Negative Bows ?

Discussion in 'Boat Design' started by Canada Bob, Apr 25, 2009.

  1. Canada Bob
    Joined: Apr 2009
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    Canada Bob Junior Member

    I've seen a couple of small {under 20ft} home built catamarans with negative raked bows, I'm trying to figure out what advantages a negative raked bow might have.

    The hull shapes were sort of like this...


    stem /...................................../ stern.
     
  2. apex1

    apex1 Guest

    Wellcome aboard to say that first!

    Looks interesting is the answer............................

    And we mortals are used to draw a boat the opposite way...stern../......../ stem in this case.

    Regards
    Richard
     
  3. rwatson
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    rwatson Senior Member

    The big advantage, as ancient greek and roman galleys found out, is that you can put a very large hole in the opponents hull without them being able to jump onto your deck.

    This is particularly useful in contentious starboard v port encounters, or when opposing boats ignore your cry of "buoy room"

    These designs have grown in popularity recently with the increasing cheaply printed boat designs, where the builder mistakenly placed the bow moulding upside down during construction.
     
  4. Canada Bob
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    Canada Bob Junior Member

    I was wondering if a negative raked bow {on a lightly built cat} might have something to so with wave peircing ? less chance of pitch polling.

    Other than that, what determines the angle of rake on the bow of a small boat {other than style}.
     
  5. lewisboats
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    lewisboats Obsessed Member

    I would think it would be to lessen weight. There is no heel to a cat ( for the most part) and therefore would not benefit from the lengthening of the waterline that a regular forward raked bow would provide at normal heel. Take a chunk of weight off the bow and it rises faster and doesn't bury as much.

    Steve
     
  6. Canada Bob
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    Canada Bob Junior Member

    Thanks for the above Steve, appreciated...

    The boats that I've seen are less than 20ft long, so I expect the weight will be very light anyway. One thing I've noticed is how they seem to cut through the water/waves rather than lift as much as a regular positive rake bow does.

    I don't know if this negative rake helps when a wave breaks over the bow, but it made me wonder why we angle a bow the way we do, at least on small / light boats that is.
     
  7. M-Sasha

    M-Sasha Guest

    As rwatson and apex mentioned, it´s a gimmick, at least on a boat that size. Show something different and sell more, thats it.
     
  8. Canada Bob
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    Canada Bob Junior Member

    Hi Sasha,

    Thanks for the above, the boats I've seen are project boats, so I guess the guy just likes the look of a negative rake.
     
  9. Guest625101138

    Guest625101138 Previous Member

    The rake in the bow is often a result of flare in the topside and a developable shape so the plate flows without twist to the bow. The rake is the intersection of the outward angled planes of the hull sides.

    The negative rake can have merit by giving a certain waterline length without much buoyancy so pitching is reduced. In my opinion this shape is more prone to pitch poling as the bow will easily bury compared with a flared bow.

    If you have a picture of one of the boats in question then it might offer more ideas.

    Rick W
     
  10. CT 249
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    CT 249 Senior Member

    If it's a gimmick then it's an awfully successful one, as demonstrated by the fact that raked bow boats have dominated the world titles in the most popular development catamaran class for a few years now.

    In A Class cats and F18s the waterline length issue doesn't exist, since they've had waterlines at the maximum 18' LOA for eons. However, the reverse-raked stems do reduce pitching while still providing sufficient buoyancy. The result is a significant increase in upwind speed.

    Small cats learned many years ago that you don't want flare in the bows to prevent severe nosediving. Submerging the flare slows the boat down, therefore (since you nosedive downwind) the apparent wind increases as the boat slows; that puts you straight in.

    It's better to reduce volume high up in the bows, because that way drag doesn't increase much when the bows go down. You can drive the bows deep down, but they'll pop back up again - especially if they have a whole lot of volume low down in the form of U-shaped lower hull sections rather than old-style V shaped sections.

    The ram-bow effect does work; we copped a former world A Class champ's bow into our port hull some time ago. If he had a vertical bow he would have spread the impact more and put some of it onto the stronger deck/hull joint; instead it pierced our hull low down.

    Some other boats have tried reverse-raked bows without success. Maybe in a slower boat, the increase in apparent wind as the boat slows under the drag of the flared sections is less of an issue, and the added bouyancy is of more benefit. It seems logical.
     
  11. Guest625101138

    Guest625101138 Previous Member

    I know there is a limit to this because I operate very slender hulls with low buoyancy in the bow. These are not sailing boats. If I press hard down the front of a wave I can bury the bow to scary depths. I have had 2m submerged on a 7.2m long hull.

    The boat I am referring to has virtually no rocker so dynamic lift is negligible. It also has no flare and there is not a lot of reserve buoyancy. Basically it goes straight through small waves to windward without any pitching.

    At speed I believe dynamic lift in the bow is more important than flare. As you point out the flare will slow things down.

    The other control on pitch poling is to prevent the stern from lifting. 'T' rudders have proven effective for this.

    Rick W
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Canada Bob

    Welcome to the wierd world of forums...

    Raked bows, as mentioned above are a combination of several factors.
    1) Gimmik..or more correctly marketing!
    2) increases the waterline length of the boat, for 'free', so boat goes a bit faster
    3) has very minor benefit in pitching..all depends on the amount of bow that is actually 'fwd' of the F/P. In practical terms (ie once the boat is actually made for its intended purpose, rather than an academic exercise), makes almost no difference.
    4)Seakeeping (as part of #3). But again, depends upon the sea state, ie quartering seas, following seas, each has a different solution. A fine "slice of cake" like bows are generally better, especially if one is concerned about bow/deck diving.
    5)Each design has its own solution, and as such is generally not applicable carte blanche, as so often in design.

    If you want more of a 'read' i can point you towards some good papers if you wish.
     
  13. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    these threads are good because it gives us a chance to say "this isn't anything new."

    [​IMG]
    WWI vintage battleship


    If it would have been successful, don't you think they'd have continued to make them that way?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Dreamer

    "..If it would have been successful, don't you think they'd have continued to make them that way?.."

    You mean like this?

    http://www.ulsteingroup.com/kunder/ulstein/mm.nsf/inpdocuments/CC610F46B5B32113C12571A7003E226A/$file/ULSTEIN_AX104.pdf

    Every design has its place and time...usualy paid for by the client :)
     

    Attached Files:


  15. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    That's pretty cool.

    There really is a trend it seems to go 'through' the waves rather than over them. I suppose a bow like this sheds water more cleenly than what we consider traditional. Notice the wave deflectors way up high. From the looks of it, this thing is expected to be part submarine. ;)
     
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