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hanschristian.org • View topic - The correct prop size
Page 1 of 1

The correct prop size

PostPosted: Wed Aug 13, 2008 5:10 am
by rick_bradshaw

Prop Size

PostPosted: Wed Aug 13, 2008 6:17 pm
by mitchk

PostPosted: Thu Aug 14, 2008 5:00 am
by rick_bradshaw

PostPosted: Tue Jul 20, 2010 7:35 am
by lonesome_boatman
During a recent slipping of Respite I found the prop stamped D21 P14 which would be a 21 X 16. It seems to be OK as far as I am aware but the engine seems under load at full throttle. Lots of black smoke. Backing off the revs it seems to run fine with no smoke. There is also heaps of prop walk, and I mean heaps. Any comments or advice would be appreciated.

PostPosted: Wed Jul 21, 2010 12:55 pm
by mimoza
Hi Mick,
There are really only two ways you can be sure of the D and P. Measure it, or have your trusty local prop shop do it. My prop is a Michigan Wheel 21x16, and I know that it is still 21x16 because I got it brand new and have never had the pitch adjusted.

21x16 is a good prop for an HC 33 with a Yanmar 3QM30 and a 3:1 gear reduction. It will push the boat through the water at hull speed at about 23 or 2400 rpm, and there will still be some reserve power for fighting a contrary wind. Pitching the prop higher won't make the boat go any faster or make the engine more fuel efficient. Pitching lower will make it hard to reach hull speed, and isn't really good for the engine because it won't work as hard.

Mine walks a little, but not badly. She doesn't smoke at full throttle - well maybe a tad. What you describe makes me think that the prop was pitched higher than the stamp indicates. Can you reach 3300 RPM at full throttle? If not, your prop may have been pitched up. Pitching the prop up can also make it walk more than it otherwise would.

Other notes:
There doesn't seem to be room for a shaft zinc.
It looks as if your cutless bearing is creeping out of its housing.


I also wonder whether your shaft was altered at some point. The originals are 32mm and have a metric taper; also there should be a good three or four inches between the exit from the cutless and the hub of the prop. Could your prop be running a little too close to the hull? and could this make your prop walk worse? I don't know about this detail, but I do know about pitching a prop too high. Yours might need to be set back to original spec.

PostPosted: Wed Jul 21, 2010 8:38 pm
by lonesome_boatman
Mimoza
Thanks for the tips. I am kicking myself now as I had the oportunity to get the pitch checked when I had the boat out of the water. A stink boat next to me was having its props re-pitched at the time. I will also check the gearbox ratio on the data plate on the box. My old yanmar would be flat out at 2000 RPM, I also need to check this as the tacho may be faulty. Not sure about the shaft setup but all appears to be in order visually. The prop position in relation to hull clearance could definately effect prop walk. I will look into this.