New propeller - strange noise?

[ThreeSummers]

I recently changed the prop on my 60hp outboard from three blade 17" to four blade 15" pitch. Looking at the new one, I noticed that the trailing edge of the blade is squared off and that edge is around 4mm thick at the widest point. It starts at the base in line with the keel, and ends near the tip near 90º to the blade - am I making sense? The old prop's trailing edge is not square.

Three things - does the square edge create drag on the prop? I imagine at high speed that water wont be able to flow around that "corner" quick enough - possibly leading to cavitation?

Two: it vibrates more than the three-blade - I'm lead to believe it should be smoother - also from the square edge, or maybe unbalanced?

Three: if I shut the throttle quickly, as the motor passes through around 3000 rpm (+/- 1100rpm at the prop) there is a shrieking noise for a second or so as it slows down - is this a 'singing prop'? This makes me think I need to grind that edge off - maybe it will get rid of the noise as well as the vibration, but I'm not willing to do without some advice or other's experience of similar behaviour?

Any thought?

Regards,

Tim.

 

[MapisM]

makes me think I need to grind that edge off

Forget it.
Propellers tuning is a form of art, it takes both experience (LOT of it) and proper equipment, to improve a prop performance just a tiny bit.
And surely it's not just a matter of grinding off a part of the blade you don't like, anyway...
If it doesn't work well with your boat, sell it, and look for something else.
What was your aim for the change, in the first place?

 

[ThreeSummers]

What was your aim for the change, in the first place?

The motor should be running WOT between 5500 and 6000rpm under normal load - I was getting 5500 with just myself on the boat - hence change pitch - now I get 5800, and a bit more with trimming on a light load and around 5700 with a few pax.

The blade change, from various forums, indicated that it would lift the back of a stern heavy boat - my boat is very light with 110kg of motor on the back - add 4 passengers, all sitting aft of me, and it struggles to get on the plane - changing the prop has sorted this out.

I can live with the noise, or eliminate it by throttling down slowly - it's only when I shut the throttle right down that I get it, ie. I think the water is driving the prop and nor the other way around.

T

 

[ThreeSummers]

Deleted - double post again for some reason...

 

[Chris_d]

Stating the obvious from what you have described it sounds like its loose, presumably it is the correct prop for the outboard with the right thrust washer etc....

 

[ThreeSummers]

Stating the obvious from what you have described it sounds like its loose, presumably it is the correct prop for the outboard with the right thrust washer etc....

That's quite possible - iirc the torque on the nut should be around 16-18Nm - that's too low for my torque wrench so I did it by guess work. I'll tighten it up by a 6th of a turn and re-pin it tomorrow. Taking the boat out on Saturday evening and again on Sunday morning - will let you know how it goes.

Tim.

 

[Spi D]

What make & model is the 4-blade?
What outboard?

 

[ThreeSummers]

What make & model is the 4-blade?
What outboard?

Outboard - Evinrude E-TEC 60, prop is a Solas Amita 4-blade...

Tim.

 

[MapisM]

The blade change, from various forums, indicated that it would lift the back of a stern heavy boat

Well, it's true that some props can provide more/less stern lift than others (for any given pitch), but afaik this can only be meaningful with very high speed boats.
Mind, it's perfectly reasonable that your new prop improved the holeshot, but I would guess that 90% of that is due to the shorter pitch, 9% to the better grab in acceleration of the 4 blades, and 1% (if that) to better stern lift characteristics.
Don't take these numbers as anything scientific of course, but you see what I mean.

Anyway, yes, based on your numbers, the 15" pitch seems indeed more appropriate.
If after checking tightening, washers, etc. you don't get it sorted, I would bring it to a prop shop to check the balance.
Not so much for the noise actually, but in your OP you mentioned also vibrations, and THAT is something well worth trying to get rid of.

Good luck!

 

[ThreeSummers]

A bit of feedback - I tightened the nut one notch yesterday morning before a run. Vibration seems to be less and so is the noise on throttle down, so maybe it is loose - but I'm tightening by feel and don't want to over-tighten... I'll see how it goes for the next few days and then maybe tighten one more notch.

As to performance, if you're not holding on when I open the throttle, you're swimming home...

Tim.