Kayak an efficient displacement stern shape?

Discussion in 'Boat Design' started by pkoken, Dec 30, 2007.

  1. pkoken
    Joined: Mar 2003
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    pkoken S/V Samadhi V

    I have been pondering the concept of a long, narrow displacement motoryacht. Steve Dashew's Wind Horse started my thinking along this path...

    But why use a conventional flat transom at strictly displacement speeds? I can see that the additional space would not be of much use in the extreme stern (the same as in the bow). There also must be some efficiency lost because of skin friction...

    Sailboats don't use this type of transom anymore, and they are usually out for maximum efficiency.

    Why don't we see kayaks with "transom" sterns?


    S/V Samadhi V
  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    I think you answered part of your question. There is no need for space at the aft end of a kayak.

    Another reason is that a kayak does not operate like boats meant to mostly run in a straight line. It needs to be maneuverable and this implies more symmetrical ends and rocker in the hull.

    A bigger reason occurs when you are running rapids. In this case the water is more often going faster than the boat. In this case, it's easy to see that a pointy bow shape is needed in the stern. A transom stern would make the kayak more liable to loose steering control and broach. Happens too often even with a pointy stern.
  3. Guest625101138

    Guest625101138 Previous Member

    There have been a few discussions around on Atkin hulls. These are a canoe under water but have a flat transom.

    There are other examples as well such as this cat that has canoe hulls:

    Modern sailing boats have power to burn so are designed to plane easily hence the wide flat stern.

    If you want an efficient hull for displacement speed then the canoe stern is best but it does not mean you have to be constrained to the canoe above the water. I did the attached hull for a speed of 6kts. It only requires 1kW to do 6kts and it displaces 4.5 tonne.

    Rick W.

    Attached Files:

  4. kengrome
    Joined: Jul 2006
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    kengrome Senior Member

    Rick, it occurs to me that for folks who are not committed to solar power, this boat might be driven via an electric motor powered by a small generator. The generator can be used for other power needs on the boat at mooring. I'm intrigued by these new generators that weigh less than traditional generators, produce clean sine wave AC and are more fuel efficient due to their inverter designs:


  5. pkoken
    Joined: Mar 2003
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    pkoken S/V Samadhi V

    I am curious about the aft deck extensions (where they overhang the stern on the sides) what happens in a seaway when these are exposed to waves? There looks like there is a lot of additional buoyancy in the stern when it is down in a wave- will this cause the bow to submerge?
  6. Munter
    Joined: Jul 2007
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    Munter Amateur

    Sailboats use beamier stern sections because they help generate righting moment as well as providing interior space (though that isn't exactly a concern on an open 60 sailed by one fellow...)
  7. Petros
    Joined: Oct 2007
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    Petros Senior Member

    Sea kayaks operate in pure displacement mode with very low power output at relatively small Rynolds numbers. They optimize the least power demand over long distances at low speed. The lower speeds they operate at (3-5 knots) means the skin friction of the stern is less significant than the drag of a flat transom (which may not be true at higher speeds).

    Also when paddling a sea kayak in a following sea a transom raises the tendency to broach and lose control. Without a deep keel and wide beam a broach unusually leads to a capsize. the sharp stern allows the swells and even waves to pass the hull with less disturbance.

    Since sea kayaks typically not do use rudders (yes I know many vender's are happy to take your money for them, but historically native watercraft do not use rudders-they are "white man" inventions), you need a skag as far aft as possible to improve directional stability, turning is accomplished by tilting the hull on edge and differential paddling. So a fine stern tapering to a point helps directional stability without a deep skag or keel.

    If you are operating in the same speed range as a sea kayak likely a "canoe" stern would be the best shape. All of the design effort put into hull design for both power and sail boats yields the transom stern designs than likely that also has the best drag and handling properties for that size and speed. You will likely not discover something new by putting a kayak stern on a larger powered hull.
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    FAST FRED Senior Member

    "If you are operating in the same speed range as a sea kayak likely a "canoe" stern would be the best shape."

    To me that would be a SL ratio of 1.1 or under.

  9. ted655
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    ted655 Senior Member

  10. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    A few canoes have been made with a transom to accommodate a small outboard motor but they are typically for flat water conditions. Saw a nice one the other day with a wineglass stern above the water line to limit drag.
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