Trailer Build...

[steve.h]

HI guys,
Ok, so I need to rebuild a trailer I use for camping, I intend to rebuild the chassis using 25mm x 50mm box tube and reuse the suspension/hub units, wheels/tyres and tow hook and so keeping it to a refurb rather than a new build. I want the body to look nice s well as last as long as possible, I am thinking of using 9mm ply for the base and 6mm ply for the walls and top.
I thought I would use the stitch and glue method as used for building boats, drilling holes along the edges of the panels and tying thm together with small cable ties, then use a resin putty to bond the panels together with a fillet joint and topping it with chop strand tape to complete the joints, this will then make the box a monocoque design without a frame.
My question to you all is how many layers of 300gms m2 chop strand mat should I apply to the outside of the box to give it strength, I intend to use a thinned coat of resin on the ply to help strengthen the ply and assist in adhesion, but I am unsue of how much fiberglass I need to apply, I intend to make a lid for it and use gas rams to keep the lid up etc, the lid will also be made in the same way.
I have looked at the stockboard and similar options but I believe this option should give a good result and be slightly cheaper and good strength to weight etc.

Thanks for your help and advice.

Steve.

 

[KemppiFrog]

Hi Steve,
Looking at Devlin's Boat Building (since you plan to use his prefered method) He suggests 3 layers on the inside on top of the fillet. First two staggered, third centred. Then one layer all over the outside of the 'hull'.

Now my thoughts It is a very messy and fairly expensive method which suits the curved chines of a boat. Much cheaper would be to use stringers in the corners and some tape on the outside. Round off the outer corners to avoid a sharp edge which the tape does not like going round.
You mention resin putty and chopped strand mat (CSM), I am assuming you are talking about polyester resin. In the boat world this is a no-no nowdays, it has poor adhesion to wood and is not very watertight. The latter can be sorted by paint/gelcoat/flo-coat. But the adhesion is a problem if you aim to keep it longterm. Epoxy resin is the stuff of choice, more expensive, but lasts forever and sticks to wood like the proverbial. Hence my comment about filleting being expensive, you need to use epoxy. If you use stringers, you need much less epoxy. BTW, while there are CSMs suitable for epoxy, most you can buy easily are not. The styrenes in polyester resin dissolve the binder in the mat so it copes with double curves. Epoxy does not dissolve the binder, so it is a PITA to try it. Woven tape is the usual stuff, and good for what you are doing.

Not sure of your trailer design, but if using 9mm for the floor, I would add some strips to double up under high load areas. 6mm sounds fine for the sides.

Alternative would be to use Polyurethane glue for the wood joints and epoxy/tape for the outside.
For water proofing the ply there are some good water based (non toxic) epoxy coatings. Get back to me if you need sources.
KF

Both my boats are completely epoxy glued, no screws or nails.

The Mirror dinghy was originally stitched and taped with polyester, not too many left built with that method.