Are you planing to race this? If so, check with the organization which sponsors or organizes the races, and see if they permit hulls which are coated with a pperformance-enhancing material. It would be awful to build the boat, win a race, and then be stripped of the honor by someone challenging the integrity of your efforts. Of course if you are just racing buddies with the loser buying the beer, well that's another matter

: I was planning on adding graphite to the lower hull panels, as it's 3mm ply
: with 4oz glass on the outside. Low weight is key. But I paddle in rivers
: with some flotsam/jetsam and snags, and hitting something at high speed is
: bound ot happen. I can't afford kevlar or carbon fibre, so was going to go
: the graphite route.

Adding graphite doesn't do a thing to protect against punctures from something coming into the hull at a 90 degree angle. But anything you encounter at a very small angle, say up to 5 degrees or so, will either snag the boat and scratch it, or, if the boat is slippery, then either the obstacle will be deflected, or the boat will slide past. The graphite makes the hull slippery, and it toughens the epoxy a bit so scratches are shallower. Combined effect is that you get less visible damage.

Think of the graphite particles as if they were a kid's marbles. Marbles are spherical, about 16 mm in diameter, and not too easy to glue down. Lets say you embed them in a 20 mm thick layer of cement, then sand that down to 10 or 12 mm thick. The marbles would be captured by the cement, and the sanding would give them flat faces. The surface would be mostly exposed glass from the marbles. The epoxy/raphite mix traps the graphite the same way.

Varnishing over this masks the effectiveness of the graphite. The opaque graphite protects the eooxy from UV.
Varnish alone would not be as strong of a 'glue' for holding the graphite.

Get a smooth hull surface with your fill coats first. The added layers of resin and graphite only go up to the waterline.

hope this helps

PGJ