Hi John,
As Keith says if you have insulation below the wooden slats there shoudn't be a problem but by the sound of your post I don't think that you have insulation below the timber and it's wood direct on to the boiler.
I had a similar problem with my Scotch return flue boiler in a tug but it was the burner that was doing the scorching of the hull as the burner went underneath the smoke box so there was no protection to the hull.
As the boiler had been in the boat for a number of years and the insulation was beginning to get a bit taty I decided that it was time for a refurb.
In the U.K. we have suppliers that stock boiler insulation for the Loco guys, it's Ceramic wool 1/4" thk but compresses to 1/8" thk so if you contact your local Model Engineering Society who run locos they could point you in the right direction to find a supplier.
I used the ceramic wool and recovered the boiler and held it all together with Stainless steel from an old pedal bin, which was .015" thk, cut all the appropriate holes and held it all together with cable zip ties, left an over lap at the underside of the boiler and used small self tappers to hold it all together before removing the zip ties.
As you can see in one of the pics there are 4- blobs of Milli Put placed in the hull and the boiler was pressed down on them while still soft, this created a cradle with a small, air space below, you can see the scorching from the heat of the burner.
The boiler is 4.75" dia and 5" between the end plates with 19 return tubes from the fire box to the front smoke box, it's all a bit of a squeeze but now there isn't any problem of heat transfer.
If you have the space you could do similar to your boiler and you would have no fear of a fire, just a suggestion.
George.
