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Old 09-13-2018, 10:09 PM   #12
hobbystuff
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City: Victoria, BC
Country: Canada
Vessel Name: Waves of Grace
Vessel Model: Universal 36
Join Date: Aug 2012
Posts: 70
Quote:
Originally Posted by LarryM View Post
My last boat was a Fales 30 which had a 13'-4"x1.25" prop shaft that was supported by two Dodge split pillow blocks that were rigidly mounted on transverse fiberglass supports under the shaft. The shaft was free to move fore and aft as thrust was absorbed solely by the engine mounts. They basically prevented shaft 'whip' which occurred at certain engine/shaft speeds as a result of harmonics and balance.

I replaced the original stuffing box with a PSS dripless shaft seal, but when I did, I actually lost some shaft support as a result. The conventional stuffing box and stiff, short rubber connection to the shaft log actually provided some support to the long, flexible shaft. I ended up with some vibration at certain engine speeds that was not evident before the 'upgrade' so I performed the bearing spacing calculations and found that the now-longer un-supported shaft length between the last intermediate shaft bearing and the cutless bearing was slightly longer than the calculated maximum bearing spacing.

I sold the boat before I got around to that item on my to-do list, and the boat has since covered thousands of miles, so I guess it wasn't a serious problem.

Here is the chart you can use to calculate the bearing spacing for your Aquamet 22 shaft. It has a modulus of elasticity of 28. I already plotted the 1.75" shaft with Aquamet 22 so just furnish your shaft rpm and follow the instructions to determine the maximum shaft spacing.

If you want to perform the long calculations, I have attached that info as well.

Easy peasy

Perfect, thank you. I'm going to sit down with some coffee and run the numbers.
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