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

[Notfast]

I just installed a Beta Marine engine into my boat this summer and had a similar situation. There was no metal available for the mounts except aluminum so I used aluminum with Monel fasteners which have little to no reaction with aluminum in terms of galvanic corrosion. It's not a frequently requested item so you might have trouble getting them but thus far, six months later and I see zero evidence of corrosion. I would place some sort of insulating material between the engine feet and engine mounts to prevent corrosion there but, assuming your engine has rubber vibration damping, the feet will probably need replacing before the metal integrity becomes an issue. Still worth considering, though.

[CarinaPDX]

I went with Orion Jim's approach. In my case I needed to raise the bedlogs by 2 inches so bought 2 inch thick G10. I drilled and tapped almost but not through to the bottom, for the engine mounts. I intend to put them down on the existing bedlogs with thickened epoxy, then add glass/epoxy tabbing all around.

It would also be possible to use bolts through the G10 and into the existing holes in the bedlogs to the steel. I would still use epoxy as above.

Greg

[Boatyarddog]

Originally Posted by 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

The AL is fine, it's just thru bolted to the steel bar in the GRP bed, a common practice, AL will be easy enough for you to tap for the mount bolts, as well makes a smooth surface for the engine alignment.
Tuff gel the Bolts, washers & lock washers to avoid corrosion.
You might consider some 4200 under the plate for vibration, if you let the 4200 under the plate firm up, then tighten the plate bolts, it will give a bit or vibration damping effect.
Boatyarddog

[Fore and Aft]

Carina PDX that looks a really good solution. I have seen similar except the client used a bit of lumber that he found in the skip bin and to top it off it must have been awkward to drill the timber so all the mounts were crooked on the engine beds.
Cheers

[RaymondR]

Originally Posted by 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?

The threads in the aluminium were there to hold the countersunk bolts in place and prevent them from turning. From the bottom, up through the aluminium, countersunk to allow the aluminium to sit flush on the old steel engine beds. The mounts sat over the bolts and there were nuts on top as you suggest.

The great thing about aluminium onto steel is that the aluminium will corrode and in doing so protect the steel. The aluminium is generally cheap and easy to replace compared to the steel.

[SV__Grace]

Originally Posted by 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.

Same here. Our new Beta 85 replaced a Ford Lehman 90 and the new engine is mounted on robust 1 inch thick square aluminum tubes like this about 4 inches higher.

They cut the ends diagonally which allows the tubes to be bolted to the stringers at each end from the inside bottom of the tube rather than thru bolted from the top.

[stormalong]

Originally Posted by RaymondR

The threads in the aluminium were there to hold the countersunk bolts in place and prevent them from turning. From the bottom, up through the aluminium, countersunk to allow the aluminium to sit flush on the old steel engine beds. The mounts sat over the bolts and there were nuts on top as you suggest.

The great thing about aluminium onto steel is that the aluminium will corrode and in doing so protect the steel. The aluminium is generally cheap and easy to replace compared to the steel.

Last year I replaced the engine mounts on my Betamarine 50. It was not an easy job to replace them with the engine in place. Access to one mount was particularly difficult and took a couple of DAYS to finally get out and replace. The engine was 18 years old at the time. I had installed it initially. When first installed I had the engine, complete with the mounts lowered onto the engine stringers by crane. Access to bolt down the mounts was relatively easy.

Why am I telling you this? Because getting in there with the engine in position was a lot of work. Don't be to sure that replacing the aluminum plates would be easy.

Steel is inexpensive compared to aluminum. It is stronger and should be permanent. As someone posted it is harder to machine. That is because it is harder, i.e. stronger. With proper tools the difference is insignificant. If the OP is making four pieces he/she can drill one and verify that it is right, then it is a drilling template for the other three.

One thing I have learned in about 50 years of boating is to strive for permanent fixes. Not something that has to be revisited every few years.

PS. The cost of materials is insignificant compared to the cost of labor. Even if it is your own time.

[RaymondR]

I'm a big fan of permanent fixes and not beyond an unorthodox use of any material in order to implement them.

I did not do it when I installed the engine on the aluminium, dummy, engine rails but it was my intention to install rubber sheet strips between the steel engine rails and the dummy aluminium rails when I carried out the next 5 yearly hull refit (steel boat) This would have sealed any water out from between the aluminium and steel rails as well as contributing to isolating engine vibration from the hull.

The use of aluminium to insulate the steel of the hull from any item which might be subject to movement and consequently might wear down through the coatings on the steel, for instance sheet winches where I used 1mm aluminium gaskets Sikaflexed to the deck under the winch, became common on my steel boat because it was the best fix to many of the maintenance problems I experienced as a steel boat owner.

[arcticmonkey]

I just read an interesting article on hardwoods... you may wish to puruse wood as an option to aluminum... I would be sceptical of using dissimilar metals

[Dieseldude]

Originally Posted by 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.

Stormalong has the best suggestion.
As for choosing bolt diameter, I would use same size as original bolts. Fine threads have better resilience against vibration. Aluminum is a very malleable material, so threads can distort over time, and your engine could loosen off. Once a tiny bit of slack occurs, bolts could quickly loosen out. Also, aluminum in close proximity to steel near the bilge is subject to galvanic corrosion. So steel would be the material of choice. In this critical situation, proper drilling and tapping technique must be used. Holes should be made in a drill press, not with hand held drill.



Precision layout is essential by making a template. Thin cardboard or thick paper pressed against original holes should make a good drilling template. This will give a precise impression of the original hole locations. I find file folder paper to be very good. Holes should be well located using good layout methods to find center points on the template. Holes should be center punched through center points on the template, then pilot drilled. First pilot hole should be made with a combination drill bit to prevent drill from skating off center. Even in a drill press, small diameter twist drills can flex and skate. Final drill diameter must be chosen with reference to a tap drill chart. High quality drills and taps should be used.


Since the adapter plate holes are through holes, one tapered tap might be the only tap needed. Hard to say if there will be need to progress from taper to plug to finishing tap. But sometimes a taper tap will not achieve full thread diameter in thick material, so may need to complete with finishing tap. A "gun" tap might be the only tap needed, as they are designed to produce a full thread diameter. It is essential to keep the tap perfectly perpendicular. This can be achieved by starting the tap in a drill press set up. If threads are slightly off perpendicular will prevent bolt head from firmly seating.



After the engine bed holes in the fabricated plates are completed, the plates can be temporarily assembled to the original bed plates. Then the engine mount holes can be marked out. The engine will need to be temporarily fitted and held in place to locate and precisely mark the engine mount holes. Perhaps a scriber can be run in the mount holes. Applying layout die to the adapter plate first would be a help. A dark felt marker can be used in lieu of lay out die. Or, if you have room, a transfer punch would be superior to scribing.


Whether to loctite or antiseize the bolts is a subjective decision. High strength loctite could make future removal a problem, as you would not want to use a torch against the fiberglass hull to soften the loctite. Perhaps low strength should be chosen. Alternately, antiseize can be used. Of course a standardized bolt torque chart should be consulted for tightening bolts, keeping in mind correction for lubricated threads. A good torque chart will provide lubed thread torque values. If a standard steel bar stock thickness is not right, shims can be made and added to slightly thinner material.



If you have good fabrication skills, please pardon all the details that you would already know. But for anyone who may have limited experience, this could help insure success. This is a critical project, as slight misalignment could cause drive train vibration and stress issues. Best to take the time and care to achieve a fit that can be trusted.

[stormalong]

Originally Posted by arcticmonkey

I just read an interesting article on hardwoods... you may wish to puruse wood as an option to aluminum... I would be sceptical of using dissimilar metals

Reread the original post. A 5/8 inch piece of wood would not be able to handle the strain of an engine. The OP wants that spacer to carry the thrust of the engine on some of the bolts without direct through bolting to the original engine bed.

I also noticed that the OP has updated his profile and the is for an HR 31.

I am guessing the engine being mounted is around 25 HP.

[Ballsnall]

Originally Posted by kjemmo

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?

No, it won't be strong enough. Great solution though.
As previously suggested countersunk S/S screws from the bottom of the ally plate will be adequate for the task. (Make sure the ally plate is threaded as well as countersunk and screws loctited so they don't rotate when fitting the nuts. Also only use steel or bronze nuts on the s/s screws so they don't gaul) Bed the alloy plate as suggested and forget about dissimilar metals corrosion it just won't be an issue.

If you can afford to use stainless steel instead of aluminium then that's what I would use but with the inverted S/S screws the ally will be just fine. A very simple and neat solution.

[VChild]

It is relatively inexpensive to have a machine shop precisely drill and thread countersunk holes on the top and bottom surfaces of mild steel plate. Only the bottom holes under the motor mounts would need to be threaded. Mine were done on 1/2" thick 316 stainless, but mild steel is much cheaper. High retail price for 5/8” x 3” bar in mild steel is less than $20 per foot and is much easier to machine than stainless (and much stronger than aluminum.) My plates had multiple bolt holes on top because I used quite a few lag bolts going down into the wood core of my engine beds. In your case it would only be a couple of top holes to line up with the holes in your embedded plate.

[Dieseldude]

Originally Posted by VChild

It is relatively inexpensive to have a machine shop precisely drill and thread countersunk holes on the top and bottom surfaces of mild steel plate. Only the bottom holes under the motor mounts would need to be threaded. Mine were done on 1/2" thick 316 stainless, but mild steel is much cheaper. High retail price for 5/8” x 3” bar in mild steel is less than $20 per foot and is much easier to machine than stainless (and much stronger than aluminum.) My plates had multiple bolt holes on top because I used quite a few lag bolts going down into the wood core of my engine beds. In your case it would only be a couple of top holes to line up with the holes in your embedded plate.

Stainless puts up the cost of fabrication much more than mild steel. Its material cost is much more than mild steel. It is much tougher to work with too. More costly higher quality tooling is needed. Machining operations must be done slower. Stainless is not needed for this application. Mild steel is fine. With a coat of good primer and paint, mild steel will not rust. Threads can be either loctited or antiseized, so will not rust. And, if you do not have a good drill press and good fabrication skills, it is quite a simple job for a machine shop to drill and tap some holes.

[Ballsnall]

Originally Posted by Dieseldude

Stainless puts up the cost of fabrication much more than mild steel.

More costly higher quality tooling is needed.

Mild steel is fine.

With a coat of good primer and paint, mild steel will not rust.

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.