I've tried various ways to weld boats, mostly using a propane torch (benzomatic type - get from any hardware store). My most refined technique is this:
Needed: propane torch, rods of plastic repair material; maybe some 'chips' of plastic, metal spatula. A piece of aluminum about 1/2" wide works for a spatula, but wrap the end you hold so you don't burn your hand. A small putty knife would also work, but be sure the it's insulated from the heat. The size/shape of plastic depends on the size and shape of repair. I've most recently done a bow crack, and it was handy to cut some 'flakes' to insert into the crack.
Here are main steps
1. Warm area to be patched, preferably in hot sun. You want to minimize the amount of heat you need to apply. When you're ready, place the boat so the patch is easy to access and be sure the boat is stable. You don't want the thing falling over in mid-process.
2. Heat the area of the repair _carefully_ with torch. You want to get the plastic hot and soft, but not scorched (it shouldn't bubble) and you don't want to open the crack by heating it too much.
3. Now heat the rod gently in the flame, and then quickly heat the metal spatula. Press the hot plastic rod into the crack with the spatula, applying heat as necessary to the spatula, melting the repair material and the boat together. Start slowly and carefully, and you'll get a feel for this. The intent is to put the flame on the metal, not directly on the boat, and use the metal to transfer the heat.
I put extra material around the crack to try and reinforce the area. Especially check the edges of the patch - it's easy to just melt the repair plastic and if it's just smeared on, it'll probably peel, potentially taking the rest of the patch with it.
I prefer to do this in a hot garage. It's a lot easier to see the flame indoors, and it seems like it's easier to get the temperatures 'right' when the boat hot. I haven't repaired any large cracks in the middle of a boat using the technique, but I think it'll work for that.