Trampoline Attachment Points

Discussion in 'Multihulls' started by Jetboy, Aug 10, 2015.

  1. Jetboy
    Joined: Feb 2012
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    Jetboy Senior Member

    I'm at the point of needing to build attachments for my trampolines on a small 18' trimaran.

    The design is pretty simple - the float has a long awning extrusion (bolt rope). The beams are alloy tube, so I can either use big velcro straps, rope, or whatever.

    The boat side is my main concern. I don't have anything built into the hull side. So my options at this point are some type of fastener spaced every foot or so like deck eyes. Or a strip of some type of track along the side with cars or slugs to tie into.

    What would be the best option?

    What about something like this:
    [​IMG]
     
  2. waynemarlow
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    waynemarlow Senior Member

    Too complicated and expensive. I plan to use a shoulder washer of say 10mm diameter ( http://www.fastenercomponents.com/washers-shoulder-washers-screw-insulators-spacers.php ) through from the inside of the hull, super glue it in or use one of the proprietary plastic to composite glues to lock it in place. Now bond a smaller shoulder washer that has an outside diameter the same as the inside diameter of the one bonded in the hull. Now simply feed a loop of D12 with a chinese knot at the inner end through the hole, yes you may get a series of chinese knots along the inner part of the hull but they are quite decorative and no more obtrusive than a nut.
     
  3. Jetboy
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    Jetboy Senior Member

    Thanks. Might be a good route.

    Basically something like this?:
    [​IMG]
     
  4. waynemarlow
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    waynemarlow Senior Member

    Yup, that through the nylon shoulder washers ( to spread the load out into the hull and to stop chaffe around the exit point ) or much much cheaper make your own loop with a chinese knot ( lanyard knot ) on the end ( easier than it looks on youtube https://www.youtube.com/watch?v=fKLLolTlvKA )

    That loop you have shown, I have lost the link for it and have been looking for the manufacturer as it has great potential for making inner jib stays for running a storm jib up.
     
  5. Jetboy
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    Jetboy Senior Member

    The trade name is "ropeye" one word. They make a range of sizes, but I have no idea what the pricing looks like. For our little boats I suspect a thick stainless or aluminum fender washer as a backing plate might make a close DIY alternative.
     
  6. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    If it is on a composite hull side I would go PVC tube glassed over with one layer of 440 or 600DB. Then cut slots in with an angle grinder to suit the net.

    Really strong on ply (not so much on foam - I have seen one tear off the foam skin) cheap and good looking. If you want to strengthen it up then stitch the high load points on with some uni passed through the hull side and back around again and again. Stitching is super strong.

    Cheers

    Phil
     
  7. Jetboy
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    Jetboy Senior Member

    I'm kinda late in the game for adding a PVC glassed tube unfortunately. Hull is already painted and basically finished. Just down to the last steps of adding the trampoline and finishing the rigging before she's ready to splash. I agree that that would be ideal.

    That was why I was kinda thinking of using aluminum track. It's easy to mount with a full backing plate and spreads the load nicely. I've got about 25 feet of 1/8" x 2" wide aluminum flat bar that I could use for continuous backing plate on an aluminum track. So it would cost about $100 - $150 for the track and attachment rings, and easy installation.
     
  8. tspeer
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    tspeer Senior Member

    You might want to use an omega track. You glass a tube to the side of the hull and cut out the tube at regular intervals. Then insert a fiberglass rod down the tube. You'll have a slot between the rod and hull where you've cut out the tube so you can lash the net to the rod.

    Or you can wrap carbon or fiberglass around a rod and then drill holes behind the rod for the lashings. Here is how it was done to attach the nets to the beam of my boat: https://flic.kr/p/56nLAF https://flic.kr/p/56nLHv
     
  9. Russell Brown
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    Russell Brown Senior Member

    I'm interested in your boat, Tom. What do you sail and does it "float your boat"?

    As far as trampoline attachment points to go on an already-painted boat, think about a heavy carbon or glass angle bolted to the hull surface with holes on the other face for tie points. I did this recently and it worked. It could also be two lighter angles glued together to make a T shape for more even pull....
    Russell
     
  10. markstrimaran
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    markstrimaran Senior Member

    Trampoline to Aka mounting . chaff problem.

    My sides are rigid enouph. I have a problem with prematurely chaffing my 5/16 cord on the fiberglass holes. Would a eyelet Chinese knot thing work?
     

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  11. tspeer
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    tspeer Senior Member

    I have a Chris White designed Hammerhead 34, built by Lonestar Multihulls. It's a good boat for me. It's a cruiser, not a racer (about the same speed as an F24). Solidly built. I did the Van Isle 360 with it in 2011, and made it all the way around but we were typically one of the last boats in each day. The monohulls walk away from me in the light stuff, but when it's blowing more than 10 - 12 kt, the boat moves out.

    There are a number of things I want to change. First up is a running baby stay to the foredeck. When using the second reef in the main, the mast pumps and inverts alarmingly, making the second reef unusable. A baby stay to the spreaders would triangulate the middle of the mast with the lowers and make it bullet proof.

    The steering is well balanced when under sail, but is way over-balanced when under power. The boat has a large foretriangle, placing the centerboard well aft. At low speed the bow will blow off the wind and there's not enough rudder authority to bring it back. As a result, in a marina it is easy to get into situations that I can't get out of (like being crosswise in an aisle, drifting toward shore), and maneuvering in close quarters can be hazardous. I'm learning long warps are my friend.

    But on the whole, I like the boat and it's one that my wife and I can handle. I'm looking forward to actually having the time to get it out sailing!

    It's in Poulsbo's Liberty Bay Marina if you want to take a look at it some time.
     
  12. paxfish
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    paxfish Junior Member

    I can definitely see how chafe would be an issue there. The cord kinda needs to wrap around something smoother and bit larger in diameter. That adjacent tube might be the answer. Spectra or amsteel may help also.
     
  13. Russell Brown
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    Russell Brown Senior Member



    Thanks Tom,
    It sounds like you have some bugs to work out. Has Chris been helpful?
    I have never seen a 34, but I thought the larger one was really well thought out and good looking.
    We just returned from a trip to the Queen Charlotte islands. A trip like that will make boat ownership make sense and make all the pain go away.
     
  14. Jetboy
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    Jetboy Senior Member

    Thanks for all the ideas.

    I might be overly concerned about tear out strength of core composite of the boat. It's only 12oz glass on each side of honeycomb core, so spreading the load as much as possible will go a long way I think.

    Lots of good ideas.

    I ordered the aluminum rail and have a bunch of aluminum flatbar sitting around that will make a continuous backing plate to see how it works. Worst case scenario it's only about $100 for the rail, end caps, hardware, and a dozen anchor rings. That should cover both sides for me. And I'm thinking maybe 2hrs to install it all and be ready to lace up the tramps.

    I'll post up some pics of how it works. It's 7075 rail and stainless hardware so I think it should hold up well to marine use. It's cheap. And should be quite strong. Maybe ugly? Maybe heavy? We'll see what it looks like in person tomorrow.
     

  15. tspeer
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    tspeer Senior Member

    I don't want to hijack this thread, but Chris has been helpful for the the items I've requested, like sending me CAD files of the surface shapes. I've not talked to him very much about the issues I've mentioned above. I may get him involved if I decide to do something more extensive, like replacing the centerboard and rudder with longer versions.

    The boat is well thought out and designed for its original intention, which was sailing in the Bahamas and Gulf Coast.
     
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