Center of Lateral Resistance

Discussion in 'Boat Design' started by Jim Herbert, Aug 23, 2009.

  1. Jim Herbert
    Joined: Nov 2004
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    Jim Herbert Junior Member

    I am wondering if the CLR would be positioned the same on a a Sailing Keelboat as it would be on a Centerboard Sailboat. I was wondering if drag would enter into the equation or other factors.

    Please see previous post with no replies:

    Winter Project

    I bought an old Chrysler Buccaneer 18 sailboat for our boat club this Summer. It is a little tippy. I had a Twin keel Westerly years ago.

    I am thinking of adding two 200 lb keels to the Buc to add a little stability, straddling the centerboard. Dimensions are about 4" x 72" x 10" ht, full of concrete.

    The question is fore & aft positioning. I am thinking that the trailing edge of the extended centerboard should be about the center of lateral resistance, which would be where I would position the center of the keels. Any thoughts?

    Jim Herbert
    Twin Cities Sailing Meetup
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Not for nothing, but these aren't things that you can guess at and have a reasonable level or expectation of success. Not to mention they information you desire is fairly easy to figure out or come by.

    For what it's worth, it's very likely your proposed ballasted, twin keel configuration addition, to your Buccaneer 18, will bring down the rig, the first time you get some moderately heavy gusts.

    Do your self a big favor and seriously reconsider your plans, or at least perform the research necessary to get it close to right or employ someone with the necessary skills.

    I'm not trying to be harsh, but adding over 20%, possibly 25% of the current design's displacement in ballast, as you've described, will have a world of issues related to it, most of which you are completely unaware of.
     
  3. Jim Herbert
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    Jim Herbert Junior Member

    Thank you for your response. 40 years of boatbuilding have brought me my share of disasters, and your post helped me recall some of them.

    Of course, we wouldn't be adding any weight. We would be moving the weight from a foot above the waterline (crew) to a foot below (who wants 6 people in a little boat?).

    I guess I was considering this research and maybe I'll start using up all the saves I have hoarded all these years in Rhino. The last NA I hired seemed to be more interested in self promotion than anything else, but thats probably just the old Architect/Engineer rivalry.

    Jim Herbert, Principal
    Marine Water Makers, Inc
    Water Treatment Engineering
     
  4. Robjl
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    Robjl Senior Member

    Ballast

    Jim, the righting moment...imagine it as a length (from the centre of gravity to the centre of buoyavcy) multiplied by the weight. If you just consider your 200 lb crew hunkered down alongside the keel, 200lb of ballast yes but of no value until they produce a righting moment. This happens in one of two ways. They move outboard... the distance to the centre of buoyancy increases and you have a righting moment... or the boat heels and the centre of buoyancy moves to leeward. The greater effect is obviously from the moveable ballast (your useless mates). To gain a similar benefit from your concrete ballast (the weight being fixed at 200 lb) you must achieve a greater distance from the CG of your 200lb to the centre of buoyancy. I think the only way you can do this is to hang it on a keel under the boat....= increased draft!
    When I was young, much younger 1965, I was real keen to look at yachts, I went aboard a 22' yacht anchored in the bay. The ballast was rocks, I was staggered but the skipper assured me that the additional ballast improved the stiffness.
    As to where to place it (if you must place it internally), my advice take no notice at all of the experts, they will charge heaps to do a half proper job and then cover themselves with a rider on their recommendations. My advice is free and absolutely accurate. Do it by trial and error. Beg or borrow some weights , even lead bars, it's only 200lb you are talking about, and try it. If you are disappointed with the result give them back, nothing lost but a bit of time. Maybe find some better crew.
     
  5. alan white
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    alan white Senior Member

    Study the Alacrity sloop, which is 18.5 ft and may have a similar underbody. It draws 1' 10" with two bilge keels, and no centerboard, yet it tacks in 90 degrees.
    If your hull is somewhat similar to the Alacrity's, the lead may be similar as well. Adjustments of mast rake can fine-adjust the rig afterwards, but only to a small extent.
    The Alacrity has about 500 lbs divided between two keels.
    Your rig is probably heavy enough for a few men positioned on the weather rail, a huge ballast on such a light boat, but do check the shroud rigging size
    because this three person strain is going to be present at all times, not just occasionally. It's inertia I'm talking about, the resistance to quick changes in heel, and not prolonged heeling angles that strains the rigging.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Maybe I didn't make myself clear, I'll attempt to do a better job. Bluntly, if you install two bilge keels, with 200 pounds at the bottom of each, you can kiss your rig good bye. I don't care what others may say about, "go ahead and try it mate . ." and all that kind of crap, it's just a lack of understanding. I'm sure they're all well meaning, but just don't know the boat or have a full grasp of the physics involved.

    This is a Chrysler Buccaneer 18.

    [​IMG]

    Speak what you know folks . . . and tell me it's okay to toss 400 pounds of ballasted bilge keels under this puppy and expect the mast not to fold up like a paper soda straw that's seen too much milk! Speak what you know . . .
     
  7. Jim Herbert
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    Jim Herbert Junior Member

    Physics was never one of my strong suits either, always more of a chemistry & astronomy guy.

    I guess I will just add a couple of sets of reef points and call it good enough. Put the hiking straps back on and be prepared to have my coffee spilled.

    The Alacrity sloop is a cute boat, I think the boat I owned when I was a kid was it's big brother, the Vivacity sloop. We just called it a Westerly so the bank would fund it's purchase. Stout build quality. Had it out in 65 Kt winds on Lk Superior when all the race boats were being dis(de)masted.
     

  8. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I didn't mean to spank you so hard, but apparently it worked.

    The little Chrysler is supposed to be fairly "sporty" and it's going to be "reactive". Just sliding your butt a few inches in any direction can make a huge trim difference. This is the way it's designed and you just don't have the freeboard or hull volume to "fix" this on this particular design. Sailed for what it is, you can have some fun. Well okay, wet fun and maybe with a capsize or two to reward any of your bigger mistakes , but that's the design brief for this little boat.

    The Alacrity 19, now this is the pocket yacht you need.
     

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