Add-on Keel bulb fairing- optimize for drag or lift?

Discussion in 'Boat Design' started by catahoula, Jun 13, 2022.

  1. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    A basic of aerodynamics and hydrodynamics: As long as the flow around a symmetric foil section remains attached the lift for a given angle of attack will be the same independent of the section thickness and shape. The lift for a given angle of attack will only depend on the dimensions and planform shape (to within a very small amount). A thicker section will usually result in the flow staying attached at a higher angle of attack, but will have higher drag at lower angles of attack. Maximum angle of attack for attached flow will also depend on thickness and section shape. Drag will also depend on thickness and section shape.

    Lift is not due to the details of the foil section. In fact a flat plate with a rounded leading edge and a tapered trailing edge will produce as much lift as a wing with an "airfoil" section at sufficiently low angles of attack.

    Option 1 will have the highest drag for a given lift.
    Option 2 will have slightly lower drag for a given lift than option 1.
    Both options 1 and 2 could be improved by rounding the sharp corners.

    I'm not willing to say anything about option 3 without doing some research.

    A keel with a rounded bulb with very smooth transition to the straight portion of the keel would probably have the lowest drag for a given lift.
     
  2. DCockey
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    DCockey Senior Member

    @catahoula What is the motivation for modifying the keel? Will you be racing the boat? If so how competitive will the racing be? I doubt differences in performance with the changes you are contemplating would be noticeable to a third part other than in very competitive racing (unless the modification is very bad).
     
  3. catahoula
    Joined: Mar 2020
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    catahoula Junior Member

    OK, very interesting. So if i'm understanding this correctly, essentially the boat is not losing lift by having the lumpy add-on bulb, but it is adding drag so lift to drag ratio is worse.

    The motivation is two part- #1, previous flaking of paint around the bulb bolts indicates that the existing bulb is the cause of some water infiltration into the keel, so I need to seal over it anyways, and it makes sense to fair it out while I do that. #2 just comes from the existing bulb looking pretty draggy and wondering if I could make it better and/or point a bit higher while making the repairs. It may well be that the potential gains are rather marginal (the boat's name is in fact "Marginal Gains" ;) ) While I may hop in a race now and then, I have no expectation or desire toward real competitiveness. I do have the keel out of the boat right now so if I'm going to do anything, now is the time.
     
  4. CT249
    Joined: May 2003
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    CT249 Senior Member

    Wow, that's a terrible looking bulb from a layman's point of view. It would surely be slowing you down a couple of minutes per race at least.

    Down here in Australia we have a similar design by Mull, the Sonata 8 (also called Sonata 26 with a different deck). For its age it's an outstanding design upwind in strong winds. The centreboard looks similar to your one, without the bulb, and they point very well.

    There's been a huge amount of work done in 25-26 footer bulbs (as in the J80 one pictured) so why not copy one? There's a lot of diversity, from the J80/Farr style to the torpedo bulbs often seen in sportsboats, but none of them are like your one which indicates how wrong it is!

    The Seascape is a nice boat but it doesn't seem to be exceptionally fast so there seems to be no indication that the bulb shape is anything special. Torpedo bulbs have the disadvantage that the part that sticks out in front of the leading edge can catch kelp etc, but the bulbs that only project aft can make it hard to raise the keel because the weight distribution is unequal, so depending on your keel case design that could be a problem. The moderate J/80 style keel can alleviate that issue. Can you just stick the extra lead on the base of the keel to increase draft?
     
  5. DCockey
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    DCockey Senior Member

    If you are sailing in a straight line the hydrodynamic lift (in the direction normal to the direction of travel and parallel to the surface of water) created by the combination of the keel and hull be exactly the same (but in the opposite direction) as the aerodynamic side force (in the direction normal to the direction of travel and parallel to the surface of water).
     
  6. catahoula
    Joined: Mar 2020
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    catahoula Junior Member

    CT249- yes! Sistership to the Sonata. That's where I'm leaning. Using a heavy steel plate as an end plate, which would increase draft by an inch or two. Then cut the bulb halves in half lengthwise, leaving bulb "quarters", which can go between the plate and the keel face. If I cut slightly more than half, plus 40 lb steel plate, this should get me back to 200 lb or so. Then wrap it in a thick fiberglass sock and fair it out. So it looks like J80 keel. That would lower the VCG a touch too. As you mentioned the balance of the lifting keel is an issue. Considering moving the bottom bulb plate forward and fairing out so the bulb area has a vertical leading edge. This will help keep weight centered, and with vertical edge should be able to ditch the kelp just by stopping. Though the keel has never been particularly easy to lift and I'm not sure how to improve that yet.
     

  7. The Q
    Joined: Feb 2014
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    The Q Senior Member

    My boat is fixed keel, shallow draft, but had a similar problem, with the requirements of shallow draft the lead needed to be low down.
    The eventual solution was similar to Keel one, but with the keel to bulb junction faired .

    The boat I've sailed most in the last few years, is also fixed keel, and is most similar to Keel 3, but because the keel doesn't have to be withdrawn into a box the keel swells out more gradually. That class is known for its pointing ability.
     
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