Learning experience/ with cost

[Avenger]

I know sky and i concure . I could swallow it if it just went one way pushing on the tapor. But with no key way in reverse the prop is going to spin. I just do not see being tightened enough not to. If it spins in reverse there goes your so called fit. Sorry sky. I kinda associate this with trick i told my wife why the car was shaking bad air in the tires!! Yes she is gullable some times about some things Tim

You may have hit on the reason they're keyed. The only way they'll come off in reverse is if the nut is loose. Ask anybody who's ever accidentally tried to pull a tapered drive without removing the nut. It's not coming off. But it may be a redundancy in case of a poor installation.

So just machining it with out lapping would be like doing a valve job without lapping Right?
Or do you blue the seats put it together and check?

If you lap a prop with fine lapping compound you will see what is making contact after you wipe it off, so using blue is not really needed. It also cleans up any scratch, burr or crud you may have missed remember this is outside under a boat not a Machine shop. It is so easy and cheap why would just go do it since every time you do it it will be a better fit.


You can machine the shaft and the prop hub and think its perfect but if you think it is perfect it just because you have not looked hard enough
It not a perfect world where theory works
Where have I heard that before

Next time you get a cylinder head machined let me know if you see evidence of lapping. Nobody does it anymore. Actually in the case of most (not all) engine valves it's detrimental since the valve and seat are purposely cut at different angles to create a smaller contact area. Lapping would widen that area and decrease seating pressure.

Using a light lapping or a blue test basically accomplishes the same thing. Verifies that the fit is correct. If it isn't, you're not going to correct any significant issues under the boat.

[Bob Bradley]

... Like cutting a piece of glass most shafts (regardless of the application) fail due to scoring or inproper machining without stress releiving the cut area...

I have to disagree with you on this point. An improperly seated prop, like one hung on the key and not on the taper puts major stress on the shaft, especially every time it goes in and out of gear. Just like bending a paper clip over and over, it will eventually fail. That's how I lost my prop. And, the other one had started to crack for the same reason. This was confirmed by Randall Hale of Hale Propellors in Old Saybrook CT. Randy knows his stuff.

[rsmith]

I have to disagree with you on this point. An improperly seated prop, like one hung on the key and not on the taper puts major stress on the shaft, especially every time it goes in and out of gear. Just like bending a paper clip over and over, it will eventually fail. That's how I lost my prop. And, the other one had started to crack for the same reason. This was confirmed by Randall Hale of Hale Propellors in Old Saybrook CT. Randy knows his stuff.

Like I said "most" but if the prop was rocking on the keyway the vibration would be horendous and the hub of the prop would get egged out to the point where the vibration would keep getting worse. If things are vibrating its time to stop and figure out what is wrong.
Now if your all getting picky about fitment and keyways dont carry any drive load. What about the muff couplings? their not tapered and no one laps them and the only thing that keeps the shaft ends from spinning is the keys.

[jim rosenthal]

Here's a case in point: My 3208 injection pump. All the drive force and pump to engine timing is handled by a tapered drive. If it slips it goes out of time and runs lousy if at all.

No key.... no keyway.

Looks kind of like a little V8 diesel itself, doesn't it...

[jim rosenthal]

If there was no key wouldn't running the engine in reverse loosen the prop and cause it to slip? then, when you put the drive in forward, it would drive the prop back onto the taper and retighten it again... most of us go through dozens if not hundreds of those cycles a year; if the key wasn't there, the prop would be alternately tight and loose on the taper, which wouldn't be good. Maybe the key is there to prevent that?

[SKYCHENEY]

Sky, I am away from the boat and my notebook today, but do remember exactly what were Jim Bircher's comments at our Hatteras school about prop fitting and lapping? As I recall he was basically against lapping as a solution and seem to feel it could cause issues. Jim is the premier machine shop in these parts and makes shafts for a lot of high end brands, and has some of Tim's former money too. He talked a long time about taper, and proper shaft to prop fit. When I get back I'll see if I can find his white paper on the subject.

I don't know what Jim said exactly. I couldn't bear to listen as I just kept thinking that he was just out to take my money. It all sounded like a big scare tactic to get very high prices for precision work that was more than necessary. I know many like him and he has a good reputation. I just would never use someone like that. It seems to be overkill (the precision and the price).

I just do what I have been doing and it works fine for my old boat.

[madhatter1]

My props are held on with crazy glue. Crazy, right??

[Avenger]

Looks kind of like a little V8 diesel itself, doesn't it...

Yep, and these vintage V-8 Macks even more so:

[Avenger]

If there was no key wouldn't running the engine in reverse loosen the prop and cause it to slip? then, when you put the drive in forward, it would drive the prop back onto the taper and retighten it again... most of us go through dozens if not hundreds of those cycles a year; if the key wasn't there, the prop would be alternately tight and loose on the taper, which wouldn't be good. Maybe the key is there to prevent that?

If they came off so easily in reverse why would anybody buy a puller? A properly made and installed taper is an extremely strong connection. Watch the next time your yard pulls your props and see how much force is required. They don't come off easily.

Also, if that happened there would be evidence of the prop and shaft "working" on each other. Has anybody seen any fretting, galling, spalling or any of a dozen other terms for wear that would be caused by that?

[Bob Bradley]

Trust me they won't come off on their own if properly seated. I have a home made puller make from 1/2" steel - one 12" diameter circular piece with a "U" cut into it and one 12" piece solid. Each has 4 holes for the long bolts that pull them together. I block the blade and tighten the bolts evenly using a very big wrench. They get so tight that the 1/2" cold rolled steel starts to bend, and still the bond of the taper does not break. I then rap the unit with a big hammer and eventually the prop comes free with a significant "pop".

The key is nothing but window dressing. The taper does all the work.