Mottle 33 to lift keel

Discussion in 'Boat Design' started by Peter6173, Jan 26, 2020.


Good idea

  1. Yes

    0 vote(s)
  2. No

    2 vote(s)
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  1. Peter6173
    Joined: Jan 2020
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    Peter6173 New Member

    I was after some thoughts on converting Mottle 33 to lift keel. I was thinking a swing keel that completely retracts into the cabin.
    How expensive would it be?
    Would it affect sailing ability?
    Who does this sort of work, guide,design?
    Your penny is in the mail.
  2. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    I've looked for a pic of Mottle 33. I found a lot. An old nice classic design, ...
    Oh man, the change from the fixed keel to a swinging keel is a major transformation. It's practically re-design structurally the whole boat as you make a slot in the hull and in all the structure that keeps the keel attached on the boat plus the structure that keeps the mast upright laterally. If the boat has inside structural counter molds that becomes very complicated.
    That will cost a lot of money. Worst, that would be not a good idea, as the shape of this classic boat is not convenient for such a transformation, you'll have serious problems of stability. That you want, a swinging keel needs a good stability by shape, that's not the case of the Mottle designed to use the stability by ballast, with smooth wine cup shapes.
    A possibility is to have a shorter, wider and heavier external ballast keel giving the same righting moment as the original keel with a swinging daggerboard in the ballast. That will remain costly but possible. But adding hundreds of kilos as the lever is far shorter.
    The sailing ability will be affected for sure, so you have to know is the smaller draft is worth the complication, money and probably lesser performance on an pretty old fashioned boat.
    Last edited: Jan 27, 2020
    bajansailor likes this.
  3. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Hi Peter,
    My initial thoughts would be to find another boat with this feature- the Mottle 33 is as similar to an Adams 35 - many of which where built in steel and sometimes alloy with this feature. Joe Adams designed lots of lift keel boats with a great reduction in draft once the keel up.
    It would be expensive and probably in excess of the second hand cost of a vessel with the lift keel already- add yard fees, design work and yard labour, add some more if casting required plus the material content which would be the "cheaper " part.
    The people that do this work are Shipwrights and Boat Builders that operate boatyards with lift and haul facilities or occasionally at yards away from the water with transport to consider. I own such but would not take this on as feel it's ill advised when the vessel would be a design "orphan", cost a lot and end as a less desirable vessel for the owner. The design work for this would be extremely important as the board slot is a massive structural discontinuity that needs to be compensated for- the work will be very disruptive to the existing fitout, the transverse floors that currently support keel loads will be interrupted, stability and ballasting must be assessed such that the vessel safe and stable in all scenarios of keel up/down and the rudder redesigned to match/be compatible for accidental grounding with the board up draft.
    Sailing ability would be affected with board up or down, it may be enhanced in some respects depending on point of sail.
    regards from Jeff.
    Rumars, bajansailor and Ilan Voyager like this.
  4. Peter6173
    Joined: Jan 2020
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    Peter6173 New Member

    Thankyou, both for your considered replies, the more I think about it the more I agree with your opinions.
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    You're welcome.

  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    I think this is an encapsulated keel boat. Cutting off the keel leaves a big hole to be filled, plus you have to rip out the interior to fit new floors and stringers. And for what? Without cutting off the skeg and installing a pivoting rudder draft is fixed by the skeg. A new rudder involves cutting into either cockpit or transom, more work. By the time you are done you could have just as well buildt a new boat, one designed as a centerboarder with inside ballast, retracting rudders and wich sits upright by itself when beached.
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