Steel prop shaft versus stainless

Any real issues with fitting a boat with a forged steel shaft, as opposed to stainless? The shaft is 4-3/4" diameter and water cooled (not grease packed). There would be a huge difference in price (at least 4X more for 304 grade). The shipyard uses forged in all their ships and if I could get a 30 year life out of it, that will take me happily to my grave.

Metalize the shaft

I know stainless-steel shafts can be metalized to bring bearing surfaces back into spec. That method may be applicable to your steel prop shaft in the areas of the shaft that the seals are installed and bearings ride to prevent erosion corrosion and just regular wastage of those surfaces. I would imagine the costs of that process would be reasonable. Reliable contact with anodes will also have to be established to provide protection from galvanic corrosion.

Don’t use 304, always 316 or
duplex 2205

Jason, I wouldn't be able to afford the 316, which is even more than the 304 grade. My budget is running tight - cost and scope creep is a horrible disease that I suffer from

On another note, all their handrails are either 304 or painted mild steel. I'm going to see if I can get them to TIG weld the stainless (for appearance sakes) and then passivate it for better durability. Still checking cost estimates.

"Any real issues with fitting a boat with a forged steel shaft, as opposed to stainless?"

If it works for other boats with that sized shaft why the question?

Use what ever system they use and your service should will be similar.

"The shaft is 4-3/4" diameter and water cooled (not grease packed)."

If the locals choose grease packed Ii would copy that.

Wow, 4 3/4" ! What HP engine is connected to this shaft? How long is the shaft? Not only is the shaft expensive but so too are the through hull and shaft support(if any) fittings.

Operating in the Persian Gulf would lead me to want more corrosion protection on a water lubed shaft. But, saying that, I've not seen a large ship with SS shafts. I was on one with a spare 12" dia shaft hanging from the ER ceiling.

Shaft length is 3.5 meters, about 11 ft, and the spec'd power is 265hp although we may drop that down to about 225hp. Prop diameter about 56" max (not finalized yet of course). These trawlers install really high torque engines, so the big shaft diameter is needed.

The shipyard that I'm working with is in China. Dave has a good point about the Persian Gulf but we might not bring it here since it's for our retirement in a couple of years and we'll be traveling. The water temp here is at least 93 degrees F in the summer and the 1st thermocline is below 15 ft (don't ask me about the time I almost passed out when at my safety stop). Very saline body of water and highly corrosive.

I'm going to ask about the grease-packed shaft. The funny thing is that the more I work with shipyards, the more I realize that they actually know what they're doing. So I'm starting to learn to trust them.

makobuilders said:

Jason, I wouldn't be able to afford the 316, which is even more than the 304 grade. My budget is running tight - cost and scope creep is a horrible disease that I suffer from

On another note, all their handrails are either 304 or painted mild steel. I'm going to see if I can get them to TIG weld the stainless (for appearance sakes) and then passivate it for better durability. Still checking cost estimates.

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You could electropolish it but if you do used 304 in a marine environment it will pit and you will get rust stain appearing and crevice corrosion Price between 304 and 316 isn’t that much... I ran a stainless steel business. With hand rails the higher the grit polish the less likely it will tea stain.. but 304 will require more maintenance.. IMO

okay thanks, I'll check out the difference in price for upgrading to 316. They also build yachts so they obviously have access to polished handrail material.

Mako, a bit of shaft sleuthing will show 2205 is better than 316 with Aqualoy 22 at top of heap - and $$$. Even the best stuff can suffer from improper installation allowing crevice corrosion. Nordhavn has seen the need to replace some shafts on the 55/60 model due to corrosion.

Nothing wrong with going with steel vs SS, especially in a shaft that thick. What needs to be done is to prevent corrosion, and there are several ways to do that. Clad welding, sleeves, seals with grease, etc. I would not want to run a steel shaft in a water lubricated bearing!!

SS shafts make sense in small diameter as there is not that much material involved. Get big and things get different.

I raised my eyebrows when I saw 275hp and a 56" prop and a 4 3/4" shaft. That is out there!!

There must be a lot of torque: I wonder what ratio the gearbox is for 225hp to swing a 56” wheel such that it needs a shaft that big.

That calculates to a 650 lb shaft, btw. Without prop, flanges, bearings, nuts....

There are lots of ways to prevent/reduce corrosion in steel.
If its over-engineered it should be fine for a long time.
But as Neil Young put it...."rust never sleeps".

Woodland Hills said:

There must be a lot of torque: I wonder what ratio the gearbox is for 225hp to swing a 56” wheel such that it needs a shaft that big.

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The gearbox that is spec'd is 5.44:1 with an engine max torque rating of 790 ft-lbs. However there are on the table other engine options which rate at 1100 ft-lbs and 1600 ft-lbs. Engine selection is fun but is driving me crazy!

I'm going through a job change right now and hopefully will fly out to the shipyard in July to see if we can come to agreement on the contract. I'm still favoring the grease-packed shaft, but since these guys have built hundreds of vessels (their way) I'm going in with an open mind.

With that mass of shaft and prop be sure to select a shift system or tranny with a built in delay , or brake , so when shifting from F too R the mass has time to stop rotating.

You can safely use a forged carbon steel shaft. However, you will have to either shrink on or clad weld 316 stainless steel bearing sleeves and stuffing box/front seal area. The remaining shaft exposed to the sea must be wrapped with epoxy and glass cloth. The threaded end with nuts are typically sealed with an end cap filled with tallow and the front end of the wheel sealed off.
Any competent propeller shop will be able to do this since this is commonly used in commercial vessels. I suggest you contact a naval architect in your area and he can make a detailed spec with a proper drawing the shaft can be built by.

With oil packed bearings could one just paint the shaft everywhere but where the seals and bearings were?
Do boats like this ride on flexible mounts that allow the shaft to move fwd from prop thrust? Or do they mount solidly to engine beds that are attached solidly to the boat hull?

This is the best photo I can find right now showing the prop and rudder (with fishtail BTW). Excuse the horrible quality as I zoomed in. There certainly isn't much shaft exposed to the water between the deadwood and the propeller.

Can't tell too much, but it looks like a streamlined cone covering the nut, so perhaps it is as bjornm stated, filled with tallow or grease or whale puke or something else slimy and protective.