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| S&S Swan Maintenance - Prop Shaft wear and bearing replacement |
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Join Date: 01 February 2007
Posts: 234 |
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Prop Shaft wear and bearing replacement I was replacing my prop shaft bearing (cutlass bearing) recently as after 12 years we had some wear. I took out the shaft to make getting the bearing out easier. The wear on the shaft at the bearing point was quite noticable, which means I will have to replace the shaft also. Is 316 Stainless Steel the only option for the replacement shaft? Can it be hardened in any way at the bearing point? Any comments on specifications for the replacement cutlass bearing? John |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear John If the shaft is worn this is likely to be the result of a too tight cutlass bearing. Bearings with phenolic (Tufnol) sleeves may develop this problem, because the sleeve can swell in water. The rubber bearings wear a little in use, and it has even been suggested that the swelling is an advantage as it compensates for the wear. If the engine is rarely used, however, the swelling may make the bearing extremely tight, to the point that the engine has to struggle to turn the shaft. Have you noticed such problems? It is suggested that when the yacht is ashore the shaft is turned by hand with the gear in neutral to find out how stiff the bearing is. An additional comment here - the bearing has to be flushed with fresh water, preferably immediately after lifting the yacht. Otherwise dried salt crystals may make it seize, and work like sandpaper inside if the shaft is turned by brute force. It is suggested you make sure that the new bearing sleeve material does not swell when immersed in water. It might be appropriate to mention here how to determine if the cutlass bearing is worn - pull and push firmly on the shaft, if you can feel play it is time to replace the bearing. If the propeller shaft is 1 ½ " or 38.1 mm, it is likely that it is original, and not of ordinary stainless but of a better shaft material denoted Aquamet 22. It would be advisable to attempt to keep the shaft provided it is still strong enough for the new engine after machining the outer end to the next bearing size. Also "mixed size" bearings are available, with metric inside, and imperial outside dimensions, and vice versa. Guess you are referring to the pitting resistance discussion in the thread "Stainless Steel trailing edge to keel" 30 Sept 09 when you ask about suitable shaft materials. PREN for Aquamet 22 is 39, while it is 25 for AISI 316. Kind regards Lars |
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Join Date: 16 May 2009
Posts: 252 |
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Here's a link to a brochure on the Aqualoy metals.... |
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Join Date: 16 May 2009
Posts: 252 |
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John |
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Join Date: 01 February 2007
Posts: 234 |
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Lars, Geoff thanks for your input. I asked my local yacht builder (they claim to compete with Swan) what steel they used for there yachts. Answer 316. I have also spoken to a specialist engineering company who say they would only use a higher grade of steel in circumstance where they wanted to reduce the weight of the shaft without loosing torque or in longer shafts. So mainly in lightweight racing yachts. The price premium is about 30% for the steel and because it is a harder metal it take more time to machine, so those costs increase. Given that my shaft is less than 2 meters and the shaft thickness 1 1/2 inches are we over engineering? The 316 steel shaft lasted 12 years. What is also a challenge is how do we know that the engine is in the correct postion when fitting the P bracket and then shaft? When fitting the new engine some 12 years ago we just used the same engine bearing positions. Is there any way we can check this Lars? The fine tunning I know we can do at the hard coupling. |
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Join Date: 16 May 2009
Posts: 252 |
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John: As you indicate, you had ..quite noticeable wear.., which means before the 12 years was up, things were not on spec. Maybe the alignment was not perfect, and the bearing swelled, and these taken together caused more of the wear than anything else. Were you experiencing excessive vibration? Any banging on changing gears? But - in this world nothing is ever perfect, and even if you start out near perfect it wont stay that way - so having a higher grade of metal there just gives you more margin, more life through longer wear. Only you can decide if the extra $$ will be worth it to you. You know what you will get out of the 316. One thing to note - 316 is susceptible to crevice corrosion, which can take place in tight spots, as when a shaft passes through a bearing, etc, and if this takes place, you can get an abrupt failure. I do not know personally of shaft failures with this mechanism, I only know that shafting installations could support this corrosion mechanism under certain conditions, probably low use/no use would be worse than anything. Maybe Lars has more to add. On a few hundred dollars, Id pay the 30% for the material and extra for machining myself, but thats just me - if you are getting up and over a grand, maybe not.... As for alignment and engine position, I have only adjusted same at last point of make up of primary coupling, using feeler gages and taking my time turning the shaft - I expect there are more refined ways of getting things dead nuts. Good luck, and best Regards, Geoff |
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Join Date: 30 January 2007
Posts: 461 |
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Hi Geoff, |
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Join Date: 02 January 2008
Posts: 1547 |
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John Thank you for the information You are right that if only torsional strength is considered the shaft diameter seems rather big, but there is also another aspect. The allowed unsupported length of the shaft is determined by the diameter and the material modulus of elasticity. The modulus is not changing with strength, but is similar to other corrosion resistant steels, and therefore the required diameter is also similar. Safety factors may also influence this. S&S Swans were built for global use, and several shaft materials were tried. Aquamet 22 was chosen based on worldwide feedback Referring to your question about the shaft line - the normal procedure is to align the P-bracket with the help of a shaft centered in the stern tube, and the engine is then aligned with the shaft. The propeller hub forward end should be at the most one shaft diameter aft of the P-bracket. Because engine rubber mounts compress slowly with the time, it would be good to sometimes check if the shaft still is centered in the stern tube. If it has dropped so much that it touches the tube wall this will certainly be noticed, the noise is impressive. Checking is done by opening the coupling bolts at the forward end of the shaft, and lifting the shaft as far as it will go, and then pushing it down to the other end position. The center position is halfway. This presumes that there is a flexibly supported shaft seal allowing such movement. If the engine mounts need to be adjusted this is best left to specialists. The principle is that equal turns are turned on both mounts at the same end. Do not adjust one mount forward and one aft, then the result may be a diagonally supported engine with strange vibration problems. Geoff and Daniel I would suggest PREN mentioned in my post Dec. 4 also gives an indication of resistance to crevice corrosion. Lars |
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Join Date: 01 February 2007
Posts: 234 |
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Guys, the level of knowledge and experence in this forum is quite amazing. Thankyou, I will be replacing my shaft with the higher quality steel, after all its a SWAN |
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Join Date: 02 January 2008
Posts: 1547 |
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Daniel Would you disclose where your shaft corrosion is located, and what it looks like? Has it changed during your ownership? In principle the cathodic protection should work like this: As long as there is a zinc it protects the propeller and shaft If the zinc is gone the propeller becomes the anode, and corrodes while it protects the shaft. Assume the propeller did not disappear, and it would be possible that the shaft and propeller corrosion was caused by an electrical leak Remark: A zinc with a thick white oxide layer on the surface does not work properly. The reason is too high iron content in the anode material. 0.005% is the upper limit. Kind regards Lars |
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Join Date: 30 January 2007
Posts: 461 |
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Hi Lars, |
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Join Date: 02 January 2008
Posts: 1547 |
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Daniel Thank you for the information The reason for the pitting you report is likely to be a very corrosive environment. I would guess these four factors have contributed - each one increases the tendency to pit - No zinc anode - Barnacles on the shaft - Quiescent water, possibly containing some chemicals. - Boat sitting in marina for long times. The propeller also suffered - this means that the effect has been exceptionally strong If the pitting then stopped this indicates that the conditions in your marina are much better, you probably added a zinc and keep the shaft clean. For a boat sitting idle in the marina the tendency to pit is reduced if the shaft is turned a few turns once a week or so. The propeller performance suffers due to the pitted surfaces on the blades. If the propeller otherwise is in good condition filling the pits by welding could be considered, depending on the areas affected. Attention John - a worn or pitted shaft may be repaired by welding provided you can find a shop capable of making a weld overlay fusion. First it would be necessary to make sure that the shaft material actually is Aquamet 22. A simple check - if the shaft is magnetic, it is not of this material, but also AISI 316 is non-magnetic. Kind regards Lars |
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