Mercury 60 Bigfoot Prop Blowing Out?

[edsjr01]

2011 60 Bigfoot EFI
18' Bennington Pontoon

The last few years I've been playing with different things trying to get more speed (I'm just that guy). Started with a 9 pitch and motor all the way down on the transom. Through a combination of raising the motor to the top position so the cavitation plate is in the correct spot WOT, cleaning the toons, and underskinning I've gotten up to an 11 pitch and doubled the speed and improved the ride tremendously.

This summer after lifting the boat off the trailer and getting the underneath side of the toons cleaned I was able to pick up a couple hundred RPM, to 6200. So decided to get a 12 pitch stainless prop.

My son ran it before I did and claims it ran fine. The following weekend the impeller grenaded (I know I should have replaced it sooner). After getting that issue fixed and all the little pieces out of the cooling system I take it back to the lake last Saturday. Get past no wake buoys and everything is normal until I get past 4K, then it jumps to 6K and I'm not even WOT and not trimmed.

Immediately I thought the used prop I bought on ebay had a spun hub. It's an older Vengeance prop with a pressed in hub. Finally I located one but wanted to confirm it was spun because the local boat shop told me they didn't think it looked like it was. I marked the hub both on the front and back then took it for a test today. Took the prop off and all the marks still line up, so I have a hard time believing it is spun.

Thing is, it's still doing the same thing....normal to 4K then acts like the prop is blown out. At 4K I looked and the cavitation plate is still slightly below the water.

Is it possible I need to put the motor deeper in the water due to the higher pitch?

Could I have done something when I remounted the lower unit to cause this? It went together nicely but when I took it off I didn't put it in gear, however prior to reassembly I did put the control all the way forward and manually put the lower unit in forward. I really think it wouldn't have gone together if the linkage and lower unit weren't matched up.

Any ideas?

Thank You!

 

[QBhoy]

I’d think the tell tale of what’s going on here is given away by the info about the impeller failing. I’d suspect you are running way too high with the motor.

Just in a side note. Don’t expect anything more out a vengeance, over a normal black max ally prop. I’d even go as far as to say that the black max of same pitch would run quicker. Total gimmick of a prop really.