How big a propeller ding does it take to affect performance?

H

hansonby

New member
We were leaving the Penrose State Park dock this afternoon about an hour before low tide. We were about 200 yards out from the dock, heading east past the little jetty which protects the dock.

I had one eye on the Garmin 7400 Chartplotter depthfinder (which said 11.8 ft.), one eye ahead for boats and debris, one eye on the grandkids, one eye on the GPS map screen, one eye on the speed [6 kt.], etc., when the motors had a little audible hitch.

I looked over the side and saw the bottom up close and personal. Obviously I had hit mud and some rocks. I guessed the correct direction to steer and quickly got to deeper water and made it home with no further problems. The depth finder stayed at 11.8 ft. for a few minutes and then started reading correctly.

4 of the [aluminum]prop blades look fine - just "polished" a bit. One on the port motor is just a teeny bit scraped/rough, and one on the starboard motor has a rough, serrated patch about 2 " along the outer circumference with the deepest gouge about 1/16".

The two blades don't look so perfect now, but I assume they'll do their job just fine. How badly does one have to ding a prop before repair/replacement is necessary?

Thanks for your comments,

Byron

P.S. I had the depth alarm set for 3.6' and it worked when I came in yesterday to the dock at low tide. I have no idea why the depthfinder got stuck at 11.8' but it's been stuck before at various depths. I was right in the channel according to the GPS, and the depthfinder said I was okay, but still I hit bottom.
 
I’ve done a lot of prop river running here in Alaska; you would really be surprised as to how much damage a prop can take before you notice a difference. If all you see is paint chipping and scratches your fine, I would not lose sleep over it.
I hit an uncharted rock last year at 19mph that nearly took off my skag, the prop lost a dime size chunk, I really didn’t notice any difference. I did replace the prop and sent the old one in for repair.
 
I spent the last two summers driving and occasionally fixing boats, especially props on our rescue boat, supply boat, and the 20 or so rental powerboats. It is surprising how much deformation a prop will take and keep on functioning. That said, if you can bend it back and file off the rough stuff, it will function more efficiently.

Our supply boat came in one night after pulling another boat off the rocks... the prop was mangled. I was amazed that the boat could even move. I asked the young man who was running the boat how it felt underway, and he said, "Kinda slow compared to normal, but it didn't vibrate."

Regarding the depth-finder: most that I've owned seem to lock, flash, or go blank when you really want to know just how close the bottom is.

Best wishes,
Jim B.
 
As Jim, I carry a rasp so I can smooth out the dings which will get there. The boat I currently own came with one new prop, and one which had a few dings. I am still running the dinged prop. I filed all of the edges back to configuration and removed any rough places.

That being said, watch out for a bent blade or enough of a ding to cause vibration. I usually just manual spin the prop, if the blades are true (use a fixed reference), then run it. The danger is damage to lower unit seals. I would replace the lower unit lube after a few hours to be sure that no water got in. This is a very easy and inexpensive procedure in comparison to replacing the seals.

As for the depth sounder, sometimes they will not lock on the bottom--either too shallow (as in your case), too deep, or intermittent results. In that case, I usually go to manual setting and put the range in which corresponds to where you are. Then go back on auto, or an appropriate range when we get in deeper water.
 
I use Piranha propellers, and recently noticed that engine would sometimes cavitate when quartering waves 2 to 3 feet in height. I found a chip in the edge of one blade, about a half-inch long by a quarter inch deep. In five minutes I had that one blade replaced, and had a propeller like new. Total cost $17.

http://www.piranha.com/
 
One thing to remember that Stainless props flex the lest, then Aluminum and finally plastic (or composite). Sometime the pitch may have to be more in the plastics, because of this flex. There is also the issue of damage if you run in sand. (which I have to do occasionally). Even more so for wood or even worse rocks, The Aluminum props will ding up, as will the composite more easily. The SS will be less likely to ding, but if they hit something hard, on occasion the prop may survive at the expense of gear teeth or other lower unit problems. I have used all 3, and prefer SS props. But my C Dory 22 came with two aluminum, and I'll use them until one gets dinged up enough to not be usable, and then buy a SS prop. But there will always be a spare aboard. That spare might be a cheaper prop to get you home. Also include a nut, cotter pin and spacer--and any other items in the lower drive train which are removable (Or can be dropped).
 
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