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

[boatpoker]

Originally Posted by kjemmo

Hi all. bolted to the glassed in steel bar.

I had exactly the same issue and did as per your drawing but had to use two plates. I used steel, just like the original steel bar that is embedded in the FRP stringer. Mild steel is fine if properly primed and painted.

DO NOT use aluminum. The steel bolts and steel plate will create a galvanic cell with the aluminum and corrode it. No matter what metal bar you use, you must not use ss bolts.

[Dieseldude]

Originally Posted by Ballsnall

You have made some good points but just to clarify for to OP.

The cost of a few feet of SS flat bar is more than MS but insignificant in this case.

Tooling cost is identical, I use the same drills, taps, hacksaw etc as mild steel. Use a different cutting or grinding disk's if you want but they cost the same.

Mild steel is certainly strong enough.

Additional cost involved for rust protection and it most certainly will rust if the coating is inadequate or gets damaged. I have yet to see a mild steel engine bed in a yacht that wasn't rusting and it is such a difficult place to repair without removing the engine.


Use aluminium if cost is critical. Stainless steel is by far the best long term option, mild steel and salt water just don't play nice together.

You make a good case for stainless. It is only a small quantity, so cost is not a huge deterrent. It can be a big job to repaint mild steel that an engine sits on. To me it seems like a luxury to set an engine is made of iron and steel on stainless mounts though. It comes down to personal preference and cost. Either stainless or mild steel will work. I would not recommend aluminum due to galvanic corrosion and due to chance of thread distortion of the softer aluminum. It must endure both high static stress and vibration. An engine doing the hokey pokey on aluminum bars is not something that I would want to face. A good torque chart for bolts set in various metals will show that torque value for aluminum will be less than for steel, so aluminum will have less holding power.

[Ballsnall]

Originally Posted by Dieseldude

A good torque chart for bolts set in various metals will show that torque value for aluminum will be less than for steel, so aluminum will have less holding power.

Correct Dude.
Aluminium is not suitable if just tapping the mounts. My recommendation if the OP was going to use it was to tap and countersink, countersunk screws from below the plate so they were like studs

I would definitely not screw into the aluminium and expect it to hold.

As for the corrosion, there is a big difference between a painted engine and an engine bouncing around on a painted steel plate. It's not the end of the world, but nice if an additional rust location can be avoided.

Another option would be to just drill and tap the existing embedded plate and use hard plastic shims to raise the new engine

[stormalong]

Originally Posted by Ballsnall

Another option would be to just drill and tap the existing embedded plate and use hard plastic shims to raise the new engine

The OP said that the new holes will be too close to the original holes so that won't work.

[Ballsnall]

Originally Posted by stormalong

The OP said that the new holes will be too close to the original holes so that won't work.

Ah yes,
That won't work then unless he adjusts the prop shaft length.

[Bowdrie]

Originally Posted by Ballsnall

Ah yes,
That won't work then unless he adjusts the prop shaft length.

That's not a bad idea, perhaps just a bit fore or aft will get the packing onto a "new" part of the shaft, and the stern bearing won't mind.
If there's no interference with anything there's no reason to alter the shaft length.

[Dieseldude]

Originally Posted by Bowdrie

That's not a bad idea, perhaps just a bit fore or aft will get the packing onto a "new" part of the shaft, and the stern bearing won't mind.
If there's no interference with anything there's no reason to alter the shaft length.

Might seem like a good idea at first, but cutting the shaft means sending it out to a machine shop. Also, getting the length precise enough for the engine to line up with the engine mount holes will not be easy.

[Bowdrie]

^ Please, I said nothing about cutting the shaft.
Depending upon clearance(s) both underwater and in the engine space, sliding the engine forward or backward an inch or so is no big deal.

In my post I said, "If there's no interference with anything there's no reason to alter the shaft length".

[VChild]

Originally Posted by Ballsnall

You have made some good points but just to clarify for to OP.

The cost of a few feet of SS flat bar is more than MS but insignificant in this case.

Tooling cost is identical, I use the same drills, taps, hacksaw etc as mild steel. Use a different cutting or grinding disk's if you want but they cost the same.

Mild steel is certainly strong enough.

Additional cost involved for rust protection and it most certainly will rust if the coating is inadequate or gets damaged. I have yet to see a mild steel engine bed in a yacht that wasn't rusting and it is such a difficult place to repair without removing the engine.


Use aluminium if cost is critical. Stainless steel is by far the best long term option, mild steel and salt water just don't play nice together.

That is correct. The higher material cost of the stainless would be insignificant for this project. Costs may have changed a little since I did my project, but I paid $0.81 per cubic inch for 316L SS.
My plates were 1/2" x 4" x 28" so about $45 per piece.

[fish53]

I did similar and the aluminum works fine just don't use stainless fasteners, I used galvanized and no problem.

[boatpoker]

1. Motor mounts are made out of rubber and mild steel.
2. Motor mounts are made out of rubber and stainless steel.
3. Motor mounts are made out of rubber and aluminum.

You chose 1,2 or 3.

Now remembering the Noble Scale and Rustoleum rust coating. You choose the plate material.
Youmight also want to remember that when steel corrodes, it is visible and easily addressed.
When stainless suffers from concentration cell corrosion it is almost invisible as each pin prick size hole is a thousand times
larger under the surafce.

As A retired Certified Marine Corrosion tech I chose steel.
Choose your poison.

[Dieseldude]

Originally Posted by boatpoker

1. Motor mounts are made out of rubber and mild steel.
2. Motor mounts are made out of rubber and stainless steel.
3. Motor mounts are made out of rubber and aluminum.

You chose 1,2 or 3.

Now remembering the Noble Scale and Rustoleum rust coating. You choose the plate material.
Youmight also want to remember that when steel corrodes, it is visible and easily addressed.
When stainless suffers from concentration cell corrosion it is almost invisible as each pin prick size hole is a thousand times
larger under the surafce.

As A retired Certified Marine Corrosion tech I chose steel.
Choose your poison.

Yes, it is possible for stainless to "rot" from within. But in this situation for engine mounts, it will only get occasional contact with water, so this should not be a problem. Personally, I think that mild steel given a good primer and top coat would be fine. But when it eventually requires repainting, it has poor access, so may require that the engine be lifted. Additionally, the most rugged coating would be epoxy primer and polyurethane top coat. If these are not already on hand, the cost could put the project price higher than just using stainless. I have only found gallon kits of epoxy primer as the smallest quantity. Traditional alkyd or enamel paints may not be sufficiently durable. A gallon epoxy primer kit will cost well over $100. Often, materials selection involves more than initial direct cost of main components. Preservation, durability, and future maintenance are important. The ability to make such decisions is part of the appeal of do it yourself jobs.

[boatpoker]

Originally Posted by Dieseldude

Yes, it is possible for stainless to "rot" from within. But in this situation for engine mounts, it will only get occasional contact with water, so this should not be a problem. Personally, I think that mild steel given a good primer and top coat would be fine. But when it eventually requires repainting, it has poor access, so may require that the engine be lifted. Additionally, the most rugged coating would be epoxy primer and polyurethane top coat. If these are not already on hand, the cost could put the project price higher than just using stainless. I have only found gallon kits of epoxy primer as the smallest quantity. Traditional alkyd or enamel paints may not be sufficiently durable. A gallon epoxy primer kit will cost well over $100. Often, materials selection involves more than initial direct cost of main components. Preservation, durability, and future maintenance are important. The ability to make such decisions is part of the appeal of do it yourself jobs.

Take a look at all the various component parts of your engine. How many of them are stainless steel, How many of them are steel ? You think cost is the only issue ?

[fish53]

Originally Posted by boatpoker

Take a look at all the various component parts of your engine. How many of them are stainless steel, How many of them are steel ? You think cost is the only issue ?

I can't comment on your experience but zinc, aluminum and mild steel are all very close together on the galvanic table, with stainless being more than four times as noble as the others. Other than immersed in seawater you should have little problem using zinc or mild steel with aluminium.

[Dieseldude]

Originally Posted by boatpoker

Take a look at all the various component parts of your engine. How many of them are stainless steel, How many of them are steel ? You think cost is the only issue ?

As an engineer, I am aware of properties of materials and cost vs. benefit and performance. Of course engine components are designed with compromise considering such factors as cost, strength, heat resistance, durability, mass, etc. That being said, the greatest mass of material that most small to medium size cruising boat engines are made of is cast iron. This would be followed by steel, then perhaps aluminum. Then smaller amounts of various other alloys. Stainless steel is generally one of the least used materials in boat engines.



Ideally, engine parts are made of the best available materials according to conditions that they must endure. But compromises must be made, and often cost is a big consideration. Not sure of the point of your question. Example: It might be great to have a stainless steel oil pan, as it sits above a bilge that can hold sea water that can slop over it. But the cost of one would be several times that of mild steel. So the manufacturer uses mild steel to keep the cost competitive.



The question of using mild steel or stainless for mounts is comes down to cost and convenience, as both are of sufficient strength and durability.