And what is your explanation for this theory? The idea behind a backing block is to distribute the loads over a larger area to prevent flexing of the deck or hull.
I was referring to elastic deformation, rather than plastic deformation. A material that deforms a little elastically
(i.e., it springs back to its original shape once the load is released) will often absorb a HUGE amount of energy (this is known as "historesis" in materials science). This is also part of the reason why fiberglass has a high strength to weight ratio. It is true that if the material is stressed too much the deformation will be plastic
(i.e., it won't spring back) rather than elastic, or it will just plain fail. This is true for ANY material, be it wood, polymer, metal, et cetera. So the "trick" is to make the object in question thick enough to absorb loads in the elastic part of their stress/strain relationship. Too thin, and it will yield and deform plastically, too thick and it will transmit (rather than absorb) the energy. In fact, I imagine that when backers are engineered by real engineers (rather than guys bantering back and forth on sites like this) they intentionally make them thin enough to elastically bend just a bit toward the high end of their design loads, thus absorbing shock loads when necessary.
Personally, I think that stainless or aluminum are probably the best backers (other than for thru-hulls, where epoxy-coated plywood would be my preference). But they're heavier than other materials, harder to drill (particularly stainless), and a PITA to fit to anything but a flat (or nearly flat) surface. I've never used fiberglass as backers, but I see nothing wrong with it. In fact, it may well be better than either wood or King Starboard. I only observed that I've seen Starboard it used on several boats. It's very easy to work with, and I've never seen it fail. I don't think I would use it for applications where the hardware is under long-term stress, however, as it is probably somewhat viscoelastic (i.e., it is elastic in the short-term, but "flows", or deforms plastically, if stressed for a longer period).
Contrary to popular misconceptions often times a properly thick fender washer can be more than enough backer depending upon the particular item being backed and the deck construction under it. Hinckley B-40's for example had the hull to deck joint bolted with regular washers but each flange, hull & deck, was over 1/2" thick solid fiberglass with no core. For a stanchion through a cored deck you want to distribute that load but for some item's a backer may not even be necessary.
I agree with you there, provided that the underlaying deck/hull is either solid or plywood core. Particularly on some older boats that were build with pretty overly thick scantlings in the first place. My Cal 2-27 had no backers originally, although I've added some (the life-line stanchions are on "the list"). Still, in her 36 year life, none of the original installations have ever failed as far as I can tell (although the aforementioned stanchions have some spider cracks around their bases).
Me too and it most often splits along the grain right under the nuts but it never should have been installed with the grain oriented that way to begin with.
Exactly. In fact, that's the basis for the trick you see in martial arts demos: the boards they break are quarter-sawn, so they split along the grain. I doubt you'll ever see anyone do such a demo with plain-sawn wood, much less plywood.
As mentioned IPE, Black Locust and a few others are better choices in a hard wood, but coated marine ply will not crack.
Just curious, have you ever used Apitong? I bought some to use as a rub-rail on my tender (a project that's still on "the list"). It's supposed to be very rot and wear resistant, although it isn't a particularly pretty wood. The guys at the lumber yard said that they mainly sell it for truck beds and the "fencing" on stake-bed trucks. So, it must be able to handle staying out in the weather.