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

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

  1. catahoula
    Joined: Mar 2020
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    Location: MT

    catahoula Junior Member

    Hi all-

    Lurker here. I have a Ranger 26-2 that a previous owner added a 200 lb lead bulb kit to. The "bulb" is two halves of a cylinder with rounded ends, one half on each side of the keel, through bolted together near the bottom of the keel. Pretty crude. I'm doing some general work on the keel, have it out of the boat, and would like to fair out the bulb while I'm at it.

    Main question: The boat was not designed for a bulb, so the bulb takes away a significant area of lift producing keel foil (approx the lowest 8"-10" of the ~4 foot deep keel). Should I re-shape the lead bulb into a foil section using something like NACA 0012, in hopes to regain a bit of lift production from the bulb area? Or should I just fair out the bulb for lowest drag, using something like the NACA 66-018, and accept the less-than-designed lift force from the keel.

    We have noticed a slightly poorer pointing ability than one would expect from an IORish boat with good fin keel and spade rudder and good sails. But, I sail shorthanded so the added bulb weight is much appreciated, I'm definitely not interested in removing it entirely. Fashioning the bulb onto the end of the keel to restore the working foil area isn't an option because it would no longer fit on the trailer. I guess the secondary question is: given any reconstruction of the bulb, it'll still be signficantly thicker than the keel foil itself, so is it possible to get good lift production from a "bulb-like" section of the foil that has a significantly increased thickness to chord ratio? I'm thinking about the style of swing keel used on the First 27 Seascape Edition which has a foil-shaped semi-bulb at the bottom. Though less extreme.

    The boat has a signficant, maybe excessive amount of weather helm, which seems to add another option: extending the bulb past the trailing edge of the keel, to move the CLR aft. This would also mean that the thickness:chord ratio of the "bulb" area could be the same as the rest of the keel, which seems like it would be effective at creating lift without excessive drag.

    Also- yes, i understand that adding keel bulbs increases righting moment and the loads on the foil/hull/keel/rig. The boat has survived at least two owners since the mod was done, and as part of the general keel work I'm making significant reinforcements to it just for good measure, as I like the boat and want it to be happy for a long time.

    Any advice appreciated. I have experience in composites and design but am totally green when it comes to foils.

    Cheers,

    Brian
     
    Last edited: Jun 13, 2022
  2. catahoula
    Joined: Mar 2020
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    catahoula Junior Member

    Oh also one thing I missed- I am willing to do some fairly aggressive re-structing of the lead. I'm figuring I can do the main profiling with a sawzall, then keep the removed lead and epoxy it on on later in other locations. The whole thing, including a foot up the keel from the bulb, will be wrapped in fiberglass/epoxy later, so that should give me some flexibility in positioning material.
     
  3. DCockey
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    DCockey Senior Member

    What do you mean by this? Circular cross section? Rectangular cross section? ???
     
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  4. jehardiman
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    jehardiman Senior Member

    FWIW, the original Ranger 26 (Mull) I had could have had what called a bulb keel developed back at the turn of the 20th century. The keel section was significantly thicker at the tip than it was at the root, which significantly lowered the CG of the bolted on iron keel. This is because the root interference is proportional to the thickness at the root (see Hoerner; Fluid Dynamic Drag) ... and lead (hah! pun!) to the later "tomahawk" keels near the end of the 20th century.
    Be that as it may, you have a Ranger 26-2 (Mull) which has a dagger board, a much lighter boat and a lot less ballast. With the vessel that light and no real mass to punch trough the chop, of course it feels much different than a heaver boat and sails more on it's ear which gives it weather helm and is most likely the cause of the added bulb to begin with. If it is a lake boat (you say you are from MT, so just extrapolating) that needs a lifting keel to get on the trailer, then you need to keep that ability. The bulb actually adds some "apparent span" to the keel, but is most likely you are gaining more drag due to the "square" foil/blub interface. I would focus on improving the blub to foil fillet rather than the shape because a "drag bucket" shape has no advantage on a vessel of this sailing speed.
    Edit: Xpost with David...we'll see where this goes.
     
  5. jehardiman
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    jehardiman Senior Member

    Yeah, photos!
     
  6. DCockey
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    DCockey Senior Member

    I agree with jehardiman about focusing on improving the bulb to foil transistion.

    The flow around a bulb is very three-dimensional while foil shapes are designed for two-dimensional and near two-dimensional flow. The pressure distribution around a bulb with a foil section will be very different than the pressure distribution around a wing with the same foil section.
     
  7. catahoula
    Joined: Mar 2020
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    catahoula Junior Member

    Thanks guys. I'll mock up a couple things in CAD tonight to illustrate a little better
     
  8. catahoula
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    catahoula Junior Member

    David- Rectangular cross section is what I'm thinking (with a fillet transition)

    jehardiman- Interesting point about drag bucket foils and slow boats. I had just searched for low drag shapes and found reference to the NACA 66. While we've had it up to 8.5 knots once, that was kind of a problem :D
    It's definitely a lightweight boat and trailer sailor. It's true that the keel is more of a weighted daggerboard, but even in stock form it has a lead plate inside at the bottom, so at 375lbs stock (575 lbs now, weighed in my pickup at the dump to verify), there is some weight down there. The remainder of the 1340 lb total ballast is laid into the bottom of the hull (stock 38% B/D, currently 42% ish). My keelboat experience is limited to this boat plus one day on a J/24 and one day on a Tartan 3400, but the Ranger 26-2 generally sails with less heel than the Tartan and maybe slightly more than the J, so it's not terribly tender. I'm indeed in Montana, but family is in Seattle so I made a two week San Juans trip on it last year and aim to do more Salish sea sailing. We sailed through some storms on that trip and it did well, though we learned to use the second reef pretty early to keep the weather helm down. The current genoa is only 130% so the sail area is a little low to start, but that's not too bad because it has an asym and retractable sprit that's great for light wind up to about 85 degrees apparent.

    Here's a photo of the keel in it's current condition plus a CAD drawing of my concept of a foil-shaped bulb. To put a fillet in the model I'd have to rebuild the profile curves, so just imagine a nice fillet blending the "bulb" to the rest of the keel ;) (I'm sure you'll also notice the drilled drain holes- I'll be replacing some core with fiberglass as there's been water intrusion) Also, I'm finishing a garage remodel before working on the keel so plz excuse the mess!

    Edit to add: the "foil-bulb" in the model is a NACA 0012 scaled 1.9x on the y-axis. A rough eye-ball approximation of the width I might be looking at if the existing bulb material was redistributed
     

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    Last edited: Jun 14, 2022
  9. catahoula
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    catahoula Junior Member

    Also a photo just for fun
     

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  10. jehardiman
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    jehardiman Senior Member

    You DO NOT want a right angle between the bulb and the keel. You want NO square corners. EVER! The rounder the better.
     
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  11. baeckmo
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    baeckmo Hydrodynamics

    Worth saying again!!!
     
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  12. catahoula
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    catahoula Junior Member

    What about a flat bottom a la J/80, J/70?
     

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  13. jehardiman
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    jehardiman Senior Member

    A parallel to flow surface is OK, as long as the radius leading into it is greater than 5% (minimum) of the breadth. 5% is minimum, 10% is better, 20% is getting towards faired. And by "parallel to flow" I mean exactly that; and falling away from the flow like the photo you show is even better. The leading shape should be elliptical, the mid-body parabolic, the trailing shape hyperbolic.
     
  14. jehardiman
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    jehardiman Senior Member


  15. catahoula
    Joined: Mar 2020
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    catahoula Junior Member

    Thanks for the links- had seen a couple of those but not all. Point taken about radii and falling away from the flow. I rebuilt some concepts in Rhino attached here but didn't fillet the bottom edge for modeling simplicity, however I'll plan on a thorough fillet and making sure the bottom surface falls away from the flow in fabrication.

    I guess to rephrase the question, and what I didn't find addressed in my searches: for a 6 knot sailboat keel, does a foil scaled only on the y axis, i.e. making a section of it thicker without increasing chord length, produce useful lift? Option #1 is what I'm thinking about here. This is probably the most straightforward modification from a fabrication standpoint.

    Option #2 has the "bulb" area foil shape scaled symmetrically, maintain the NACA 0012 proportions of the original keel foil so the "bulb" section becomes longer in chord. My assumption would be that this does produce useful lift but with somewhat increase in drag, too. Similar in concept to the First 27 Seascape keel. Probably the best option hydrodynamically? but I'm not sure I want to commit to changing the underwater profile/CLR that much. Also the most difficult in terms of fabrication.

    Option #3 is a slightly more extreme version of what you described from the earlier Ranger 26, where the keel section is thicker at tip and blends to a thinner root, this just makes the transition more quickly. This might be reasonable from a fabrication standpoint, seems fairly conservative from a design standpoint, and does restore a fair amount of the original foil "working" area, while lowering VCG without adding more weight.

    Thanks again, the feedback is very helpful

    Also, to the moderator, I wasn't sure to put this in the sailboat forum or hydrodynamics forum so just put it in general- if it should be moved please feel free to do so.
     

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