailor Marine Surveyor

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  3. Barry
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    Barry Senior Member

    You are referring to Chine Width, not beam
    The Cheyenne chine width scales to 43 inches at the transom. I would try to find a plan that uses as much as you can to the width of a sheet of plywood, say 45 to 46 inches.
    The wider it is, the easier it will be to plane.
     
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  4. cracked_ribs
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    cracked_ribs Senior Member

    Honestly... unless you build really heavy and pack enough fishing gear to troll deep in strong currents with a dozen rods simultaneously, a flat bottomed 12*4 skiff that planes with 6-8 hp and does 10mph gives you a fair amount of leeway.

    My current boat is a fairly big v-hulled 17 footer with a 25 horse motor and does twice that speed, easily, with two large adults and gear. I could probably do 10mph with 8hp in that thing, and it's twice the size of your planned craft. More, really.

    Actually, come to think of it, the first engine it had was the same block, but had a restrictor plate and small volume intake and would only produce around 15hp at the prop, and I could get that setup to run around 13 knots IIRC. And since it was a temporary arrangement, I never bothered to dial it in at all.

    A 12*4 flat bottomed boat with the power you're talking about should have no problem hitting 10mph even if you don't really maximize the hull efficiency.
     
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  5. BE_
    Joined: Jul 2021
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    BE_ mr. man

    thanks for the feedback!

    this is what i have planned so far new short wide boat.png
     
  6. messabout
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    messabout Senior Member

    You are are wasting water line length with the bow raked so severely. You have more flare than you need or is advised for a boat of this size. Move the widest part of the boat forward to about the middle. Make the bottom wider without increasing the gunnel width, thus less flare. Leave 30 to 36 inches of bottom width at the transom. Consider the weight of the outboard and the flotation needed at or near the transom. If you build a well designed 12 foot planing dinghy, the 8 HP motor will drive it into the low 20s unless you have overloaded the boat. If you anticipate a heavy load then increase the bottom width to as much as the width of the ply, at 45 to 48 inches.
     
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  7. Barry
    Joined: Mar 2002
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    Barry Senior Member

    WC-12-Overhead.png
    This is a Lund 12 foot with a 61 inch beam. Almost flat bottom, or maybe flat. The the chine width works out to be approximately 43 inches.
    This company has built tens of thousands of boats and has been around 70 years. If a narrow chined hull design would be more easily driven with low horsepower, Lund and the myriad of other short length boat builders would have
    followed your design.
    Note that the chines are pretty much parallel over the latter 2/3 of the boat.
     
    Last edited: Apr 5, 2022
  8. BE_
    Joined: Jul 2021
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    BE_ mr. man


    something like this?
    new short wide boat.png
    (i didn't move the widest part of the boat forward because i wanted to keep the classic "skiff" shape)

    (edit: also fixed transom angle)
     
    Last edited: Apr 4, 2022
  9. BE_
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    BE_ mr. man

    i made the sides slightly taller, and readded an inch of flair to the sides.

    this is probably my final design here- new short wide tall boat.png
     
  10. bajansailor
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    bajansailor Marine Surveyor

    @BE_ re your two proposals above, try including an 'average' 5' 8" / 180 lb man on board who moves towards the bow to recover the anchor after a day of fishing - with that very narrow hull up forward, I wonder what will happen?

    Take note of Barry's comment above -
    "This company has built tens of thousands of boats and has been around 70 years. If a narrow chined hull design would be more easily driven with low horsepower, it and the myriad of other short length boat builders would have followed your design.
    Note that the chines are pretty much parallel over the latter 2/3 of the boat."

    That Lund 12' will work well with a small outboard motor, and the design is well proven.
    Be cautious about trying to invent something new.

    In addition, it would be prudent to carry a pair of oars in case anything happens to your O/B motor - and I think that you would probably find that the Lund 12' would be easier to row than your design.
     
    Last edited: Apr 5, 2022
  11. BE_
    Joined: Jul 2021
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    BE_ mr. man


    i have caved

    new short wide tall boat.png

    it is starting to look more like a dory than a skiff, though the narrower transom should make it easier to row
     
    Last edited: Apr 5, 2022
  12. BE_
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    BE_ mr. man

    Last edited: Apr 5, 2022
  13. Dolfiman
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    Dolfiman Senior Member

    There is something unusual in the drawing of your sheer line : it is because, geometrically speaking, its max curvature seems positioned like the max beam, i.e. here at about 28% L from aft. To have a better more usual shape of your sheer line (in my opinion), you should disconnet these two maxima, i.e. to put the max curvature more forward. Here attached a proposed alternative with this max curvature at ~ 55% L while the max beam 52'' is maintained at ~ 28% Lw, you can see the difference.
     

    Attached Files:

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  14. messabout
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    messabout Senior Member

    BE, none of us intend to be picking on you. That said, it appears that you are more interested in what the boat will look like than your interest in best and most proven lines for a boat of that type. That mindset is a familiar one here on the forum. You seem determined to have the boat to look like what you want it to look like. I say go for it then.

    Please do not take offense for what is written above. I repeat, we are not here to give anyone a hard time.
     

  15. BE_
    Joined: Jul 2021
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    BE_ mr. man

    this helped a ton! thanks!
    new short wide tall boat.png
     
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