prop nut not staying tight

Are you using a Nylock nut or a castle nut? Nylock should stay put and if it doesn't it is worn out and you need a new part. The pin is located at the end of the shaft? Sounds like someone might have replaced the shaft with a newer style correct craft shaft set up for a Nylock nut. The castle nut is supposed to have a hole positioned so that the cotter pin runs through the castle portion of the nut to keep it put. Photos help.

Pics would help but here is the long and short of it:

If a propeller is fit to the shaft taper correctly, you wont even need the nut. (figuratively speaking, of course) It aint coming off without a puller.
There is more wrong than just a nut.

What Pete said. Tapered fits are designed as a fail safe for safety related parts.

Automotive uses lots of tapered fittings. Ball joints, tie rod ends, Safety parts etc.

My boat ran for a month or two with no prop nut because it lives on a lift. I bought what was labeled as a stainless steel cotter pin but it was standard steel and rusted away in a few months.

89Martinique wrote: So then how did the prop fall off of my grandparents (now uncles) 87 Martinique, never had the prop messed with, only 1200 hours, we were cruising, starting to come off plane, and had a really bad vibration. So we all checked out the drive shaft behind the tranny. Then we put it in gear, and nothing! Except for a fast moving shaft... That boat went in with a nut and cotter pin...

random guess, but this would make sense to me...

probably was seated nicely and hit something in the water hard enough to knock it off of its taper, or came unbalanced when it lost a blade. when it loosened up, the spinning prop twisted the nut hard enough to shear the cotter pin, then the nut came off as it spun.   

haha, navy seals...

The vibration wasnt shacking the boat violently, just a vibration that was never there before...

Well, the new prop is too small the motor winds well up into the 5k+ rpm range... But, the motor has a really awesome throaty sound to it! We dont go up past the 4500 rpm mark as higher than that on a 1200 hour motor that has been in a boat that has been abused needs to be babied a tiny... But, the new prop doesnt load the motor up near as much, therefore a new sound...

It's a castle nut and not the nylon nut and yes the hole for the cotter pin is at the end of the shaft. I've had this boat for almost 20 years and have not replaced the shaft, I did whoever put a 4 blade prop on there to replace the three blade that was the original and had a heck of a time getting it off. As I'm writing I remember that after I changed out the 3 blade for the 4 blade that it ended up coming off because the cotter pin was not stainless and so I put the 3 blade back on. Our lake dried up and fortunately I found my 4 blade prop that's when I had it refurbed and put it back on. Now noticing that the nut doesn't stay snug against the prop no matter how tight I get it. I'll do a search on how to lash the prop then see if I can locate a lock nut that size.

^^^^^^^ this

A prop that comes off it's taper was not lapped in and seated. It comes down to a poor install.

Thanks guys. I just went back and read the "lapping" process, I skimmed right past it when I read the post the other day, and will have to get under there and do this so my prop stays on. As far as it being the right prop, I believe it is, I bought for my year and make, because it does get snug buuuut works it's way back down the shaft.

Prop shops will ream the taper bore to true them up before truing up the prop pitch. This will allow the prop to sit deeper on the taper. In this case, I have had to use washers to bring a castle nut aft so they align with the cotter pin hole. I like to use brass thrust washers since they are 1/8" thick and their diameter is closer to the prop hub diameter.
Mark,
BTW, you may want to go back and edit the title of this thread. I'm sure you wanted to say nut and not bolt!!!