how do I repair my motor mounts?

[23marathon]

I have a 23 foot Marathon Cabin cruiser I/O with a 260hp Mercruiser Alpha 1 outdrive that is being repowered upon inspection with the motor out of the boat I noticed that that wood and fiberglass wrapped motor mounts were soft already having cut out some soft spots in the floor out inside the cabin I cut the tops off the fiberglass enclosed motor mount to find a bunch of rotten plywood and some sort of 2x4 material So i cleaned up everything and degreaesed and powerwashed the entire boat.


I need to repair the mounts or maybe they are called beds I am not sure on the correct term? any help? I have some Ideas that I am sure would work but I am really looking to do this job right so that I can enjoy my boat for many years with out having to worry about this.


Before I took the cutting wheel to the the tops of the motor mounts I took the proper measurments so that I know what my finished height will be I also left all of the fiblerglass enclosure that was the old motor mounts next to the stringers which only have about a 1/2 in of soft wood a the very top I am not planning on doing any work to the stringers I have thought about repairing the mounts this way...



Use some 1st growth douglas fir and make one piece wood inserts and cut them to fit my existing fiberglass mounts I would like to wrap these pieces with fiblergalss and resign before installing them I have read that these mounts are not susposed to touch the hull of the boat I believe my original ones were so some help here would be great I would like to make these mounts slightly longer than original and have them attach to the stringers maybe 6 inches to a foot in front of the originals. I was thinking on thru bolting these, by using the douglas fir mount then the stringer then a piece of preasure treated 1'' my 4 1/2 wrapped in fiberglass making a sandwich persay, then I was thinking about enclosing every thing with fiberglass and then applying paint please see drawing.

fiberglass wrapped 1x4.5----================================
existing stringer ------------XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
new douglas fir -------------""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
motor mount ---------------""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

I was thinking about using galvanized thru bolts dipped in silicone to sandwich all of this together and then fiblerglassing it all to gether for a nice finished look.

This is just my idea I m sure there are much better ways or the right way to repair this common problem I am sure. If you need any more info or anything like that I will provide it Thank you

[TeddyDiver]

Get a copy of Dave Gerr's Boats Mechanical System Handbook and/or Elements of Boat Strength.. :)

[pistnbroke]

Without a photograph we carnt see exactly what the situation is
But I can say is be sure to get all the wrotten wood out ..I use an air chisel...maybe take the whole section of ply out....personally I would use hardwood if you can get it for replacement timber and bolt as you say....I would wrap the new timber in 250g matt and resin then wrap in cling film (cooking film) to pull the mat tight on the corners ..when dry remove film inspect for holes and then glass it in to the hull wet pulling all the bolts up to squadge it together ( not too tight or you will squeeze the resin out ....fully tighten when set .Where the old and new glass come together wipe the old with acetone just before putting in the new . this softens it and makes sure there is no grease

Note ..if you have used fg befor you will know if you try to go over a 90 deg angle it bulges up ..the cling film will hold it to the wood with no air pockets

[2farnorth]

Take a few pictures and post them. I did the same to my 35 year old Larson last year. I just 'glassed in a couple of new pieces of wood then covered them with a sh*tload of epoxy and bolted her down. But I dont know what you are looking at.