Bulbous Bow applications

Discussion in 'Boat Design' started by juiceclark, Oct 25, 2007.

  1. kach22i
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    kach22i Architect

    I remember that one, a large freighter ( w/bulbous bow).................the title of the thread was "Will It Sink?"

    Right?
     
  2. brian eiland
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    brian eiland Senior Member

  3. kach22i
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    kach22i Architect

    The orginator of this thread (not me) wants to plane on the surface, not foil or hover (those ideas were just my ramblings).

    Question; besides helping save fuel, will a bulb bow help stabilize the boat or help performance in heavy seas?

    If so, this alone might be a good reason to have a bulb bow.
     
  4. No, Bulb wont help stabalise, just reduce wavemaking and bit of viscous resistance hence less power needed.
    When doing stability and response calculations you just factor in the bulb, adding one after production might destabalise/oscillate it's frequency response in certain wavelength seas.
     
  5. kach22i
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    kach22i Architect

    If you assume a rough sea condition (affects wavelength, right?) and therefore need to use less power or have a "reserve" of power in which to fight other forces such as headwinds, then in a round about way since you "designed for it" the bulb bow will help, or at least perform as intended...............right?

    By the phrase "stability in heavy seas", I meant or intended to mean that the ship/boat could continue to operate safely as long as conditions did not overcome design parameters. Stability in this context just means (to me) that you will be coming back, as a unstable ship voyage could mean capsizing or worse. Having the bow rise up and flip the ship over might not happen if the bulb bow maintains an attitude where it splits the waves at the bow in lieu of riding on top of the wave.

    Any of this make sense or did I just use the wrong term/word?
     
  6. Mmmm, the bulb is intended to create a wave in advance of the ship whose trough coincides with the bow waves crest thereby destructivly interfering and reducing the overall waves magnatide reducing resistance, the bulb is in no way a wave piercing mechanism and also it will only operate efficiently at the designed draught, either attained by a fully loaded ship or filling ballast tanks etc. All ships, in order to get classified by Lloyds etc. have to have a minimum GMt/metcentric height which will ensure a certain stability, which will almost certainly ensure it's survival in almost any conditions except for maybe HUGE rough waves, but they will heel it over not pitch it over, ie. it will roll over completely rather that pitch end for end.
    Margins are also added to power estimations usually in the range of 15-30% depending on windage, expected route and allowance for time delays in voyage, so in severe weather conditions the ship will almost always be able to attain service speed.
    Also the bulb is not solid lead like yachts etc, in some cases it may form part of the tank arrangement whereby it can be flooded to cause parallel sinkage to attain aforementioned designed draught, to ensure the bulb is at it's designed attitude.
    Bulbous bows are VERY specific things, in order to get any advantages out of them you MUST travell at deigned speed at deigned draught etc..
    Hope this helps...
     
  7. nordvindcrew
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    nordvindcrew Senior Member

    wave breaker

    this is something i've been wondering about. would a bulb bow help on a row boat designed for open water rowing races? this whole thing about waterline length is open to question for me. If I add a slender extension to bow and stern of, say 3' does that effectively add 6' to my hull length and change its basic hull speed? (IE: like a bow-sprit under water and an elongated rudder) On Aluetian Bardikas, they use some thing reffered to as a wave breaker. it is simply a slender piece of wood the extends from the base of the stem forward and then curves up out of the water and is held to the stem head by a light piece of cord. I have read that this is what gives Bardikas such great speeds. Always looking for a new "trick" to wring a little more speed out of my row boat. Any thoughts or ideas?
     
  8. kach22i
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    kach22i Architect

    Thank you for the explaination, it seems I tried to make a connection which was not accurate. Perhaps part of the reason I tried to connect a bulb bow with rough sea states is a video which was posted in this forum last year. The video compared a ship model in a wave making tow tank to the actual ship in rough seas. The video was later taken off Youtube, it had a South American or Spanish name to it.
    ....................................................

    Row Boat;
    Good questions, I'd like to read a good answer about it.:)
     
  9. kach22i
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    kach22i Architect

  10. kach22i
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    kach22i Architect

  11. juiceclark

    juiceclark Previous Member

    These are all awesome posts. I couldn't draw a picture to save my life, but I envisioned something a little different. Why couldn't the extended bulbous bow have the same shape as the front of the hull? Thereby, at displacement speed a plate resting flat against the bow at the waterline could telescope forward a short distance to break the surface. At speeds not condusive to a BB, the deal could retract back to sit flush at the point of the bow. Since the plate would have a v-shape, it would add slight lift as well as the functions of a BB.

    By the way, if you understand that rather sad description you should get some kind of medal!

    Tony
     
  12. kach22i
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    kach22i Architect

  13. haha thats a good thought!

    ''plate resting flat against the bow at the waterline could telescope forward''

    The added resistance of that would be HUGE, there would be a massive pressure drop just after the plate where it would be conned to a hydraulic ram and the wave shape would not be nice, ie no real trough so to speak of. The point of the bulb isn't to break the water so the bow doesnt have to, it's just to create a smallish constant depression on the free surface at the bow so therefore the bow wave is reduced and hence viscous and pressure resistance.
     

  14. juiceclark

    juiceclark Previous Member

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