Normally, I don't write about maintainence things on the sail trim forum but this one is important. Last week I was at an Alamitos Bay (So Ca) boat yard visiting a friend. The mate with the boat next to us was installing a through - hull. What caught my attention was he was scurrying around looking for a piece of plywood to use as a backing plate. He told me any piece of wood would do. That is a BIG mistake and here is why. Not all pieces of plywood, even marine, are the same. Regular plywood is full of voids and is not bonded together with marine glue. Plywood marked marine could be made from a lot of things (Douglas fir, tropical hardwood, Okoume or Mahogany). For plywood blocks, I prefer Mahogany plywood. Why do you need a backing block in the first place? The reason is to spread the load. Without a backing block, you are creating a hard spot in the hull which can easily be overloaded beyond a safe limit. If the sea cock becomes stuck and you try to force it with a wrench you could crack the laminate on the hull. That is not a good thing. What shape of block (round or square) is best for a fiberglas hull? The answer is round. On wood boats, square would be the best choice. With the round block you should take it one step further to improve its usefullness by tapering the edges. Now that you have cut the block to match the seacock - are you finished? The answer is NO. A seacock could last 20 years but an untreated piece of wood may last only 5 years. To extend the woods life you should saturate the blazes out of it with several coats of epoxy. Actually, I don't like to use plywood as backing blocks. My wood of choice is either mahogany or white oak. Do you think the sailor installing the sea cock listened to my backing plate advise? Not at first because he told me he was in a big hurry (like most folks in So Ca) to get the boat back in the water. Fortunately, his wife was listening to my 2 minute pitch and she thought it made sense. A couple of days later my friend called me and told me the husband and wife installed the mahogany block and they both said to tell me thanks for the advise.
