Backing Plate size and material??

[Bill g]

I am rebedding my mid-boom traveler bridge risers and when I took it all apart I was surprised to find that the backing plates were the same dimension as the riser bases. I had thought that backing plates were supposed to be 30% or 40% bigger than the item it is backing, but I am not sure where I heard that.

I did not see anything in the Apprentice discussing this, and I searched the SparTalk archives to no avail. Don Casey emphasizes the importance of backing plates in This Old Boat but he does not discuss sizing (pp 134--135).

The existing plates are aluminum, and were tapped to accept the fasteners. Remarkable, I found no corrosion whatsoever, despite the dissimilar metals.

Now that I have the opportunity to upgrade the plates, are there any suggestions for a better installation? Re-use the same plates? Bigger aluminum plates? Stainless steel? How about fiberglass (I have a sheet of 3/8" prefab vinylester layup that is very hard and flat and very strong--is this a good material to consider if through-bolted?)

Thanks in advance for all input.