As for rigidity and structure, the plywood is equal in useful strength to the fiberglass, meaning, they are both absolutely solid and rigid for the purpose. And finally, as for longevity and strength, I have a boat that is 23 years old with untreated plywood in various places, including the old 1/4 inch puny backing blocks, and none of it showed any signs of significant decay.
I would disagree with this...
Here is a picture of the sink drain through hull that was on my '87 O'day 35:
I removed it because I noticed several small cracks in the bottom paint around the fitting. After I removed it I saw evidence that this through hull had been replaced at least twice (Black unknown sealant, which had been covered with White 5200). I presume that the reason for the earlier replacement was galvanic corrosion, and explained why the bonding wire (visible on the left) was not attached to this fitting. I also found that the genius that installed the latest through hull, put the NPS
thread of the through hull into the NPT
thread of the seacock...
The ½" marine plywood backing plate, however, had been mushed down by the nut and allowing the through hull fitting to wobble slightly, thus causing the little cracks.
While the plywood had not decayed, it did not support the compressive load that it was subjected to from the through hull fitting and the flange nut. It was definitely weaker than what I used to address the problem. Had I left this, I believe that I would have soon had a leak around the through hull, through the plywood. I believe that it would also have invited osmotic blistering around the through hull.
Because it was relatively new, I re-used the seacock, and the through hull, but added a flange adapter, which adapts from NPS to NPT.
I installed a new backing block made from 3/8" thick G10 (GP03) fiberglass cut into a 6" circle. You can buy a 36" x 6" sheet ready made for less than $25 here
. You could make 6 of them from one sheet, and your cost/per would be $5... I could have purchased the same material in ½" thickness for less than $50, but a friend gave me the 3/8" scrap that he had for free!
Because 3/8" is thinner than ½", I had to cut some of the threads off of the through hull. Because through hulls use NPS thread, you can easily do this. Just remember to put the flange nut on before you cut. Use the flange nut as a guide while cutting, and to help clean the threads after the cut is complete.
I used black 4200 to seal around the flange fitting, and you can almost see that the bonding wire has been reattached.
Here is the result;