Long stern tube with dual cutlass bearings?

[RyanN]

I've got a shaft problem I'm trying to fix. The vessel is a long keeled sailboat with the prop in an aperture. The 1.25" (32mm) shaft is about 7' (2.1m) long and is currently supported at the aft end by the cutlass bearing and forward at the engine. Max rotational speed of the shaft is 1200 rev/min. The unsupported length is obviously far too long.
Previously there was an intermediate pillow block bearing, but this is/was inaccessible and was never aligned correctly. It seemed an unworkable solution and has since been removed.

I have two ideas to fix this situation:

• Get an aquadrive and spend a lot of money.
  
• Replace the current 1' stern tube with a much longer (3.5') stern tube and put a cutlass bearing at both ends.
  The stern tube would be pre-made heavy wall fiberglass and glassed to the hull at both ends. I have a dripless shaft seal with water injection so the bearings should have a supply of lubricating water. This setup should be much less expensive and I have no problem with the labor.

Am I missing any downsides to the dual cutlass bearing stern tube?

[TerryKing]

Sounds workable.. Only downside I see is that the engine-end of the shaft will be much more rigidly supported and the engine will have to be even more precisely aligned (centerline to centerline) than is the case when some small side-to-side and up-and-down movement is possible with the rubber tube on a typical shaft log.

Are you trying to solve a vibration problem? Could the shaft be slightly bent?

[lazeyjack]

brg lenmth is by formulae, so many times the shaft D, and so is the length between brgs, and you know what i forget , the formula)

[RyanN]

The formula for bearing spacing is at the bottom of page 3.

http://www.marinehardware.com/downloads/catalog_v4.pdf

You need to know the modulus of elasticity in tension or the weight of one cubic inch of material.

A guideline is 40 times the shaft diameter. which is much less than I have now.

The symptom I'm experiencing is vibration at higher RPM's. The engine is a new 4cyl Yanmar on the stock (soft) mounts. The shaft is brand new.

Even when I lengthen the stern tube, the distance from the engine to the forward cutlass bearing will be fairly long.

[mydauphin]

Your shaft is not that long. But remember that when you align shaft, and tube the shaft will bend a little. Hold in center with string. Shaft will straighten out when it starts to spin.

Have shaft checked, but it may just be out of alignment

Aline tubes first, then shaft then engine. Dont try to do it all one time.