8m trailerable displacement powercat design

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

  1. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    20200425_161902.jpg Hi all.
    Very longtime lurker of these forums, and new member with my first post, and a couple of questions too.

    I want to build a 8m x 3.6m lightweight displacement power cat, using sliding aluminum beams to reduce the beam to 2.4m for trailering
    • Usage will be 2-7 day fishing and surfing trips,
    • very minimalist, camping style
    • 1 - 4 people
    • Economical to build and operate
    • Light and easy to tow
    • Comfortable at anchor
    • Under 1750kg dry on the trailer
    • 16:1 hulls, 8.0m x 0.5m
    • Plywood and fiberglass construction
    • 2 x 20-30 hp 4 strokes
    • High bridge deck for ride comfort
    • Sliding beams
    In a previous life I was a custom alloy boat builder, with experience in composites as well. I am well aware of build times and this limits the boat to this size.

    Previously I was looking at this build through Aluminum tinted glasses but 4 weeks of Covid19 enforced isolation has changed my thinking into using plywood for weight savings and quietness at anchor.

    I was struggling to get the numbers to work in Alloy without being to chunky underwater, and high on the trailer.

    Working on a displacement between 1000kg and 1300kg depending on final layout and load.

    I have made a very rough scale model to see how it would look, along with some very rough sketches.

    So my questions for those that know this stuff;
    • Will 3 x 150mm alloy tubes be enough to stop twisting of the hulls in rough conditions. Mounts will be suitably strong tied into full bulkheads.
    • Where is the best place to find and calculate ply scantlings? Aluminum I would design this in my sleep, but wood is foreign stuff....
    I have read all the threads on this topic, great inspiration from Richard Woods, Groper and others. Plenty of stuff bouncing around my head, if anyone has questions.

    Cheers Brendan
     
  2. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    Some more pics 20200425_162357.jpg 20200425_162405.jpg
     

    Attached Files:

  3. Ad Hoc
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    Ad Hoc Naval Architect

    Welcome to the forum BrendonfromNZ,

    You should read the article on Proboat Magazine, Dec/Jan 2019, issue 182. It discusses all these issues.

    The main issue for you is this:
    upload_2020-4-29_8-44-44.png

    This beam doesn't do much. It is too close to the centre of rotation of the 2 hulls; as they rotate in opposite direction in a quartering sea.
    upload_2020-4-29_8-48-34.png

    Which tends to be worst case.
    It provides stiffness only in a transverse bending moment case, but not in the pitch connection moment.

    Thus it is better to make the 2 outer beams take the loads for the PCM and then check against the TBM.

    But your key driver will be displacement, not stress.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Ad Hoc, if his central tube is locked in place, it would contribute by torsional resistance, would it not ? But I am curious about the yawning gap when at wide spacing, what kind of concertina arrangement do you envisage, Brendan, and if that locks into place, would be a big help in tying up the structure.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Of course it would, that's blatantly obvious, any structure held in place, provides resistance to an applied load. But that is not addressing the question....since without being secured in place, what is it doing anyway ...nothing, just rotating!

    The question is, THE TUBE.. the method of attachment is secondary.

    The torsional resistance of the tube given the applied loads is insufficient by a long shot... If you calculate this yourself rather than guess, then you'll know...
     
  6. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    Thanks Ad hoc + Mr E for the replies.
    Sorry I meant 3 tubes at 150mm dia. Hadn't decided on a thickness yet.

    I see your point, the centre beam doesn't do much, only as the fore/aft beam distortion would allow it.

    Hull beam at sheer would be 600mm, but the anchor bin structure included would allow tube connections spaced up to 1100mm.

    I like the idea of cross beams fixed near the transom, and fwd of the cabin as they can bolt through into drained cavities, not into the sealed hull structure.

    I thought long and hard about other ways of folding, but felt simple tubes would be easiest, short ones for transport, and long ones once launched. Admittedly I did most of my thinking about an alloy version, so I could revisit this.

    Cheers Brendan
     
  7. clmanges
    Joined: Jul 2008
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    clmanges Senior Member

    Brendan, I'm curious; how will you fill the gap in the cabin? Also I'm curious about how you'll control the steering for both sides in unison.
     
  8. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    Hi.
    I'm thinking composite panels folding/ sliding in and fastened down. 2x 600mm wide panels per section.
    Fwd would be tramp.

    Cabin I'm thinking a framed tent for weight reduction and watertightness. The sliding panels will be hard to seal, particularly around the cabin angles.

    Steering and remotes should be easy, cables and hydraulic lines will loop up slack when hulls are retracted, but still operable.

    Brendan
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    So, the reason for the adjustable beam ( a rather big complication, construction-wise) is what ? To garner more internal space ? To spread the hulls further apart, to reduce interference drag ? Sounds live a nightmare to me, but we know there are some around, and have you studied any of them ?
     
  10. fallguy
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    fallguy Senior Member

    If you go away from the tube; it is easier to avoid the rotational issues; done right.

    many designs utilize a rectangular beam

    round is perhaps the most difficult choice for this issue
     
  11. BrendanfromNZ
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    BrendanfromNZ Junior Member

    Mr Efficiency - yes it does create some issues....
    Factors;
    • Main cruising area is 200 km north of here, windy NZ roads
    • No desire to moor an old boat, or travel at 6 knots
    • $2 - liter unleaded
    • Ability to sleep (up to) 4 without V Berths
    • Stability
    • 10 knot + cruise
    • Soft ride, I'm not that old, but already my back hates planing hulls
    Last years design spiral solution was a 7m x 2.5m along the same lines, built in light aluminium.

    My calcs for that are 4000 miles away at work, but the limitation was to keep a sensible L/B and draft (@ 1100kg) with such a short hull.

    Worked fine with 1 or 2 persons, but 3+ really killed it. Would have been easy to power, with a single 40hp four stroke

    All this talk of Interference drag scared me....
    I think I've studied most of what's out there, Richard woods designs are good, but not quite what I'm after.

    I have a small plate alloy boat already, that's my go to for local trips, so the cat would only be used for 2 +day trips, and 30 min setup wouldn't be a problem.

    Fall guy -

    I have looked at scissor type, telescopic tubes and straight tubes, and that seems the most simple with less risk of binding up.

    Small sail cats seem to have crazy B/L ratios and still work, but I imagine the mast stays create triangular stability?

    Again most of my design stuff centered around alloy construction, thinking now in Ply composite, could revisit the scissor type arrangement. Fabbing alloy components is no problem.

    Cheers Brendan
     
  12. fallguy
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    fallguy Senior Member

    I, personally, would opt for a plywood Wood's boat with folding hulls.

    The Skoota 24 is probably my all time favorite boat. I am building bigger for a trip to Alaska and to keep wife happy.
     
  13. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Nice model! You could check if you can get a oversize permit for a trailer. Up to 3.1 meter might be relatively painless. Maybe that is wide enough?

    If you're worried about wave interference you could try testing this in prelimina.com or michlet.

    Have you considered a power trimaran? Smaller amas are easier to fold.
     
  14. BrendanfromNZ
    Joined: Apr 2020
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    BrendanfromNZ Junior Member

    Dejay -
    yes considered the overwidth trailer, can be done semi legally here, but the roads we travel are very windy and narrow, it wouldn't be safe.

    I did sketch ballasted mono's and little trimaran as well as part of this. I think if you were solo, the tri would be hard to beat.
    but the cat wins for me on big deck space and wide comfortable sleeping area.
     

  15. brendan gardam
    Joined: Feb 2020
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    Location: east gippsland australia

    brendan gardam Senior Member

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