TOPIC: Inboard/Inboard Outboard engine mounts

As part of my continued desire to re-engineer the boat industry, I'm currently studying engine mounts. My boat was delivered with 4 long lag bolts screwed into the stringers, 2 port, 2 starboard. I'm considering boring the original holes larger, filling with epoxy and then fitting inserts of some sort which I can bolt the engine mounts to.

What has been successful for your projects?

You can glass in thick aluminum plate if you want and drill and tap for bolts. You can make extra long aluminum plates which can be screwed into the stringers and then drilled and tapped.
One thing you have to remember is that the stinger height is controlled by a transom jig and must be maintained at that height no matter what modifications you end up making. This Height controls shaft alignment in Mercruiser/Volvo Penta in particular. For the old Stringer mount OMC Stern drives it is not quite so critical.

Thanks Pappy.

To clarify, the plate sits on top the stinger, and does not wrap around in an L-shape for instance.

I believe you're also suggesting the plate should be mortised in, so the height is not impacted?

That is correct. Your stringer height should not be altered.

I'm not sure the installation details for a MerCruiser 2 but I'm sure Pappy is correct the mounts need to be the approximate height as original for engine alignment purposes. If you look at the engine mounts (the Mercury portion, not the boat's portion) there is an amount of adjustment available.
On a MerCruiser 1, the bellhousing to transom plate is where the majority of the torque is transfered. Your engine block wants to twist clockwise looking forward when you accelerate, trying to lift the left forward mount up. I assume your #2 drive has a similar arrangement. (If you look at an inline 4 or 6 cyl the front only uses a single center mounted stud to hold the engine up, it does nothing regarding twisting forces).
For clarity regarding OMC Stringers, some early ones actually use a transom mount and front mount arrangement. My 69 Shell Lake uses this type. My 67 Evinrude uses the 4 floor mounts with 4 lag bolts each.

Jim, thanks for the explanation of the Mercruiser mount function. My 215E is configured as you describe, with the clutch bellhousing bolted directly to the transom plate.



The front mounts bolted the block of the engine use these 1/2" lag bolts that go through the mount, and a 2 x 4 before tying into the stringer. Perhaps 1" to 1 1/2" of the screw actually penetrated the stringer.



I'm considering trying to source a 1/2 inch brass threaded inserts like those supplied by E-Z Lok. (3/8 inch largest I've sound so far)
Would drill the original mount holes larger, fill with epoxy, bore/tap and install insert.



Another option is the more difficult to install Barrel/Dowel Nut

Thoughts?

Yep, the ears on the transom plate/bell housing are similar to all the MerCruiser 1 drives.
I would use stainless lag bolts if you go with the original design. Longer if possible, you will need to evaluate if you will end up coming out the bottom.
Like I said in last post, my Evinrude has floor mounts. Original design was a hocus picus plywood block design with horizontal steel rod running laterally drilled & threaded for 3/8 x 16 bolts. Wood was rotten & steel rod & bolts were steel, not stainless. Rebuild used 6" of pine stringer & 3/8 stainless lags. Here are pictures...of original mounts with floor removed

New stringer & mount build up....

Engine install, flat floor with spacer blocks vs built up stringers protruding thru 6" holes in floor boards

Shell Lake with transom mount design

View from outside as I was installing ladder last summer. 2 vertical bolts at each upper mount & 3 in a 90 degree pattern for lower mount.

Posting pics is a PIA and I have no idea why some post opened & some need to be clicked on to view.

I included these examples because:

1st, most people believe OMC stringers are all mounted to the stringers & that isn't so. In the case of transom mount engines, the transom was laminated with 1 1/2" of plywood core. The engine & drive torque is transferred to the transom.

2nd, on floor mount engines, the transom technically needs no core except for the tie down/ski tow areas. OMC used one sheet of 1/2 ply. Engine & drive torque if transferred to the floor. I used 16ea 3/8 stainless lag bolts in my rebuild.

BTW, if I recall correctly, the late 80's SeaRay Pachangas had a problem with the fwd mounts sagging and engine alignment going bad. I don't remember the cause or correction. But, I seem to think they used an aluminum bracket (2 actually, one on each side) that was bolted into the stringer/floor but did not rest on hull bottom & began to sag.

I think in your quest to re-engineer the industry, an aluminum bracket idea would be good IF you build it like a truss or bridge so both sides are tied together (think old RWD car forward cross member under the engine design). Then you could thru bolt the engine mounts to the aluminum structure. This would require verifying oil pan & bilge clearance. Would be strong as hell but it would however probably make maintenance & cleaning access more difficult.

Jim, thanks for the write up and all the pictures. I envy your Quonset hut. Looks like a great place to work on your boat. Really helps me better understand the solutions you deployed. After considering the information you provided, I believe the front engine mount on my tie in point to the stringers are primarily locator pins to maintain engine/drive alignment. No need for the engine cradle. The stainless hardware is good advice. Funny that the drive itself is assembled entirely using stainless hardware. (more a reflection on Larson)