drive shaft straight?

[seaesta]

My drive shaft does not seem to run quite true - it has been like this for 15 years and has not (touch wood) been a problem presumably because the flex coupling can cope. The shaft is solid 316 stainless and is connected to the engine via a flex coupling and out through a standard stern gland with grease pipe/nipple and a relatively long stainless tube. The engine runs slowly (old MD1).
I am planning to replace the engine with a Beta which will run up to 3600 rpm and even when reduced through the gear box the shaft is likely to be running much faster than at present. How do I check that the shaft is true and if not how can I get it straightened.
Advice greatly appreciated
Martin

 

[Avocet]

If you have access to a dial gauge, put the point on the shaft and turn it by hand. You'll see the pointer move backwards and forwards indicating how far in and out the plunger is moving. If you don't have one, try to rig up a RIGID pointer as close to the shaft as you can. Again, as you slowly turn the shaft by hand, you should see a gap between the pointer and the shaft. Turn it until the gap is at its largest and measure that gap (maybe use a feeler gauge or something like). Do this as near the end of the shaft as you can.

 

[discovery2]

Continuing with Avocet's suggestion......
If the rigid pointer could have a sharp point on the end (a metalworker's scriber?) use a permanent marker to 'colour' the circumference of the shaft where the pointer will touch. When the pointer makes contact with the shaft, a line will be marked on the ink. This will help you to measure the gap at the widest point - half way round the unmarked section!

David