8m trailerable displacement powercat design

Discussion in 'Boat Design' started by BrendanfromNZ, Apr 28, 2020.

  1. brendan gardam
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    Location: east gippsland australia

    brendan gardam Senior Member

    these sliding beams on my boat are elliptical mast sections. very simple but easiest to open with 2 people. they bind if you push the hulls out uneven.
     
  2. fallguy
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    fallguy Senior Member

    Elliptical is good as they can be strapped better than rounds.
     
  3. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Brendan, that's interesting the way that slides out, have you had it in rough conditions? No issues with retracting it in after?

    I sat down with a beer and my model and pondered a few things, like shorter with reduced beam, and came up with a question

    IF I was to make this 2.5m wide ( or even 2.7m) what would be the penalty with the narrow tunnel?

    Assuming say 1275 kg
    Hull w.l. 7.5m
    Hull beam 0.5m
    Draught 0.3m
    Ca 0.85
    Cp 0.65

    What would be the difference in hp requirements at 10 knot cruise given a beam of 2.5m versus 3.5m?

    With a single 40hp or 2 x 20hp, there's going to be plenty of power, with a narrower hull, I can drop the bridge deck a bit, but would lose the big spacious rear deck....
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You want something like the mechanism of a scissor-lift ?
     
  5. brendan gardam
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    Location: east gippsland australia

    brendan gardam Senior Member

    I only opened it up once and ot was a pain on my own because I had to keep pushing each end out even but would have been easy with 2 people. I haven't been out in it yet but the previous owner said it was solid. I have actually set it up with the beam at 2.7 mt so i can tow and launch easy. even though it is 200mm overwidth. I am copying a waller 570 in sail area and plan on fitting a 15 to 20 hp outboard. It only has a 6 atm but that pushes it pretty good. sorry i can't be more help but I have not had it long.
     
  6. fallguy
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    fallguy Senior Member

    Hull wake drag, but you basically have identified the classic reason for the planing 2.5m powercat.. personally, the beer was a good idea.

    the sliding beams less so

    that Woods 24' cat is the way to go

    if you don't like the aft cabin; you can build benches instead for the aft cokpit
     
  7. brendan gardam
    Joined: Feb 2020
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    Location: east gippsland australia

    brendan gardam Senior Member

    there are thousands of 2.4 mt beam powercats so i don't know why you think its a problem.
     
  8. brendan gardam
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    Location: east gippsland australia

    brendan gardam Senior Member

    the woods idea of folding the hulls under is nive and simple but deaws far too much water. in my area anyway the trailer would be off the end of the ramp before the boat floated.
     
  9. fallguy
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    fallguy Senior Member

    read up on wave interference in catamaran hulls; the planing powercats just apply more power and fuel to the situation
     
  10. fallguy
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    fallguy Senior Member

    ah...yes, the way Richard dealt with this is a retrievable trailer or by extending the reach at the landing...

    honestly a reach extension seems better than beam extensions; you get a piece of tubing and the extension just has a home on the trailer...

    but I so feel your pain, I really wish my boat would trailer simply instead of needing cranes to assemble.. I started to draw up a design for a trailer that would accomodate it all, but simply not practical. It would be more practical for a trailer that keeps the cabin and lifts it up and to trailer the hulls with a hauler
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    It isn't an issue at the speeds planing cats cruise at. But, the spacing of the hulls is also best kept within limits, to avoid some very lively motions at high speed, that would create some high stresses on the structure, not to mention the occupants.
     
  12. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    It's that this one is designed to always run at med / high displacement speeds.
    I notice those little cats that the use in rowing are about 7.5m x 2.5m and seem to go well, but those are ultra light displacement
     
    fallguy likes this.
  13. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Yes that makes sense, no need for too much beam as there are no sails, but 3.6m would be nicer to lounge on than 2.5m. The narrow hull is nearly taller than it is wide....
    Another thought is asymmetric hulls, straight on the inside face - as Richard Woods has done
     
  14. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    Yes that's a big part of keeping the design the way I have, it will be getting launched straight off the sand at times

    The boat you are building is big! I would love it, but would need mooring in my case. We tend to use boats in bursts, as NZ s crazy weather allows. 2 weeks of wind and rain, followed by 4 epic days of sun and light winds. Need to get on it!
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You might get away with a little asymmetry, but the flat inside face, seems not popular, and I suspect would weave around a bit in quartering seas.
     
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