Prop Pitch

[BillBerner]

I've read all of the posts about props, cavitation, etc. and found something interesting today when I was putting on a new cone zinc
The boat has the original 15X9 sailor prop.
BUT:
From an additional stamping I found on the prop, it seems to have been repitched to 7.5.
I get that the change should theoretically give the prop more thrust (torque?), at the expense of of speed. I'm told the equivalent of about 150 rpms worth.
So my question is why did one of the PO's make this change. Would pitching the prop lower cause any less cavitation in general. If the boat is going slower I assume there ought to be less cavitation, but couldn't that be achieved just by lowering rpms?
I had an occasion last fall motoring into a 25-30 knot wind and chop and the boat barely made headway with crazy amounts of cavitation at around 2500-2700. I actually wound up bearing off and sailing a few tacks close hauled to get back to the slip.
Comments?
Thanks everybody.

BB

[wolfe10]

Bill,

Let's define ideal prop/"rear axle ratio" for your boat. This is something you can VERIFY in 2 minutes the next time you are out and the engine is warmed up.

Assuming clean bottom, not towing a 30' dingy, calm water, calm wind, etc:

1. With transmission in neutral, go to full throttle just long enough to read max (governed) RPM. Should be a little over 3000 RPM.

2. Put it in gear and go to WOT (Wide Open Throttle). Note RPM after speed stabilizes.

Ideal prop/"rear axle ratio" would be for the engine in gear to turn approximately 150 RPM less in gear than governed redline.

If RPM is much lower than that, prop has: too much pitch, too large a diameter or too many blades= it is "over-propped".

If RPM immediately jumps to governed RPM, you are "under propped" and can increase pitch, diameter and/or number of blades.

Brett

[russp]

Hi Bill,

I have just put mine back in the water so can't check for an extra stamping, but I do have the original 15 x 9 sailprop and like others have experienced exactly the same problem pushing into a 40 knt headwind and sea, I thought the boat was coming apart and had a diesel guy in straight away to realign the shaft, but in the end it was purely cavitation at around 2600-2800 rpm. That prop is just dangerous under certain situations and I am considering, like many others, putting a feathering or folding prop on so I can have more thrust and less cavitation if and when I need it. Haven't decided what yet, am leaning towards the simplicity of the Kiwiprop.

Russ Peel
Avalon #150

[leigh weiss]

To all who have prop cavitation,
The solution for me was a KIWI feather prop. (Used for 2 seasons)
Easy install, three blade design = lots of thrust both forward and astern. You can use the design guide on line and or send them an e-mail. I found them to be very helpful.
I have made small adjustments to the pitch ( In the water)
to get the correct WOT in gear RPM.
I ordered three extra blades which can be changed quickly in the water.
If you have any questions please drop me a note.

Leigh Weiss Brisa C350 #155