Bulb keel shape

Discussion in 'Sailboats' started by rcnesneg, Oct 25, 2014.

  1. rcnesneg
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    rcnesneg Senior Member

    I am building an 8' pocket yacht. It basically looks like this:

    [​IMG]

    I want to put a lead bulb keel on it, target of 50-60 lbs. The portion of the daggerboard below the waterline is about 3 feet.

    I realize the daggerboard/keel won't be able to be removed from above, it will have to be slid out from underneath, but I will be able to raise the daggerboard up until the keel touches the bottom of the hull. The original design had 30 lbs of lead inside the daggerboard, but I want to increase it to make the boat stiffer and able to carry a taller rig.

    What shape should I make the lead keel to go on the end of the daggerboard? I want to maximize upwind performance.

    Can someone direct me to some resources or summarize pros and cons of different designs, like these? :confused:

    Simple bulb keel
    [​IMG]

    Flattened bulb. That is just to stop flow around the tip, right? Why is it so far back?
    [​IMG]

    What is the theory behind this strange shape? Does it work well?
    [​IMG]

    What does this little winglet on the bottom do?
    [​IMG]

    Are the wings worth it? Do they pull up or down?
    [​IMG]

    How do those perform different than these?
    [​IMG]

    Why are the shapes so different? What is up with the tiny wings on the 2003 AC yachts?
    [​IMG]
     
  2. gonzo
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    gonzo Senior Member

    Have you calculated the extra stress on the boat? The centerboard needs to be able to handle the extra force produced by the ballast being concentrated at the end. Also, that will generate extra force on the mast, chainplates, rigging, etc. Otherwise, the wing type bulbs will give more lift to windward and also dampen the pitching of the boat a bit.
     
  3. TANSL
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    TANSL Senior Member

    It is very possible that I'm wrong but I think there is no physical connection between the bulb and the mast, rigging, etc., of a vessel. Therefore, in my opinion, the bulb produces no extra effort directly on the mast, rigging, etc. However, as much as the curve of GZ of the ship changes, the righting moment at 30 degrees, which is what is usually taken to calculate rigging, may increase, so will have to check whether it is need to reinforce something.
    It must be checked, and possibly strengthen, the anchor of the daggerboard in the hull and to check the pure bending of the keel due to the apparent weight of the bulb.
     
  4. rwatson
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    rwatson Senior Member

    The big question is Why ?

    Most of those keel shapes were off the cuff experiments that didn't do anything for performance.

    If you don't know what the optimum shape is, then casting around for public opinion one a subject that millions of dollars of research has been spent on, is an exercise in futility.

    If you just go back to basics, getting weight down low, as Tansl has said, a whole new study in hull engineering.

    But even more basic, the original weight study for the hull had the maximum desirable ballast, so you must understand the effect of the slight lowering of the weight with the extra drag and upwind performance degradation that putting a 'blob' on the end of the fin will produse..
     
  5. rcnesneg
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    rcnesneg Senior Member

    I'm not modifying an existing boat, I'm building a new boat, so I'm not too concerned about the stress on the hull, more about stress on the daggerboard itself. I designed and built it from scratch, copying the idea, but none of the structure of the real minuet(pictured). I'm using an unstayed larger diameter mast, and off-center daggerboard, instead of the centered one and stayed mast on original. I also sit 2 inches higher.

    I'm just asking which ones do best where. I read the page for one of them, the flat-tailed lead bulb. Originally it was a solid fin keel, but he wanted less draft, so had it cut off and extra weight added in the bulb. It improved his pointing performance dramatically. Are you saying it doesn't really matter what shape the bulb on the end is, and wings don't do anything? Australia II seemed to prove wings can help in that case, although its keel has a much lower-aspect ratio.

    Simply put, I believe that amount of ballast is inadequate. My boat is slightly larger, and sails with a much taller rig. The production boat is not very stiff, and cannot carry much sail. I did density calculations, I can't fit enough weight in the end of that thing without it going halfway up the daggerboard. Are you suggesting I just make the whole bottom half of the daggerboard lead? I think I might have trouble with structure where the lead part met the wood/fiberglass part. Maybe I could run rebar down the center of the wood core into the lead.

    Another problem, lead is expensive and somewhat hard to find.
    I am considering making the ballast out of Concrete with chunks of scrap steel in it. If I go that approach, I would definitely need a bulb just to get the volume needed for that weight.
     
  6. gonzo
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    gonzo Senior Member

    The wings increase lift. When the boat heels they get more vertical. The projected area is larger. If the increased stress on the hull and rig is not a concern, you only need to calculate the stress of the bulb on the board. You can fiberglass over the bulb and make the skin transfer the load. If you are using metal bars or straps, it is better to offset them as close to the side as possible.
     
  7. rcnesneg
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    rcnesneg Senior Member

    I am planning on glassing the outside, to keep all that nasty lead from leaking out and make it stronger.

    I did a little bit more research, and it looks like the fin keels point a little bit better than shorter winged keels with wide wings. The fat-winged keels are to maintain ballast when reducing draft.

    So the next question, if I won't be doing wings, is which shape to make the bulb? blended into the daggerboard, or a separate bulb? Still focusing on upwind performance... It seems like most of the modern racing yachts have bulbs like the VOR yachts, instead of blended shapes. Do they perform better?

    [​IMG]
     
  8. gonzo
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    gonzo Senior Member

    You need to compare apples to apples. Are the wing keel and fin keel the same weight? If so, the draft will be much larger on the fin. If you are putting a bulb of some kind, the wings will provide lift, while the "torpedo" type won't.
     
  9. rcnesneg
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    rcnesneg Senior Member

    I see. Ignoring the winged keel, and using the same weight distribution in the same places on a bulb/torpedo keel, how does the shape affect things, with the two shapes from above? I guess the teardrop/torpedo bulb on the tip is marginally the more efficient of the two?
     
  10. gonzo
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    gonzo Senior Member

    It is more efficient downwind and less efficient upwind.
     
  11. PAR
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    PAR Yacht Designer/Builder

    There's isn't (usually) a physical connection, in as much as chain plates attaching to keel bolts, but on some designs the keel support grid or structural elements do in fact connect the appendage(s) to the rigging. Even if no physical connection is made, a load transmission linkage is made, as forces are transferred through the structural elements (directly or indirectly).

    As you surmised, changes to the CG and GZ will affect the rig, it's attachments mostly, though stay sizes, as well as mast dimensions will also be affected.

    Bulb keels are difficult to design well, particularly as a retro fit and especially on a small boat like this. These bulbs usually create a lot of drag, especially compared to a regular fin, so it's only real advantage is what it does to the overall package. In other words, unless the boat can take full advantage of the lowered CG and possibly higher ballast/displacement ratio, the drag penalty isn't worth the bother. A well design fin will kick a bulb's butt, assuming all else is equal, which is pretty rare. The biggest advantage of a bulb is to get a bigger rig on the boat, because of the lowered CG, so unless you're willing to make full use of the advantages and the design has the performance potential to utilize these improvements, there's no good reason to under take as radical an overhaul.

    As to configuration of the bulb (torpedo, wings, winglets, beaver tailed, etc.) well this is a study in hydrodynamics, typically based on the boats performance envelop, which simply put means you can design a real crap bulb, really easily, if you don't know what you're after. In other words, the bulb is shaped to work in a specific range, so you need to know what this is and the hull and rig need to be able to exploit these gains, or it's a draggy, hunk of weight, the boat is now burdened with.
     
  12. SukiSolo
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    SukiSolo Senior Member

    Also worth bearing in mind that weed and other gunge is much harder to remove off a bulb that sits forward of the leading edge of the fin.

    Maybe check how the K1 carries it's bulb, that works quie well on that particular design. Note there is a weed clearing hole at the front edge of the fin to poke obstructions off!.

    http://www.k1sailing.com/
     
  13. TANSL
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    TANSL Senior Member

    Thanks for the clarification. Naturally I could not talk about the exceptions, there are always.
    It is obvious that any effort anywhere in the boat may indirectly affect other parts of the boat. So I was saying : "the bulb produces no extra effort directly on the mast".
    Always good to clarify things. Thank you, again.
     
  14. PAR
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    PAR Yacht Designer/Builder

    This depends on what your definition of "directly" means. I know I've done some conversions, where the GZ was significantly changed, which required a new mast and related equipment, which to me is a pretty direct impact on the design as a whole. This might just be simply semantics though . . .

    I think to gain any real benefit, you'll need a good hard look at the particular design you want to do this with. I don't know if plans, kits or hulls are available for that Minuet Yachts offering (who seemed to have dropped off the face of the earth), but without a close look at it's abilities, weights and general volumetric and dynamic figures, this type of speculation isn't going to help much. My understanding of this particular little boat was it was overly tender.

    Our member Doug Lord likely has a lot more information about these little puppies than I do. Maybe drop him an IM.
     

  15. TANSL
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    TANSL Senior Member

    Well, we are talking about similar things. I'll try to explain. I mean bulb weight is not transmitted directly to the rig and so I say that it does not directly influence. Now, as you and I know, the weight of the bulb low center of gravity of the ship but also increases the displacement and the KC also changes. (How the KC changes with the weight of the bulb depends on other factors, such as the hull form). There is a relationship, which is not direct , between the weight of the bulb and the righting lever GZ. So I say that there is not a direct relationship between the weight of the bulb and the righting arm at 30 °, and therefore on the loads used to calculate the rigging.
    If, by varying the weight of the bulb increases the righting arm at 30 degrees, we have to recalculate the rigging. Otherwise it may not be necessary.
    I hope I have managed to explain properly.
     
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