Aluminum or Steel?

[jr438234606]

Ok all you metallurgist out there: In the process of doing my repower I have found it necessary to adjust the engine bed in order to get the engine to the right height relative to the shaft. I don't want to use GRP to modify the existing engine bed, which was constructed when the hull was originally laid up, so it's very strong. I would rather adapt it to what I have now using metal of some sort with heavy fasteners.

I've designed a bracket to fit perfectly. I'm going to bring it to the machine shop tomorrow and have them fabricate a pair. I just need to know what to tell them to make it from: Stainless steel, or aluminum?

Aluminum would be lighter and cheaper to fabricate. But would it stand up to the vibration? Steel would be stronger (I think?), but would it be overkill?

I've including some photos. Any advice is appreciated!

[Faster]

You'll have fewer corrosion issues down the road with SS, and since you're on the isolated side of the engine mounts I don't think vibration will be a huge issue

[Rockter]

Design to get them out of there fairly easily. Given the choice I'd go for stainless, but don't paint it. Watch that the vibes do not loosen those stud fasteners. Keep checking.

[sailingdog]

You really want to use stainless steel. Two major reasons: 1) it is more corrosion resistant and will generally have less problems in that area of a boat; 2) it is far more fatigue resistant. Vibration may cause aluminum to fatigue rather quickly.....and lead to the bracket failing.

[Sasha_V]

Get it machined in a solid engineering plastic like vesconite.
Better at coping with vibration, dampens sound instead of transmitting it. Totaly corossion proof and lighter then either stainless or aluminium. Stainless will work harden fomr the vibration and eventually become either brittle or lose its chromium and rust (depending on type). Aluminium will be like a sacrificial annode in that location and application...And you would want to get it in a soft temper like T3 or T4...T6 will workharden and crack too.

[sailingdog]

I wouldn't recommend Vesconite due to some of its limitations:

Vesconite is suitable for continuous use at 100ş to 120°C (212ş to 248°F) in dry conditions and 60° to 70°C (140° to 158°F) in wet conditions.

What do you think the average temperature of an engine room in a small sailboat is when the engine is running??? Considering that the thermostat on a raw-water cooled engine is set for 140˚F... it is probably a bit warmer than that near the engine.

[Valiente]

True, but at the bottom and aft, away from the block and the oil sump?

I simply don't know the answer for that area. I do know the answer for the head: friggin' hot!

[beej67]

I sure as hell wouldn't want a plastic engine bracket in my boat.

[pfatyol]

316 Stainles or 17-4 ph SS (17-4 annealed not heat treated) would be a good choice. 316L is not worth the extra money for an engine mount.

[rheaton]

You might consider fabricating the piece our of fiberglass and epoxy. machine this piece to fit then use 5200 to attach it. This would be strong and would not rot or corrode. I saw this described for use as a backing plate for a thru hull fitting.