Adding vee to cat hulls

Discussion in 'Multihulls' started by mikereed100, May 22, 2011.

  1. mikereed100
    Joined: Jan 2007
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    mikereed100 Junior Member

    My 46’ cat has an unusual construction. The designer used a single mold for the deck and hull which are then joined by a horizontal seam at mid-hull. Thus the hull bottom is a mirror image of the deck. Unfortunately, the properties that make a good deck do not necessarily make a good hull shape. I have found that due to the flat hull bottom the boat will pound mercilessly under some conditions. I am planning to add a vee section forward, tapering to flat in the sterns in an attempt to alleviate some of this pounding. Construction would be a timber stringer over the centerline of the existing hulls with 3mm marine ply skins covered with 450g biax and epoxy. The void between the hull and the addition would be filled with high density pour foam.
    If my calculations are correct this will add 160kg weight, 800 liters flotation and move the center of flotation 20cm forward. It will also add 12cm of depth and move the point of greatest depth from about two thirds of the way back to midships, just aft of the daggerboard adding a bit more rocker.

    Disadvantages would include a net loss in speed and maneuverability due to;
    extra weight
    increased lateral resistance due to the vee forward
    increased wetted surface

    Advantages include;
    decreased pounding
    the vee would provide a virtual stub keel for taking the ground and added protection for the props
    added protection and flotation to the hull in event of collision or accidental grounding
    better tracking

    My reason for posting here is that I would not like to inadvertently damage the sailing qualities of the boat so am looking for some unbiased opinions on the changes I have planned. Here are some line drawings showing the present hull shape and the changes I have planned.

    Mike
     

    Attached Files:

  2. Alik
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    Location: Thailand

    Alik Senior Member

    What boat is that and who is the builder?
    To me, this construction technique does not make too much sense.

    Why should one get weird hull shape just because someone wants to save some money on moulds? Original shape and Your V-shape are evidently lacking volume aft; CP seems too low.
    Yes bow sections are too flat, there is no flare and the boat will be very wet. Deck is something but not practical and comfortable.
     
  3. redreuben
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    redreuben redreuben

    I would avoid the plywood, stringer is good with H80 foam and glass over with bit extra glass and overlap on keel. Pouring foam below waterline, especially around keel is invitation for disaster, chopped out tons of the stuff from inside leaky "buoyancy" tanks absolutely sodden !

    RR.
     
  4. redreuben
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    redreuben redreuben

    Reminds me of an IOR boat I sailed on, rating bumps were epoxy bog and ping pong balls !
    RR
     
  5. mikereed100
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    mikereed100 Junior Member

    Alik,
    The designer is a fellow named Copelli, formerly worked at Jeaneau according to the PO. 3 boats were built in Sete, France, one of which was later destroyed by fire, the other was last seen cruising the S. Pacific.

    Red,
    Ping pong balls, why didn't I think of that! Yes, foam would be my preference over ply, but sourcing the stuff in Thailand can be a challenge. I've done it before but would not like to repeat the experience. I will try again, this time through the boatyard, hopefully with better luck. The ply and polyurethane foam is plan "B".

    Mike
     
  6. Alik
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    Alik Senior Member

    Thanks Mike; my first impression was that You are going to build Your boat using this technique. I have seen some in operation and there are a lot of disadvantages...
     
  7. redreuben
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    redreuben redreuben

    Just get it sent from oz ?
     
  8. sabahcat
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    Location: australia

    sabahcat Senior Member

    I take it the Arnie Duckworth sailing in Thailand

    is the same Arnie Duckworth from ATL Composites in Australia

    They have Corecell Foam as well as Airex and Coosa whatever that is.

    They also have distributors in
    THAILAND
    East Marine Co.Ltd.

    Contact : Scott Bradley

    23/159 Moo 2
    Tambon Koh Kae
    Phuket 83200

    Ph: +66 76 239 113
    Fax: +66 76 239 132
    Email: info@eastmarineasia.com
    http://www.eastmarineasia.com/
     
  9. redreuben
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    redreuben redreuben

  10. mikereed100
    Joined: Jan 2007
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    mikereed100 Junior Member

    Sabahcat,
    Thanks for that. I tried to order some Corecell through East Marine a few years back but did not have any luck. I got the impression that while they are technically distributors they weren't much interested in retail, although things may have changed since then. I did get some Divinycell h-80 from the distributor in Chonburi but it was a nightmare and, as Chonburi was a duty-free zone, I had to pay duty, which is always as painfull as possible in Thailand. I plan to haul out at PSS in Satun and they tell me they can get foam so hopefully they are right.

    Alik,
    I hear you concerning the disadvantages! The flat bottom planes easily and it tacks like a fin keel monohull, but the pounding really is unacceptable. I will happily sacrifice a bit of speed and slower tacking for a little peace and quiet. Hopefully the addition of some vee forward will help in this regard.
     
  11. Alik
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    Alik Senior Member

    Proposed difference in deadrise in original shape and modified V will have almost no effect excerpt of extra weight on speed. But You are getting a headache with attachment part that is joined by secondary bonding. Also resale value of boat will be lower once it is not original and looks 'modified'.

    Materials - no plywood; use foam sheets (can be DIAB also produced in Thailand). We did similar surgery on multihull but that one was on wet deck - adding wave breaker wedge.
     
  12. Alik
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    Alik Senior Member

    East Marine is a subsidiary of Precision Shipwright Services specialised in refit and repair; the build some cats also (2 from us now).
     
  13. sabahcat
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    sabahcat Senior Member

    I'll be keen on a report on PSS

    Its where I was planning on getting my boat to after launch so as to get the finishing touches internally after a 6 mth shake down cruise from Brisbane to there.
     
  14. cavalier mk2
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    cavalier mk2 Senior Member

    I'd think about making the stem deeper yet with more v in the forward section. In any case your center of lateral resistance will move forward a bit so you may have to adjust the rig forward. Rake might do it but your stays may need alteration if it is a big change.
     

  15. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Basically agreed. I think, I once saw Farrier doing it similar. However: Even then, you will need two moulds - and it would be possible to shape them the right way.
    But the boat here and now was already made. :(
    To my understanding you are familiar with buoyancy and trim calculation.
    Did you consider to simply remove the wrong shape and replace with an improved body? On first sight that looks like much more work, but building up a new shape around the 'old' hulls also requires quite some time for adjustment, alignment and fairing.
    The idea of casting foam contains traps: You will not be able to discover voids and there is a clear risk of over-expansion when too much foam is poured, or when it meets narrow spots.
    Plywood: can be tricky, because(in 'our regions') you must be faster than the termites. And local 'Marine Plywood' is a somewhat relative description of what you REALLY get ;)
    - - -
    Getting material from overseas: We recently received a complete container full of nice things. The local retailers could not offer us a price advantage.
    'ATL' was mentioned before, and I must say that they are extremely supportive. That does of course not mean that the others are not...
     
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