Round stern vs box stern??

Discussion in 'Boat Design' started by Deadrise13, Feb 8, 2009.

  1. liki
    Joined: Nov 2008
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    Location: Finland

    liki Senior Member

    Here you can see a bit more sophisticated retrofit:
    http://www.friddemartin.net/fanny/picgallfi.html
    :)

    The texts seemed to be only in Finnish but a traditional wooden displacement hull was turned into a +40kn performing monster powered with 440hp yanmar and a 2-speed marine gear.

    I've heard rumors that the engine already into one with well over 500hp.
     
  2. BMcF
    Joined: Mar 2007
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    Location: Maryland

    BMcF Senior Member

    Miami Brookerage Show I'm guessing?..missed that this year but have some associates hanging arounf there...
     
  3. Deadrise13
    Joined: Feb 2009
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    Location: Sebastion inlet fl

    Deadrise13 Junior Member

    Got it PAR, Thanks I will look forward to hearing from you.-----Nice pix Liki----Thanks for the input, I will check them out tonight.-----
     
  4. kmorin
    Joined: Apr 2005
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    Location: Alaska

    kmorin Senior Member

    Stern Shape

    Deadrise13,
    I'm not a wood boat man so I haven't a clue about building these hull forms in wood, but I do have a few notes to add here about his particular stern's behavior at sea.

    Almost without regard to exact type- net fishermen face a gear related displacement event when they have lots of fish. Even drift gill nets get a large concentration of fish in one location, needless to say trawls, draggers and seiners all concentrate their catch too. So net fishermen often have to load their catch in a large bunch instead many single wts added over time like the hook and line or long liners.

    If you're working the net over the stern and have to add lots of wt to the hull in a short time then a following sea can board more easily if the hull shape has less reserve buoyancy like a squared off, plumb transom. A rounded shape or conic form increases the RATE of increasing buoyancy as its settled deeper in the water.

    Many traditional commerical fishing net fishermen found the fan tail, conic, or rounded and flammed stern to be drier and safer when working their gear. This is one reason for the shape in older working boats.

    In a following sea, especially with a loaded fishhold, these shapes are much more steady on course. The transom stern power boats require a good deal more helm, more often as the waterlines aft are less rounded and therefore more vector is imparted to the hull's movement by overtaking seas.

    Rounded, fan tailed, or conic stern forms generally have a less angular vector to yaw the helm about the keel, as the form provides a more uniform lift and less 'one side or the other' thrust by the oncoming (following) sea. When loaded a fishing boat's helm can be a bit of a chore, the crew is often tired and the run might be at night, so contributing to the success of these shapes was also the conditions in which they were most used.

    I use these forms in metal boats because I like their looks, they are less tiring to fish in the open ocean, and with a simply angled step plate you can transition from square stern line of planing shapes to the nicely rounded stern.

    And who among us doesn't enjoy a nicely rounded stern?

    cheers,
    Kevin Morin
     
  5. Deadrise13
    Joined: Feb 2009
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    Location: Sebastion inlet fl

    Deadrise13 Junior Member

    I for one Love a nicely Rounded, Firm Stern!!! Ow you ment the boat, forgive me. Thanks for all the input. I will not be comercially fishing this boat. I will be hook and line fishing off shore. I Have heard much of what you are saying from some old fishing buddys who did it all back in the day. I would probabaly think long and hard if I was going to use the boat for other reasons. Could you explain the angeled step plate idea? I think I know what you mean but would like to be sure! Thanks for all the input----
     
  6. kmorin
    Joined: Apr 2005
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    Location: Alaska

    kmorin Senior Member

    Example of a metal step stern

    Deadrise13,

    I only build in metal so I'm only discussing metal in this reply. I don't know about wood construction well enough to predict if there's any way to do this reliably in wood.

    [​IMG]


    First is an image of a little round stern tug shape without the keel to keep the image simple. The sketch shows her from a few views confirming the more or less rounded stern but because of her rising butt lines the water line is rounded too. Many designers want a more squared off after surface for planing hulls.

    [​IMG]

    This just confirms that the chine line rises above the waterline at the stern, leaving the waterline 'under' the hull and leaving the after edge of the running waterline to the shape of the bottom at this level.

    [​IMG]

    Here is a little 14'er shown in various views to help make her more clear. The step I've referred to is the 90 degree pair of plates at the waterline under the transom. The vertical plate is at 90 to the bottom, and the horizontal is the bottom curve of the 'rounded' or curved transom.

    [​IMG]

    This photo will be confusing if you don't look carefully as its not well taken or prepared to post. The skiff in the green model just above this image is being built. The stern is curved not fully rounded, and the transition step is shown. The problem in the the photo is that he female jig holding the bottom's shape has a straight edge stiffener at 90 degree to the aft most transverse former. This shows as a vague gray line, instead of clearly, but the bottom's section is visible farther forward where the shape is more distinct because of better contrast. You can see the after seam, curved; and you can see the horizontal transistion seam tacked too. What is hard to see, and I dont' have a better pic, is the lower seam of the step piece(s) that is where the vertical piece meets the hull bottom. This line is just barely visible at the upper corner (starboard) just near the chines.

    [​IMG]

    the method works for fully rounded sterns as well. This shot shows a full round stern on a skiff that has the transition step well under the bottom. An outboard T bracket in common to the bottom plane so the image is not exactly informative of the idea but just here to confirm that planing shapes that move at good speeds can have rounded sterns so they handle well in following seas at slow speeds but they don't lose anything at full speed.

    again, I don't advocate this in wood since I've only done it metal, but I've done for nearly 35 years so I'm also pretty sure it works as shown. I hope this explains what I meant by a step at the stern to convert from a planing bottom to a counter/curved/rounded/fan tail stern?

    cheers,
    Kevin Morin
     
  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    All those shapes can be handled in wood and commonly are. Looks like nice work Kevin.
     
  8. Deadrise13
    Joined: Feb 2009
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    Location: Sebastion inlet fl

    Deadrise13 Junior Member

    Kevinn nice work!! Thanks for all the input. Very informative, Do you have any examples of larger boats? I would love to see more. Thanks again----
     
  9. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Hm. Stitch and weld! Nice work!
     

  10. Deadrise13
    Joined: Feb 2009
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    Location: Sebastion inlet fl

    Deadrise13 Junior Member

    Hey Par, Never heard back from you!!! Hope all is well-----G-----
     
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