Displacement speed HP requirements for 20' flat bottom skiff

Discussion in 'Boat Design' started by YotaTruck, May 28, 2015.

  1. WindRaf
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    WindRaf Senior Member

    and...genius, why you diden't made the calculation of the YotaTruk design?...but you mde a wrong calculation of my example?
  2. YotaTruck
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    YotaTruck Junior Member

    Given your comments I reached back and dusted off two designs I had considered a while back before monkeying around on my own-Jeff Spira's Carolinian:


    And Ablemarle:


    Obviously the Ablemarle is a true displacement hull while the Carolinian is a flat bottomed planing hull but with the line pinched in toward the stern. The Ablemarle would do fine with a 1000lb load and a 15 HP-I'm not sure about the Carolinian though.

    In an ideal world, I'd love to build a boat that I could putter around the local restricted lakes with a 15, but also be able to put a 30 on it for trips to the lower Delaware River or Barnegat Bay. I guess if this is possible with any boat it would probably be the Carolinian. The alternative of course would be to build both the Ablemarle AND the Carolinian and fit each with a motor befitting their strength. My wife may have something to say about that though :D
  3. Easy Rider
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    Easy Rider Senior Member

    For more capacity, seaworthyness and efficiency you may look at the 17' Banks Dory w OB well. (Spria) Should be a much dryer boat as well. Won't even need 15hp. Perhaps even a larger Banks Dory.
  4. WindRaf
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    WindRaf Senior Member

    Mr Efficiency,
    my example has two longitudinal position: one, that visible in the drawing with using 7 hp-7 knots, and another slightly more sitting at incresing the power to 14 hp - 12-15 knots.
    This causes a gradual shift towards to the stern of the center of bouyancy.
    The piercing bow is fundamental because there is only one way to overcome the speed of displacement without planing with a displacement boat: pierce gentle the wave.
  5. WindRaf
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    WindRaf Senior Member

    TANSL, why you want to hurt you?
    in addition to not understand, Your attitude is ethically wrong: when I have spoken ill of your designs?
    You want that I to begin to analyze your designs?
    And if I do, the moderator delete my post?
  6. TANSL
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    TANSL Senior Member

    WindRaf, dear friend, I would love you to be able to discuss my designs. For it would be best for you to start a new thread, not to jeopardize this. Please, do it. If you maintain a polite tone Moderator does not have to delete your comments. I sense that many people besides myself, might be interested in reading what you write and engage, with this excuse, a good discussion on issues of theory of the ship, naval architecture and marine engineering. You had a good idea. Thank you for your interest in my designs.
    In the current thread I will not answer you more to not continue spoiling.

    P.S. : You can find most of my designs in My Gallery. I can send you the reports and calculations you need via email.
    Last edited: May 30, 2015
  7. messabout
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    messabout Senior Member

    ^ Tansl, thank you for your efforts toward being a peace maker. For the most part we are gentlemen here. We do endure some less informed individuals who are a bit hard headed. That we generally behave with civility is a testament to our desire to rise above the ordinary.
  8. whitepointer23

    whitepointer23 Previous Member

    Having 2 different engines doesn't make sense when a 30 four stroke will potter around as slow as you want anyway. Unless you have size restrictions on your lake.
  9. Kevin Morin
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    Kevin Morin Junior Member

    Simple Mod's to get your design

    Yota, to adjust your existing design to the speed you're describing ( if that is in fact a goal?) simply pull the keel down amidships so the bottom is not flat except in section; transversely. In Profile View your keel would be deepest just aft the (longitudinal) center of buoyancy, in rough terms.

    Then, lean/flam/'tip out' the topsides' sheer line so the boat's waterlines increase as they rise from the baseline. The Plan View sheer would be outside the chine everywhere around the hull but more so in the forward 1/3 of the hull. (so) Add more flam forward than aft, rake the transom to 18 deg.s (aft) and increase the chines' BOA a foot.

    An after raking transom will lift the entire boat in a following sea/chop/lake swell and avoid shipping water in the engine cut down. (If you want real following sea safety put a splash well/ slop tray/coaming around the low point of the transom's engine mount area. Or put a cantilevered engine mount and rig some controls, keeps the engine cut out from lowering the transom in regard following seas.

    Rake the bow stem forward at the sheer so the topsides have more spoon, and will lift in a swell/chop/head sea. Camber the bow stem to stiffen the topside forward 1/3 with convex section.

    Taper the (Plan View) chines from the Master Section aft at curve of 1.365"/10ft. of chine (not much more or she may squat) and she will turn well, track well and run with displacement speed horse power.

    Engines, especially small engines, do not have linear power curves, ask dealers for the torque curve graphs so you can plan the skiff's power to have peak torque at hull speed and the basics you show will work fine.

    Small outboards are often available as 'high thrust' engines where the lower bulb gear ratios are 3:1 or even greater. This will allow the engine to turn up into its native torque curve while still turning the wider/large dia./'bigger' wheel with more diameter than pitch- which is what you're saying you want.

    Deeper topsides will carry more, make the passengers 'feel' safer but will make more windage (fighting the helm) in mooring or docking when in a cross breeze.

    IF these steps were followed as modifications to the design you posted; the skiff will work fine, and be safe.

    Remember; those first couple dozen skiffs are the greatest learning experience for a new builder.

    Kevin Morin
    Last edited: May 30, 2015

  10. fredrosse
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    fredrosse USACE Steam

    Back to the topic of this thread: "Construction will be screwed and glued plywood, glassed on the bottom and chines. Flat bottom, plumb sides, high sheer height"

    The hull panels can be made as very nearly rectangular slabs and will have rise at the bow and stern if the sides are not plumb, but have slope at the midship section. The sharpie hull I built has a 4 foot wide flat bottom, and 5 foot hull beam and a plumb stem. The side panels are cut at 20 foot x 2 ft rectangles from butted plywood panels. The attached section drawing shows the rise attained at bow and stern with no extra material beyond the rectangular panels. I understand you want more sheer height, and more beam, but the principles are the same.

    With plumb sides the rise of the sheer line fwd and aft would require extra material height. Also a small boat with straight plumb sides would be an unusual, generally this feature is found on much larger oceangoing ships, and is less appropriate for a small boat.

    Get some stiff paper to make a crude model of your hull, and tape together the sides & transom. Push the hull wider at the sheer line than at the chine to check this out.

    Attached Files:

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