Max beam forward

Discussion in 'Boat Design' started by 23feet, Feb 19, 2017.

  1. 23feet
    Joined: Dec 2014
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    23feet Junior Member

    Just looking at the Vivier Ebihan 15, 16 and 18 designs. They are described as being designed for open water. The thing that is most noticeable about the hull shape is that the max beam is forward. Most monohull hulls are designed with the max beam aft of the center of the boat. I notice that many older (British) designs had max beam forward of center.

    Can anyone tell me what the advantage of having the max beam so far forward is? I assume it is something to do with its open water "sea worthiness"?
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You're jumping to uninitiated conclusions. The beam at the rail looks to be forward, though her waterlines are pretty conventional, which is all that really matters. This series of boats are designed to look like the old north sea working craft, which followed some old and less than desirable shapes. Fran├žois has made these shapes more acceptable in modern terms, while retaining a lot of their charm, so yeah the beam looks forward, but really isn't, unless compaired to a modern dinghy or sportboat.
     
  3. 23feet
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    23feet Junior Member

    OK thanks. None the less, can anyone comment on what the perceived advantage of the bluff bow (mackeral shape) of earlier British boats was?
     
  4. philSweet
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    philSweet Senior Member

  5. CT249
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    CT249 Senior Member

    I can't recall whether it was WP Stephens (late 1800s designer and sailing historian) or someone like Dixon Kemp, but recently I was reading that part of the shape was created by the very heavy displacement required in those days of primitive construction and internal ballast. Combined with the low speeds of such craft, the result was that having a clean run aft was critical. Given that the heavy displacement required considerable underwater volume that had to be put somewhere, the result was a bluff bow.

    Another thing I picked up from DK Brown (naval ship designer and historian) is that the very poor diagonal strength of traditional construction (before diagonal straps were introduced by Stebbings) meant that each fore-and-aft section of a ship had to be able to effectively support its own weight; you could't have a fine bow with ballast moved further aft to compensate because the whole craft would just hog and leak. Therefore the bow, which was heavy because of the bowsprit, primitive anchors etc, had to be full enough to provide a lot of buoyancy.

    Dinghy and yacht designers also note that you can't have a fine bow if the construction is very heavy, because a narrow bow will not be able to lift a heavy displacement boat and therefore she will nosedive. In those days of long bowsprits, which would nose into a wave, that would regularly mean broken gear. A fuller bow could lift to the choppy tide-ridden English seas. If you read old copies of Hunts Yachting Magazine etc you see many accounts of broken gear when racing; construction was too weak to withstand further strains.

    In racing boats there was a rage to refit old yachts with longer, finer bows after the schooner America proved her performance, but some observers noted that many modified boats were not improved.
     
  6. PAR
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    PAR Yacht Designer/Builder

    The forward sections of this boat aren't particularly bluff. They do pick up volume rapidly as they emerse, but this is expected and necessary on traditional builds, especially with rigs in the eyes of the hull. As CT249 points out, you're going to make a significant hole, so the idea was to offer a reasonable way for flow to reattach and separate aft. In terms of speed, it wasn't effective compaired other approaches, but at the speeds these were intended to move, not as much of a determinant.
     
  7. Earl Boebert
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    Earl Boebert Senior Member

    If you google on "cod's head and mackerel tail" you'll find a great deal of material on what the designers of that day were thinking. Model yachtsmen moved away from the shape very early, another example of model yachts being the fruit flies of sailboat design because so many generations can be experienced in a short period of time. Plus, of course, not having to deal with structural issues or where to put people.

    Cheers,

    Earl
     
  8. PAR
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    PAR Yacht Designer/Builder

    This particular design isn't a cod's head approuch, though is intended to mimic the design aestedics of an antique workboat.
     

  9. Earl Boebert
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    Earl Boebert Senior Member

    OK, I see. Thanks.

    Cheers,

    Earl
     
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