Motor Mount Repair - Rotten Stringer

I have been a fiberglass hobbiest for many years and in 1976 worked for 4 months in a fiberglass shop making master molds for sinks, tubs and shower walls from woven roven and epoxy resin. I have owned several I/O boats with rotten mounts and fixed them all the same way. If you pound a boat with bad mounts, the engine will crush the mounting floor and sink the boat. I felt if I had to go to all the trouble of removing the engine to fix the mount, I might as well sell the boat for junk, therefore I came up with a creative repair method.
I've seen repair shops rip out and splice in a new wood stringer and then glass it up. Well that's great but a lot of unecessary hassle and expense.
1. Take the motor mount off but dont remove the engine. This allows you room to work. If you repair the mounts one at a time you probably don't need to support the motor. Take a wood bit about 1.5 inches or larger depending on the boat size and drill one or more holes in the mount as deep as possible without going through the boat bottom.
2. Get a one gallon bucket a mix a thick but semi pore-able slurry of shredded woven roven and polyester resin. Pour it into the hole you drilled while using a stick to stuff the hole until it wont take any more of the slurry. On my under 20 ft boats I fixed, they took from 1/2 to 3/4 gallon per mount.
Let it set up over night and drill new motor mount holes the next day. Use a good grade of stainless or the original screws and drill the hole smaller then major diameter, but not smaller then the minor diameter of the screw. If the hole is too small you will crack the glass. On a 140 hp Volvo Pinta I used a 5/8 screw and torqued it to about 125 ft lbs, I was amazed it did not strip. Whatever you do make sure that the mixture does not cure at a higher rate then recommended. If done correctly this will be stronger than the original. Assuming you have room to work you should have about 4 hours into the job four four mounts.
Fiberglass is nasty to work with, so buy a gallon of acetone for cleaning up and a particle mask to avoid inhaling the glass fibers as you shred them. The fiber glass itches so minimize contact with the cloth, especially on your forearms.
If you want to check to see if your motor mouunts have any integrity, check the wood screws that penitrate the stringer with a wrench. Just remember they are very easy to strip in an old boat, so don't use hardly any force at all. In fact, if you see that the screw at not screwed all the way down, this is a sure sign that your stringer screws are stripped.
When I fixed the Volvo Pinta, I had already crushed the rear motor mounts in a wave jumping storm and did not know it until I got the boat home. Another five minutes of pounding would have sank the boat because every time I jumped a wave the motor would lift out of the rear mount about six inches and slam down crusing the floor.

 

my boat is a 78 marquies and has a 305 198 horse io engine and the moter is a little down on the one side because the mount / floor is rotten what should i use to fix this thanks

 

I have been a fiberglass hobbiest for many years and in 1976 worked for 4 months in a fiberglass shop making master molds for sinks, tubs and shower walls from woven roven and epoxy resin. I have owned several I/O boats with rotten mounts and fixed them all the same way. If you pound a boat with bad mounts, the engine will crush the mounting floor and sink the boat. I felt if I had to go to all the trouble of removing the engine to fix the mount, I might as well sell the boat for junk, therefore I came up with a creative repair method.
I've seen repair shops rip out and splice in a new wood stringer and then glass it up. Well that's great but a lot of unecessary hassle and expense.
1. Take the motor mount off but dont remove the engine. This allows you room to work. If you repair the mounts one at a time you probably don't need to support the motor. Take a wood bit about 1.5 inches or larger depending on the boat size and drill one or more holes in the mount as deep as possible without going through the boat bottom.
2. Get a one gallon bucket a mix a thick but semi pore-able slurry of shredded woven roven and polyester resin. Pour it into the hole you drilled while using a stick to stuff the hole until it wont take any more of the slurry. On my under 20 ft boats I fixed, they took from 1/2 to 3/4 gallon per mount.
Let it set up over night and drill new motor mount holes the next day. Use a good grade of stainless or the original screws and drill the hole smaller then major diameter, but not smaller then the minor diameter of the screw. If the hole is too small you will crack the glass. On a 140 hp Volvo Pinta I used a 5/8 screw and torqued it to about 125 ft lbs, I was amazed it did not strip. Whatever you do make sure that the mixture does not cure at a higher rate then recommended. If done correctly this will be stronger than the original. Assuming you have room to work you should have about 4 hours into the job four four mounts.
Fiberglass is nasty to work with, so buy a gallon of acetone for cleaning up and a particle mask to avoid inhaling the glass fibers as you shred them. The fiber glass itches so minimize contact with the cloth, especially on your forearms.
If you want to check to see if your motor mouunts have any integrity, check the wood screws that penitrate the stringer with a wrench. Just remember they are very easy to strip in an old boat, so don't use hardly any force at all. In fact, if you see that the screw at not screwed all the way down, this is a sure sign that your stringer screws are stripped.
When I fixed the Volvo Pinta, I had already crushed the rear motor mounts in a wave jumping storm and did not know it until I got the boat home. Another five minutes of pounding would have sank the boat because every time I jumped a wave the motor would lift out of the rear mount about six inches and slam down crusing the floor.

 

Humm, are you fixing only the bolt holes? If the engine stringers are rotton, then what supports the repair? Since one of the one goals of a stringer is to distribute the thrust load of the engine (and the pounding load) over the length of the hull strucuture, I would think that what will happen is that you will have failure somewhere else, maybe more spread out, possibly causing hidden damage that will be wide spread over time, and possibly also eventually catastrophically failing - somewhere, later... just my thoughts...

 

Hauling an engine isn't really so bad. It gives you the opportunity to clean up and make 'er perty again. I just did the same repair on my starboard side engine stringers. I removed all the old wood through some rectangular cut-outs, cleaned, and poured the "Seacast" system. No more wood, and I preserved the engine mounting holes so I didn't have to mess with the alignment too much. I put the rectangular cut-outs back on before the Seacast cured. You can knock the stuff - won't hurt my feelings - but it was 10,000 times stronger than what I had. I think it is just PE resin, but cheaper for me to buy that stuff than to buy the resin by the gallon at Jamestown Distributors.

Do you know a less expensive source for the fibers and resins? I'm doing port side next year.

Mike