Bent prop shaft repair- will it be good as new?

[wsmurdoch]

As Billinghamster said, "...a slight corkscrew in my shaft. Local shops said it couldn't be fixed but I had very little money and access to a shop with a lathe and press and plenty of time. With careful measurements, marking and pressing using a dial indicator to determine precisely when "yield" was met, repeat many times, turning slowly in a lathe between centers. I was able to get it true to better than .002 runout anywhere along it's length."

I have watched the process. It is not a hit. Nothing that savage. It is a careful and progressive series of presses. A simple bend would be easy. Multiple bends hard. A corkscrew harder still. It is slow and patient work. The artisans use heat as well as force. That steers the bend to the desired place and spreads it along a length. It is planning, thinking, maybe a smoke or two; all before doing anything.

[Prairie 36]

Guess what. Even a brand-new shaft must be trued. Why, because prop shaft material comes in long lengths which get bowed from handling and stowed on a rack. Once it is cut to length it must be trued before it is machined for proper taper and threaded.

[Goodxcharly]

Just by new and you won’t have to worry about it. The shaft is a valuable link when motoring away from a lee shore.

[MicHughV]

How did it get bent in the first place??

[Distant_Shaws]

I have straightened many shafts not just prop shafts.

I use a set of roller Vee blocks to allow the shaft to rotate and a dial gauge to measure the out of true and location.

I the use a hydraulic press to deflect the shaft i the opposite directing by twice the out of true measured.

Test again until the out of true is small.

A similar rig can also be used to balance a prop or coupling on a shaft

When I worked in an Engineering shop we would do this obtain when making shafts from stock material which is not always straight enough when supplied

[sailcrazy]

Just after we bought our sailboat-25 years ago- we learned that at the PO last haulout he allowed the lift strap to bear on the shaft, and not the hull. Yup, bent shaft. He had it straightened and reinstalled...it was OK for a while, but got worse. I pulled it out, and was told by a prop shop the best soln was a new shaft. We did replace it, and no issues since. I'd replace it!

[chriswheat]

MicHughV nailed it. What caused the shaft to be bent? If the cause is identified and remediated then a straightened shaft will probably be fine assuming it has not suffered traumatic injury. It could well be an alignment problem, or maybe something else. Basically a straightened shaft is almost as good as a new one if the bend was slight.

[svmatilda]

I had my prop shaft straightened about 6 years ago and havent had a problem since. Wha have evn had a heavy line around the shaft (line cutter chopped it) and a ding in the prop from a fishing float, still no run out and no vibration. An experienced shop should know the limits of run out and advise accordingly.

[MicHughV]

On my first boat, the prop shaft was not properly aligned with the engine and cutlass bearing. Over time, the cutlass bearing wore out, and the prop shaft would wobble around in there when the engine was engaged.

At my first haulout, I decided to take a closer look and pulled the shaft.

Much to my amazement, the shaft was quite worn where it passed thru' the cutlass bearing and a slight bend could be seen. The worn area was clearly seen, and I was flummoxed to understand how rubber could wear out stainless steel, but it did. Additionally, the shaft was worn at the flange coupler.

Long story short, I spent considerable time to straighten out this dilemma, which involved the purchase of a new prop shaft, as the worn shaft was un-fixable.

It is quite an art and procedure to get an engine, shaft and bearing, all perfectly aligned and I wonder just how much attention is given to this matter in the initial build ??

[BBill]

It depends on how bad it was bent and IF you can find a shop that can do it. But yes shafts can be straightened just like new without problems showing up later. We had a 27 twin straight drive inboard and curled the props and bent both shafts on a coral bank. Had the props reworked and shafts straightened...all bronze. I think its a lost art anymore since straight drive runabouts are far and few. Good luck finding a shop that has a clue other than telling you to buy a new shaft.

On another tack...I had a badly scored SS drive shaft and had a machine shop weld in material, grind it smooth and polish to a mirror finish. It was their suggestion, not mine but it worked fine just like a new shaft.

[skipperpete]

Originally Posted by AKA-None

What I learned some time back was to use v blocks and a dial indicator to check runout

Yes Aka-None,( and Distant_shaws), spot on. The first and simplest way to check a prop shaft for true. I use a pair of Vee blocks with 1 ½ “ diameter ball races to spin the shaft and a dial indicator to measure runout..... no way a bent shaft can escape detection, and the whole operation can be done on any reasonably flat surface. The guys that make ( and straighten) the prop shafts for the passenger ferries where I worked don’t even use a dial indicator, just a steel pointer on a metal block. Interestingly, often a new length of stock can have substantial runout and require either straightening or return to sender, delivery drivers are often less than gentle with their cargo and senders less than fastidious with protective packaging.

[BBill]

Originally Posted by skipperpete

Yes Aka-None,( and Distant_shaws), spot on. The first and simplest way to check a prop shaft for true. I use a pair of Vee blocks with 1 ½ “ diameter ball races to spin the shaft and a dial indicator to measure runout..... no way a bent shaft can escape detection, and the whole operation can be done on any reasonably flat surface. The guys that make ( and straighten) the prop shafts for the passenger ferries where I worked don’t even use a dial indicator, just a steel pointer on a metal block. Interestingly, often a new length of stock can have substantial runout and require either straightening or return to sender, delivery drivers are often less than gentle with their cargo and senders less than fastidious with protective packaging.

What diameter shafts were on those passenger ferries?

[skipperpete]

Originally Posted by BBill

What diameter shafts were on those passenger ferries?

Mostly 3” on the smallest and 4” on the larger vessels but the Roro’s had bigger shafts. Bent shaft/s was a very common event, almost exclusively from prop strikes.
The father and son team made a huge variety of shafts from as small as 1”-2” for cruising yachts but the main business was ferries, both slow Roro’s and high speed passenger vessels.
They used a forklift, chain blocks and a huge hydraulic press to do the work....... but I watched a machinist at Tropical reef shipyard in Cairns straighten a very large diameter shaft with heat alone between centers, he’d heat one small spot with an oxy acetylene torch and I watched the dial indicators register the movement, very slow work.

[CarinaPDX]

Clearly repair is a reasonable possibility. You might put a bit of effort into pricing for a new shaft - the prices can vary widely. There are companies that do high volume computer-controlled lathe work on shafts. Since you have access to the old shaft you can easily collect the information required, and especially the taper for the prop, and order direct. This might be an attractive proposition. If you do buy a new shaft then be certain to get a high quality alloy for ocean use. The largest local prop shop stocks the marine alloy based on 304 stainless - appropriate for fresh water - but only special orders the alloy based on 316 for salt water. (Shaft alloys use additional metals in the alloy but start with the standard formulations.)

Greg

[skipperpete]

If I had a choice, I’d generally opt for a new shaft rather than repair it (in the small diameters). A lot depends on which part of the world you happen to be transiting when the propshaft becomes a problem. Asia is good for lower cost labour but the quality of stainless steel is highly variable, Europe has expensive labour but far more reliable regarding the identity of the stainless steel.....Australia is similar in this respect.