Loading on U Shaped Engine Mounts?

[76SeaRay]

While waiting for delayed shipment (seems to be common these days) of parts for other areas, I am trying to sort out whether to change to U shaped front motor mounts (heavy 5.7L vortec with closed cooling system). The originals were boxed in old style. After watching all the videos and reading all the threads, I didn't see much discussion about the point at which the U mount contacts the hull and whether point loading there could cause stress cracks in the hull without some underlying wide piece to distribute the load horizontally along the length of the mount before being absorbed by the hull. What I have seen (unless I missed something) is that the legs are typically 1 1/2 inches or so thick, about 8 to 10 inches apart front to back, 4 to 5 inches wide side to side, and set directly on the hull with PB and tabbing. I am using all marine grade 3/4 inch plywood in this area so thinking that maybe I should lay down (brick mortar bedding style) a piece of the plywood over the hull area slightly longer front to back under the legs of a U mount. I know the tabbing will help distribute the load but don't you risk the tabbing eventually cracking and increasing the point loading without extra structure under them? Am I overthinking this or being overly cautious?

 

[Bondo]

Ayuh,..... Got a picture,..?? I donno what yer talkin' about,....

So far, I've always used the oem motor mounts,.....

 

[76SeaRay]

Sorry, no pic since I haven't built them yet but here is the video link from another thread to show the mounts that I am working on.

 

[Bondo]

Oh, Ok,.... The wooden part,.... I was thinkin' slightly higher up, the bolt ins,....

Pretty much all I've seen, tie into both the hull, 'n the stringers,.....
Either with a laminated stack of plywood, or a big piece of white oak, or a box affair like in the video,.....
If yer able to tie into both the hull, 'n stringers,..... Yes, yer over-thinkin' it,....

 

[76SeaRay]

Thanks. The original design had them boxed in with lags. You can see the old ones where I tentatively marked the landing points for the engine. I had originally intended to just replicate the original design but I still can build the U shaped ones and attach them to the stringers. I would have to cut out the short stringer near the center of the engine bay to put them in. But I already had to cut down that inside stringer to provide clearance to the starter behind the engine mount and clearance to the seawater pump in front of the engine mount.

 

[Bondo]

Ayuh,..... Have you hung the motor on a chain, 'n aligned it to the gimbal, to see exactly where the motor mount blocks gotta be,..??
It looks like it'll be awhole lot easier to mount it on solid wooded blocks, than skeletonized wood mounts,....

 

[76SeaRay]

It is hanging from a gantry crane on a chain hoist. The rear mount bolts (new original style spring and washer) are installed. I was trying to align the engine (hours of trying and unsuccessful) to get the final landing pad for the mounts but having problems with the gimbal bearing (in my other thread) as I don't think it is moving at all with all the whacking. I do want to get the engine aligned on shims to determine the final height of the wood mounts and then build them to that.

I am looking at doing the U shaped ones for through bolting instead of lags. This due to comments about water intrusion with lags.

 

[todhunter]

You could do solid mounts out of a rot-resistant wood like mahogany, then encapsulate the mounts in fiberglass separate from the stringer system, that way if they do get wet, the water won't get into the stringers. That's what I did.

 

[Bondo]

You could do solid mounts out of a rot-resistant wood like mahogany, then encapsulate the mounts in fiberglass separate from the stringer system, that way if they do get wet, the water won't get into the stringers. That's what I did.

Ayuh,..... Or p/t framin' lumber, or boiled white oak, or locust, or,......