Prop damage threshold

[guitarplaya39]

Went over a couple small logs that were lurking just under the surface of the water and two out of three blades on the prop appear to be slightly bent. Not a big deal and I've straightened out blades before, but I've always wondered - at what point does an unbalanced prop start to cause damage to the lower unit?

 

[Chris1956]

Bent does not mean unbalanced as the blades can all weigh the same. Losing a piece of the blade often causes unbalance of the prop.

Aluminum props w/o hubs are pretty cheap, so when you notice unbalance or rough running, it is likely time to replace it.

I chipped a couple of blades on such an alum prop. I use some JB Weld to fill in the chips, and it usually lasts the season.

 

[guitarplaya39]

Considering I'm running an old 75hp 2-stroke 3cyl Merc, I'm afraid I wouldn't feel any prop vibrations lol. The weight balance would be fine, but a bent blade would still cause an uneven load while rotating I imagine...
I like to overthink things especially when it comes to my beloved boat!

 

[flashback]

Bent prop is similar to mis-placed frets on your guitar

 

[JimS123]

A little off topic, but it kinda fits in.

Due to some slob that lost a mooring line, our PWC sucked it up. I knew that it "could" be a problem, but wasn't sure in this case because she ran well and couldn't detect any vibration. Had the dealer check it out and he proclaimed it all well.

A year later the boat sank. Turned out a few head bolts vibrated loose and the tight one sheared off while running. The boat filled up with water in a no-wake zone where the auto pump could not keep up.

I monitored it closely and the bolts kept coming loose. Finally went to another (more reputable dealer) and he found that the impeller was damaged. Had the drive system replaced and all was well after that.

Grandpa always told me to fix my prop if I hit something. I guess the old fogies knew much more than us last century.

 

[airshot]

Yep, bent blades can be just as bad as out of balance as the load on each blade changes so some serious load changes are not gonna be good !!!! At slow speed you can limp back to the dock, but prolonged running at speed will just put excess wear on the entire engine. So which is easier to replace/ repair, the prop or the lower unit ?? I am that guy that fixes things when they go bad or need repair ... Be it boat motor, car or whatever, when something goes wrong it gets fixed !! Over my 60 plus years of driving various vehicles, I have never been stranded or needed a tow....knock on wood....must be doing something right !!

 

[Scott06]

Went over a couple small logs that were lurking just under the surface of the water and two out of three blades on the prop appear to be slightly bent. Not a big deal and I've straightened out blades before, but I've always wondered - at what point does an unbalanced prop start to cause damage to the lower unit?

You can try running it and see how much vibration is present. You also can do a static balance pretty easily. I started doing this with my mower blades and it does seem to help keep vibration down.

 

[guitarplaya39]

Thanks everyone for the input. I think I'll just buy a new prop and before installing it, use it to make some kind of template/jig to measure blade deflection for the next time it happens (which is inevitable in the lower James River).