Does a shaft log require a bearing when using a dripless shaft seal

[theoldwizard1]

My friends' Carter 33 is "up on the hard" in the Dominican Republic. They are replacing the cutlass bearing (appears to be the original form when the boat was built in the 1980s) and most of the components of the dripless shaft seal.

The question is should there be some kind of a "bearing" (similar to a cutlass bearing) in the shaft log ? Currently it is just a solid chunk of 'glass and resin with a good 1/8" - 1/4" play on both side of the shaft. Is this the typical setup for a dripless seal ?

I know water has to come through the shaft log to lubricate the dripless seal, but that amount of play seems excessive. (The cutlass bearing did require replacement but it was not so far "gone" that the shaft would have enlargened the shaft log.)

[Emmalina]

The shaft should be held at the gearbox coupling and the cutlass bearing not in the middle. The play around the dripless seal is irrelevant ! Unless is has replaced a stuffing gland. Then there may be excessive movement in the shaft.

[RaymondR]

The clearance between the shaft and the shaft log is a deliberate design feature. Because of the use of flexible engine mounts the shaft needs room to move around in the shaft log otherwise the engines movement will be restricted and damage to the output shaft bearing will occur.

[theoldwizard1]

Originally Posted by Emmalina

The shaft should be held at the gearbox coupling and the cutlass bearing not in the middle. The play around the dripless seal is irrelevant !

Originally Posted by RaymondR

The clearance between the shaft and the shaft log is a deliberate design feature. Because of the use of flexible engine mounts the shaft needs room to move around in the shaft log otherwise the engines movement will be restricted and damage to the output shaft bearing will occur.

Thanks to both of you ! After pondering the design, I understand your answers perfectly. Kind of makes you wonder how old stuffing boxes held up so well !

[DougR]

Most ( but not all) sailboats are designed so that the shaft is supported at only the cutless bearing and the prop shaft coupling, and as noted by the earlier posters the amount of clearance at the dripless stuffing box/ shaft log is relatively unimportant.

Whether there needs to be another bearing on the shaft is a function of the “critical” speed of the shaft. All rotating shafts experience a critical speed where the rotational forces of the shaft overcome the ability of the shaft to remain straight, and at this speed the shaft begins to “whip”, or look like a jump rope. This whip is, of course, undesirable as it causes noise, vibration, fatigue, etc.

The major factors that determine the critical speed are:
- unsupported length of the shaft
- rotational speed of the shaft
- shaft diameter

Some other factors can be shaft material stiffness and whether the shaft is supported at the transmission in a solid coupling or a flexible arrangement.

You can readily see that a long, thin shaft rotating at high speed and supported only at the ends could go into a whip situation, whereas a short, larger diameter shaft turning at the same speed would not.

We see this same thing frequently on “box” trucks running around our cities making deliveries. The drive line from the transmission to the rear axle may be divided up into three short sections of shaft with steady bearings between them. This makes the shafts short and stiff enough that they don’t approach the critical speed. If the drive line was made up of only one shaft length of the same diameter, it would whip so much it would try to jump out of the truck. Or it would have to be so large in diameter that it would be difficult the get in place under the truck.

DougR

[theoldwizard1]

Originally Posted by DougR

Whether there needs to be another bearing on the shaft is a function of the “critical” speed of the shaft. All rotating shafts experience a critical speed where the rotational forces of the shaft overcome the ability of the shaft to remain straight, and at this speed the shaft begins to “whip”, or look like a jump rope. This whip is, of course, undesirable as it causes noise, vibration, fatigue, etc.

In the mechanical engineering world this is called a "harmonic". Yes, VERY BAD !

Anything that is rotating has a harmonic at some speed. Longer shaft likely will hit a harmonic at a lower speed. Shafts that experience a wide range of RPMs will have more than one harmonic.