Why does multihulls have pointy bows?

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

  1. bjn
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    bjn Junior Member

    Wouldn't it be better with a more rounded bow than the typical pointy bow?
    Like a NACA 0010, on a hull with 10:1 slenderness ratio, for example?

    Im suspecting a pointy bow is creating drag from detached flow when the hull is making leeway?
    That would be the case with a foil with a too small leading edge radius.
     
  2. upchurchmr
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    upchurchmr Senior Member

    Do you have any idea what the angle of the bow is on various multihull bows?
    My old Tornado catamaran had a total angle of 18 degrees, so the angle of one side to the centerline is 9 degrees.

    A severe leeway angle might be 5 degrees.
    So there is still a 4 degree positive angle to the waterflow.
    No separation.

    If this idea had any possible chance of being right, why do you think the modern racers have "pointy" bows?

    Have you ever measured the bow shape on any multihull?
     
  3. Emerson White
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    Emerson White Junior Member

    He is asking a question not trying to press a position in an ongoing thread.

    One reason is because the blunt bow provides a lot more forward resistance in water than it does in air, the sharp transition can move the area of delamination forward and create more parasitic drag on the skin. A second reason is that it's easier to reinforce a narrow bow than a wide blunt one. A third is that a wide bow gives you a higher Cp with a shallower draft and more wetted surface area, which aren't necessarily good things because the boat can lose its footing and slide sideways down waves, and has trouble tracking.

    The front portion of NACA 0010 is probably okay for a multi, especially if it is not intended to race, but most fish that swim a lot and rapidly don't have fat round heads like that for a reason. It wouldn't be likely to improve boat performance under most circumstances.
     
  4. upchurchmr
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    upchurchmr Senior Member

    And I answered his question.
    There is no issue of separation with a pointy hulled multihull.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Speed would be the main factor to consider, and since catamarans have potentially less speed restriction imposed by boat length, more likely to be going at a speed where a bluff full bow could bog and become very wet also.
     
  6. DennisRB
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    DennisRB Senior Member

    Its amazing how much a naca 0010 actually looks like a normal waterplane for a cat. But not on the direction bjn suggests. Blunt shapes like this do work well, but only when they are underwater. Hence the exact use of this section for rudders etc. The issue is if you use it at the waters surface it will generate unnecessary large bow wave. There would be next to no stern wave though.

    [​IMG]
     
  7. patzefran
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    patzefran patzefran

    Don't you think also bows are pointed, not only on multifulls, because of wave drag. A wing is in a single medium, a hull is at the interface of a light medium, air and a heavy one , water, suject to gravity, which gives wave drag. Blunted hulls (at waterline !) increase wave drag.
     
  8. Manfred.pech
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    Manfred.pech Senior Member

    Yes, I think so. A large stagnation area produces a big splash causing hydrodynamic drag.
     
  9. upchurchmr
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    upchurchmr Senior Member

    Dennis,

    Perhaps you should post a picture of an actual waterplane of a typical multihull as comparison, so we could see your point.
    I don't think they match at all.
    The Tornado I referred to would look more like that picture with the bow on the right and the left side more blunt.
    Of course the water plane tells you very little about the 3D shape of the hull in the water.

    Your comment about rudders is quite appropriate - the rudder is used to provide lift, and the hull does not (mostly) so your picking the shape for the wrong reason.
     
  10. bjn
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    bjn Junior Member

    I had never thought about it that way, but I think it's not only about angles. The foil has to have a significant leading edge radius. Otherwise the "backflow" from the high pressure side to low pressure side, which is flowing in the opposite direction of the fluid and back around the leading edge of the foil, will cause the flow on the low pressure side of the leading edge to separate, if the radius is too small.

    See attached pictures. Last picture shows the "backflow", at an exaggerated 20° angle of attack to see the effect more clearly. The NACA 0010 has a 1.1% leading edge radius. I compared it with 0.11% leading edge. As you see the in the polar diagram, drag increases a lot at 4° angle of attack on the pointy foil, which I assume is because flow separates. This doesn't happen until 10° on the normal foil. When I tried with a smaller radius, it happened at 2°.

    But I understand from the replies that the shape of a hull is much more complicated than a foil in 2D shape. The flow is 3D, because is moves also under the hull. And the volume distribution is an important factor. So there are more things to consider than the shape of the hull when seen from above.

    The tornado has a leading edge radius though. 30mm or more, from what I recall. It might be enough for the leeway angles it has when sailing, as you suggest.

    The reason I started to think about these things is that I made a hull (proa) with very sharp bows. And now I'm afraid it will be draggy when making leeway.
    Picture:
    [​IMG]

    Thanks for a good reply! good points!
    I'm not sure I understand or agree with the bold part though. I understand the NACA foils are used as rudders and centerboards, even though they have this blunt leading edge. Airplane fuselages aren't pointy, except the hypersonic jets.

    Could it be that when swimming fast, fishes are using short strokes / low angle of attack?

    That sounds very plausible, thanks. So the bows on my hull is actually very good?
     

    Attached Files:

  11. upchurchmr
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    upchurchmr Senior Member

    The Tornado bow sitting in my yard has no more than a 7mm radius.

    If the boat shown is going to have a good centerboard or daggerboard, where most of the leeway prevention is going to be accomplished, then the hull will be fine from a drag on the bow standpoint.
    If you don't have a board then there is probably going to be much more leeway- I still guess there will be no excess bow drag, mostly because you have the pressure of the incoming water to suppress any separation, not like on the stern.
     
  12. Emerson White
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    Emerson White Junior Member

    When the hull is acting as a foil and generating lift it's extremely low aspect ratio, so it's water flowing under and around it that's going to do most of the equalizing, not backflow forward past the bow, unless it's an extremely narrow beam deep draft hull, in which case excess wetted surface area is going to be a bigger concern (also lack of righting arm in a monohull). Leave the lift to appendages under the water, they are much better at it than hull form.
     
  13. DennisRB
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    DennisRB Senior Member

    I did try to find an actual waterplane and struggled. My point was not its identical, but a striking similarity. My last boat had a long fine entry more like the pic showed, but most would be fuller. This is one I found. Its not 10 to 1 but shows my point.

    http://www.marineinsight.com/wp-content/uploads/2016/12/stability5.png
     
  14. upchurchmr
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    upchurchmr Senior Member

    Dennis,

    What boat was that taken from?
    Which end is the bow?
    I don't recall any boat that would have a tapered to a point stern waterline, except a Hobie 14, 16. The shape of a Hobie 18 is semicircular at the rear. Even if you had the transom out of the water it would be much fatter than the bow - since it is a pointy bowed boat.

    The boats I'm familiar with would be strikingly not similar.
     

  15. DennisRB
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    DennisRB Senior Member

    That is just a random waterplane I found. My argument is the pointy bit is the front not the back.

    Hulls do not come to a point at the back. If the section is rounded and the transom is not immersed, that is the waterplane shape that will result, roughly. If the rear sections are flat then the rear shape will be fatter.
     
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