Replacing Encapsulated Keel with Bolt On Keel

Discussion in 'Boat Design' started by FirstLight, Nov 19, 2010.

  1. FirstLight
    Joined: Sep 2007
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    FirstLight Junior Member

    Here comes a crazy, crazy question.

    Currently we have a 40' sailboat built in 1984. We love the boat. It is solid, well laid out and has been brought up to current spec in every way possible.

    We have out over $50,000 in systems into the boat and feel a certain kinship towards her.

    However, we are beginning to have some problems with the encapsulated keel where the glass is separating from the lead\resin mixture and has several bubbles in other areas. Grinding, drying it out and repairing is doable.

    However, with the boat previously having been a Bahamas boat with a 4'5" draft the upwind performance lags horribly and the stability is a bit tender with an average righting moment.

    I was thinking of chopping the keel, adding a stub and re-enforcing with "sump bulkheads" and having the folks at Marsmetal design and pour a new keel that bolts on.

    Was curious if anyone has tried this or if there are other options to consider??

    Any thoughts greatly appreciated..

    Cheers,

    R
     
  2. PAR
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    PAR Yacht Designer/Builder

    Upwind preformance is dependent on several factors, the arrangement of the keel is part of the equation. I would approach a designer, familiar with the design you have, for this type of upgrade. Marsmetal can follow plans well enough, but I don't think the design attributes should be left up to them.

    Having sailed and worked the Bahamas, 4' 5" of draft is a luxury and you should be grateful for it, if you expect going into shoals again.

    In the end, your request is doable, though how much of an improvement a new $15,000 lead casting (as an example) might bring your yacht's preformance envelope, is the true issue. What is the make, model and year of your boat? Does it has the "stock" rig? Some designs have the room for considerable improvement, others not so much.
     
  3. Stumble
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    Stumble Senior Member

    I would actually first see if there was a designed keel with a deeper draft. Most production boats of that size would have been designed with at least 2 keel configurations. If it was, then just find a an example, or find the designs.

    If that is not an option I would second Par's advice. Find a designer with experience with the boat and hire them. Compared to the cost of a new keel the designers cost will be minimal, but ensure you get actual performance instead of just a deeper draft with no advantage.
     
  4. PAR
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    PAR Yacht Designer/Builder

    The bottom line is after reviewing the polar plots for the current and proposed appendage configurations, will the costs and difficulties be palatable, given the changes to her preformance envelope. This is not as quantifiable as it might seem.
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    A deeper keel, well designed, perhaps with a bulb to keep the weight low, is most assuredly going to improve boat performance, that's a given. A VPP study will likely not be all that sensitive to show really reliable results, and you have to have the lines plan of the hull anyway to run a VPP. Is the lines plan available? And is it worth it to go through a few thousand dollars of time to program the hull shape and rig into the computer to get results of questionable value? Probably not.

    The devil is in the details, how to modify the boat to build on the new stub and to bolt on the keel is the hard part--you have to survey the existing structure, design and engineer the new stub structure and keel bolt arrangement so that it can be easily built and assembled. It can all be done--just has to be done carefully and with a good boatyard as well as a good designer.

    Eric
     
    Last edited: Nov 22, 2010
  6. gonzo
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    gonzo Senior Member

    Is the keel fiberglass skin structural or just for fairing? Some keels are bolted and then skinned over. In that case, the delamination is mainly cosmetic.
     
  7. FAST FRED
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    FAST FRED Senior Member

    4'5" draft the upwind performance lags horribly and the stability is a bit tender with an average righting moment.

    1 /7 the LWL is normal draft for a cruising boat , sounds right to me on a 40ft LOA..

    And "tender" is frequently only initial stability , and is sea kindly.

    Perhaps a more cruising cut of sails (less 12 meter round the buoys) would help?

    Opening sheeting angles would probably help too.

    FF
     
  8. idkfa
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    idkfa Senior Member

    What's your keel/hull weight ratio, and the aspect ratio of the keel?

    This is more about increasing power than lessening drag.
     
  9. michael pierzga
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    michael pierzga Senior Member

    As Mr. Sponberg states..the issue of any keel modification is an engineering one in which your present structure must be evaluated ..... Keel, mast and standing rigging included, before a keel change, modification is contemplated.
    Be aware that ballast..in a keel, does not give you Upwind performance...a keel, "ballast " generates righting moment..stiffness. Upwind abilty is generated by the efficiency of your foils.

    A boat with 4 and a halve ft draft can have very good windward performance if her foils are optimised. The whole purpose behind wing keels , the sheel keel, end plates , centre boards, leeboards, variable depth rudders is good bite and good upwind performance for reduced draft keels..

    If you contracted a designer like Mr Sponberg he most certainly could come up with simple appendage modifications that generate greater lift and lateral resistance. This will be the best bang for your buck
     
  10. idkfa
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    idkfa Senior Member

    "A deeper keel, well designed, perhaps with a bulb to keep the weight low, is most assuredly going to improve boat performance, that's a given."
     
  11. FAST FRED
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    FAST FRED Senior Member

    "is most assuredly going to improve boat performance"

    And your definition of "performance" for a cruising boat is?

    FF
     
  12. PAR
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    PAR Yacht Designer/Builder

    We all know (hopefully) how subjective a term performance is, particularly on a cruising boat. I too am curious as to the real meaning of Idkfa's post.
     
  13. idkfa
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    idkfa Senior Member

    You know I was quoting Eric Sponberg post #5?


    Performance is the same for all monohulls: improved VMG, which is effectively improved upwind angle, that hull speed thing again...


    Lets say you are weekend car racer and you ask for help to improve performance. One guy asks "do you have a turbo charger and if yes, what psi boost are you at?", the other guy suggest you change your spoiler to a more sleek one....

    The one makes more use of your engine's disp, the other lowers drag, you do both, which will have the bigger effect?

    We need more info, like ballast ratio (potential for better use of disp) and aspect ratio.... it's not complicated.
     
  14. michael pierzga
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    michael pierzga Senior Member

    I could probably go to any cruising boat in the harbour with a drill, a pad eye , move the genoa leed inboard, narrow the sheeting angle of the genoa, fit a new genoa, tighten the backstay and improve up wind performance. Ballast ratio, sail carrying ability and foils are only part of the equation. Many times the simple things greatly improve performance. Fit a duckbill or max prop...clean the bottom... new sails, get weight out of the rig, make sure the boat is floating on its lines and improve your own ability....
    A good designer will know this...and recommend accordingly
     

  15. Eric Sponberg
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    Eric Sponberg Senior Member

    By itself, "performance" can be interpreted many ways, certainly. In sailboat design, in my mind, improved performance means better boat speed, better speed made good to windward (VMG) and better pointing ability. These are all interdependent, but they are three measurable quantities that determine sailboat performance.

    In the case of keel design, which we are talking about on this thread, FirstLight complains of a shallow keel and that the boat's "upwind performance lags horribly." He also states that "stability is a bit tender with an average righting moment."

    First of all, anytime you can improve or increase righting moment, you are going to improve sailing performance. Practically all boats will sail better (be faster, point higher) with increased righting moment. How do you get that given any single hull? Make the keel deeper, perhaps heavier--get the weight down lower. Also, the single most important factor in keel design is draft--a boat with a deeper draft keel will always perform better than a boat with a shallow draft keel. The Next most important factor is planform shape. A shallow keel is going to have a not very efficient planform shape, but a deeper keel is--it will have a higher aspect ratio and, if the designer is good, a better sweep angle (if any) and tip shape, perhaps incorporating a bulb to get that center of gravity down low. Section shape also figures into this, but it carries less importance than all the other factors.

    So, all these very positive known effects, taken together, say, as I did in my first post, that "A deeper keel, well designed, perhaps with a bulb to keep the weight low, is most assuredly going to improve boat performance, that's a given."

    Eric
     
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