They have some a test plant doing this already (Italian outfit IIRC). Fairly successful with polyester resins, and re-used 50/50 with virgin resin gave good results. They were investigating epoxy pyrolisis as well.
This was done many years ago on the bank of the River Severn near sharpness , using redundant barges some made of ferro cement pulled onto the bank at an angle to prevent erosion .
It's been used in the Walton Backwaters, too - notably the NE corner of Horsey Island.
I think that the problem is that although we see it as being a big problem, in fact the quantities involved are too small for pyrolysis or even shredding for use as feedstock for cement because the economics just don't work out. How many new boats are built each year? Thousands, but probably not ten's of thousands. The weight of GRP in each hull is perhaps 2 tonnes, so we're looking at maybe ten thousand tonnes of waste per year, But that waste isn't concentrated in one place; it's distributed round the coast in large lumps that are difficult to transport, and mixed with other materials that would need manual labour to separate. The quantities wouldn't justify more than one or two processing plants, so you've got transport costs. If the end result was a high value material it might work, but it isn't - it's low-value feedstock. So the problem is an economic one - in general the cost-benefit analysis doesn't work out.
But you've still got the issue of transporting large, unwieldy lumps of material. Given that lifting and moving a boat any significant distance is a 4 figure sum, not to mention stripping other material from them and breaking the GRP up for feeding into the reactor, your other sources of plastic waste would have to subsidize pyrolyzing boats. It's an excellent solution for non-recyclable domestic waste, but I don't see it for boats. Very happy to be proved wrong!
