How to modify polyethylene hulls?

[llamalookout]

I really want to add dagger boards to my polyethylene windrider 17 foot trimaran, by sticking a couple of boards from a 16 to 18 foot beach cat through the outer hulls.

Since nothing sticks to polyethylene I'm guessing the best way would be to make the dagger board trunks out of polyethylene and then have them professionally plastic welded in place. But I'm not sure if its practical to create extra parts out of this material or if the plastic welding is good enough.

Alternatively the trunks could be made from composite, but then there would be issues of how to bond them to the hulls.

Any ideas appreciated!

[MikeJohns]

In my experience they do not weld well. Probably best to cut the slot and fabricate the trunks out of metal or FRP and fasten them through a generous flange bedded with sealant. For fasteners use dome headed bolts or even some decent rivets.

[waikikin]

Just to add to mikes suggest' I reckon in addition to the "generous flanges", generous matching backing plates/rings(that go on the outside) so the PE is sandwiced in between flange & ring, obviosly either the upper(probably best) or lower cut out would need a bigger flange & cutout to allow easy insertion of the case & inner flange arrangment, you could "save" some of the deck cutout to make a "make up" positioning packer or if using composites mold a rebate to do so. Also an alloy tapping plate could be incorperated into the inner flanges for easy outside fixing of the outside to inner allowing for some countersunk to flush metal theads to go thru the nicely faired outer plates of alu or glass or more glamorous composite of choice. Hope that reads sensibly & regards from Jeff:)

[llamalookout]

Well the rest of the boat is in aluminium so that would work well.
But it would be difficult to get it to match with the complex hull shape.
Composite would be easier from this point of view and could still be attached using bolts.
I'll get in touch with a poly welder and see what they think about doing mods in polyethylene,
if they have ultrasonic weld inspection equipment then it might be OK.

[marshmat]

Composites in general don't get along too well with concentrated point stresses, ie. bolts. You'd still need backing plates
I'd say your best bets are metal trunks with bolts, or polyethylene ones thermo-welded in place. Afraid I'm not that familiar with this type of thing though.