Replace C/B with Shallow Keel

Discussion in 'Sailboats' started by Flumixt, Feb 12, 2007.

  1. Flumixt
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    Flumixt Junior Member

    A friend is replacing a centerboard with a long shallow keel to sail in very shallow water. Since I can sail and he's a rookie I get all the questions.

    Well my first guess is that if the keel has the same area as the C/B it should work. Will it?

    This is a modified El Toro (8 footer). The original C/B is about 198 sq in. So just scaling arithmetically an 8 foot long keel would be 2 inches deep (ie 198/96). He'd like a 5 foot long keel so that'd be 3.3 inches deep. Long (undefined length) shallow rudder about the same depth. Would either of these keel shapes function properly?

    How would this keel affect the steering? Seems to me might be tough to tack. Will it go to windward reasonbly?

    This boat is strictly for casual sailing; just has to move along. I will NOT be in it :)
  2. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    A shallow keel is less efficient than a deep keel (of the same area).
    For moderately low aspect ratios you can improve efficiency with more sweep back. See picture from (Ted?) Brewer.
    For very shallow keels, I feel they work better if they area is almost triangular, deeper at the aft end. Any comments from others on that?

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  3. messabout
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    messabout Senior Member

    Rig some lee boards if you want to play in shallow water. If you put enough keel on the Toro she won't tack very well. You are going to need a lot more area if the keel is shallow. The rudder will look like an oyster sharpie type. Long shallow rudders are not fun to steer. One other possibility is to use a pair of bilge keels. Better still, leave the boat as it is.
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  4. Mikey
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    Mikey Senior Member

    Both advice are good :) Use lee boards if you have to change

  5. Flumixt
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    Flumixt Junior Member

    Gentlemen: Thanks for your comments and insights. I'll pass em along to my friend.

    Based on the few pics we found on the net it looked like the proposed keel should be aft. Never found any pertinent discussion of it tho. Its tough to sail in 6 inch deep water with an 18 inch centerboard - tends to slow the boat. He doesn't want lee boards - hates em. I intuitively figured it might be tough to tack or round up for a landing with the long keel but there's lots of ways to get it around - a paddle being one.

    He is also thinking of twin keels. Not much info on them either for a monohull except maybe in some academic publication. I've no feel for that at all. Would 2 shallow parallel keels equal one deeper keel for lateral resistance? Presuming they were both in the water of course.
  6. PI Design
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    PI Design Senior Member

    He wants to sail in 6 inches of water?!
    Essentially, long, shallow keels are extremely inefficient (and the dimensions you quoted are very, very long and shallow), so you would need greater area to achieve the same effect as a deeper, higher aspect keel. Changing direction would also be harder.
    Twin keels are less efficient than one, deep, keel but in this instance they are probably better than one that is 5ft long and 3 inches deep.
    Why the desire to change? A boat that small doesn't need a keel - as suggested previously, a centreboard or leeboards would be fine.
    I can't believe a keel/centreboard less than 6 inches deep would be any good at all.
  7. PAR
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    PAR Yacht Designer/Builder

    You bath in 6" of water, you sail in a little deeper. Sailing in 6" of water is purely unrealistic. With your planned 3 1/3" deep keel you'll generate huge leeward slip and very little progress to windward (if any). Frankly you need a foot of appendage, to really do anything at all to weather, in that boat. Sailing in calf deep water is about as silly as angle deep water, but you'd have enough lateral plane to do so (just barely) at 12".

    When sailing in these shallow waters, there's not much better then a centerboard, which can bounce along the bottom, like an automatic depth sounder, easily retracting into the case when hard on or with bumps around the shoals.
  8. Ike
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    Ike Senior Member

    To put this in plain English, it will go like hell down wind, probably won't tack upwind at all. With that long shallow keel it will steer a very staright track, but will also be hard to turn. Frankly if you want to sail in skinny water, raise the centerboard.
  9. water addict
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    water addict Naval Architect

    Forgive me, but a keel on an El Toro?
    Good lord why? What is wrong with the centerboard? I used to have an El Toro when I was a kid, so am familiar with the boat. Putting a keel on it is like putting a 75 hp outboard on a 26' sailboat- oh wait a minute...
  10. Flumixt
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    Flumixt Junior Member

    Genthlemens: Please.

    This friend WANTS to do it. That's all. Telling him something is "just not done" is like expecting a dog to back out of a meat market.

    He intends to try sailing this in one end of a tidal bay where the watah is thin anyway and when the tide goes out even thinner. As his "chief technical advisor" I try to be encouraging and posative while at the same time stifle my groans of dismay.

    The main thing here, he is creating a custom vessel for his own amusement. Most of the amusement is in the creation. I won't describe the whole thing - I just can't. Any attempt to do so causes my vision to dim, breathing to become labored and fingers to flop lifelessly on the keyboard. :)

    It should launch early this summer, maybe in April. I am looking forward to informing you how it sails. I sincerely hope it functions properly. I anticipate that it will sail albeit not perfectly. Initial trial will be in a regular lake.

    Thanx for all your inputs. Tho I have sailed for 61 years there are some things for which I have no feel or experience and this is one of em.
  11. PAR
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    PAR Yacht Designer/Builder

    It'll sail down wind, but with progressively less ability as you come up, eventually "stalling" and skidding sideways. It's hydrodynamics, not us or you. After the first few sails and your friend can't get past a beam reach you'll be "discussing" the options again.

    In all honesty, you don't need the appendage as you've described, just go sailing without the board and get an idea of how the boat will handle.

    Have you also discussed the placement of the CLP on this new, abet real shallow appendage with your friend? What lead did you decide to use over the CE?
  12. Flumixt
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    Flumixt Junior Member

    I encouraged him to eyeball a picture of a Pelican and proportion it out that way. I recommended he take a 'try it and adjust it' approach with locating the mast etc figuring the 2nd adjust would make it good enough. A single trial and failure (or success) will teach him more'n any amount of figuring.

    I concluded long ago that when it comes to subsonic fluid dynamics that what makes sense to the "man on the street" is probably rong. It certainly is in his case. CE? CLR? CLP? WHA? :)

    This is a bit off subject but I agree with all your assessments because one time I built a 2 masted sail rig for a 17 foot aluminum canoe. Made a lee board. Had a jib, main and mizzen. 70 sq ft. Rudder straight off the stern. Marconi type sails with roach luff and foot to shape em, booms on the masts. Masts about 8 feet tall. Eyeballed the whole thing. It was rather a busy sail.

    Well it absolutely would NOT come about. Took a couple damn good sweep strokes with a paddle. Wouldn't even come up into the wind. I spect the lee board shoulda been farther forward. Tahell with it.
  13. Ike
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    Ike Senior Member

    When I was 14 or 15, my dad, brother and I took an old 14 foot boat, put a mast and sail on it and leeboards, and added a rudder. It took a couple of sails to figure out where the leeboards should be. It was slower than molassas but sailed well. It went upwind pretty well considering it was never intended to be a sailboat. It was so heavy it was almost impossible to capsize. I learned a lot about sailing in that old boat, and if I wanted to go somewhere in a hurry, an outboard would make it plane.

    My next sailboat was an OK dinghy, what a difference.

    Anyway, let him experiment. I suspect he gets his jollies out of trying things out. As long as he doesn't do anything stupid and kill himself, well, what the hell?
  14. alan white
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    alan white Senior Member

    My current boat is of mysterious origin, a 15 ft daysailer with a Bahamian dinghy underbody, meaning no centerboard, and a full (with moderate cutaway) keel deepest (22") at the stern.
    I was genuinely surprised at the performance of this boat. While heavy (maybe 1100 lbs) she carries a 155 sq ft cat rig. The boat ain't fast, but I absolutely love the simplicity of the keel. I can launch from the trailer unaided too.
    Tacking is no problem, and windward performance is perhaps 5 degrees worse than I'd expect if it had a board. Tracks beautifully as would be expected.
    How this relates to the El Toro: The customer obviously wishes to sail in very shallow water. My boat does sail pretty well in two feet or so when heeled, and maybe someone could scale that down to about eight inches in a small, light dinghy. A miniature Bahamian dinghy. Less I couldn't imagine would be very practical. Bolger once designed and had built a peapod with a full length lifting keel of extreme shallowness and there might be some commentary on that boat somewhere. The complication of it was severe in any case.
    If speed isn't an issue, twin bilge keels about 3 ft long (fore and aft) would work, would give lateral plane, would tack okay, and would beach better. They might be about the same depth as the lowest point of the boat's bottom and still be about 4" "deep". I don't know the El Toro's deadrise (if any) however, so maybe this won't work.
    Any case, that rudder is probably almost as big as the centerboard, and as far as the water is concerned, it IS a centerboard. It drags and lengthens whenever it gets kicked up, so converting to a full keel is only replacing one of the two "centerboards".


  15. gggGuest
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    gggGuest ...

    Guys, plenty of sailboards go upwind adequately that will float in six inches of water. There's no magic rule that forces you to have a foot of plate under the boat. And those of you who read Arthur Ransom as kids should know a deepened keel on a 14ft dinghy can give adequate grip for upwind recreational sailing. Of course it will go upwind fairly poorly. But better than not sailing at all...

    Give it as much depth as you can, run it from bow to stern, vanishing near the bow, deepest in the middle, but keep it going as a skeg aft to help out the rudder. It will probably be pretty insensitive to CLR etc anyway, but you can always chop bits off if its impossible. Be prepared for it not to work, but you ought to be able to get upwind to a reasonable extent, if at maybe no better than about 50/55degrees. Probably good to sail it heeled so the lee chine is in the water helping too...But take some oars and the ability to take the mast down, you can always get upwind that way!

    I think I'd try and arrange a pivoting rudder so that when in the up position the blade is still immersed, parallel with the extension of the skegline. That will give a very long blade fore and aft, inefficient but with loads of authority for brute forcing the boat through a tack. Down it would be more normal for deeper water. Try not to have much of a gap between blade and skeg when rudder up. The stock will have to be insanely over built because steering with the blade up the loads will be huge.
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