Can a prop shaft move forward.

[atlowers]

Boat just been lifted. Noticed a worn shiny groove on props haft about a centimeter out from the p bracket and cutlass bearing. Also had a bit of rope wrapped around the shaft. Assumed it was this that had caused the wear but thought this excessive. Then noticed a difference in the gap between back of p bracket and the prop when compared with my other engine shaft. Measurements reveal a 13mm difference in length of shaft from stern gland exit point to back of prop. Is it possible for a prop shaft to move forward 13mm and the groove in the shaft is from where the shaft was previously sitting in the cutlass bearing?
Also what effect would this have on the boat. Been in canals for 6 months at tick over speeds and bit concerned about later return to normal cruising.

 

[sailorman]

Boat just been lifted. Noticed a worn shiny groove on props haft about a centimeter out from the p bracket and cutlass bearing. Also had a bit of rope wrapped around the shaft. Assumed it was this that had caused the wear but thought this excessive. Then noticed a difference in the gap between back of p bracket and the prop when compared with my other engine shaft. Measurements reveal a 13mm difference in length of shaft from stern gland exit point to back of prop. Is it possible for a prop shaft to move forward 13mm and the groove in the shaft is from where the shaft was previously sitting in the cutlass bearing?
Also what effect would this have on the boat. Been in canals for 6 months at tick over speeds and bit concerned about later return to normal cruising.

your boat ( as most are ) is being pushed forward by your engine mounts

 

[Tomahawk]

Suggest you look very carefully at the clamp at the back of the gearbox...

It will have two halves. One on the shaft coming out the gearbox. The other at the end of the prop shaft... It is possible that the latter clamp has allowed the shaft to move forward... If somyou shouold also notice a mark of some sort on the shaft just fwd of the stern gland... even a slight polishing mark wil show that the whole shaft has migrated fwd... not good but not nevessarily a big problem...

god luck

 

[scottie]

If it can move at all then it can also move back in reverse fouling the rudder or even coming right out the tube in nothing to stop it.
It should be secured by at least recessed pinching screws or pinned through the side of shaft never through the middle .

 

[Neil_Y]

Assuming you have what is known as a soft drive (thrust loads are taken on the engine mounts and these are rubber) then shaft fore and aft movement is normal (max we see is 9.00mm forewards and 9.00mm aft total 18.00mm)

This will polish the shaft and we often use this polishing as a guide when fitting rope cutters as to how much shaft movement there is to determine spool length.

It is also normal for boat builders to get shafts different lengths, I have seen some with 30.00mm difference.

As you mentioned possibly getting rope/debris in the gap from wrapping round a prop, this causes a huge force trying to pull the shaft aft which can stress couplings or even pull the gearbox off the engine so check anything that could be effected by this sort of force. Check things like oil lines where one end is on the engine and one end is fixed I have come across boats where oil lines have failed when debris was caught as the engine moves.

 

[Topcat47]

Yes it is possible. My puny little 1GM10 still managed to push the shaft forwards by about 6mm, causing the shaft seal to fail. You can drill the shaft and fit a cotter pin.

 

[prv]

Worth checking the coupling, but it may not be a problem.

As others have said, most shafts (and engines) move forward a bit when you start pushing the boat with them. Mine certainly does - if I open the inspection hatch in the cockpit sole I can watch it clunk forwards as I put it in gear. So I would absolutely expect any polishing marks to be slightly aft of what was making them, when the shaft's not turning.

Was a bit concerned when you said the two engines were different, but if Neil's right that boatbuilders don't always get them identical, then that's that explained.

Pete

 

[david_bagshaw]

I would suggest you check the p bracket as well for displacement.

 

[atlowers]

Thanks all for the responses. I had another look today at the gearbox end. There is evidence on the stuffing box that the shaft has moved forward by about 13 mm. Which agrees with the worn groove 13 mm forward of the p bracket. Only just had chance to view replies. I will check the gearbox coupling tomorrow. Trouble is I am returning uk on Monday and boat is staying here in France for winter. Can someone explain the first contribution re boat moving on engine mounts as not quite sure of the meaning. Do I need new mounts? They appear to be in the same position it's the shaft that appears to have moved. Sorry if I am missing something obvious.

 

[prv]

your boat ( as most are ) is being pushed forward by your engine mounts

Can someone explain the first contribution re boat moving on engine mounts as not quite sure of the meaning.

Much the same as what I said. Think how the force is transferred. It's generated at the propellor, but the propellor isn't attached directly to the hull. It's attached to the shaft, so it pushes the shaft forwards. The shaft isn't attached to the hull either, it's attached to the gearbox, so it pushes the gearbox forwards. The gearbox isn't attached to the hull, it's attached to the back of the engine, so it pushes the engine forwards. The engine is attached to the hull, so finally that forwards force that was generated at the propellor starts to move the boat forwards. The boat is, as sailorman said, being pushed along by the engine mounts.

If the mounts are quite soft, then the engine can move forwards a bit relative to the hull before the force is transferred. It's like the whole thing is leaning forwards to push the boat along. This happens even with new mounts, a bit of movement doesn't mean they're knackered.

Pete

 

[atlowers]

Thank you Pete. Clarified and now understood. The p brackets are fixed to the hull. So am i right in thinking that this normal movement of the engine forward when forward drive is applied moves the shaft forward for those moments before the mounts take up the pressure and start to move the boat forward and that when drive is stopped the shaft would normally return to its static position. So if the shaft static position is now forward by the 13mm does this point to the shaft actually moving through the coupling which i understand should not be able to occur suggesting that this is more likely the problem rather than the mounts?

 

[Tranona]

Depends on the type of coupling. If it is just a plain clamp then the shaft could move in the coupling. However, many couplings have a cotter pin, role pin or set screws to lock the shaft to prevent this. Assuming the shaft is locked properly into the coupling, then the whole engine, box and shaft move in and out when you engage gear. As Neil said with very soft mounts this can be up to 8mm either way, but more commonly only 3 or 4mm. Any more suggests the engine mounts have failed.

First job is to check the clamp on the coupling.