Stability of power dory

Discussion in 'Boat Design' started by davflaws, Feb 13, 2006.

  1. davflaws
    Joined: Jul 2004
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    davflaws Junior Member

    Hi Folks,

    I recently spent lotsa hours and $s on a trailerable power dory which I hoped would be capable of going 10-15 nm offshore and still coming home if things turned nasty. She is GOP, 18ft with 4ft chine beam at the transom, with a cuddy and lotsa built in buoyancy underfloor and in side cockpit tank/lockers.

    The sea trials were good in terms of speed and economy (12kt with 15hp OB fully laden, adequate ability to maintain it in a short head sea, and she lay well to wind and swell both at anchor and while drift fishing.) She is very tender, but I expected she would be and was willing to accept that in the interests of an easily driven hull. Since she has lotsa flare in her topsides, I thought we would be able to perch a small elephant on the gunwale without putting water over the cockpit coamings. We can't. A standing, lurching, listing, slipping, falling mishap with crew weight and trim managed to put water in the cockpit on a flat calm day.

    She won't do. We have to get her terminal stabilty up, and altering the hull shape seems the best way. Would it be practical to:
    1. Attack the area immediately below the gunwale beltings with a grinder to prep it.
    2. Glue (epoxy) on planks of Airex or similar foam, bending them as we go.
    3. Fair with a surform or similar.
    4. Glass and resin over the faired foam.
    5. Sand, fill, sand, paint etc.
    6. More sea trials with small elephant.

    I would appreciate any advice
    Thanks
    DEF
     
  2. terhohalme
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    terhohalme BEng Boat Technology

    Some pictures to analyse?
     
  3. safewalrus
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    safewalrus Ancient Marriner

    Apart from the thought of anyone wishing to proceed 10 to 15 miles offshore New Zealand (or offshore anywhere else straight into an Ocean) scares the sh*t outa me, what about ballast? Put a few bags of 'beach' (stones etc. not sand) on the bottom and try that! Play until you get the right amount; the idea being that you can dump ballast as you catch fish! if it's 'beach' you don't lose much! [also stay inside the boat of course!]
     
  4. FAST FRED
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    FAST FRED Senior Member

    The old Grand Banks dories would be flooded with water for stability, the water bailed as the fish catch added to the weight.

    There terrible empty , thats just life in a Dory, learn to utilize the design.

    FAST FRED
     
  5. FAST FRED
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    FAST FRED Senior Member

    The simplest MOD for more stability would be a Proa Arm, say a 10ft hunk of 6 inch aluminum pipe with a set of outrigger arms.

    FF
     
  6. safewalrus
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    safewalrus Ancient Marriner

    The mind boggles Fred but yeah your right of course!:eek:
     
  7. davflaws
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    davflaws Junior Member

    Thanks guys. The idea of outriggers doesn't appeal much in view of considerations around trailering and launching, but ballast will be trialled in the next week or so. We are currently considering lead slugs along the insides of the chines. I have been thinking about the terminal stability (which was more of a worry than the initial tenderness). I had thought of foam sponsons and was working along those lines in my first post, but I have also been thinking that the same volume of glassed over foam buoyancy distributed over the whole side (from the gunwale down to the chines) would have a (fairly) similar effect in increasing terminal stability and at the same time would improve the initial tenderness. Any thoughts/advice.
     
  8. safewalrus
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    safewalrus Ancient Marriner

    The slugs are OK but go for the 'beach' first, it's cheaper and of course environmentally friendly if you decide to dump some once your at sea! Just a thought is all!
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The dory hull form has a long and interesting history. I consider the Swampscott the best of the breed, though you need a modified dory, not one of traditional proportions, for the tasks you're asking of one.

    Dories were typically well laden with fish or cargo, where they became quite stable. Lightly loaded they are very tender and usually shipped water at the drop of a hat. Most carried ballast stone or water in the bilge to account for good sea keeping manners when reasonably empty. The shape of a dory lends itself well to fast sailing light and takes a burden nicely, picking up stability as she drops into the water. Most folks don't feel to comfortable, in a traditionally shaped dory, unless it's getting near it's maximum loading.

    It would be helpful to know what type of dory you have, as there are several types, each having particular traits and requirements for adjustment to a modern sailor's liking.

    The usual modification (not one taken lightly as it takes a good deal of skill to do well and not destroy the qualities of the craft) is a wider bottom, an additional strake, deadwood for inboard engines (skeg for outboard), bearing area added aft (usually in the garboard) to hold the weight of the engine, plus a few other shape changes to suit local conditions and owner desires. These changes would be typical for say a Boston fishing dory of the early 20th century.

    Taking a guess at a better hull shape will surly result in a poorer example of the type. Strapping 55 gallon drums to her flanks will dramatically increase her initial stability (just like outriggers), but you can forget economy or performance. In short, she needs to be operated as she was intended, or reasonably informed changes need be made to her shape, to increase her usefulness in a new role. I'm pretty good at guessing, knowing where to nip, tuck or add displacement to get the desired results. Your suggestion of just adding displacement and buoyancy will likely just make her worse, requiring more weight taken aboard to "calm her down". Try adding weight low, on the centerline and midship until you have more satisfaction with the roll moment, then adjust the ballast fore and aft until she trims up properly. This ballast should be removable.
     
  10. sharpii2
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    sharpii2 Senior Member

    This flooding of dories to make them more stable is news to me. My understanding of the type was that it was to be easily rowed to its fishing grounds and often after being hoisted from its mother ship's deck. To be easily rowed, it needed a narrow bottom and it needed to be light.

    Of course it was very tiddly in that condition, but so is a rowing shell or a kayak. My guess is such tiddlyness was hardly notice by those who knew her just as the tiddlyness of a bicycle is hardly noticed by an experienced rider.

    After being loaded down with fish (some times 1,000 lbs or more), the dory was difficult to row, so a crude sail, usually a sprit sail, was fitted to help it return to the mother ship which was usually parked down wind for that reason.

    I never heard of water ballast being used. And if it had been, it would have had to be contained somehow in special tanks, bladders, or other kind of containers to keep it from flowing to the low side.

    Free water aboard any mono (especially a flat bottomed one) subtracts from rather than adds to its stability.

    I even read somewhere that if a laden dory got seperated from her mother ship and got caught in a blow, much of her catch would have to be jettisoned to keep her afloat.

    Bob

    PS- I think the outrigger float suggested in your later post is a good idea. I've often thought of that tactic myself as way of converting a rowing dory into a sail boat.
     
  11. safewalrus
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    safewalrus Ancient Marriner

    Didn't some guy suggest using bags of pebbles off the beach for ballast? :p it's movable, easily altered and environmentally fiendly when dumped! Also surprisingly cheap as long as you don't ditch the bloody bags! :p

    I reckon you should listen to that guy as he sounds pretty sensible :!: and cuts through all the double talk and **** of them as want to justify their existance! :cool:

    Off some mild interest don't some modernish 'cathedral hull' plastic, outboard driven vessels get called 'dorys' these days - we're all presuming this guy has an old style 'Grand Banks' dory - is it?
     
  12. PAR
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    PAR Yacht Designer/Builder

    Power dories differed considerably from their sail counter parts. They didn't row worth a hoot, just too much drag. The modifications to the dory type for power usually had a fore, side and aft decks, the aft deck had a sliding hatch or low house to cover the engine (inboard), an amount of deadwood (with drag) to house the shaft, less rake in the stern post and widened garboards aft to provide bearing for the additional weight of the engine.

    The rowing/sailing versions are different animals, though related in early versions (powered attempts with sailing hulls) It became quickly realized that the sailing hull couldn't support the heavy, turn of the century power plants, so hull shape changes were made. The rowing examples of this type performed very well and sail racing at the end of the 19th century was quite heated and lively.

    We do need to find out what type of dory this one is. I can think of at least of dozen distinct dory types. A photo will help a great deal.

    Stone ballast was common, water in a midship hold was also done. These holds had drop boards, similar to what crabbing boats use today, which divided up the hold into small (baffled) compartments, helping control the sloshing of water.
     
  13. michael-compwes
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    michael-compwes Junior Member

    ...a bit off-topic,,but I'd be interested to know of various sailing or power dory designs available....gotta link??......thanks.
     
  14. PAR
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    PAR Yacht Designer/Builder

    I have several dory designs, some of classic proportions others modified for modern use. If you drop me an email I can fill you in on the details. A generic web search for "dory" will account for many hits, though I think you'd have better luck nailing down a type for more application specific results. Most every plans house will have a dory or two in their stock files.
     

  15. Jolly Roger
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    Jolly Roger Junior Member

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