Dory - Flat Bottom - J. Spira - Claims

Discussion in 'Boat Design' started by Standpipe, Jun 10, 2015.

  1. Standpipe
    Joined: Dec 2014
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    Standpipe Junior Member

    Hi all,

    Mr. Spira claims that small, flat bottomed "dory style" boats are safe and seaworthy. Mr. Spira is a professional engineer.

    Aside from the uncomfortable pounding - firmly in the "against" column ;what do you guys make of his claims?

    Correction -> I should have been more clear. By "small" I was referring to his 25' .. and larger designs.

     
  2. Jamie Kennedy
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    Jamie Kennedy Senior Member

    A Grand Banks Dory has more veed sides, and quite a bit more rocker and sheer and overhang than a sharpie. It was certainly designed for open water, given its name and purpose. A large part of its design was low cost and stackability, but was also very stable in rough water, both light and loaded, but not so well behaved when light. The Grand Banks Dory shape has been adapted to cruising with the addition of a cabin and keel and seem well suited for sea keeping if the total displacement is comparable to a fishing dory when fairly full, not empty.

    Here are some Grand Banks Dories in use by the White Fleet of Portugal.
    https://www.youtube.com/watch?v=-zpMPmhPdWI

    and another great documentary, from 1966...
    https://www.youtube.com/watch?v=Git-48_CPww

    The Dory Shop in Lunenburg...
    http://www.doryshop.com/dories/
     
  3. Standpipe
    Joined: Dec 2014
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    Standpipe Junior Member

    Very interesting links Jamie , thank you.


    ----------------------------

    Getting away from the ideal of "stack-able" boats used by a ship.....

    As a novice I think his "selling points" are interesting - a simple but rugged boat for the masses.

    ----------------------------

    On a separate point...

    I wonder how many get half way into their Home Depot-build and find themselves overwhelmed by a 25+ boat.

    Feeling duped into a fools errand .........left eyeing the chainsaw in the hanging in the garage.
     
  4. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    "Vee Bottom or Flat Bottom

    The controversy rages on about which is the better boat. I've had people look at me in disbelief when I
    suggest they can use a flat bottomed boat, such as a Grand Banks dory, for deep water ocean use. They've
    always been taught that flat bottomed boats flip over. I don't know where that idea came from, but on several
    occasions, I’ve heard, "You'd actually take a flat bottomed boat to Catalina?" (an island about 25 miles off the
    coast of Southern California.)
    The secret of boat stability is more a function of the boat’s center of gravity and loading than the shape of
    the hull. In truth, a flat bottom hull is more stable at rest than a vee or round bottom hull, when it comes to
    shifting loads from side to side. That's why big ships like freighters and tankers are all flat bottomed. The flat
    bottom hull tips far less than the vee bottom. This is called “initial stability,” and it's why flat bottom hulls are
    often a better choice for fishermen and people who bring along shifting loads, like children or dogs, who cant
    seem to sit still.
    The one thing left unsaid so far, though, is that once a flat bottom hull passes it's stable range, it does tend
    to become unstable, for instance, when struck abeam by a breaking sea, more quickly than a vee bottomed hull.
    In conditions like these, a round bottom or vee bottom hull with a weighted keel has far more tendency to return
    to upright than a flat bottomed hull"

    Whoever wrote that is confusing flat bottom and dory types of hulls. Yes, a dory is flat bottomed but it has lots of flare and the bottom is relatively narrow which is completely different from a flat bottom boat with a wide waterline beam. His claim of high initial stability of the flat bottom does not apply to a true dory which has very now initial stability.

    A properly designed and loaded dory is a good sea boat and arguably more seaworthy that a similar displacement V hull with wide waterline beam. There are many factors that go into making a good seaworthy boat and the writer made a mess of his. Spira boats are similar to the Tolman Skiff and Simmons Skiffs even if the latter are V hulls, albeit with very shallow V.
     
  5. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    A professional engineer is not a Naval Architect. The above statements are all incorrect.

    Stability of any hull is dependent on hull shape and center of gravity position. Depending on the boat, it's shape is usually a larger factor contributing to the vessel's stability.

    It makes no sense to first say the boats stability is dependent on her COG and then state that it's shape (Flat vs vee) dependent......Well make up your mind.....

    Freighters and Tankers are flat bottomed because that's cheaper to build, but more importantly because that shape allows max cargo volume within given dimensions, importantly draft. It has nothing to do with stability which he already stated is COG dependent.....

    Whoever wrote this statement knows a bit about stability, but not enough to write clearly on the subject.
     
  6. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Thanks. Gotta love the old footage eh.

    I have to agree. A dory is very simple, but 16 feet is much much simpler than 26 or 32 or 40. There was a youngster from Guatemala that posted a few days ago about starting his boatbuilding with a 8 foot pram. Smart kid.
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    As an engineer, but more an amateur sailor and naval architect than engineer, I wholeheartedly agree. Engineering and Naval Architecture have a lot in common, and are fundamentally the same, but if an engineer hasn't grasped the concept of metacenter and how the center of buoyancy shifts about trying to keep itself under the center of gravity, he might want to hide the fact that he is an engineer until he does so, ;-)

    The dory has a flat bottom, but is perhaps the best example of low initial stability and high secondary stability, and not the other way around. This characteristic has more to do with the flaired sides than the flat bottom. Low center of gravity also helps, but a vee bottom does not have a low center of gravity until you add ballast. A dory has a relatively low center of gravity because of it flair, rocker, sheer, and its having no deck. It's carrying capacity lends itself to a good cruising boat because that carrying capacity, normally reserved for fish, can be utilized by adding a ballasted keel to compensate for a cabin and accommodations. A ballast ratio of 50% is quite achievable, like a folkboat, and if you took a dory and rounded off the chine and added a full keel it would pretty much have the same shape as a folkboat. It is the flare and sheer and, for a boat under 30 feet, lack of headroom, that makes a good sea boat. Herreshoff, not sure which one, said headroom was for people that like to dance or sleep standing up. A deep vee than merges into a full keel makes it easier to have headroom in a boat under 30 feet, but having that extra buoyancy down low does nothing for stability and actually detracts from it, until you add fish. ;-)
     
  8. Rurudyne
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    Rurudyne Senior Member

    I'm guessing Herreshoff wasn't a stocky 6'4" (weird proportions, legs about right for sub 6', big and tall torso, hat size 8.25) who weighted 275 with a sub 10% body fat but now has a weak back and one trick knees (thank you football).

    Of course my 16EEEE feet have never met this "dead pedal space" car people talk about either. ;)
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Glad I don't get the bill to feed you, Rurudyne ! :eek:
     
  10. whitepointer23

    whitepointer23 Previous Member

    My doctor said I was short for my weight. My correct height should be 9 ft 6.:D:
     
  11. SaltOntheBrain
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    SaltOntheBrain Senior Member

    I'm picturing Larry Csonka from your description, Rurudyne.

    LF
     
  12. Rurudyne
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    Rurudyne Senior Member

    Yeah, I wonder what Herresoff would say about large refrigerators on relatively small boats?

    Be nearly 7' based on my noggin.

    Being stocky didn't make me slow either. Back in the day at SMU, while still on lower string, the defensive back coaches used linemen like myself to run pass patterns in practice and I routinely burned the starting DBs even with my knee braces and other linemen stuff on. That had to have been pretty embarrassing for them....
     
  13. SaltOntheBrain
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    SaltOntheBrain Senior Member

    North Texas is pretty vague.
    I'm in Crosbyton, east of Lubbock.
    Where are you?
     
  14. Rurudyne
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    Rurudyne Senior Member

    Lewisville.
     

  15. loucarboni
    Joined: Sep 2015
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    loucarboni Junior Member

    I have a 6metre ply dory with a 40hp yamaha on the back
    I have logged up over 10000 nms in protected waters.
    She has all the theoretical attributes when the whether is fine but when the wind tips 20 knots i long for a vee bottom. I normally motor at about 15 knots but have to slow right down going to windward. And with a small following sea is very unstable with a tendency to
    Broach suddenly. I would never have a flat bottom boat again
     
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