If your boat is anything like my B36(2002), the hull thickness varies from one place to another. The average hull thickness may not be what you need to know rather than the specific thickness in the locations where you intend to fit your new through hull fittings. The existing hull fittings are easy enough to remove for you to measure that thickness at a time when your boat is out of the water, but what I can say about the Tru-Design through hull fittings is that they are more than adequately long enough to go through a modern glass fibre hull. Years ago glass fibre boats were considerably over engineered with hulls even on a 27 footer being in excess of one inch in thickness as I found on a boat I had that was built in 1969. Back then it was unclear how long a GRP hull would last, so they built it strong and added a bit more for good measure. The result was a boat built to last practically for ever.
Going back to your request for information, don’t worry about whether the new fittings will be long enough, the people who make them will have done their homework and figured out that if they weren’t long enough they would get a lot of return parcels. I have installed these valves and through hull fittings on my B36, and the only comment I have was that the fittings, particularly the valves themselves, were of a size that it was tricky to get them in without some imaginative thinking resulting in tightening the through hull to the valve after the valve was in place, and then tightening the internal collar for the through hull fitting last !!!
As for those backing plates, the through hull comes with a backing plate that is screwed up from inside the boat. The collars are up to you, I didn’t fit them as the fittings struck me as being of sufficient strength on their own and too complicated to fit in the very limited space provided on my boat. In addition and because of the very limited space, it is highly unlikely that I would want to attempt any modification that might impose any loading on those fittings - my choice !
