Damaged prop on Merc 125hp - what else to check?

slesns

Recruit
Joined
Sep 9, 2009
Messages
5
Hello. I am brand new to boating. I recently purchased an 18' dual console with a 125 HP Mercury 2 Stroke outboard motor. Apologies for the long post, but want to be clear in my info and request...

The boat is in a small slip on a small lake. On my first trip out all went well until docking at the slip. It's shallow, and I trimmed up, but it wasn't enough. In focusing more on not hitting the dock or boat in the slip next to me, I was moving forward, then reverse...going slow. In reverse the motor stalled a couple of times. It wasn't until the 2nd (maybe 3rd) time it stalled that I realized it was because I was hitting the bottom.

I was going slow, so didn't expect to really see much prop damage, but when I trimmed it all the way out of the water, 2 of the 3 blades were damaged. 1 had a small nick, but could be bent a bit too. The other was missing about a nickle size piece off the end and some bend to it.

I have a spare prop and plan to replace it, from the water (obviously shallow enough for it). So, here are my questions:
- what else should I be concerned about?
- bent shaft I suppose. Should I invest in a dial gauge clamp, or just eyeball it?
- if the shaft is slightly bent, will it do much damage to put just a few more hours on the boat, getting used to it, before I pull it out of the water for the season next month and get the shaft repaired?
- from what I've seen online, it looks like a pretty straight forward job to replace the prop, but while the water isn't the best place to do it, is there a concern I'm missing about doing the swap in 2-3 feet of water? I plan to rig up a tarp on the back of the boat and under the motor in case I drop the cotter pin, etc.
- Anything else I should be checking?
- If I end up having to replace the shaft, is that a DIY job or needs a mechanic? If a mechanic, any sense for how much $$? I'm fairly handy, but have absolutely no experience with boats and boat motors.

Thanks for any advice. Great forum.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
There is no real concern with changing a prop in shallow water. Just go slow and don't drop anything (like the prop nut into the water.Keep a firm grip on your tools. Sometomes the thrust washer will stick inside the prop hub so before pulling the prop all the way off double check this.

If you are in an area with a lot of traffic and wakes, this can be a bit bothersome.

If you do not have gauges or dial indicator, install the new prop and slowly spin it by hand. measure the tip of each blade to a known reference point like the anode. If the shaft is bent the blades will each have a different measurement. However, with such an easy hit I doubt that the shaft is bent. You need to really smack a prop into a rock or hard bottom to bend prop shaft and with a hit like this, you will know it.
 

slesns

Recruit
Joined
Sep 9, 2009
Messages
5
Thanks for the response, Frank. Appreciate the advice.
Other than this upcoming holiday weekend, the slip is quiet. I'll repair mid-week sometime.

Just fyi as I forgot to mention originally, the motor is a 1999 with 350 hours on it, always run in fresh water (seems really clean to me, though I'm no expert). So, should have lots of life left, but I do plan to put it in saltwater next year.

I assume that I can get details on the prop when I remove it (or from the markings on the replacement that came with the boat), but googled and found something on the boat manufacturer's site that stated the prop for that year boat and motor model should be a 12.75 x 21 aluminum Black Max. If anyone thinks that sounds off, please let me know.

Thanks again.
 

fishin98

Chief Petty Officer
Joined
Nov 28, 2009
Messages
521
Whew....Pull prop....Run a straight edge along the shaft and slowly rotate, look for ALOT of shaft moving back and forth....Better yet put it on the trailer and look for oil leaking from prop shaft seal. The prop hub should have had rubber insert to prevent excessive shock from strikes. I know we all want to DIY to save money...this is a time when a certified Merc Mechanic needs to get involved to be sure....just my 2cts
 

garbageguy

Lieutenant Commander
Joined
May 8, 2012
Messages
1,531
If your slip is in an area (marina?) where there are boats plugged in to shore power, there could be stray voltage that could pose a safety concern. Someone please correct me if I am off-base here.
 

slesns

Recruit
Joined
Sep 9, 2009
Messages
5
It is near a marina. The slip agreement states that we can work on the boat, though nothing specific about in/out of water. There are paddle board, kayak and slide rentals right there by the slip, and folks launch from that spot. So, seems ok to be in the water, but I will check on that. Thanks for mentioning.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
If you hit bottom at slow speed with an aluminum prop, you probably did not damage the gearcase or bend the shaft. Spin the new prop and look for a wobble.

If the bottom of the lake is mostly rock, it probably makes sense to stay with aluminum prop and replace them whenever you need to. If the bottom is sand/silt/mud, you could consider a SS prop. SS props will stand up to those kinds of lake bottoms w/o damage to themselves, and won't damage the gearcase as well.

SS props that hit rocks can damage the gearcase as well as themselves.
 
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