Why does multihulls have pointy bows?

Discussion in 'Multihulls' started by bjn, Jan 22, 2017.

  1. Corley
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    Corley epoxy coated

    So OP when are you going to start building this boat? With its bluff bows and central mounted centreboard? Is a real design process underway or just some renders of a boat like object and a load of conjecture.
     
  2. bjn
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    bjn Junior Member

    I assume you are referring to a Canadian gentleman, and not me. I was initially interested in the form/induced drag caused by pointy bows when making leeway. And I think that question is still to be answered from a scientific point of view.
     
  3. Corley
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    Corley epoxy coated

    Fair point I was intending to reply to Valery's thread. At least your question is more theoretical in nature and therefore ok. My objection is to people who come onto the forum and insist all current designs are flawed or conservative and their own personal vision is better. I want to see faith in their concept translate into an actual, physical boat.
     
  4. upchurchmr
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    upchurchmr Senior Member

    I think you have demonstrated your inability to recognize simple geometry if you still think there is an issue with sharp bows and drag during leeway. Post # 2.
    You certainly are one of "those people who come to the forum and insist all current designs are flawed or conservative and their own personal vision is better."


     
  5. bjn
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    bjn Junior Member

    I don't agree that it is an insult though. At the most you could say we are ignorant. But I'm sure none of us are writing here to insult designers. Certainly not me.

    I still think there are ways to improve performance. And I started this thread with the idea that this could be one area where we could reduce drag.

    The hull is a hydrodynamic shape. I refuse to relinquish the idea that it shares some physics with hydrofoils. I have been ignoring the wave making, but have understood I cannot do that, and have learned something about waves in the process.

    But if we go back to the topic, I think you are wrong. I wrote why in a reply to your post.
    A hydrofoil with a sharp leading edge does not perform well at all. Flow will separate, even if the angle of the incoming flow is less than the angle at the leading edge, which results in very poor performance. So that theory simply can not be right. Has to do with local flow directions and inertia of the fluid.
     
  6. UpOnStands
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    UpOnStands Senior Member

    check out the bows of catamarans, not the bows above the static waterline but those under the waterline. The sharp edge above is usually gone below the waterline. If bows were sheer and carried straight down for over meter (to ease tip losses) the point about flow stall/separation would be moot as the boat would refuse to slip -- impossible to tack as well. Your argument is germane with regard to LAR profiles, but no one builds LAR with sharp leading edges for exactly the reasons you note.
     
  7. bjn
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    bjn Junior Member

    How about the modern wave piercing hulls for example?
    When they sail with some heel and pitch down they have a sharp edge deep into the water.

    I have never seen this addressed in text before. So either the drag is neglible, or the designers haven't though about it.
     
  8. UpOnStands
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    UpOnStands Senior Member

    the key selling point of wave piercing bows is that they shed water quickly after diving into a wave. if your were to fatten up the sections they would not rise quickly. The drag losses are not an issue given the very small profile lengths involved, free surface effects, and lack of end plate.
     
  9. valery gaulin
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    valery gaulin Senior Member

    @Corley: Look dude my first intention was only to ask if someone as tried Round blunt bow hull shape for a catamaran. I am intrested in learning from others good and bad design, before trying it myself. Where else do you want me to ask this question than on a boatdesign forum???

    Second when the time will come I will try a model to see if it coild be a reasonable compromise with all the pluses and minuses of such a design. But if someone has the experience with this shape I would like to know.

    Third, It did built a trimaran. The design needed to be capable of sailing in a bay where there is only 2 feet of water, single hand, capable of trailering and putting in the water by myself in less than 30 min. Capable of going 8 knot with a minnkota 12vdc 55 lbs trust. Relying only on hull shape for leeway because of the shallow water and the single handling at the same time being responsible for a 3-4 year old son while sailing. All this was achived with this design. Also it needed to be built with aluminum, with simple tool, for strenght if it hits bottom, and for weight, all this was done without being welded with very simple tool in my Garage.

    It is not a perfect design but for where I am using it and for my need it is very clise ti perfect for me.
     
  10. valery gaulin
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    valery gaulin Senior Member

    Here are some pictures i found on my cell.
    @Corley: Before being rude to someone for asking questions on this design forum website ask yourself if it is a constructive comments or not. If your answer is no than keep your comments to yourself.
     

    Attached Files:

  11. bjn
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    bjn Junior Member

    Very nice boat Valery.

    Impressive!
     
  12. Corley
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    Corley epoxy coated

    I was just interested to know whether you are serious about building something. Theoretical discussions are all very well and good but boat design questions are generally intended to work towards a practical conclusion.

    I'm not completely against the idea of a round bow but think applying it to a concept like the Baltic proa (tacking outrigger really) which was linked here at one stage might be a better approach. You would be able to keep your LoA under control and still end up with a decent boat for flat water sailing. I don't think I'd go with the junk rig but a more conventional rig could work well, by all accounts it sails quite well. Ok it's not strictly a bluff bow but the main hull is quite voluminous.

    http://tacking-outrigger.com/baltic_proa.html
     
  13. Rastapop
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    Rastapop Naval Architect

    bjn, the primary function of an airfoil is to produce lift (usually while also trying not to increase drag too much).

    The primary function of a bow is not to produce lift, it's to minimise drag (among some other things not relevant to the discussion).

    The differing functions require different shapes.
     
  14. bjn
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    bjn Junior Member

    I'm concerned that a sharp edge on the bow will create turbulence and drag. Since the flow will never be perfectly parallell to the hull.
     

  15. UpOnStands
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    UpOnStands Senior Member

    sigh, we have argued this too long. Go build your hulls with round bows to minimize turbulence and drag. Take it sailing and report back to us.
     
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