Should a thin hull’s freeboard be vertical near the bow wave?

Discussion in 'Boat Design' started by Paddlelite, Feb 6, 2013.

  1. Paddlelite
    Joined: Feb 2013
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    Paddlelite Junior Member

    In watching videos of kayaks, canoes, and paddleboards, a big hump of a bow wave can be seen building in the first third of the hull as speed picks up. For the same reason that reserve volume in the stern is suggested to avoid sinking into the trough of the transverse wave train, I ask whether flared freeboard should be avoided in the area of the bow wave on a thin hull in order to avoid excessive lift.

    For example, if the waterline width is only 18 inches wide a third of the way back from the bow, and the hull is flared at 45 degrees above the water, then a 2” tall bow wave increases the waterline to 22” or 22%, meaning significantly more lift on the bow than if the sides were vertical. I realize that other design considerations may come into play, but this seems like it could have significance
  2. gonzo
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    gonzo Senior Member

    Bow design comes and goes with fashion. On working vessels there are regulations and specific uses that also affect bow design. On a kayak you will be hard put to paddle at a speed where the stern starts to sink, so a very wide stern will probably create more drag than lift.
  3. Petros
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    Petros Senior Member

    I have built a large number of kayaks and canoes and I am familiar with the design decision made by the many manufacturers in the industry. Most of them do not have a clue about what you are talking about.

    The bow wave is not the reason you but flare in the hull of a kayak, it will make little differnce in hull drag. A flare sided kayak will have more drag in heavy chop or rough water, and give you a rough bouncy ride, than one with vertical sides. But the flair sided kayak will have more reserve carrying capacity and more reserve stability on smooth water, and it will give you a more dry ride in most conditions. A slim bow profile will slice nicely though heavy chop and waves, but you will have a lot of water washing over the fore deck, much of it breaking against the spray skirt (if you use one).

    If you are designing for recreational use on inland waters, flare sides are generally desirable. If you are an experienced and skilled kayaker that intends to go long distances in ocean conditions than a slim bow with not a lot of flare will be better. Not having flair will also be better for rough conditions, but it means there will be more demand on the kayaker, more skill required. Of course, low experience paddlers should stay out of rough ocean conditions anyway, and stay with inland waters until they get the necssary skills.

    Drag reduction comes from having smooth transition from bow to the max beam, and than back to the stern. A rounder bottom will have less wetted area (for less drag) but will be less stable. Kayaks hulls are very efficient compared to most hulls because the length to width ratio is so high (fineness ratio), so there is not a lot of direct improvements to make them more effieincet.

    BTW, I think you have been mislead by the way people will describe the wave drag on small boats, they say that the boat trys to climb the bow wave and it take more power to drive the boat forward. This is silly and nothing to do with drag, the boat is moving forward, not climbing a hill of water. the size of the bow wave is related to the amount of energy it takes to form it, on a kayak that energy comes from your muscles. Hull length and very narrow hulls take less energy to push though the water than short fat hulls. If you can learn to balance well in an 18 or 19 inch wide kayak, it will have noticeably less drag than a 23 or 24" wide kayak. Learning to balance a narrow kayak comes quickly, it is like learning to ride a bike; once you master a two wheeler you would never go back to tricycle. Narrow kayaks are the same way. I have built both the Greenland type and the Aleut type, I like the Greenland type, light, fewer parts and assembles faster.

    Good luck.

  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Traditional kayaks were all flat bottomed. Almost all had some kind of flare to the sides. The bows or decks were usually high to help recover the boat from a capsize.
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