Bow Shape

Discussion in 'Sailboats' started by SuperPiper, Aug 23, 2013.

  1. gggGuest
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    gggGuest ...

    Thing is there are always rules and limitations... Even if they are not racing rules. When boats are berthed in marinas at so much per foot length that's a powerful incentive to cram as much volume into a given length as possible, and I'm sure that's a reason why boats tend to have much shorter ends now than they did say in the 30s, where the last few feet even of cruising boats tended to be unused (and unusable) space.
     
  2. SuperPiper
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    SuperPiper Men With Little Boats . .

    As a plumb bow becomes more submerged, the immersed volume will increase somewhat linearly. A flared bow will increase the immersed volume somewhat exponentially.

    Which of the 2 will result in more hobby-horsing?
     
  3. michael pierzga
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    michael pierzga Senior Member

    The more volume forward, the more energy needed to push thru the waves.
     
  4. capt vimes
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    capt vimes Senior Member

    you could actually have those two worlds combined in one, even if someone might think of it as exploiting a loophole... ;)
    it is just a question of rules, ratings and speed in the racing world and those race-designs then become 'fashion'...
     

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  5. michael pierzga
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    michael pierzga Senior Member

    Fashion... style driven design never works well on a multipurpose boat.
     
  6. HJS
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    HJS Member

  7. SuperPiper
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    SuperPiper Men With Little Boats . .

    Incredible videos JS.

    So here's what I've concluded. Flared bows are more seaworthy. Plumb bows are for rule beating.

    Agree?
     
  8. Mike Graham
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    Mike Graham Junior Member

    There are examples of plumb bows in vessels which had no need for rule beating (Earthrace and copycat high-speed tris and various pleasure yachts, among others). I really don't think we've discussed sufficient evidence to conclude that.

    (By the way, I think that saying 'flared bows' could possibly be a little confusing, as it could be taken to mean bows with bow flare, not just bows with conventional profiles.)
     
  9. michael pierzga
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    michael pierzga Senior Member

    You cant say just rule beating. The axe bow offshore supply boats arent beating rules, they are chopping thru the North Sea.

    Does a sailboat need a plum bow ? Good question. We know that with thier fine entry they will slice thru the chop better and we know that you are going to get wet while you chop thru and sail faster than your competion.
     
  10. rwatson
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    rwatson Senior Member

  11. MikeJohns
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    MikeJohns Senior Member

    There's no hard fast answer, every vessel can be quite different and you have to look at several factors.

    Generally a raked stem provides a lot more reserve buoyancy for a given waterline with less water over the decks and less likelihood of plunging. A plumb stem provides the best performance and interior volume for a given length on deck but some plumb stems are basically truncated raked stems vertical in profile but still a V section.

    V section raked stems also limit the climb of the bow wave. Some slab sided plumb bows have a bow wave that climbs almost to deck level and this increases resistance

    You'll see some daft statements claiming plumb bows are more seaworthy which is a nonsense without considering all aspects of the vessel they are fitted on.

    Also have a look at what I wrote here:

    http://www.boatdesign.net/forums/mu...uising-cats-pros-cons-46412-8.html#post623287
     
  12. gggGuest
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    gggGuest ...

    There's also a big relationship between the bow profile and the shape of the rest of the boat. If you avoid distortion and simply draw parallel waterlines then a slab sided boat wth a flat bottom will tend to a plumb bow, a boat with U sections a spoon bow, and one which has flaring out topsides a clipper bow.

    Then, if you decide you are not going to have parallel waterlines, then that too has effects. Reverse bows are trendy at the moment, and people talk about wave piercing, but if the result of piercing the waves is that an increasingly blunt entry angle is presented to the water, well, that may not be a good thing. By contrast with an exaggerated clipper bow, then as the bow goes deeper the entry actually gets finer, and that may have its advantages.

    And you can't separate any of this from the location of the waterplane and buoyancy distribution. There are no easy answers, just difficult compromises, so be *very* wary of the glib tongue that purports to offer ideal solutions.
     
  13. PAR
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    PAR Yacht Designer/Builder

    As has been eluded to, there's a lot more to consider in a bow shapes, than a first glance might suggest. The profile is just a part of the picture.

    As with just about everything else, the shapes employed, including the profile will address the SOR goals for the particular design. Hull material choices can affect the shapes, as can performance expectations.

    Ideally, you'll want as much LWL as you can get, on low power vessels, though this too is governed by other SOR restrictions. The only time you can have a free hand is in a racer, where little but the performance envelop is addressed, assuming structural issues are entertained sufficiently. Even at this, you'll have various hydro approaches open to you. The usual choice will present itself with some testing.
     
  14. tugboat

    tugboat Previous Member

    I agree with Par.

    "...you all know me- all know what I do" (ok that's a quote from "jaws"- remember the blackboard scene?)"

    so you'll probably guess what I have to say about plumb bows. :D


    Its funny how many hulls I have looked at -that the owners classify as a tug simply because the bow is plumb- and going along with that,
    is the amount of owners that seem to think they have a "tug" boat because the stem is raked too!...what bull *****! I've heard a lot of people talk about the hydrodynamics of a plumb bow but not much on what its functionality is- Ill be willing to bet that a plumb bow, as far as trends are concerned comes from the influence of tugboats and workboats on yachting. many a trawler design is based on tugs and workboats, where a wide, plumb bow is NECESSARY. obviously a plumb bow is suited to pushing duties.

    let me help to define for others some traits that MUST be included in order to be classified a true tug. some of it my opinion.

    a true tugboat:

    1. PLUMB BOW!

    2. counter stern a must! not a curved transom- not a flat one- not even a fantail necessarily, but a true counter stern that has some depth to it. if it aint got a plumb counterstern- it aint a tug!! plain and simple.


    3 high D/L ratio- full displacement hull...if not forget it its not a tug.

    4. wide foredeck area to have room to handle lines...

    5. rounded edges to allow the tug to act as a barrier between vessels and wharfage - in order not to snag a line or itself.

    6. a tow bitt located at least 1/3 rd aft of the stern. ideally about 2 ft back of the LCB



    I have even seen towbitts at the after most end of a quasi tugboat this is so wrong on so many levels! after all squatting is what women do at the roadside not what a vessel should ever do!

    of course only rule #1 here applies to the topic but its fun to define it- since there are too many twits who think they own a tug when it is not...see pic of one in particular .

    the nerve!!- the owner calling it a tug replica! it does have a plumb bow though..
    sorry I digressed...
    :p
     

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  15. MikeJohns
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    MikeJohns Senior Member

    tugboat
    You'll find plenty of harbour tugs and a lot of offshore tugs with raked stems.

    Remember a lot of working boats and fishing boats are built as rule beaters and that drove their hullform as much as any other requirement.

    Cartoon tug characters and the leisure tug parody types might fit your rule #1 but real tugs don't necessarily have plumb stems some have both pronounced rake and flared bows.
     
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