Bilge Keel Question

Discussion in 'Boat Design' started by flathead65, Nov 4, 2016.

  1. flathead65
    Joined: Apr 2014
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    flathead65 Jeff Thompson

    I have a question regarding bilge keels. Those being placement and profile. I helped launch a friend's 34' x 11'-6" sailing house barge today and he has concluded he will go the leeboard route (it is being finished in the water). Pictured is my design for a 32'x9' scow motorsailer that I intend to build in steel. I would like to avoid leeboards or a centerboard of any kind. I am willing to sacrifice some performance as a result. The rationale is a coastwise cruiser with comfortable accommodation for myself and a companion that can also navigate the Fraser River up into Harrison Lake in the shallow summer conditions. I would like to keep the draft as minimal as possible, but also expect reasonably good sailing performance. It will be rigged a junk yawl. The shallow center keel could be made more parallel to the flat run going forward to help prevent leeway. What sort of dimension would the bilge keels have to be to work effectively? Where should they be placed in relation to the CLR and or the CE? Finally, at what angle should they be set considering the long flat run? Thanks for any input you can offer.

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  2. Angélique
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    Angélique aka Angel (only by name)

  3. Tad
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    Tad Boat Designer

    Those two are mutually exclusive.

    What do you mean by "minimal". Three feet? Four feet?

    One of your problems is that "reasonably good sailing performance" is an unknown standard. It means different things to different people. Usually it's based on the speakers previous experience. Do you want a boat that sails to windward? How close? Tacking through maybe 120 degrees? Stopped dead and going sideways at what windspeed? Or, are you speaking only of offwind sailing and always motoring to windward. Then your keel can be truly minimal.
  4. b1ck0
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    b1ck0 Senior Member

    Frankly I didn't understand your question, regardless of my education in Naval Architecture. The solely purpose of the bilge keel is to increase you roll damping and decrease the roll motions and accelerations in seaway. Regarding the size of the bilge the more is better of course. However you would have some "natural" limitations. When you place the bilge keel on the bilge you should try to get the maximum height of the profile such as it does not go beyond your molded beam neither lower your keel (applies for a commercial vessel). Another "strong" requirement for the bilge keel profile is that it must follow (be parallel to) the stream lines in the area of the bilge (so it's effect on the resistance is smallest). For example if you have a fast boat and your stream lines tend to follow the buttock lines of the boat then your bilge keel would be more "vertical" compared to a bilge keel for a slow boat, where the waterlines tend to follow the waterlines. However it really depends on what hydrodynamic information you have for the flow around your vessel.
  5. SukiSolo
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    SukiSolo Senior Member

    For reference, I would suggest asking a lot of bilge keel boat owners. IMHO the performance of bilge keel sailing boats varies dramatically depending on the design and optimisation done by the original n/a and yard. Some are pigs, others quite well mannered. Some of the better UK ones are the Hunter range.

    You never quite lose the nasty traits of the arrangement such as overpowering the rudder when heeling (a little beyond optimum) on a puff, but the best allow quite reasonable vmg upwind. Even better if you can get a sail on a few as this will illustrate in short order exactly the compromise that bilge keels are.

    Some designs are offered in both fin and bilge keel options, if possible try both, it might be quite insightful.
  6. Angélique
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    Angélique aka Angel (only by name)

    That sounds to me you're assuming the post #1 bilge keel question is about a power boat.

    But post #1 stated it's about a 32'x9' Scow Motorsailer rigged as a Junk Yawl.

    Which would make providing leeway resistance the main purpose of the bilge keels in this case I think, besides the things you named.

    But there's little chance we'll ever learn more about the nature of his question since the OP has a track record of asking a question and then isn't replying at all to the answers he'll get . . :rolleyes:

  7. sharpii2
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    sharpii2 Senior Member

    Hi flathead.

    I looked at your sketch. I don’t think the extremely long-shallow keel you drew will be effective. It is just too long and shallow.

    Most keels you see today work like underwater airplane wings. One side, the leeward side, is the high pressure side, as the sails try to push the boat sideways. The windward side is the low pressure side.

    As the keel moves through the water, the high pressure side, the leeward one, pushes the boat upwind. The windward side, the low pressure one, tends to pull the boat the same direction. This is because both sides of the keel tend to divert the water flow across them to leeward, causing the reaction of the boat moving to windward. This is especially true if the keel is nearly as deep as it is long, if not deeper.

    As the keel gets longer, there is still the upwind push on the leeward side, but a much smaller upwind pull on the windward one (for its area). This is because some of the water cheats. Instead of following the length of the keel, it takes a short cut by slipping under it. The longer the keel is, I relation to its depth, the more true this is true. After a certain point, the keel is so long in relation to its depth, the windward side provides no upwind pull at all. Instead, it is buffeted by a swirling vortex caused by the water slipping under the bottom edge.

    After the keel gets longer and shallower still, even a lot of the leeward side upwind push is lost. This is because the water would act like pedestrians encountering a long, low fence. Instead of going out of their way to go around it, they will simply step over it. In order to make up for this loss of effectiveness, more area must be added. But this is only partially effective. The rub is that a greater angle of attack must used as well. This means a loss of pointing ability, as the keel gets longer and shallower, as well as more whetted area. In essence, a long keel works more like a snow plow than an airplane wing. After a certain point, so much more area must be added to the keel that the actual depth decreases very little, as it gets ever longer.

    Over the years, I have created a mathematical model for designing such keels. This model is based on theory only, and has not been tested, as I have no means to do so. But it has somewhat successfully predicted the length and depth of keels on existing long keel sailboats, so it may not be far from the truth.

    With your sketch, I tried two different configurations with my model. The first was a single keel as long as the waterline, 24.5 ft. This keel needed a depth of 1.34 ft (16 inches or 40.6 cm)

    The other was a twin keel arrangement with two 11.0 ft keels. These needed a depth of 1.41 ft (17 inches, or 43.2 cm).

    The keels in both cases would have a straight, level bottom edge, as the rig you chose for this boat is a ketch. With an aft downward slope (Drag) the Center of Lateral Resistance (CLR) would probably be too far aft. With the shorter twin keels, it would be possible to move them further forward to allow this Drag. But this would mean added depth, which you have expressively said you want to limit as much as possible.

    Being a scow with very blunt ends, your boat would need little or no Lead, as when the boat heels, the leeward bow and stern start to immerse, creating more drag on the leeward side. This additional drag will cause the boat to want to point down wind. If your boat had a finer up and down curve (rocker), along its length, this would be less true. If it had a pointed bow this would be even less so, as the bow wave would tend to push the bow upwind, making it want to round up.

    Your sketch does not make clear what the bottom width would be. In your post, you said the boat would be 9 ft wide. In the sketch it is 8.5 ft wide. Do you mean the bottom width (between the bottom edges of the sides) will be 8.5 ft and the Sheer would be 9 ft across?

    By my estimation the Hull itself would have a Draft of around Ten inches, at 9400 lbs. Adding the single keel would make the total draft around 27 inches with dead level trim.

    As for the performance, I can only speculate. My guess is that would do somewhere between 120 and 126 degrees, with 0 degrees being dead down wind and 180 degrees being straight into the wind, so your tacks will look more like glorified close reaches. The engine of course could improve on this considerably. But a real sailboat must be able to make windward progress with its sails alone. So that is the performance which is counted. It would be very wise to make a scaled model complete with the cabin structure represented to test the concept. The model could be as little as one inch per foot.

    I have a strange visual affinity for scows. Some would say it is a warped aesthetic sense. I hope you go forward with this project and I wish you the best of luck.

    Attached is a sketch of a scow designed to sail on a lake which had a typical depth of just 4 ft.

    Attached Files:

  8. The Q
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    The Q Senior Member

    You dismiss lee boards, but have you considered asymmetric Boards located just inside the hull, they will give you a much improved windward performance. You loose little space inside, they are sealed from the inside of the hull and you keep ultra low draft. here's a somewhat different boat with asymmetric boards of the type I'm suggesting.
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