Need help with my Yamato 302 gearcase

[Systemoverload]

I was going ~54mph when this whirring sound started and so I shut off the engine. I then restarted and the engine ran but the boat was no longer moving so I paddled back to shore.

Upon inspection my locally-made 3-blade prop had thrown 1 blade and the cone nut on the Y302 worked itself loose apparently due to the resulting vibration. Because the cone nut had moved the gears un-meshed and scraped against each other and mangled all of the teeth (the whirring sound). The resulting debris from the gears lodged in the bearings and did some damage there also. Gears are still useable but I'd rather replace them along with all the bearings.

Does this loosened cone nut happen a lot on this engine and if so are there measures to prevent it? My prop shaft is about 1" below the bottom, running a 3-blade cleaver at ~7,200 rpm on a 13-ft vee-pad boat. I have installed a home-made tuned-pipe and was trying it out when this happened.

Please help.

[zul8tr]

Because the cone nut is a left hand thread (righty loosy, lefty tighty) and the prop shaft rotates lefty it seems even with one blade gone the lefty rotation of the prop shaft would aid to tighten the cone nut.. When was the last time the nut was checked for proper tightening? I have never had a cone nut come loose but will say I have never thrown a blade.

A tip, When I torque the cone nut I use thick wheel bearing grease to protect threads and then place a black 1" strike mark with a sharpie pen at right angle to the threads from the cone nut to the foot. Then at a glance I can tell if the nut moved.

[Systemoverload]

I was about to put on some thread lock adhesive to make sure it stays in place! So just plain torque only according to the manual.

Thanks!