motor mount modification for OMC V8 + Stringer 800

[MrMilbe]

I'm in the process of restoring stringers and transom for a 20ft cuddy cabin. The old engine mounts are closed boxes made out of 1/2"ply. 3 sides + lid glassed to stringer. A 6x8 wood core snuggly sitting in there ( but not attached to anything, I think its only purpose is to hold the lag screws and press the engine against the mount box).

When opening it, the wood was soaked/rotten but the fiberglass completely fine ( no cracks, mounts where completely filled with water ). I think it entered over time through the mount screw holes.

Do you have any tips or a link for a better technique ? Something where the wood is protected. I saw a couple mounts that where like bridges, not closed boxes. Ideally pre drilled glassed holes and then using an aluminum back plate + machine bolts + nuts instead of screwing directly into the wood. But idk if that bridge looking design is sufficient for a stringer 800 outdrive as it doesn't push the transom but directly the motor mounts.

I also got 5gal of seacast standing around. I don't want the whole mount made out of seacast but maybe only the parts that are under the motor feet holding the screws.

 

[MrMilbe]

Nobody? I added 3 pictures. One is the old mount. You can see that those are enclosed boxes, I removed the core out of one already ( its a solid piece of wood, not tabbed or screwed to anything).

The other pic shows how I would do the new mounts. Main intend is having a solution that has completely encapsulated wood and bolts+nuts that are accessible. Any opinion if not doing the "box design" is a bad idea ? It is a OMC stringer so the mount will see some heavier load. I'd try to tab that bridge design heavily to the stringer.

I was wondering if I should add at least half of the other side ( see orange box, so that i still can access the bolts but have more structural stability).

 

[Scott Danforth]

in addition to the water in your mount boxes, you will find that your stringer wood will be wet, the transom most likely is wet and the flotation foam is wet. the two choices are to cut the wood out and wait a month for it to dry out, then re-tab..... or simply cut the wet or rotten wood out, and replace with new and start tabbing this weekend.

beyond that, once you fix all that, how its attached wont matter, because you will most likely do a better job than the factory did 35-40 years ago when your boat was built. either a big chunk of wood encapsulated by plywood and fiberglass for lag bolts to go into, or a nut plate or seacast........ however remember, your boat slapped together over a period of 5-10 days lasted 35-40 years and it was only designed to last 15. what ever you do, you will probably sell the boat in another year or two an move on to the next boat. My vote would be not to over-think it and build it as original.....especially if you are married.

and you are correct, the stringer drive will push the boat with all 4 mounts, so they need to be secured to solid stringers, which should be secured firmly to the hull.

 

[MrMilbe]

your assumptions are correct, that picture is from an older step to show the original mounts, most of the back part was rotten, the foam soaked and the actual hull is already completely stripped of floor, stringers, mounts, foam and the transom plywood ( there is one 1/4" layer of thin wood + the outer fiberglass left, I want to attach the new transom ply to this instead of glassing a complete new back ).

I just wanted to make sure I don't overlook something when changing the mount from a solid box to a bridge with a nut plate and end up with a V8 jumping into my back when pushing the throttle. But you make it sound like its not much of rocket science. I already got that impression when removing all that sloppy attached fiberglass (I think its just sprayed on). If I use the recommended 1.5oz CSM + 2x 1708 for stringers and transom this will be way thicker and stronger than the original build.

Was everything build back then crappy or could at least the hull be fine? In case I do a good job putting new stringers and transom in, is it worth keeping it, maybe switching the OMC for a 4-stroke outboard one day or will it never be a good boat ?
At that point I already spend more money on the boat and material than someone will pay, so whatever I do now I do for my own building experience and hopefully using the boat.

 

[froggy1150]

This is how mine was. I rebuilt it like it came to me. I made the solid mounts, wrapped them in glass then glued them down with epoxy and 3 or 4 layers of 24 oz woven. I had to bed with epoxy because of a previous repair that would have not been possible to grind out. I then had to get creative to transition back to poly. Poly don't stick to epoxy. Motor mounts are in with good quality 7/16 diameter 8" long lags.... I think

 

[MrMilbe]

thanks froggy, that's the original design which will inevitably lead to water leaking into the mounts through the holes at one point. But probably Scott is right and I'm overthinking a problem that will occur earliest in 10-15ys for an engine that's going to be toast in 5.

 

[froggy1150]

My mounts will come out one more time before I am finished. When they go back in I will seal off with 3m 4200uv. I might also dump a little penetrating epoxy in the holes first. Not sure about that yet. Do some experiments first:noidea:. But I do plan to seal it well

 

[Scott Danforth]

Was everything build back then crappy or could at least the hull be fine?

boats are still slapped together today......however there is less wood in many cases. today, the issue is the plexus joints that they slap boats together with today last about 15 years prior to the fiberglass cracking.

boats are made to make a profit for the company and to then wear out about 10 years after the warranty period so that people keep buying new boats.

the water comes from all the screw holes in the bilge not being sealed. it comes from the drain plug area of the transom and wicks forward. it comes from water in the bilge and poor encapsulation of original stringers. it comes from owners drilling holes to mount stuff on the transom (depth finder transducers, etc). remember, boats have water outside of them, they were never designed to have water inside of them