It happened like this. We approached my brother in law who is a retired professional painter and decorator and asked if he would be interested in earning some money by painting some rooms in our house. We would of course pay the going rate. He was interested but suggested a different plan. If I would fix his beloved little fishing boat he would do the work on our house. We agreed as it was a 'win - win' situation and a fair trade in terms of work time equivalence. Each party would pay for the materials required; we for our house; he and his wife for his boat.
The only solution I could think of apart from filling the whole area in with huge amounts of 'bog' (filler), was to build up the hollow areas with plywood (laminated in strips and held down with temporary screws) then plane the plywood down, fairing the offending areas into the lines of the hull.
I completed the job by giving the area a coat of resin.