Drill and fill transom holes questions

[bigtalljv]

Hi,

I am getting ready to mount my outboard bracket and had a question related to drill and fill. With the exception of the plywood plug that I made the transom of my boat is fiberglass with a foam core. Should I be thinking about drilling and filling the holes through that? I have 24 bolts to mount my outboard bracket. Is it a strength issue or a water intrusion issue that the drill and fill is used to solve? I wasn't too clear on that.

Thanks,
Jason

[Cracker Larry]

It is primarily to prevent water intrusion into the plywood core. As an alternative you could give all the holes several coats of epoxy, but I would drill and fill for the motor mount because it has so much stress on it.

[jacquesmm]

Larry, thank you, you are correct for plywood core but I read that he has a foam transom. In that case:

Look at this tutorial:
http://www.bateau2.com/howto/foam1.php

I wrote it 10 years ago and it covers most foam sandwich questions.

Go to page 10:
http://www.bateau2.com/howto/foam10.php

It shows different ways to handle bolts through foam, depending on the load.

Water intrusion is not the main problem, it is compression strength. Without the inserts I show, regular marine foam will crush, the bolts will come loose, water may enter etc.
Now, transom foam with high compression strength exist. If the boat was well built, it should have transom foam.
If you are certain that you have transom foam, a small epoxy plug is sufficient. Otherwise, do as in the tutorial.

And for a bracket, always wide backing plates etc. as per manufacturer instructions.

[Cracker Larry]

Sorry, I didn't catch the foam core

[bigtalljv]

it's an ex-military Zodiac so not sure what the foam material is, all i'd have is a picture at this point. the bracket is also zodiac surplus so no instructions to go with it.

Here is a cross section of the transom. The white was a filler they had in around the hole for the original outdrive. The foam in the transom was denser that other foam used in the console and such but beyond that I can't tell you much about it. Wide backing plates are already in the works.

[jacquesmm]

If you are not certain, I would use the techniques listed on the last page of the foam tutorial: inserts, large ones.
Either pieces of plywood or better, epoxy putty made with milled glass.

[bigtalljv]

Thanks Jacques. Is 1" putty insert for a 1/2" bolt big enough or larger?

Jason

[jacquesmm]

I would go larger, 2" diameter for a 1/2" bolt plus a nice backing plate, preferably AL, 1/4" thick, either to cover all bolts or 4" square per bolt.
All that also depends on the engine size.

[bigtalljv]

Jacques,

the holes are 3-4" apart and there are 24 of them. Also since we are talking holes that big I think I'm looking at a hole saw for that. Any positive or negative if I leave the inside skin intact? Seems like I should be able to but i'm not sure if that gets me anything.

I'm going with a single 2005 suzuki 250. Bracket was designed for twin 150s.

The bracket looks like this

[bigtalljv]

and, of course, there is one more little problem. The two lowest holes on the outside of the bracket hit squarely into the end of the stringers. Not sure what my options are there. Is 20 bolts enough just leave the four out? Lag bolts are just wrong. Epoxy in some sort of nut sounds incredibly hard to line up. Cut into the stringer and then repair?

these four:


Thanks,
Jason