Bow shape and keel arrangement

Discussion in 'Sailboats' started by Lee01, Oct 18, 2014.

  1. Lee01
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    Lee01 Junior Member

    Hello, the argument of this post is not about another reverse bow and a kanting keel, I'm at work on a design with twin keel, my doubt is about the bow shape.
    Most yachts have full rounded or straight waterline figure forward, in this case however there is a bulbous bow for a narrow entry, the hull is not for surfing speed but a cruiser. Why designers don't use this shape today? Is it slower?
    Thank you in advance for any comment!! :cool:
     

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  2. PAR
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    PAR Yacht Designer/Builder

    Okay, I'll bite, how is it you're "designing" a yacht, yet asking questions a first year student would ask? I'm not trying to pick on you, but the answer is very obvious to someone capable of designing a significant yacht. Lastly, I hope you aren't particularly serious about those bulbs at the end of those fins, as they'll have so much drag, you might as well just tow the rig astern, rather then use it and mother nature to propel you.
     
  3. Lee01
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    Lee01 Junior Member

    Thanks you for your comment Par but if you look for twin keels yachts you see it is not so uncommon where tides have big shifts, and a famous french shipyards already produce this kind of hulls without much advertising, the proportions of the appendage are not my point here.
    About the bulbous waterline you can see the attachment or google for some dinghy 12 original plans and even if they are not recent they are both taken from well-known designs. Frigates use bulbous bow with success and while they don't travel inclined in this case you just have to consider changes in the immersed volume parameters. Axe bows have a concavity as well.
     

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  4. Lee01
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    Lee01 Junior Member

    and to reduce drag one rudder has less drag than two!!
     
  5. gonzo
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    gonzo Senior Member

    PAR is talking about the cylinders under the keels. You will have enormous drag. Also, the number of rudders alone is not an indication of total drag. I am not sure what you mean by "bulbous bow for a narrow entry". They are a contradiction, it is either bulbous or it has a narrow entry.
     
  6. PAR
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    PAR Yacht Designer/Builder

    It appears a translation issue Gonzo, but Lee, if you can't see why a larger entry 1/2 angle, is less beneficial then a narrower one, then you've slept through large portions of basic hydrodynamics classes. Also as Gonzo noted, those bulbs at the ends of stubby fins aren't beneficial in any regard. We understand the benefits of bilge keels, but considering their draw backs, further increasing their already large drag element, just doesn't make any sense. Simply put, those ballast bulbs are just about the worst set of shapes you could employ on a fin, suggesting you just need to study a lot more, before considering "designing" anything other than a hypothetical project.
     
  7. TANSL
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    TANSL Senior Member

    No, Gonzo, you're wrong. Sometimes we need bulbous bow with narrow entry.
     

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  8. Lee01
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    Lee01 Junior Member

    I should have posted this in the motorsailer section to find less radical opinion about the twin keel ;) I have considered those cilinders because they have a neutral drag and are symmetric when heeled, and can hold a good amount of ballast inside, but I agree with you now that drag would be too much. Just to say they could support a foil between them?
    I have also the idea that a single rudder with twin keel would be very stable and good manouver, but there is no redundancy in the rudder and it would be not safe.
    The only advantage remains it would be less draft especially useful in bigger designs.
    About the bow (on the right side of the waterline drawing) I have some difficulty with all the transactions but, if you confirm that those depressions marked with a red arrow are faulty i will make them straight! I suppose what i see in those dinghy linesplan is derived from bildge keel boats. And yes i was late that morning at university and the dog ate my book :rolleyes:
     
  9. gonzo
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    gonzo Senior Member

    TANSL: seems like you are on rampage again. Read my post first. You tend to get your panties all in a knot whenever I post something. The bulbs are NOT in the bow, but are at the bottom of bilge keels.
     
  10. gonzo
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    gonzo Senior Member

    Lee01: There is nothing wrong with ballast bulbs. It the cylinders with full ends that give them a huge drag. Twin keels have the advantage of shallow draft, at the cost of higher resistance. I have sailed several bilge keeled boats. When the keels are angled out, the boats have less initial stability than if they are both vertical. However, they generate a bit less resistance and may have more stability when sitting on the bottom at low tide.
     
  11. Lee01
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    Lee01 Junior Member

    I have learned something and I didn't consider this advantage at low tide :D
     
  12. TANSL
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    TANSL Senior Member

    What you do not understand what you yourself have written?. Lee01 is refering to "bulbous bow", isnĀ“t it?
    Where is the contradiction in what Lee01 says?
    I beg you, although I do not think it's possible, to be more educated in your forms.
    I have seen exactly where the bulbs of the figures are, however I appreciate your timely explanation. Not talking about that, what I mean is that what you say, once again, is incorrect. You can not know everything, and when someone does not know something, the best thing he can do is shut up.
     
  13. TANSL
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    TANSL Senior Member

    That's interesting as long as you tell us what is the initial stability for you and how you measure it. Without knowing that, no one is able to know when and how it is major or minor.
    No need to have sailed at all to know that the same hull, same displacement, with a lower center of gravity has a greater GM. But I do not know if that means anything to you.
     
  14. Lee01
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    Lee01 Junior Member

    "clipper bow waterline" i think is the word that describe better the shape, and google has many nice images on the subject, they're all vintage I am really missing a lesson or two, you guys wasn't so pessimist about the clipper bow rather than the twin keels though
     

  15. PAR
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    PAR Yacht Designer/Builder

    The clipper bow refers to the bow's profile, not it's WL's. Maybe you're trying to describe the hollowness seen in some displacement bow WL's, but again, if you had a better understanding of the disciplines involved here, you'd realize how poorly thought out your appendages are and how much more you need to learn about preferable WL arrangements. As far as hollow WL's, you have to work around the goals established with the SOR, which sometimes forces you to make these decisions about the entry WL shapes. FWIW, making hollow entry WL's can be a bit of black magic, to get it right, so stick to the SOR goals and let the entry fall where it will produce the best performance attributes it can, give the restraints of the SOR.
     
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