Prop spinning under load

[garytally]

Have a 1988 Merc 175 Blackmax. My son and I just put on new flo torq hub and put her in water. Idled out a ways and then slowly throttled up. Ran like a dream for about 45 seconds and then prop started spinning. Idled back to marina, pulled boat out and checked prop. Apparently my son (cause I never would have..ha) didn't tighten nut anywhere near 55 lbs. the nut was very loose, after slight pull was loose. Unfortunately we have now a stripped shaft and can't get nut off. So I have two questions...would a less than fully tightened nut cause the prop to spin under load. The castle was still in the slots. Secondly, how big of an issue do I have with that stripped nut? Thanks

 

[Texasmark]

The splines on the prop shaft and the inner diameter of the prop lock the twisting action of the shaft to the prop. The nut just keeps the prop ON the shaft and when tight keeps it from banging around which will sooner or later cause interface component damage. 55 ft-lbs IS the number and 55 isn't finger tight meaning that Mercury wanted the prop tight on the shaft. A stripped nut is just that and cannot be relied on to hold.

Is the nut the only thing stripped, aka the shaft threads are still intact? If so go to Merc and get another one or a high quality hardware store will have one which is where I bought my last one, just like the one Merc sells. If just slightly marred, go online or machine supply store and get a "dye" and chase the threads before you install the new nut.