Prop balancing/vibration

[mlines]

We have a vibration on our Regal 2250 above 3000 RPM that cause loose items in the boat to buzz and rattle, you can fell it through your feet. I have been told it could be the UJ, a bent propshaft or props out of balance. Today I took a look at the props, what does the panel think??

I have a dial gauge so went over to the boat to take some measurements, first the props. This is not very scientific, I just chose a high spot on the blades to measure the difference.

The rear prop of the Duo Prop pair showed two blades on "0", the dial gauge is a 0.01mm gauge, and one on 0.58mm, so one is offset slightly.







​I then did the "front" set of blades, again choosing a random high spot on the blades and checking the difference. Starting with a "reference" blade, one was 0.10mm different and the other was 0.36mm different.








I then removed the props and measured any rotational run out (i.e. shaft bent) on the concentric shafts. The first (inner) shaft was a steady 0 all round and the second (outer) varied by 0.1mm but it was on what appeared to be a dirty, slightly unfinished surface so probably can be excluded.



So it doesnt look like a bent shaft but is the sub-millimetre differences on the prop blades going to be a balance issue and do they "balance" to this level of accuracy??

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[Ianfs]

Although the props are turning at 1200 -1600 revs depending on the gear ratio of the leg, It might be as simple as the propellers cavitating.

Do the edges/tips look as if the paint has worn off?

[mlines]

They are highly polished stainless duo-props so no paint.

The opinion on the Regal forum is tending towards a UJ issue. They feel the 0.5mm difference on the prop blades is ok.

So I think we are going for the UJ next.

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[mlines]

The Regal forum has also mentioned indexing the props which is interesting as I have wondered how the front and rear props should be aligned.

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[shibbs]

The Regal forum has also mentioned indexing the props which is interesting as I have wondered how the front and rear props should be aligned.

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Just for my own interest Martin, what do you mean by indexing the props?

[Ianfs]

I have had 2 Gimbal bearings go bad and as you might know from your previous experience, they have a low pitch rumble but more so at full lock. Although I've not had a UJ problem, I've asked about these previously and was advised by several mechanics that these also rumble rather than a higher pitch vibrate.

I suppose UJ's are a more obvious choice because they were heavily oxidised from the leak, but that was back in October last year and Tom checked the needle bearings and all were ok.

You're a methodical gentleman Martin.

Has this suddenly come to light?
Does it get worse above 3,000rpm?
Is it worse on full lock?
Does it subside or increase on certain trim levels i.e. if you trim in does it get worse, or out is there a sweet spot?
Is it possible the alignment is out, maybe because of an engine mount?
Also, now I remember you polished your props a while back and are SS, did you put them back so that they don't cross at the Ventilation plate at the same time?

[mlines]

Thanks for the replies.

It has become noticeable in the last week or two but in reality the Isle of Wight last week was the first proper opportunity for a sustained series of high speed runs which is when it shows.

It appears as you go through 3000 rpm and is a consistent, sustained vibration, it does not really get much worse as the RPM increases above this. You can feel the vibration through your feet and fittings are buzzing/rattling. James played with the trim and did not notice any particular change but I am not sure he carried out any turns that would test it a lock. We tend to do full lock turns at very low speed anyway (Turn then power rather than power then turn - I recall from my training )

The engine mounts were checked as part of the last service and re-torqued and alignment checked. The Gimbal bearing was changed at this time.

SHIBBS - Ianfs mentions the indexing in his post, I hadn't heard of it before the Americans and Ian mentioned it. Apparently when you install Duo props you should ensure that the trailing edge of the props do not cross each other as they pass under the cavitation plate as this causes some sort of resonance (ie vibration) in the system. I guess it is similar to older multi engined propeller aircraft having to play with propellor syncronisation to stop resonance (the old WW2 pulsing drone of the bombers). I will give it a go although Tompamarine is going to look at the UJ again for us when he passes the marina.

Martin

[betty boop]

seems youve go the opposite of what Ive got, a low pitched hum/whine but at low revs, as Ive been going on about the last week or 2 . havent had the chance to blast thru the revs yet, so keep us informed as I'll be interest to find out if it is the gimble. Like you mine was changed last season but Im not convinced its not the fault. my prop shaft is definatly running straight i used the steady finger nail on low revs trick but Ive no reason to suspect it wouldnt be.

[mlines]

I "indexed" the props today. Not had the chance to go out and see if it works but it seems sensible anyway.

To do this they say to set a rear blade at 12 o'clock and a front blade at 6 o'clock. Then each blade sits in the gap created by the other sets of blades. The photo is not 100% clear as I could only take if from a slight angle without moving the boat but the principle is clear.

[ChrisH]

So what engine and drive combinations have the duo prop configuration and why?

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