Just before a propeller shaft meets the propeller it passes through a bearing known as the cutless bearing, which supports the shaft and helps it rotate smoothly. Cutless bearings are tube shaped and typically made from bronze, marine grade brass or stainless steel and are lined with nitrile rubber. The rubber is grooved where it meets the propeller shaft to enable water to pass through the grooves in order to lubricate the bearing. The bearing is usually located in a strut or bracket suspended from the hull or in traditional deep keeled vessels may be enclosed in the deadwood part of the stern through which the propeller shaft passes.
Cutless bearings can last for many years but if the propeller shaft is out of alignment they will wear through more quickly. If you have noticed a clunking sound when motoring then it could be a worn cutless bearing that is causing the problem. With the boat out of the water it is a good opportunity to check. To do this:
- Try moving the propeller shaft or propeller itself from side to side and up and down.
- There should be a very small amount of movement – about 2mm, to allow the propeller shaft to be able to rotate inside the bearing.
- If the shaft moves more than 3mm this indicates either the cutless bearing is worn, or possibly the propeller shaft and cutless bearing are both worn, which is a more complex problem. Either way, further inspection is required.
- The condition of the cutless bearing can provide a clue as to what the exact problem may be. If the bearing is worn unevenly, on one side only for example, this indicates the shaft is likely to be out of alignment. You can only check this by removing the shaft.
- A worn cutless bearing can be relatively easy to replace but if in doubt it will be a good idea to ask your boatyard’s advice to help ascertain exactly what the problem is.
While on some boats the procedure for replacing a cutless bearing is relatively straightforward, on others it involves specialist tools and several pairs of hands to do what amounts to a lot of hard work – followed by a trip to the chiropractor. Being aware of what is involved will be of benefit before deciding on your best course of action.
Cutless bearing replacement
The typical procedure for replacement of a cutless bearing is as follows, noting that the first big challenge is to get the old cutless bearing out:
- Remove the propeller.
- Remove the propeller shaft (necessary in most cases).
- Remove the stern gland (sometimes necessary).
- If the cutless bearing is embedded in a strut or bracket then it should be held in place by some locking screws either side of the strut. These need to be located and removed.
- Old cutless bearings can be very hard to move as when installed they have to be a very tight fit. There are several ways to do this – a bearing puller tool can be used, in some cases without having to remove the propeller shaft, so a good option if you can get hold of one. Another method is to use a section of tubing with an inside diameter equal to the shaft which is then hammered to force the old bearing out – but caution is required to avoid damaging the bracket. Alternatively, cut lengthways into the old bearing using a hacksaw, taking great care not to cut through the bearing into the bracket itself. The old bearing can then be prised inwards but will usually still need knocking through with considerable force and skill to prevent damaging the bracket.
- Once the old bearing is finally removed the bracket or shaft opening needs to be cleaned up and checked for condition – any damage or corrosion will need to be dealt with. Also clean and polish the shaft using fine grade wet and dry paper.
- Meanwhile the replacement bearing will need to be ordered with highly precise measurements and specification given for length, outer diameter and inner diameter. Given an option, it will always be worth going for high quality here as you want the next bearing to last as long as possible.
- The last part of the process is to refit the new bearing. This should be a very tight fit but not so tight that it might be damaged by being forced into place with a mallet. Professionals sometimes leave the new bearing in a fridge for a few days to contract the metal a fraction to help it slide into place.
- With the bearing in place, the locking screws are inserted and the propeller shaft and propeller re-attached, remembering that aligning the propeller shaft itself is a skilled procedure.
Cutless or Cutlass?
The name cutless is derived from the “cut-less” design of these bearings which enables small particles to pass through grooves in the bearing without cutting or wearing the rubber surface. Not to be confused with a “cutlass” type sword which has zero association with the design.