Boat Doctor ( AKA Michael Hannock ) Dear Mick,I’m writing you for a friend of mine who has incurred a problem for the last two seasons. We are located in Mentor Ohio so our boating season is only 5 - 6 months long. My friend Dan has a 2005 Regal 35ft cruiser powered by two 8.1 Liter 420hp Mercruiser inboards. Two years ago he took his two props to a propeller specialist and had them profiled and checked for correct pitch and balance. The specialist told him to lap the tapers in the props to the shafts with lapping compound for a better, fit which he did. Last season when we pulled the boat, he found the starboard prop was loose on the shaft and thought maybe he just didn’t snug the nuts down all the way. We just pulled the boat again this season and upon checking the props found the starboard prop was loose again. Dan told me he was going to replace the keys as he thought maybe they were part of the problem. But upon buying new ones they turned out to be too high so he used the old keys again. He assured me he had gone thru the same process of lapping the props to the shafts and making sure he had wiped them clean so there was no left over lapping compound on the shafts or in the prop tapers. He also said he had really reefed the nuts down tight seating the prop securely before putting the boat in the water this spring. I had called him after I got home and asked if it was possible the nuts had bottomed out on the threads and he just thought he had them tight. He said no way, they were tight when he put them on this spring. The boat runs smooth with no vibration and he had the alignment checked on both engines this spring. So what are we missing ?
Which nut is he putting on against the prop? I bet he ijnstalls the big nut and then the small nut, right?
Many people do. But that's wrong--or at least, not the ideal. The half-height nut should be the one against the prop hub. Torque that down. Then install the full height nut and torque that down. Once you torque the second nut, most of the load ( in tension) transfers to that nut. And since that nut has more threads engaging the shaft, it locks the prop in place better.