There's one thing you guys are forgetting:

Rum !!! Who knows how much of it...

On a more serious side-

Plastics are combustible, fibers are not, they may burn but won't sustain the fire.
Rolf -if you are to make a spontaneous fire scenario you should take into account:

Source- maybe engine fuel, rum bottle or any other combustible material that can catch a spark and ignite

Materials that will sustain the fire - plastics, sails, gear, hull epoxy

Oxigen - if an opening provides fresh oxigen - it will raise the temp of the fire.

Temperature of the fire - once a certain temp is reached a "flashover" phenomenon occurs, at that point combustible materials starts to burn instantaneously, like paper, cloth etc

At some point the materials of the hull will deform and the structure will collapse very quickly, so the whole "thing" won't even have to burn because the hull integrity will be compromised, leaving afloat only the foams that have not burnt completely, and the other incombustible stuff.

For such a fire to grow inside a ventilated **** with some combustible materials close by, it won't need too many minutes to develop enough heat to deform the hull, that might appear as the hull burns much quicker, but actually a part of it sinks.

It's not necessarily the fire itself doing most of the harm, but the hot gases and raised temp that deforms the structure.

And don't forget the rum.