prop shaft - is it bent?

[homer]

Having had a bit of vibration when motoring (but not in neutral), I have been checking the prop shaft. The outboard end of the prop taper is 0.09mm out of true; the inboard end of the taper 0.03mm. About half way along the shaft is also 0.09mm out - elsewhere, less. Any expert advice as to whether this is likely to be significant would be much appreciated?

 

[mtb]

has it always been like it or just suddenly occured ?

What's you prop like ? any damage or is it the wrong size cavitating ?


have you got plumber blocks ? is the block either up or down re misalignement

mick

 

[Norman_E]

If those figures are correct and the 0.09mm figure for the bend at the shaft centre is total indicated run out it is only 3.5 thousandths of an inch, meaning that the shaft is bent by half that amount. It is not enough to cause bad vibration. To measure that amount of bend in the shaft accurately you would need the shaft set up on vee blocks, preferably on a large surface plate, and measure the run out with a DTI. Any other method and the limits of accurate measurement are probably bigger than that.
You don't state the shaft diameter, but if you supported a 1 inch shaft about 3 feet long by its ends, it would bend under its own weight by close to that amount.
The fact that the shaft taper is very slightly out of true may indicate a slight bend, but is more likely due to the shaft not having been running exactly true in the lathe when the taper was machined, though it is a bigger error than I would like to see in a new shaft.

If you have vibration in gear, and it varies with engine speed I would suggest you examine the propeller for damage which has put it out of balance. Also check for play in the cutless bearing. Another possibility is that the engine seems smooth enough with no load, but is vibrating under load. A lot of faults could cause that including for example one cylinder with poor compression or a bad injector, not pulling its weight.

 

[roly_voya]

Other area to check is engine mounts, when worn the can cause both poor alignment and vibration and if wt comes on te end ofthe shaft can flex it making it appear bent.

 

[Bilgediver]

Having had a bit of vibration when motoring (but not in neutral), I have been checking the prop shaft. The outboard end of the prop taper is 0.09mm out of true; the inboard end of the taper 0.03mm. About half way along the shaft is also 0.09mm out - elsewhere, less. Any expert advice as to whether this is likely to be significant would be much appreciated?

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I would suggest you clock the shaft at the point it emerges from the gland inside the boat.

The shaft will not suddenly bend at the point it emerges from the cutless so what you are measuring is the end of a bowed shaft. but turning the shaft with it coupled to the engine and clocking it about mid length you will then get an idea of how bent it is.

If the shaft is bowed as you indicate then the coupling on the inner end will also be trying to wobble just like the propeller and this will excite vibration in the engine and gearbox unless you have a flexible coupling fitted.

 

[homer]

Thanks for feedback so far. A few more details:- shaft is 25mm diameter and about 1200mm long. I measured it using vee blocks (one at position of cutlass bearing and one near other end) and DTI. Cutlass bearing is good and I can find nothing wrong with prop. Vibration is annoying rather than bad.