Power Dory seaworthiness

Discussion in 'Boat Design' started by makobuilders, Jan 20, 2014.

  1. makobuilders
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    makobuilders Member

    I am considering a steel power dory design in the 45 to 50 ft range, but I have concerns about comfort at sea. Plenty of stories are told about how typical displacement dories are very tender but then become solid and stable when loaded down with a 1000lbs of fish. Being that my boat will be a personal ocean cruiser, how would one assess, design and build the boat properly ballasted and loaded from the very beginning to obtain the necessary comfort at sea? There are some old threads from member Kelp about a 36ft Thomas Colvin power dory that he built and which obviously had bad motion problems that even the designer couldn't resolve.
     
  2. gonzo
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    gonzo Senior Member

  3. PAR
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    PAR Yacht Designer/Builder

    Dories in relatively small sizes can be somewhat seaworthy, but this set of shapes has limitations. Once you ask these to be this big (45' - 50') you lose most of it's advantages and none of it's disadvantages. Simply put, if you're looking for a custom or semi custom design deep blue water craft, forget about dories and select a hull form that is proven in the size range you're after.

    Colvin is certainly capable, but pretty old school. I don't know if he's still active (or even alive), but at 90 years old, I would suspect not so much.

    The obvious questions are: do you have building experience - a fully developed SOR - sea going experience - have you a set of plans - are you looking for stock plans or custom?

    I ask because going to sea in a flat, narrow bottom boat, with relatively slack bilges and likely of shoal draft, isn't the best approach. Powerboats rely nearly exclusively on form for stability, so picking an appropriate design, for the goals of the SOR, should be paramount. If you're absolutely in love with the dory shape, it can be styled like one, with the benefits of more modern underwater features, appropriate for deep water powerboat work.
     
  4. tom28571
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    tom28571 Senior Member

    The best advice you are going to get. The only reason power or sail dories are popular is that many are attracted to the visual appeal. Dories are very good at the job they were originally built for. Anything else, not so much.
     
  5. rasorinc
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    rasorinc Senior Member

  6. tom28571
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    tom28571 Senior Member

    The Big Hunk has been modified from a true dory shape to give it acceptable handling characteristics. Specifically, the waterline beam is much greater than a true dory of the same LOA and the width is taken all the way to the transom. A very common practice as most designers recognize the dory limitations. What it is now is a flat bottom skiff with a generous amount of flare. Designers/boatbuilders know that the dory has a certain cachet and use the term a lot-----semi dory, modified dory, etc.

    OK if you are willing to either like or tolerate the pounding that such a hull offers at speed in any kind of chop. If run at "hull speed" or a bit above, narrow bottom dories are acceptable and very seaworthy if there is adequate ballast down low and you can tolerate a limited amount of usable space inside relative the the size of the boat. The St Pierre dory is one of these.
     
  7. PAR
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    PAR Yacht Designer/Builder

    The only large dory I've seen is about 40', but also has a 6' deep, lead fin dangling below it, so it's stable. Unless traveling at displacement speeds, I can't see any logical reason to use this hull form on a blue water powerboat, particularly with the other shapes available.
     
  8. makobuilders
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    makobuilders Member

    As this would be a displacement speed powerboat then no 6' deep keel of course.

    I spoke to Colvin years ago and he said in his 50' design that the bottom plate of 3/8" steel provided the necessary ballast.

    The 50' dory pretty much has the interior room of a 40 footer, but it is the simple, ease of construction that is attractive. Of course I don't have to pay dockage fees here.
     
  9. tom28571
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    tom28571 Senior Member

    Volume yes but really usable volume is even less. You should concentrate anything on the interior near the centerline. Good practice on any boat but moreso on a dory. If you have ever had a powerboat lay over on its side and become stable there, you might understand some negative feelings toward that hullform.
     
  10. frank smith
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    frank smith Senior Member

    How about a big sharpie? get rid of most of the flair in the dory, and widen the stern. Then add a keel and vee the bottom.
     
  11. gonzo
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    gonzo Senior Member

    In the South Atlantic they are used to be able to haul them out at the beach.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Agreed, there are lots of much better hull forms, some of which can have flaring topsides, so it'll still have the dory look. A skiff or modified sharpie would be better choices. A relatively narrow, firm bilge design, shaped for the speed and stability range you're targeting, is the logical route. Anything less and you're hampering the design, with aesthetic considerations, that aren't going to help your other SOR goals.
     
  13. Tad
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    Tad Boat Designer

    Unless massively ballasted a dory (or any flat-bottomed hull) will have too quick a roll response to be comfortable. Carrying a bunch of ballast is inefficient. If you cut the corner of the chine off (sinking the hull deeper), creating a double chine hull, she will be slower rolling and more comfortable in a sea.
     
  14. tom28571
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    tom28571 Senior Member

    I suppose that depends on which kind of roll a person considers the most uncomfortable. A low deadrise hull will have a quicker motion as the hull attempts to follow the water level in a wave. A rounded hull will tend to roll slower but deeper and not usually in concert with the water level. Neither is particularly comfortable in cross seas but I find the round hull to be more likely to induce seasickness.
     

  15. FAST FRED
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    FAST FRED Senior Member

    Dories were the fish boat of choice as they were cheap to build , and would NEST so dozens could be carried in a small deck area.

    Neither attribute is useful for your proposed cruiser.
     
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