Sunday, October 14, 2012

On Props and Rudders and erudition

I never knew my readers were such an erudite lot.  Thanks to comments from Brian & Diana, Adam, Simon, and Carol, my knowledge on the sunken boats, the mini Sea Otter and the wheelbarrow boat has taken leaps forward.  See the comments and follow the links for yourself if you are curious. And thanks to our recent cruise, these are now all people that we have met face to face, so I can confirm that they are not only knowledgeable but also a fine looking lot.

Meanwhile I have not been idle in improving my own erudition, for I have been reading up on propellers and rudders.  Now that Herbie’s engine is running smoothly again it leaves me free to notice that the tiller is wobbling more than I would like.  You know the feeling you get when the prop picks up a bit of weed.  I keep getting that and when I check the prop, it’s clear.

It could be that the prop has been damaged or bent by hitting an underwater obstruction, although groping around down the weed hatch I can’t see or feel anything obvious.  It could also be that the prop is not in it’s ideal position fore and aft.  Our recent engine remounting may have shifted it a bit, although not much I would have thought.

Here’s what I have found out from reading up. 

1. Quite a small amount of prop damage can throw it off balance. At cruising speed our prop is rotating at about 700 rpm, so dynamic balance is important.  There may have been a time in my engineering days when I might have calculated the off balance forces. One suggested remedy (from Oakie if I recall correctly) is to replicate any obvious dings on one blade by grinding a similar ding in the other two.  Of course if a blade is bent, then unbending it accurately might be quite a skilled job.  You can send props away for redressing and rebalancing.

2. The free length of shaft between the stern bearing and the prop should be about 1.5 times the diameter of the shaft.  So for us that would be 2.25 inches.  Space forward of the prop (as well as the hydrodynamic shape of the swim and the evenness of the uxter plate above) affects the amount of cavitation you get.  We do certainly get the gravelly noise associated with cavitation at some speeds and conditions, although I understand that’s not unusual.

3.  The distance between the prop and the rudder is also important.  I reckon Herbie has less than two inches clearance here, which might be a bit too close, apparently some folk have a lot more gap.  Too close and the vortex behind the prop can throw the rudder about.

4. The proportion of the rudder blade forward of the pivot should be around 25% of its full width, mainly for steering purposes, especially in reverse.  When the tiller is fully pushed over, and observer standing  a few feet behind the boat should not be able to see the propeller.  Don’t try this in the water!  This can also affect how heavy or light the tiller feels.  Herbie’s tiller is comfortably light.

Of course, being such an erudite lot, you probably knew all that, but I didn’t.

In December when Herbie comes out for blacking, I mean to check all these points.  All I have to go on at present is what I can find out through the weed hatch and two not especially helpful  pictures I took when we were last out of the water.  The rudder dimensions look about right but I fancy the prop might have room to back away from the rudder a bit.

034 (1024x768)

032 (1024x737)


Adam said...

Most of us have been commenting for years -- and it's only now you've realised the quality of the comments??

Oakie said...

"all people that we have met face to face, so I can confirm that they are not only knowledgeable but also a fine looking lot."
Does that include me Neil? If so, you have a friend for life. I've not been told that for years.
Just looked at old pics of my boat out of the water and the prop boss is very close to the stern bearing. Nut to front of rudder, about 2" to 3". Any out of balance on the prop will be exagerated by the overhang on the shaft out of the bearing. Will e-mail pics when I get your address.neespic

Neil Corbett said...


I'd like to see your rudder photos. my e mail address is

my first name dot my surname at