how about a 25hp outboard on a planing sailing dingy?

Discussion in 'Boat Design' started by Squidly-Diddly, Sep 16, 2011.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    A large dingy like a Johnson 18, but with a little bit of vee-hull for planing with an outboard, and a minimal cuddy cabin up front.

    Maybe water ballast like a MacGregor(although that would be for "semi-displacement").

    A sturdy dagger-board with ratching adjustment on height that could serve as a Spud-Pole to easily anchor the boat in swallow water without making a mess with wet lines and muddy anchors.


    Basically, a Utility Skiff with some very good sailing ability(but not a racer) to save on gas and for fun and for "Green"-ness. Pre-engineered for Junk-rig option.

    I figure between the water-ballast and dagger-board/spud-pole it should have a lot of stability and sea-kindness for a planing skiff its size.

    Similar in concept to a MacGregor, but in a mostly open utility hull, and of course much smaller overall.

    Easy to trailer, of course. Some built-in ability to use the sails as sun-shade AND as general Boat-Cover.

    Include "hike out" boards on the side for improved sailing performance without the water ballast, and other utility uses such at mounting fishing poles, or use as boarding planks with docking parallel along muddy banks.

    And I guess it should have provision for at least two sliding seat rowing stations, or 3 fixed seats, and maybe even a transom "scull" oar...and I guess it would HAVE a transom instead of 'self bailing' like a Johnson18.

    Probably epoxy/wood stitch and glue.

    Any body got any pre-existing starting points?
     
  2. keith66
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    keith66 Senior Member

    Why not have something really radical, a nice wholesome Rowing Skiff with a dagger board & standing lugsail to give good performance off the wind.
    Who needs a stinking outboard. The exercise will do you good as well.
     
  3. PAR
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    PAR Yacht Designer/Builder

    A powerboat planning hull is a fair bit different then a sailboat planning hull. The 25 HP outboard will cause a sailboat hull to nose up uncomfortably, because her planning patch is much farther forward. If you want powerboat preformance, pick a powerboat hull and put a rig on it. It'll sail like crap, but it'll plane off nice. In short, pick one a sailboat or . . .
     
  4. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I think it might be pretty good at semi-displacement speeds, but get to a speed length ratio of 3.0 or more (Fn>0.90) and PAR is right - anything with convex buttocks immersed at that speed is likely to behave problematically.
     
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  5. Easy Rider
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    Easy Rider Senior Member

    Stephen,
    I agree but the speed depends on the degree of convexness. Almost flat and you could still go 20 and possibly 25. More convex and we'd be out of control at anything near 20.
    Here's a thought. How bout a 10hp on a modified Thistle? Modified rig and stern for the OB.

    Easy Rider
     
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  6. pistnbroke
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    pistnbroke I try

    why put the outboard on the stern ..mount it midships in a box ..the it becomes the dagger board .....
     
  7. Easy Rider
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    Easy Rider Senior Member

    pistn,

    Then it becomes a dragger box.
     
  8. Squidly-Diddly
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    Squidly-Diddly Senior Member

    could someone explain why a planing dingy's hull works

    but isn't good for motor planing, or why having the plaining patch more forward isn't good for motoring?

    Are we assuming very different speed ranges?

    Is a plaining sail boat hull more efficient in the 0-20mph range than a motor hull?

    I don't think the hull "knows" if it is sail or motor driven due to suction from prop, does it?

    What part is Convex vs motor planing hull?

    Are you guys talking about looking at it from stern, were it is rounded but a motor planer will be either flat, or have two flats making a Vee?

    Or is it when looking from the side and even planing sailboats don't have that dead straight plan at the stern?

    Is it just that when we talk "motor boat planing hull" we mean "over 20mph" and a rounded hull doesn't like to do that?

    Is the sailboat hull rounded so it will be roughly the same at a range of angles, and because rounded is lower drag even at lower sailboat planing speeds?


    Is the reason a motor boat hull wont sail well because it will be heeled over and outside its 'comfort zone', because it is heavily optimized for running level(port to starboard)?

    Would a motorboat hull(Vee or Flat transom) sail well if it was kept unheeled with counterweights/ballast?

    I'd be fine with 20mph top speed on a 20' Utility MotorSailer Dingy/Skiff.

    If Plaining Dinghies can do 20mph I'd just stick an(optional) outboard on that, but build the hull a little more "utility" on the insides.

    Are those Plaining Dinghies just beastly for anything but a couple of wild-eyed 'sportsmen' for short races?


    and why is "plaining" flagged with red lines by the spell checker???

    Source: Webster's Revised Unabridged Dictionary (1913)

    Plain \Plain\, v. t. [imp. & p. p. Plained; p. pr. & vb. n.
    Plaining .] [Cf. Plane, v.]
    1. To plane or level; to make plain or even on the surface.
    [R.]
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

    EasyRider, you saying those outboard boxes cause a lot of drag?

    I always wondered about that, and couldn't see how they couldn't.

    I guess they just use them on big double-ended dories at low speeds?
     
  10. Easy Rider
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    Easy Rider Senior Member

    Yes. Too much for sail power. And I don't like them anyway but to power a double end dory it's almost a must. And I'm not a sail boater either. I live in Alaska and have places to go so sailing's out. But I love good semi-disp hulls. Keith says "who wants a stink'in OB?" I do. OBs are great. The Thistle is a beautiful and slick looking boat that would go 10 knots all day w a little ballast. Here's a good application of the OB well. Picture was taken at the NW end of Prince of Wales Island at Point Baker in Alaska.
     

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  11. PAR
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    PAR Yacht Designer/Builder

    Squidly, your understanding of sail and boat design in general is causing you some issues. A 20 MPH sailing dinghy is screaming along, with a ridiculously huge rig and white knuckled sailors aboard.

    There are too many questions in you post to answer without the comment to pick up a boat on design. The are substantial differences between powerboat planning and sailboat planning and the two don't interchange well. Explaining them all isn't as important as acceptance of this as true.
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Thx, Easy Rider, so I guess the boat I'm talking about is a

    20' semi-displacement round hull sailing dingy with as much outboard as she will take and where the outboard isn't just an afterthought but central to the design, like a MacGregor.

    20mph in a 20' boat sounds fast enough and I figure would be harsh pounding in most water, even for a deep Vee.

    I was thinking of a Sports Utility boat mostly for swift sailing for fuel savings but with same "motor is always an option" like the MacGregor.

    Now the project is to design dual use sails, for both sailing and sun/rain canopy, and "mothballed" boat cover.
     
  13. Squidly-Diddly
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    Squidly-Diddly Senior Member

    thx PAR, I'm thinking motoring at 15mph plenty fast enough.

    And sailing with guys on the outriders at 10mph would be very fast.

    Is a 'up to 15mph' motor/sail hull of 20ft doable, especially one that can haul alot of cargo or passengers?
     
  14. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Is a 20' planning dingy hull a nasty ride at 14knots and generally a squirrellybeast?

    Is a 20' planning dingy hull a nasty ride at 14knots and generally a squirrellybeast? Like a actual racing motorcycle only guys in the 15-25 age range can stay on for more than 10 minutes?

    I posted about an all purpose boat based on a sailing dinghy, but with enough outboard to get it to whatever the hull can reasonably do. Anything wrong with a planning dingy hull at 'top speed' but powered by an outboard carrying a lot more weight than two crew, or in mild chop?

    Are those planning dinghies fast and fun, but only good for white knuckle performance, and bad for everything else?

    Will a Planning Dinghy hull hobby-horse or otherwise snap around at 10-15 knots under power on anything but smooth water?
     

  15. PAR
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    PAR Yacht Designer/Builder

    I had a 5 HP on a Laser style dinghy many times, though I didn't bother with the rig, just used it to fish with. You can do this, but at some point (speed) you'll find out why it's not commonly employed.

    As to any particular 20' dinghy having a nasty ride, well each will have a comfort zone. Above this speed it will get progressively worse, though some find this fun.

    Most preformance dinghies suck at everything, except having them in the groove. They do poorly at displacement speeds, because their under belly is designed to be half out of the water, not full immersed, probably dragging her tail because of the outboard weight.

    Will it hobby horse? Probably, until you find a trim point it likes. Just scoot your butt around and she'll settle down.
     
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