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Thread: Bluff bows

  1. #1
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    Default Bluff bows

    Can anyone please shed some light on the nature of bows.

    Why do so many boats and especially yachts have a blunt bow? At the extreme 'chin' or waterline (what's the correct name for this part?) there is usually a fairly sharp entry for the water, but as you look higher there often gets progressively more radius, and presumably, more drag. I can understand that a yacht rarely travels perfectly straight forwards which results in a slightly cross ways flow over the bow so a rounded bow will improve flow, but why make it such a big radius higher up?

    In fact somewhere there's even a picture of a footy made out of flat plastic panels and taped together, (great idea) and it has a simple flat triangle forming the extreme bow, with the thin end at the waterline. The top edge must be about 3/4" or more across, a flat plate to cut the water. It would have been much easier to make this hull with a sharp bow by just pulling the sides in a little further at the top.

    Surely making yacht hulls this way only increases drag, especially when downwind or punching through waves, so it doesn't look like a desirable feature to my untrained eye

    There must be a technical reason for it but it's eluding me

    Thanks!
    Last edited by MeGrimm; 05-11-2007 at 03:02 PM. Reason: typo

  2. #2
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    From my experiences with trying to shape wood plugs to get a sharp bow all the way from the deck to below the waterline, it has to do with wanting/having a plumb bow. Without drastically changing the lines of the boat in the bow sections you cannot have a sharp bow all the way from one extreme to the other. The only way this will work is if the extreme bow sections have vertical sides from the waterline up. Does this make sense? I'm not even sure if it does so please let me know.

  3. #3
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    Yes thanks, it does make sense!

    So really if you weren't lmited to a certain LOA you could just carry the upper lines forward a bit to give a sharper entry and an overhanging / swept forward bow?

    Sorry but I'm not familiar with the term 'plumb bow'. Does it mean dead vertical?

  4. #4
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    The main reason for the vertical bow is to gain maximum sailing length in a class that limits overall length, eg IOM, Marblehead, Footy. Contrast that with the long graceful bow (and stern) on a 10R or A class, that measures waterline length as an input to calculating sail area. These boats gain 'free' length when they heel.

    The main reason that these 'vertical bow' boats are wider at the deck, is to provide volume forward, to resist driving the bow underwater when pressed running downwind.
    John Ball
    CRYA #895
    IOM CAN 307 V8

  5. #5

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    The most extreme example I know of were the "Tucker Ducks" of the 1950s in the Marblehead and 36R classes. Tucker took a 10-rater with a 50 inch Waterline and lopped off the ends, yielding a "bathtub" bow. They were moderately successful.

    Cheers,

    Earl

  6. #6
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    i figure it's because of the angle(s) of the hull sides, flare if you will.
    two triangles coming together, are sharp at the intersection (the chin part you mentioned) the more the flare, the more blunt the top gets.
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  7. #7
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    to my understanding and study, nigel has it spot on. the extra volume in the bow, [which leads to a "blunt bow"] is definetly a superb way to resist diving. it also sometimes is just easier to build that way... as is the case with the "Bug" [which i think is the boat you are noting as being a pram.] it is of course a trade off, but if you can get a fine bow entry, then the rest of the bow above the water line can become somewhat "blunter" so as to add bouyancy, and to add waterline length when heeled...

    i guess i could have just said "i agree with what he said..."
    the pessimist complains about the wind, the optimist expects it to change, the realist trims the sails.

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  8. #8
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    Thanks for the insight!

    I did wonder if it may help with forward bouyancy, particularly in the smaller boats like footys, but then dismissed the idea on the basis that the extra volume is so minute that it wouldn't make much difference. My Mk2 MM has this too, but the difference in volume must be so small that I find it hard to picture it doing much to hold the bow up

    But a large and blunt bow or 'pram' (such a fitting description, though I actually like that design!) is going to increase the drag quite a bit, especially in waves and when diving downwind, and both situations seem to want a much cleaner hull form to reduce these effects

    So in a perfect world (or at least with a non-class boat) you could just bring the deck edges further forward and sweep the bow forward to maintain the fine entry and reduce the 'bluffness'?

    Hmm, just repeated everything that's already been said... Doh!

  9. #9
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    Quote Originally Posted by MeGrimm View Post


    But a large and blunt bow or 'pram' (such a fitting description, though I actually like that design!) is going to increase the drag quite a bit, especially in waves and when diving downwind, and both situations seem to want a much cleaner hull form to reduce these effects
    I don't see the wide bow as a problem.

    My IOM is a Triple Crown design. Not only is the upper bow wider than the lower, there is a distinct flare at the deck from the bow, back to station 3 to add even more volume forward.

    In flat water, the bow just 'kisses' the water. The wider upper part is dry. In heavy conditions, my boat resists diving the bow, and readily pops up onto a plane .

    By contrast, I have watched another 'famous' IOM design that has a narrow bow all the way from waterline to deck. In the same planing conditions, this boat will dig its bow in quite deeply and stop dead.

    Every decision is sailing is a compromise. I think the full bow, above the waterline is a good one.
    John Ball
    CRYA #895
    IOM CAN 307 V8

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