Silicon bronze hydraulic ram shaft

[Mischief Managed]

My dinghy has a CMC PT-35 tilt and trim. It worked well for 3 years and then started leaking oil from the hydraulic ram shaft. I took it apart yesterday and discovered the ram is made of untreated aluminum and after 3 seasons of salt water exposure, it is horribly pitted, thus the leak. It cannot be "cleaned up", it's too far gone. CMC does not sell the ram and want to sell me a complete pump/ram assembly for 600 bucks, yeah, um that's a no for me...



A friend of mine has a machine shop and will be making me a new ram out of a $123 chunk of solid silicon bronze 655 alloy (the best for marine use). I cannot think of any downside to this, but I'm curious if there's some dumb thing I have not considered. I know there will be some galvanic action and plan to add a zinc. I also know verdigris will eventually cause problems, but I suspect it will work 5 times longer than the aluminum part it replaces.



Your thoughts?

 

[Bmarler]

Why not stainless?

 

[Fleming]

Agree stainless steel would be better. The nearby aluminum will become the anode to the bronze cathode in that galvanic circuit. You'll want to isolate it is as much as possible and probably add extra anodes to the lower unit.

 

[Mischief Managed]

Why not stainless?

Solely due to the ease of machining bronze compared to machining 316 SS, which is much more challenging. The machine work is a favor, after all and I know how awful it is to work with 316 SS and how pleasant it is to work with bronze...

 

[OldDan1943]

Why not stainless?

 

[jungpeter]

Hi Mischief,

I'd be inclined to look into adapting a filament-wound fiberglass cylinder. Something like this:

https://polygoncomposites.com/tailored-solutions/cylinder-tubing/

Regards,

Pete

 

[Mischief Managed]

Hi Mischief,

I'd be inclined to look into adapting a filament-wound fiberglass cylinder. Something like this:

https://polygoncomposites.com/tailored-solutions/cylinder-tubing/

Regards,

Pete

I wish it were that simple. The pump and cylinder all all one unit.

 

[Bmarler]

I wish it were that simple. The pump and cylinder all all one unit.

I see. If the housing is aluminum I probably wouldn’t use bronze. I know it’s mostly isolated from the housing, but aluminum and bronze aren’t a good match for marine service.
Stainless isn’t tough to machine, I do it all the time.

 

[Fleming]

but aluminum and bronze aren’t a good match for marine service.

That's an understatement. I just finished reading Nigel Warren's "Metal corrosion in boats" and he made it pretty clear that any of the copper based metals will make short work of nearby aluminum.

I'd love to see what it looks like after a year of use. Will be an interesting corrosion experiment.

 

[OldDan1943]

I am an old fashion guy. Bronze and stainless.

 

[Mischief Managed]

I was looking at it this weekend and realized that the aluminum piston was acting as a sacrificial anode for the rest of the tilt/trim. Must be a different alloy because there's no corrosion on the rest of the mechanism. I'm interested to see how it works out. If corrosion becomes a major issue, I will address it.

 

[C lectric]

You might consider chrome plating the rod. That was done for all kinds of industrial machinery that was exposed to the full weather.
That may be supplanted now by SS. but plating would protect the rod.


Don't really know if it is practical so just a suggestion.

 

[jungpeter]

I was looking at it this weekend and realized that the aluminum piston was acting as a sacrificial anode for the rest of the tilt/trim. Must be a different alloy because there's no corrosion on the rest of the mechanism. I'm interested to see how it works out. If corrosion becomes a major issue, I will address it.

Hi Mischief. Given your CMC tilt mechanism employs mixed metals in a saltwater environment, you're pretty much doomed to failure, no matter what. Simply disassembling aluminum parts with stainless fasteners is typically an exercise in frustration. While attempting a DIY repair may gain you some time, it's a bit like putting lipstick on a pig. It's gonna get ugly again real quick.

You may be better served by trading in your existing outboard motor for one recommended for salt water use, with an OEM power tilt. While not the end-all for corrosion issues, at least the OEM outboard manufacturers recognize salt water use by various measures (plating/painting/isolation/material choices, etc.) to at least minimize corrosion. It appears from their advertising (and your experience) that CMC does not, and expect this product is more suited for fresh water use only.

Regards,

Pete

 

[OldDan1943]

You might consider chrome plating the rod. That was done for all kinds of industrial machinery that was exposed to the full weather.
That may be supplanted now by SS. but plating would protect the rod.


Don't really know if it is practical so just a suggestion.

Crome plating will change the clearances ….. keep that in mind.

 

[Steve DAntonio]

The ram is aluminum, or the cylinder? Rams are usually stainless, with AL cylinders.

These pump mechanisms are aluminum alloy on thousands of outboards and they hold up pretty well for years and years, even when not maintained, and if the tender spends most of its life out of the water, this should not be an issue. Are you changing the anode regularly?

As others have noted, bronze is like kryptonite to aluminum, it will need to be completely isolated from the aluminum.

https://stevedmarineconsulting.com/understanding-and-preventing-aluminum-corrosion/

 

[Mischief Managed]

The ram is aluminum, or the cylinder? Rams are usually stainless, with AL cylinders.

These pump mechanisms are aluminum alloy on thousands of outboards and they hold up pretty well for years and years, even when not maintained, and if the tender spends most of its life out of the water, this should not be an issue. Are you changing the anode regularly?

As others have noted, bronze is like kryptonite to aluminum, it will need to be completely isolated from the aluminum.

https://stevedmarineconsulting.com/understanding-and-preventing-aluminum-corrosion/

The ram and the "cylinder" are aluminum. The "cylinder" is just a hole in a box shaped machined billet case that also contains the motor and pump. The seals should isolate the piston and the shaft of the ram from the case and I can easily engineer a method to isolate the skinny end of the ram, it's just attached with a roll pin, and replacing it with a nut and bolt with an insulator sleeve like PEX would be a breeze.

 

[OldDan1943]

How long has the current ‘set up’ worked?
It may be worth putting this on a scheduled replacement

 

[Mischief Managed]

How long has the current ‘set up’ worked?
It may be worth putting this on a scheduled replacement

Bought it new in 2020. Used it for 3 seasons. At 600 bucks for the repair part, I won't be buying another one. I originally bought it solely because my engine does not have the option of factory tilt and trim. My "new" engine does have the option and if the bronze ram wrecks the CMC PT-35 tilt and trim, I will just install factory tilt and trim on the engine and be done with it.



When I leave the dinghy in the water, I will strive to leave the trim all the way down so that very little of the bronze is exposed.