I don't think there should be any perceptible movement of the shaft through the stern tube (shaft log) bearing. Beware of “ovaling”, particularly on the lower half of the bearing. Does the
boat suffer from vibration, underway?
The most common cause of vibration are:
motor, shaft, or stern-bushing misalignment, followed by
motor mount, and coupler problems.
The most common reason why drive systems go out of line is that over time, the motor mounts or bushings wear. Progressively the motor or shaft settles into an off line position until it is eventually bending the shaft. This can also be cause if the
hull changes shape.
Improper alignment, weak
engine beds, or loose couplings can cause incredible Cutless Bearing wear. A properly installed Cutless Bearing should last many years and should be checked yearly upon
haul out by grabbing the
propeller and attempting to shake it in order to judge the Cutless wear. When you grab the shaft or
propeller and shake it you should feel
virtually no play or motion. If you remove the shaft, the bearings
interior should be smooth, without cracks, cross wear, low spots, tears or indents. Cross wear on the bearing indicates a major alignment problem. The shaft log or shaft tube is just that, a hollow round tube which penetrates the
hull through which the propeller shaft goes with no bearing surface, hopefully. Ideally the propeller shaft should be perfectly centered.
The general
rule is, the longer the shaft and the larger the diameter, the more critical proper alignment is required. That's because larger shafts tend less toward self-centering than small ones, because they're more rigid.
ABYC standard P-6.5.5.3 refers to the alignment tolerance between the parallel flange of the coupling with the connection the bolts loose. That tolerance is defined as 4 thousandths of an inch. The alignment is specified by the ABYC to be done with "the
boat floating". This allows the boat to obtain its normal flex so that an accurate alignment can be done. The measurement should be made by feeler gauge with the bolts loose, four times, 90 degrees out each time.
From
ABYC P-6 table III, for shafts over 15/16" to 8" in diameter, and with a length of over 4' to less than 8'; you are allowed three thousandths of an inch tolerance. Table II calls out that for shafts supported at 42" where the shaft diameter is over 15/16' and under 1 15/16" the permissible variation is .006".
Shafts 1 15/16 to 2 1/2" the variation is .007".
Some good resources:
Cruising World published (Sept. & Oct. 1966) a couple of good articles by
Steve Antonio:
Heavy Metal Part 1 - Careful attention now to below-the-waterline running
gear can prevent problems later on - By Steve D’Antonio
http://www.cruisingworld.com/article...=396&catID=569
Heavy Metal (Part 2)
In the second of a two-part series, we look at the tools and techniques needed to pull the shaft, replace the cutless bearing, repack the stuffing box and put things back together - By Steve D’Antonio
http://www.cruisingworld.com/article...=396&catID=569
SHAFT
LOGS AND STERN
GEAR (from Reading College):
http://www.reading-college.ac.uk/marine/10sgear.htm
FWIW,
Gord