efficient 10m displacement powercat (build thread)

Discussion in 'Boatbuilding' started by groper, Apr 15, 2012.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, so you are getting some pounding in head seas, presumably from the transversely flat, and rising bottoms forward ? I was a little bit concerned about that in post #19, It may have been better to go deeper at the forefoot, and maybe fit shallow skegs aft to counter any bow-steer, but you would have lost speed doing that, though better motions going into the seas. Ya can't have everything.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect


    Keep it round bilge all the way matey.


    That would be because of this region being:

    GOPR0631_1465051086546_high_zpsxiykypp5.jpg

    1) so blunt and 2) so low down, the wet deck freeboard is too low. It’ll always slam. If you need it for the accommodation…then make it like a pointy 3rd bow…and add a nice Vee into it to prevent heavy slams.

    Nope.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    He did not say the slamming was under the deck, though.
     
  4. brian eiland
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    brian eiland Senior Member

    CS hull form

    Are you saying that the stern sections of the hulls pound?

    If that were the case then I would assume a number of the Malcom Tennant CS hull designs would do the same,...but I have never heard of negative comments as such on those hulls??


    http://www.webring.org/l/rd?ring=multihulls;id=1;url=http%3A%2F%2Fwww%2Ecatamarans%2Ecom%2Fnews%2F2006%2F04%2Fcatcomparison%2Easp


    http://www.boatdesign.net/forums/motorsailers/cs-hull-sailing-performance-35387-2.html

    http://www.boatdesign.net/forums/multihulls/hull-shape-cruising-power-v-sail-cat-44023.html

    [​IMG]
    CS hull form.jpg


    MotorSailer:!:
     
  5. groper
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    groper Senior Member

    Adhoc,

    The bridge deck almost never slams, that's one thing I got right was the high clearance. It looks low and blunt because it's actually very deep in the accommodation area so we can sit up in bed etc and it made the cabin look lower and sleeker by having the deck level higher. The hulls are very deep, I have nearly 7 feet of headroom whilst standing on th4 false floor inside...

    The pounding comes from the bottom of the front 1/3 of the hull which is flat bottom with rounded chines. It's just noisey. Don't get me wrong here, the seas which gives us trouble I doubt you would get any boat of similar length to go fast in, unless you like getting fully airborne and have your belongings flying all through the cabin you c a nt go launching off 2m waves at 20kts fly thought the air and go crashing into the next one... so you have to slow down no matter what your hull shape is...

    For the most part, it goes really well, just a bit noisey like banging sounds, when we get conditions where the forefoot comes clear of the water and punches into the next wave. If the forefoot was round or veed, I think it would still pound in th3 same conditions but do it with less noise...
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Does trimming the motors in help at all, Groper, with this ? Should do a little, imo.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Gotchya...

    Ok..in this case yes, making those fwd sections into a Vee will help and keep it round bilge not hard chine. Beyond that not much else you can do. Unless you wish to experiment with a fwd fixed/passive fin.
     
  8. Barra
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    Barra Junior Member

    How did your children find the motion?. I'm assuming you didn't leave them at home alone all this time.:)
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    One difference would be noticed in nudging onto a beach, it would ground further out.
     
  10. brian eiland
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    brian eiland Senior Member

    Thanks for that clarification. Somehow when I first read your first posting on this slamming, I thought you were saying that some of it came from the aft portions,...."The main thing I would change is keeping the round bilge further Aft before flattening the bottom".
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    A greater radius would have achieved that.
     
  12. groper
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    groper Senior Member

    Near the stem it's round, and transitions to a flat bottom U as it runs aft. What I should have done is kept it semicircular until mid ships, before makin the transition to the flat bottom... hope that makes sense? That's one thing I would have done different which is what I was asked...

    In all but nasty headseas conditions, she gets along nicely. We have never had to slow down in following seas or seas from the sides, we can surf down 3m waves in 25kt winds at 22kts all day long and do it very comfortably with autopilot on all day. We made 200NM in under 10hours on 1 occasion in PNG outside the reef I'm these conditions. Occasionally we woukd see water over the deck and up onto the windscreen, but otherwise very comfortable and tracking very straight.

    The other thing I woukd change is the method of propulsion- wish we had sails , but we might have to save that for the next boat :)
     
  13. brian eiland
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    brian eiland Senior Member

  14. groper
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    groper Senior Member

    No way... I was thinking more like the Sig45 by lebreton or the flying fish 50 design by grainger :) superlightweight , powerful rotating rig, minimalistic construction using a couple small outboards for berthing...

    This boat was always about learning to build boats and proving methods. I've learned enough to build a really good sail boat and do it quickly, so building catch 22 has more than served it's purpose...
     

  15. brian eiland
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    brian eiland Senior Member

    My error, I thought you were primarily power oriented.
     
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