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
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-126.96.36.199
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:
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
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
AND STERN GEAR
(from Reading College):