Engine bed: Fix mounts to aluminum bar - will it work?

[kjemmo]

Hi all.


I am preparing the install of a new engine.



The existing engine bed is to low for the new engine and mounts. So I want to raise the it a bit.



Also the the engine bed is constructed with glassed in steel bars. I can not drill and tap new holes in existing bed as the new holes will be too close to existing ones.


So the two things I want to achieve is to raise the bed a bit and provide new tapped holes to fix the engine mounts.


My idea is to bolt a 15mm (5/8") high aluminum bar on top of the existing engine bed. The aluminum bar will be thoroughly bolted to the glassed in steel bar.



Then I will bolt the engine mount to the aluminum bar.



Any downside to this approach? Better alternatives? Will 15mm (5/8") thread in the the aluminum bar be sufficient to hold the engine mounts in place?


Please let me hear your comments.



Thanks

[Orion Jim]

Why not use structural fiberglass, such as G 10 (there are many others available) so there will be no dissimilar metals bonded by fastenings?

[kjemmo]

Originally Posted by Orion Jim

Why not use structural fiberglass, such as G 10 (there are many others available) so there will be no dissimilar metals bonded by fastenings?

I like the idea, but how will I attach the engine mounts to the structural fiberglass? With aluminum I can tab threaded holes for bolts.

[Orion Jim]

You can drill and tap structural fiberglass products just like aluminum.

[RaymondR]

I had a similar arrangement on my previous boat, which was steel, to allow me ti lift the engine to allow maintenance of the hull under it without disturbing the alignment with the prop shaft. I used countersunk bolts threaded into the aluminium rail from below to fasten the engine mounts to the rail.

[jt11791]

I don't like steel bolts threaded into aluminum for holding the engine mounts. They will work loose and/or corrode over time. Either find a way to through-bolt with nuts, or don't use aluminum. Maybe you could use counter-sunk bolts from the bottom of the aluminum with nuts on top?

[Bycrick]

When I wanted to replace my OEM Yanmar motor mounts, I found some third-party mounts that looked good, but were obviously far shorter that the originals. They also had a "special Yanmar version." Just the regular shorter mount attached to a 1-1/2” x 1” x 7” block of aluminum with the right Yanmar holes. Put some Tefgel or other anti seize compound on the SS screws.

[osprey877]

Originally Posted by kjemmo

I like the idea, but how will I attach the engine mounts to the structural fiberglass? With aluminum I can tab threaded holes for bolts.

You could use stainless T nuts fastened to the underside of the structural fibreglass before you attach the fibreglass to the existing mounts.

[Manateeman]

You are thinking layer cake. A big 90 degree angle in aluminum with unequal legs let’s you bolt to the old engine beds at 90 degrees to the bolts holding the new engine mounts onto the aluminum. Now you can adjust the heights and the angles. You can through bolt everything.
We have used aluminum 90 angle, thick u shapes and box sections.
Layer cake is not the only system.
The manatee crew

[Ken Z]

I added a aluminum bar5/8” thick and 5” wide. I bolted it through the fiberglass engine stringer. You might consider thru bolting, may require countersinking, would be stronger. It works fine for me used ss bolts that need cleaning occasionally fo mild corrosion.

[Scubaseas]

Originally Posted by Bycrick

When I wanted to replace my OEM Yanmar motor mounts, I found some third-party mounts that looked good, but were obviously far shorter that the originals. They also had a "special Yanmar version." Just the regular shorter mount attached to a 1-1/2” x 1” x 7” block of aluminum with the right Yanmar holes. Put some Tefgel or other anti seize compound on the SS screws.

This.

[stormalong]

You do not say the size of the boat or engine. Your profile says Drabant 22. Either that is 22 meters or we are talking about a different boat.

Regardless, use steel. Drill and tap the steel to take bolts or studs to fasten the engine mounts. Being dependent on threaded aluminum long term is not a good idea. Where the bolt holes can be drilled through to the original engine bed without overlapping existing holes do so and through bolt the engine mounts to the original bed.

Keep in mind that engine mounts are where the thrust from the engine is transferred to the hull.

[DougR]

Storm along has the correct answer......just make your plate from steel.

[Bellinghamster]

If you want to stick with aluminum, consider using threaded inserts.

You tap an oversize hole, locktite the insert in the alumimum, then screw your engine mount bolts into the steel inserts. Spreads the loads over a larger area/coarser thread and doesn't wear out the aluminum.

I used E-Z Loc brand (https://www.grainger.com/category/fa...ategoryIndex=8) to secure a whole lot of 7ft tall 3500lb telecom battery racks to a 2" thick 6061 T6 aluminum adapter plate on my lab's hydraulic shaker table to perform seismic testing (~8.3 equivalent on the Richter scale). I wouldn't hesitate using them to bolt engine mounts down to aluminum bar.

[stormalong]

Originally Posted by Bellinghamster

If you want to stick with aluminum, consider using threaded inserts.

You tap an oversize hole, locktite the insert in the alumimum, then screw your engine mount bolts into the steel inserts. Spreads the loads over a larger area/coarser thread and doesn't wear out the aluminum.

I used E-Z Loc brand (https://www.grainger.com/category/fa...ategoryIndex=8) to secure a whole lot of 7ft tall 3500lb telecom battery racks to a 2" thick 6061 T6 aluminum adapter plate on my lab's hydraulic shaker table to perform seismic testing (~8.3 equivalent on the Richter scale). I wouldn't hesitate using them to bolt engine mounts down to aluminum bar.

While the idea of threaded inserts are doable, IMO steel is stronger and less of an issue over the long term. You can't beat the strength or the durability. Once properly painted and installed you can forget about it for the life of the engine, other than checking the bolt torque periodically.