Bent prop shaft vibrate less at high speeds?

TrenchFisherman

Petty Officer 1st Class
Joined
Apr 7, 2002
Messages
213
So I got a 03 Merc 90hp ELPTO that I replaced the swivel pin on over the winter because it was really worn out. It wasn't too bad considering i had to remove the power head, midsection and lower unit but now its all back together and I had it back on my pontoon boat and this weekend I was able to take it out for the first time. It fired up fine but I noticed when I am at a trolling speed it seems like it had more vibration than last year, I went out to the deeper water and opened it up and it seemed fine at WOT. The only thing that has changed since last year other than the pin is I put on a larger diameter prop on it. My question is if I had a bent prop shaft would I feel it at WOT as well or does it smooth out? Could it be since I removed all the play from the swivel pin now it transfers that energy through the boat? Any opinion would be helpful...thanks!
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
Maybe pop your old prop back on and go for a ride to determine if vibration is in new prop....while prop is off check shaft for being bent...any reason I might be bent??.... Did you hit something in the water?...
 

TrenchFisherman

Petty Officer 1st Class
Joined
Apr 7, 2002
Messages
213
nope didn't hit anything, i had to replace the prop because i was sucking a lot of air last year under a heavy load so much to the point i had small pin holes in my prop. i dropped the motor down one hole and put on the largest diameter prop that would fit. up on plane it feels smooth as can be its just when we are going slow through the channel that i felt it.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Prop-hub could be out of alignment. I had a loaner SS prop from a prop shop for a 115 hp like that once. The guy wanted to sell it to me and I said that I would try before the buy as it was used. Good that I did. I didn't buy it.

Prop could be malaligned. Quick check on both counts: Get some sort of a clamping device and something to clamp extremely close to the flare on the rear of the prop barrel....the exhaust comes out of the barrel part of the prop. Ensure that whatever you have adjacent to the prop can't move. Slowly rotate the prop and watch or measure the distance between the prop barrel end flare and the measuring device. If you have an up and down motion on the dimension as you rotate the prop, something is out of alignment. If not forget your original question.

If you do have a variance, remove the prop and do the same measurement with the end of the prop shaft where it's threaded. If it moves your shaft is bent. If not it's not and the problem is prop misalignment. Might remount and recheck before you go to the next step. Prop nut torque on that engine is 55 ft-lbs. Put a 2x4 between the anti vent plate and the prop blade and get after it with your torque wrench. If you don't have one, put 50# (a sack of feed is 50#) on the end of a 1' wrench handle.

Next Step: Take your prop to a prop shop, with the hub and ask the tech there to check the runout.

Next Step: If that passes fine, let's move to yet the next step.

Next Step: How much turbulence is around the prop when you have this problem? You probably have a lot at the low speeds and this turbulence is varying the density of the water the prop is trying to push against hence it will push hard at times and miss when air comes by making the engine jerk fore and aft; water just boiling around the rear of the engine. The turbulence is probably not prop related, but changing props changed the sensitivity to the turbulence.

"If it ain't broke (busted) don't fix it!"

I had a reaction to a certain prop I ran on an '02 year model of that engine. I knew what it was and that nothing was being hurt by the noise, but the prop was my favorite and I ran it every chance I got....boat ran fast. Problem occurred at slow speeds. So I just minimized the time I spent at the speeds where it made the noise. I'd intermittently go faster or slower to get the boat momentum I needed at the time....driving on my trailer. Problem solved.

HTH,
Mark
 

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
7,985
It could be because the swivel pin was so worn out the looseness was isolating some of the vibration from the motor. Now that it is tighter you may be feeling the natural vibration of the motor.
 
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