Planing St Pierre

Discussion in 'Boat Design' started by JWill, Dec 21, 2016.

  1. JWill
    Joined: Dec 2016
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    JWill New Member

    Hello everyone
    I've come to you for advice after many years of lurking to get my answers. I'm interested in a St. Pierre dory that would achieve 20 knots or so. I'm aware of the 27' model by Nexus from the Pac NW that comes close. When I get to fish and cruise, time is often limited so a good turn of speed is desired.
    I would like to take a st Pierre dory similar to those found in Gardener's Dory Book and straighten the run of the bottom aft and install something like a 115 in a well or a straight gas inboard. My concern with these alterations is how the lack of bearing from the narrow bottom aft will react when trying to reach plane. With such little planing area will there be serious squat in the stern? Also, one of the desirable features of the st Pierre as designed is capability in a following sea, I imagine with no rocker or close to it the stern is more likely to be pushed around.
    I'm sure there are other designs that fit my needs but I'm interested in hearing opinions on this idea. I assume since I've never seen it done, there is good reason. I look forward to any comments.
    Jon
     
  2. DCockey
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    DCockey Senior Member

    An Oregon coast dory would be a better choice if you wan to go 20 knots. John Gardner called boats with dory bows and wide sterns "semi-dories". If you start with a St Pierre dory, flatten the bottom aft and widen it for planing the resulting shape will be closer to the Oregon coast dory than a St Pierre.
    https://www.youtube.com/watch?v=SuOKC7TupgU

    For a planing double-ender with an outboard in a well built with plywood have a look at the Bartenders designed by George Calkins. https://bartenderboats.com/
     
  3. JWill
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    JWill New Member

    Thanks for the reply David.
    I'm familiar with bartenders and have seriously considered building one. I have owned a semi dory and loved many things about it. The only drawback besides pounding in a chop (which a little self control on the throttle solves) is that I didn't like the wide transom getting pushed around by following seas. That boat only had 3-4 inches of rocker forward (less it seems than most Oregon dories) which may have added to the forefoot rooting into waves as the transom was pushed forward and sideways. It did have the benefit of splitting a chop if you didn't go fast enough for it to raise out of the water. This is one reason that I'm interested in a boat with a narrow stern.
     
  4. DCockey
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    DCockey Senior Member

    I've been studying the "literature" on the design of planing boats for several years, and recall having seen anything about double-ended boats or boats with transom width a small fraction of the maximum beam. So you may be largely on your own if you decide to build a boat such as you are considering. My concerns with high speed performance would be more about avoiding bad behavior than getting the boat to plane. And you will need sufficient volume aft so it does not trim too much by the stern with the large outboard installed.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Anything with a flat bottom on a planing hull will bash you senseless on anything other than a rare flat day. So whilst it might have the virtue of a simplified build, it has little else to recommend it. And if you must have such a boat, start with plans for that, don't attempt to modify an existing displacement hull, the odds are it will be a botch.
     
  6. PAR
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    PAR Yacht Designer/Builder

    The inclined bottom experiments in the early 20th century may be helpful, which used narrow beam/length ratios and modified double enders. They do plane off differently and they do squat a fair bit, but it can be manageable, given the other tradeoffs. These didn't have the burden of an outboard, but a much better iron hunk placement midship. On the size boat you're looking to do, I'd be surprised if you can offset the weight penalties with sufficient trim attributes. It might be possible to get it to plane (much like the inclined bottom designs) on a more forward located patch and dragging a bunch of eddies aft of a well mounted outboard, but retro fitting this feature into an existing hull, would be problematic at best. Simply put, you could do this, but considerable testing would be necessary to get it right.
     
  7. JWill
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    JWill New Member

    Thank you for the insight PAR. I've long been a fan of your cooper and digger designs, and have appreciated your knowledgeable posts over the years. I do like the idea of a centrally located onboard to help trim and vent of gravity. I agree a lot of trial and of course error would be involved in getting this to work, meanwhile there are many proven designs that will work for me. I may just carve a half model to better view this idea, I'm stuck on it because I love the St Pierre lines. I remember reading about a herreshoff "torpedo" double ender that was generously powered and had to use horizontal fins to increase bearing aft to prevent squat. It was around 50' so twice what I'm looking to do.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The only thing that you might be able to survive at planing speeds that would still be a simpler design, would be something with a constant-vee of about 10 degrees, over the whole bottom, still retaining your dory style.
     
  9. PAR
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    PAR Yacht Designer/Builder

    The V bottom will detract from initial stability and increase power requirements over a flat bottom dory shape. The Bartender also needs to use horizontal foils to hold her butt up. I've often though a step could be used to help hold a double enders stern up, but again, you'll need a considerable amount of fiddling, before you get it right.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course, but I wasn't thinking of retaining the double-ended configuration. There seems no point in a double -ended planing boat, the Bartender is a pretty normal planing hull with a rounded stern, it does not taper to a point aft, trying to make a planing hull like that would make little sense to me.
     
  11. PAR
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    PAR Yacht Designer/Builder

    The Bartender is a double ender, though the aft sections are quite full, it does come to a point. Initially, the Bartender didn't have the anti-squat wings and it still got up on plane, though it took longer and it flopped around in turns, falling off and climbing back up, until the foils were added. The waterlines on a Bartender look like a fat PeaPod. The dramatic flare aft and well rounded sheer, does make the hind quarters look much wider and fuller than it actually is. It's the chine profile and the way the buttocks are kept down, that really makes the Bartender work.

    [​IMG]

    The hull form is very speed limited, but respectable S/L's can be reached. Further investigation shows this 18' 6" LWL boat is about 3,300 pounds displacement, making it heavy for it's length, compairtivly. With 200 HP installed (max recommendation on the 22' model shown) she'll top out around 30 knots and will have started to pound pretty good at this speed too. Driven reasonably, in the mid to high 20's, she'll be a well mannered little boat, with enough mass to crunch through modest chop too. Because of the steeper than typical buttocks, they tend to run bow high and with the dramatic sheer sweep, can make vision a problem. The plane patch is odd looking and larger than typical and this drag is one of the speed limiting factors, along with it's warped bottom and fat entry with its modest deadrise.
     
  12. FAST FRED
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    FAST FRED Senior Member

    "I didn't like the wide transom getting pushed around by following seas"

    At 20K????
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    You got me, it does come to a (blunt) point aft ! :) Still, I cannot see much point to it other than appearances, if you wait around for breaking waves to pass you from behind on a bar (it is a bartender !), in a boat that can out-run the swells, it is not a sound practice. At best, you might say it is marginally better that a squared-of stern if you are limping back in at low speed. The dory-style of boat was a cheap and simple design that arose before planing boats became the thing, I think it fair to say it does not really adapt to the new circumstance, mainly because flat bottoms pound, and the double-ended configuration squats badly. However, it is still possible to have the front half of the boat resemble the traditional dory, and construction can be kept simple, but the end result is still a sub-standard boat.
     

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Certainly that Oregon dory is the obvious "solution" to his problem. But it must be awful thing to go far in.
     
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