Good point. I had to modify a bunk to give some space for my transducer and saw on my Pacific trailer the wood was untreated. Glass boat so it doesn't matter for me but I wonder if the manufacture does this on all of the trailers for this reason.Not recommended for aluminum boats. The PT lumber has metals in it that are dissimilar to aluminum and can cause corrosion. PT is fine for FG boats.
Do not use preasure treated lumber it is made from Hemlock Fir and is not as strong as reg Doug Fir and it also has been compromised by the chemical and the holes put into it during the process and it flexes to much and can break. Carpet holds water and takes for ever to dry under a boat and has a lot of friction for launching and hauling. Cheers HD
Unless you're getting your PT from the east coast it's not Hemlock, it's Douglas Fir. I built docks and piers for years here in SoCal and it was always specified PTDF (Pressure Treated Douglas Fir) on the plans. You can either get it SFS (surfaced four sides) or incised. We used incised for under cribbing for better treatment penetration and SFS for decking, facia and hand rail material because of esthetics.