Strut Backing Board Replacement

I am in the process of bringing my Cal. 34LRC drivetrain back to good shape after an obusive trip up the Mississippi against a flood current. The 6 to 10 mph current was bad enough, But the incredible amount of floating debris was worse. Try as I might, I didn't get to steer around all of it. I thumped a few things hard that I never saw before or afterward. By the time I got home, both engines were vibrating like a paint mixer if I pushed them beyond 1500 rpm. As a result both props were bent, miraculously, the shafts check straight. During the trip, I was down in the bilge, trying to figure out why the gen set quit, while my Dad was at the helm. I glanced down at the bottom of the boat and realized the strut bolts were loose and moving around. Definately not a good thing, I gave up on the genny for a bit to get a wrench and tighten up the strut. I realized when I tightened the nuts that the washers were sinking into the 30 year old backing boards (plywood). So it is time to replace the backing boards. I wanted something that wouldn't split like a solid board, and wouldn't rot. I considered 3/4 treated plywood but decided against it as I don't see treated wood holding up the way it did before they stopped treating with arsinic. I also know from my engineering background that in order to keep threaded fasteners tight in vibration they need to have and maintain adequate bolt stretch. This is not possible in a 'soft' joint. Thus I looked beyond wood for the boards. The guy who runs the yard where my boat is on the hard suggested Starboard. I read somewhere that is a bad idea as the bedding compound doesn't stick well to the slick polyethelene. What I decided to use is GCO3 electrical grade fiberglass. I couldn't find 3/4" in a 12 x 36 piece, so I settled on 1". I had planned to float these in thickened epoxy then fiberglass tape the edges to the bottom. I was surprised when I lifted the first plywood board off the bottom that it wasn't attached at all except for the strut bolts and a thick layer of bedding compound. Now I am thinking that my fiberglass replacement is several magnitudes stronger than the original, as well as several magnitudes stronger than the six bolts that hold on the strut. I will forego the tape and just use the thickened epoxy underneath. The fiberglass is absolutely flat but the boat bottom is not. I don't want to create the sanding dust in the engine room that would come from grinding it flat. I am figuring that if I coat both surfaces with unthickened epoxy first, then use the thickened epoxy to fill the voids I should be good to go. I have wondered if doing this could trap air voids that have nowhere to vent, so I drilled 1/8" holes thru the sheet on two inch centers or so to let the air out from underneath. I plan to vacuum bag the boards to give what ever air is present motivation to leave.

I invite comments and suggestions as you folks are my sole source of boat repair second opinions.

as long as where the bolts go through the new FG panel is faily close to the bottom...you should be fine and forget the tape on the side...as long as you think your epoxy goo is gonna hold.

I know that's not all that technical...but if you need more info as I do lot's of glass work...call me some night next week... 609-780-2728

I use plywood for all my backing plates. I like the ability of wood to absorb some imperctions in the mating surface, plus the ability to absorb some shock load.
I have also used a disc grinder many times inside the boat and clean up is not as bad as you think.
But there are many ways to skin this one.

See if you can get a copy of the October/Novemeber 2010 issue No: 127 of "Proffessional BoatBuilder" magazine". Bruce Pfund does a detailed article on backing plates titled; Back It Up.

Refitting the engine to a friend`s Resort 35 trawler,we needed to raise its height to align the prop shaft. We used inexpensive kitchen cutting boards from a local supermarket,made of some sort of nylon or resin material,to "shim" or pack the engine higher on the mounts.The material does not seem to compress, is easily to cut to size,and readily available.We did not need to glue or fibreglass it in place,that might be a challenge,sanding the smooth finish to rough may help. Even if it doesn`t work in this instance,it is a useful material to keep in mind. BruceK

Thank You to all who posted. I think this will work out well. I paint remover on the area that the old backing board covered. afterward I scraped up what I could get. The paint remover did well on the old bedding compound as well as some paint. What was left was some gel coat and bare glass. My 5" round Milwaukee Sander is almost dust free with the shop vac hooked to it. I couln't get to about a third of the area due to a height restriction. I used the Fein Multi Tool there. It doesn't stir much dust. I got the whole thing pretty sanitary. I have good confidence that with the WEST system pre thickend epoxy, The new glass board and the glass boat bottom will become one. Thickened epoxy is an excellent gap filler.

Larry,
Thank's for the advice about Professional Boatbuilder. I had no luck accessing the old issue, but I did get myself signed up for a free subsciption. I love free. Once the subription starts, I'll be able to access the archive. I was contemplating paying for a subscription when I read about free subsciptions for industry people. I thought I have a John Deere Email address, we mfg. marine diesel engines, I ought to qualify. I think it will be a great resource.

Bruce,
Kitchen cutting boards are typically high density polyethelyne or H.D.P.E. It is essentially the same thing as Starboard. Compare the cost of a piece of Starboard to a WalMart cutting board and you can see what a deal we are offered for 'boat' stuff. Poly cutting boards are a great source of plastic to make stuff around the boat. It is tough to glue, but there are some newer adhesives that will work.

Once you get confirmed with Professional Boatbuilder you will be able to go back in the archives and find and download that article. It's a fine publication.