8m trailerable displacement powercat design

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

  1. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Have you considered a trimaran? ;) A central hull would allow a corridor with standing headroom, desk and bunk space to the sides and you could be bridge it for a big sleeping area. Sorry, I'll shut up now haha.

    Ok, one more idea, you could do something similar with a catamaran:
    CatamaranMinimal v0 2.jpg CatamaranMinimal v0.jpg CatamaranMinimal v1 3.jpg
    Doesn't solve the sliding problem but removes the complication of a central cabin.
    BrendanfromNZ likes this.
  2. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Yes Dejay,
    I have considered a tri with fold out ama's. Means going from a huge rear deck to a tiny one. Great if seaworthiness is foremost, but this build will be a coastal boat.
    Tri was my favourite last year in my neverending circle of what should I build. If was to build something 12 meters long capable of heading to the islands for winter - that would be it!

    That cat is nice, good idea, but a bit large.

    My way of removing the complication of a central cabin was to use a heavy duty tent, I'm not a sailor so when travelling or anchored it acts as a nice wind break

    I've built a few cats over the years and wouldn't consider using the hulls as accommodation until about 11 meters.

    It's a fishing boat so I have to keep the windage down where possible, also on road as well.
  3. Dean2014
    Joined: Jun 2020
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    Location: Arkansas

    Dean2014 New Member

    First post noob so take this with salt... I've pondered a trailerable houseboat with two trailers, boats launched independant, then joined on the water. With your proto:
    Interlock beams slide to span the middle section. Cabin designed to join at center. Center sponsons could be separated slightly for a single center motor. etc..
    You might even make each half only 2m wide and really easy to trailer on narrow roads. I'm just so jealous of the big >20m ocean going solar catamaran beam widths...
  4. Steve W
    Joined: Jul 2004
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    Location: Duluth, Minnesota

    Steve W Senior Member

    I just stumbled on this thread and have not had time to read it all so can not offer much except that for perspective i used to own a Macgregor 36 sailing cat with 18 ft beam which had three beams in similar locations as those in the first post. The middle and aft beams were 6.625" od with 1/4" wall thickness, the fwd beam is the same od but 1/8" wall thickness. The aft beam has the traveler on it so has to take significant mainsheet loads loads which a powercat does not. The fwd and aft beams sit in troughs molded into the deck with ss saddles beach cat style with one bolt through the trough and beam wall to prevent them from rotating whole the center beam has just 2 x large bolts horizontally all the way though the trough and beam. There were over 300 of these boats built and have been raced hard at speeds into the 20 knot range, carried up to 16 passengers as day charter boats and one has crossed the north atlantic solo so are very well proven so I would think your 150mm beams suggested in the first post would be more than adequate. I built a telescoping trailer and was able to slide the boat closed to trailering width by myself relatively easily. It did require walking around the boat and working the ends in a bit at a time rather than pushing from the center but not a lot of effort involved.
  5. JoanTheSpark
    Joined: Jun 2020
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    Location: Australia

    JoanTheSpark New Member

    Have you considered hinging instead of sliding?
    Would give you the option to increase stiffness of the connecting beams, by using space frame sections (structural frame)..


    Only problem there is that it gets longer when you fold it to put it on a trailer.
    To solve that one could add another hinge point - which naturally makes the whole thing more complicated, heavier and adds more play - but maybe worthwhile:

  6. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Hi all,
    Thanks for all the new input, there's some interesting ideas in there, Joan I've contemplated centre hinged beams before, they have been used before with good success.
    Thanks Steve W for measuring those beams.
    I got frustrated with the sliding beam cat option , and descended into a spiral that ended back at the Trimaran folder option again.
    But now I'm back with a fresh idea.
    I pinned down some requirements...
    • Single larger central outboard instead of two smaller ones
    • Aluminum construction for toughness / faster build time
    • Preferably a hard cabin and fixed decking which was problematic with the sliding cat idea.
    New idea is a folding cat, 2.5m folded beam, where the hulls tuck under for trailering, but rotate out 90 deg to increase the beam to 3.35m.

    (Rough) drawings to follow....
  7. BrendanfromNZ
    Joined: Apr 2020
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    Location: New Zealand

    BrendanfromNZ Junior Member

    20200626_120007.jpg 20200626_121014.jpg 20200626_120330.jpg 20200626_120018.jpg

    Attached Files:

  8. BrendanfromNZ
    Joined: Apr 2020
    Posts: 15
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Hopefully that make sense.
    Apart from #3 that was weakness on my part.
    Cat has pontoon style hulls, fully sealed, pressed ribs to minimize welding where possible. Very stiff.
    Centre hull, with inbuilt cabin for diagonal stiffness, 4 x hinges per side, one at each structural crossmember. A locking system and some sort of diagonal cross brace to lock the transom end in.
    It ends up a little heavier, so have widened hulls to 0.55m beam giving a loaded weight of between 1350kg - 1600kg at 0.25 - 0.3m draught.
    Bonus is it drops the boat height on the trailer
    Negative is the hull side height is limited by the the folding geometry, I'm still working on this, but have added 0.5m extra hull fwd of the cabin to add more buoyancy
  9. BrendanfromNZ
    Joined: Apr 2020
    Posts: 15
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    Location: New Zealand

    BrendanfromNZ Junior Member

    Pic #4
    Shows the hull in the folded position (in red)
    Dotted red line shows the hinge / fold line.

  10. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Just buy Skoota 24 plans and find someone to adapt them to ally.
    brendan gardam and bajansailor like this.
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