Installing Motor Mounts to Stringer

[delmarrey]

Originally Posted by James S

I figured he would still use the isolating motor mounts between the engine and the stringers.

Sorry for the miscommunication but I did mean the motor mounts mounted to the motor bed stringers. I would think wood between the two would dampen the sound. As well, wood would be sacrificial or EZ to repair.

[James S]

Agreed...in that case it seems that it would have been no big deal to change the wood instead of trying to fix the holes in the wood...unless they are fairly good size ?
Nicks a clever guy...I'm sure there is more to this....
Nick...ping...ping
Inquiring minds want to know.

[s/v Jedi]

ping ping? ;-) Nick is trying to switch over to MacOS-X from Windows XP !! Man, this is worthy of a big long thread but anyway, engine mounts:

We are plastic fantastic: glass + vinylester + balsa everywhere except high stress locations. Only the first Sundeer was aluminium.

Now, I've never done this before that I can remember (but I should have done this when sailing the same boat for 17 years.. right?? but I can't remember ;-)

So I have been thinking about this, reading everyones methods and thinking about those. Now, the tapping in a metal engine bed with an intermediate metal plate is a no-brainer but all the rest I think can be improved The system with the dowel has probably been done for ages and is proven to work. But I am always looking for innovation. If that dowel is just a tad bigger than the bolts, the threads of the bolt will end up in the area where the dowel is glued into the hole. When epoxy is used as the glue it will have reinforces that area by penetrating the wood fibers. This will have caused the wood to expand but after you drill the new hole you take away part of that tension and rely on the strength of the epoxy again. If you use a much bigger dowel ,you are back to the original wood scenario as long as the dowel isn't ripped out, which is easier the bigger it gets because of the glued surface area which increases with diameter times pi. However, the dowel will have the grain vertical except when you make your own (is that true?) I think I would laminate something myself and use a hole saw to create my own dowels for this.

But I am looking at more innovation. If I will take machine tools to the job, this is what I would do: find access to the side of the stringer. I would take a hole saw 2 or 3 times the bolt diameter and drill into the side right where the bolt comes, going through the old hole but not all the way through the stringer, stopping 1/2" past the bolt hole. I would leave at least an inch of stringer intact between the top and this hole in the side. I would vacuum that, and do the heat & impregnate with epoxy thing. Then I would take fiberglass tape, soak it with epoxy and ram it into the hole using the core I got from the hole saw and a mallet. When almost filled up, some plastic wrap over it, the core against that and some way to keep pressure on while it cures. After that fair it out and drill the new hole through that plug and tap it for a machine bolt. I think it's overkill but it'll hold for ever and can be reused.
Instead of a hole saw you could use a Fein multimaster to cut slits and use lag bolts. This wouldn't be overkill but only practical for us who have this super tool.

What I hope is that the wood is in perfect condition. In that case, vacuum, heat & impregnate with epoxy should result in a better than original strength. I know that epoxy shrinks during cure but as it gets sucked into the wood with the vacuum that is created by cooling, it saturates the fibers so the wood swells a bit. The question is even if the hole doesn't become too small for the bolts.

cheers,
Nick.