How common is a spun prop?
 

Hey all,

In addition to some regular service, I had my mechanic put my spare 14 x 21 prop on my Mariah 1900 Z. (19' open bow, 4.3L Mercruiser.) I bought this prop used, never used it until now, and never knew its history. But the metal was in nice shape--much better than the 14.5 x 19 that was on the boat--so I decided to switch.

I'm sure a kid at the shop did the install, and they made a comment afterwards about needing a few additional parts. (Seemed odd, but OK, whatever.)

With the 21p now installed, I immediately found I couldn't get the boat on plane without the engine revving out. The first time it happened, I thought for sure the prop fell off! Slow speeds were fine, but I could never get the boat up on plane without the engine racing up to redline-level RPMs.

So I checked with my mechanic, expecting him to say that the prop was spun. Although I've never experienced a spun prop before, it sure seemed to have all the classic symptoms. Instead, he said the 14 x 21 was totally wrong for my boat and that the problem was a misapplication of the 14 x 21.

So what do you think:

A. It's certainly possible that a 14.5 x 19 will work well, but a 14 x 21 won't even get the boat on plane.

B. My mechanic is covering for a kid who installed an incorrect thrust washer or otherwise messed up the installation.

C. The 14 x 21 prop is spun.


Any thoughts or insights GREATLY appreciated.

Re: How common is a spun prop?
 

If you were running a 19p prop fine before, you certainly won't over rev while running a 21p prop that's only .5" smaller. Symptoms sounds exactly like the hub has been spun.

How common is it... I grew up around boats, still have boats now and I've never experienced a spun prop. I've only heard of that happening. I've certainly knocked my share of blades off though :shakehead: Because of the number of rocks where I usually boat, I've always run aluminum props though. That might be the difference.

Re: How common is a spun prop?
 

It only happens at the moment of the most stress on the prop...just before planing. That's why I tend to think the prop is spun.

I mean, if the propshaft ISN'T spinning inside the rubber bushing of the prop, then where are all the engine RPMs coming from? Is there some sort of safety mechanism in the lower unit where, if the stress on the prop is too great, the transmission disengages, effectively leaving the boat in neutral? If that's the case, then I stand corrected (and hopelessly clueless, apparently.)

Re: How common is a spun prop?
 

No, there's nothing in the outdrive like that. The safety mechanism is the hub in the prop :rolleyes:

If you're able to get just about to planing speed, it's either spun the hub or the prop is ventilating. If you're trimmed all the way down, I wouldn't expect the smaller prop to be sucking air which would mean it's spun.

Re: How common is a spun prop?
 

Re: How common is a spun prop?
 

My guess is option "B" and / or "C"..... :shakehead:

Unless there was something wrong in the installation or in the prop itself, a change from a 19" to a 21" pitch would not cause the problems you incurred. It's possible too that the 21" prop was not being used because of a problem (such as a spun prop) and the prior owner didn't tell you about it.....

Personally, I would be looking for a new boat mechanic.

Re: How common is a spun prop?
 

I'm betting the prop was spun prior to my purchasing it. (Note to self: From now on, just buy the new one and be done with it.)

I know I can test this by reinstalling the 21, putting a mark of paint on the propshaft and the prop, taking it out on the water, hitting the throttle, and seeing if the marks still line up afterwards. Maybe I'll try that on a warm summer day.

Or, can a prop shop test it?

I guess if it's spun, then my mechanic's explanation is just wrong.
If it's NOT spun, then he's just covering for a screwed-up install.

Re: How common is a spun prop?
 

It's pretty tough to screw up the install of a prop on an Alpha 1. Put on thrust washer, prop, hub kit, lock washer, tighten nut, bend tabs on lock washer.

Hopefully the mechanic isn't a good buddy of yours. It's going to be awkward when you have to tell him he doesn't know his head from a ... :D

Re: How common is a spun prop?
 

Nope... not a good buddy. But he's been in the biz for about 20 years. I'm equally torn between two scenarios: I have a suspicion that the thrust washer that was installed with the 21 wasn't the right one, OR the 21 was spun before I bought it.

I'll get to the bottom of it -- the prop is either spun or it isn't, and it won't take all that much effort to figure out.

I just have no idea whether a spun prop is a relatively common occurrence or something extremely rare. Knowing that would help me determine whether the dude is incompetent and lying, or just incompetent. :rolling_laugh:

Re: How common is a spun prop?
 

Spun props aren't 'uncommon'. I had it happen to me a few years back and i know of 3-4 others who have had it occur over the years. The symptoms you state are a textbook case of a spun prop. Its possible (maybe even likely) the prop was already buggered & the installer had no idea there was a problem and just as likely the regular mechanic is covering up for him (& his business) by putting you on the defensive. Anyway...if it runs fine with the 19" mounted then i have no doubt the 21" prop hub is buggered.