One more idea....
Doug,
I’ve been following this post pretty closely as adding a 4th engine mount is something I’ve considered doing in the future. My M-25 vibrates a lot at low rpms. I’ve always wondered if those vibrations are working loose what’s left of the old sealant on all my deck fittings.
I thought it a little unfortunate that you’re considering going back to the 3 point mount, after all the progress you’ve made in the other direction. I get what the Isoflex guy said about weight on the mounts, but it sounds like the stiffness of the “rubber” material can be varied, so you may be able to compensate for the light weight.
People have voiced many ideas about ways to address your problem. I’ll just add one more:
It seems that the real strength in these engine stringer design comes from the box-beam type construction. Any one of the 3 (top, front, back) stringer surfaces is only ¼” (if FRP) to 1” thick (if FRP and wood). Join three of these surfaces close together in a box-like fashion, though, and they becomes extremely rigid (particularly at their corners). But, because they are thin surfaces, they don’t hold heavily loaded fasteners (bolts, lag screws) well. A way to exploit the strengths of the design, might be to simply surface-mount a u-shaped bracket around the three FG surfaces. Something like this, onto which you can attach the motor mounts:
8 to 12 screws holding a 3 sided metal plate against 3 FG surfaces should be extremely strong, and the forces would be spread out evenly over the 3 surfaces and the many screws. You could have threaded studs welded to the top plate to hold the motor mounts, or you could have threaded nuts welded below the top plate to bolt the mounts down into.
A mount like this would let you combine your options 1 & 2 from post #19 if you
skewed the attachment of the top plate in relation to the side flanges. This would allow you to align the front mounts with the rear mounts. You could also extend the top plate fore and/or aft of the side flanges to adjust the fore-aft placement of the mounts. Might look like this:
i.e., the side flanges lie flat against the
angled sides of your engine stringer, but the top plate is in alignment with the rear engine mounts as in the picture below.
Just some thoughts. Good luck!