Why does multihulls have pointy bows?

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

  1. valery gaulin
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    valery gaulin Junior Member

    Yes that would probably be a couple of years at least. But maybe a model would come eventually.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    If blunt bows were always a resistance disaster, bulk carriers and oil tankers would not have such full bows, which are obviously a compromise where internal volume has been prioritized. Speed, of course, is not that great, a discussion point that seems to be lacking here. If you want speed, there will be a penalty.
     
  3. valery gaulin
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    valery gaulin Junior Member

    Thank you MrEfficiency,
    This is exactly the point i am trying to understand myself. Speed is not the main priority, basically what i have in mind is to get the most volume out of a 30ft x 14 ft catamaran.

    To get that extra volume I am ready to make a compromise on speed. Of curse not ready to compromise to the point were it would sail slower than a similar size 30ft sailboat.

    In full round blunt bow i see these advantages: increase Volume amd displacement , shallow draft, no hobby horsing, might be able to surf in the right condition.

    disadvantage : confort in short frequency waves, slower in light wind, unorthodoxe solution.
     
  4. redreuben
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    redreuben redreuben

    Valery;
    Explain no hobby horsing ? A full bow is going to pitch a lot more than a wave piecer surely.
    In the 6.5 mini video the sea is pretty flat, whats going to happen in a swell ?
     
  5. Emerson White
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    Emerson White Junior Member

    "unorthodox solution" isn't a penalty. Surfing can be a disaster though and can be a high penalty, greatly amplifying the damage done and risk to life in a storm. Surfing can be good if you want speed around the buoy, but I wouldn't call it an advantage when viewed in the light of your other statements.
     
  6. valery gaulin
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    valery gaulin Junior Member

    Well, no hobby horsing! Because from what I understand when the hull midsection has to much volume compare to the stern and bow it tend to create a pivot point in the mid section most boyant part of the hull with the stern and bow going up and down around that center point. Therefore the fuller round blunt bow as I suggested as a high primatic coefficient, high volume in the stern and bow.

    Also weight distribution can also exacerbate hobby horsing.

    Now most design try to prevent hobby horsing by creating a more volumious stern than the bow.

    On the earliest design of catamaran they also realize that adding some more volume in the bow it helps prevent pitch polling.

    Basically the fuller the end of the hull are compare to the midsection it help prevent those two condition, hobby horse and pitch polling. But don't get me wrong I don't believe it is a necessity to go all the way to a full round blunt nose to achieve this goal.

    The full round blunt nose that I am suggesting is mainly a solution to increase volume for a maximum size of a cruising catamaran 30ft X 14ft without too much penalty from this choice.

    Basically I believe that the pros surpasses the cons.

    What everybody said in the reply make sense and I understand their points but they seam to forget that everything is about compromises.

    As I shown by video, mini transat, this choice cant be as bad as they make it sound. Now I am not trying to say that this option will win a race on a cruising catamaran even if it did for the Mini transat.
     
  7. semelis
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    semelis Junior Member

    Right, it had to do with the assumptioin that he would be planning more often than it could in reality.
    Planning was the thing needed to reduce Parlier's cat drag and is the thing needed to get the blunt bow out of the water as so nicely works in the mini, and it's the thing that is not going to happen often enough, as Parlier tested in a real life 60 feet boat.
     
  8. bjn
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    bjn Junior Member

    The results showed the opposite. High drag at low speed, low drag at high speed. So if he wants speed, it might be a good idea. But he can only reach that speed in rare conditions.
     
  9. bjn
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    bjn Junior Member

    I asked him in a PM about the hulls going forwards and backwards. He says the Michlet results were correct! Backwards and forwards gets the same resistance, but different wave pattern!

    Here is the PM, and don't think he minds that I put it here in the thread:

     
  10. DennisRB
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    DennisRB Senior Member

    If the wave pattern is different, it will effect the total difference in a catamaran form due to different interference. Have you tried it yet as a cat?
     
  11. bjn
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    bjn Junior Member

    If you read his pm, he says it's also true for multihulls, which I interpret as meaning that the wave interference drags will also end up being equal.

    I haven't tried it yet as a cat. I will see if I can make it work today.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm not sure what results you are alluding to, but I can't recall seeing any fast boats/ships with that bow shape, except possibly modern subs, and they are optimized for submerged running.
     
  13. bjn
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    bjn Junior Member

    I did a comparison in Michlet with this result. The blunt hull was as good as the best hull (symmetric proa) above 10 knots. But much worse at low speed.

    [​IMG]

    And if the blunt flat bottomed hull would plane at high speed, it would get less resistance.

    But this is all simulations and theories, so maybe I should not put so much weight on these results. But it seems plausible that the resistances at high speed doesn't differ a lot, since it aligns with the chart Dennis posted:
    [​IMG]
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    With planing hulls, the difference between a very full entry and a more slender one can become very noticeable, as a distinct braking as the bow dips in following seas. This "bog" is mainly a result of shape, not an increase in wetted area, a good hull will slice through with little wash-off of forward speed.
     

  15. Kestrel
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    Kestrel Junior Member

    A sharp bow (pointy and about vertical shaped) entrance angle is one of the factors which reduce resistance in displacement end pre-planing conditions, and wetlenght could be greater a bit too, so as hull displacement critical speed. In planing, sharp bows could be usefull if a wave piercing effect is desired.
    K
     
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