Plastic welding

Not sure this is going to help much, but I have welded rigid fuel and spray tanks using my Leister hot air gun. The big advantage is precise temperature control which is essential to match the welding stick and the tank so that both materials flow together. Check the plastic type letter on the stick and the dinghy to be the same.

As expected, Youtube has a number of videos, e.g.

welding pvc dinghy at DuckDuckGo

Lots of ventilation !
 
In general I have found that the thinner the material, the harder it is to successfully heat weld. The margin between hot enough to weld and hot enough to melt a hole is small.
The difficulty of getting heat, and particularly pressure, to the back of the joint also works against you.
 
I wouldn't bother without the right tools and knowledge, the PVC fabric is pretty delicate if not done correctly. Even if it was originally welded, most repairs would be done with the appropriate 2 part glue and patches.
 
I fitted and welded up my swimming pool liner. Solid 3mm thick PVC fabric. I've also welded plastic bumpers and other things needing repair. Very different from the thin pvc membrane on a dinghy. I'd stick to 2 pack adhesive.
 
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