Learning experience/ with cost

[Tim Powell]

every since my go cart days racing induro at (vIr) Viginia International Raceway I have never seen a taper without a Key. Please Please not to say you are full of muschutto but i tell you that seemd like a stretch. I wounder through the boat yards all the time getting idears. I see all types of boats at Jarret Bay from their famous 30 footers to 70 footers to some honking big boys. With some funny props 8 and 9 bladed stuff sporting 1000 plus horse power per side and sometime 2000 per side awsom stuff burning 120 gallons per side making 50 plus knots. To yet i have not seen a shft without a key way. I understand your thinking but just can not get it down. One of those 1000 plus
hp engins spinning on a fish i can not belive the prop will not spin on the shaft... If there was some merit to your thinking Hinkley or hatteras or someone would be doing what you are thinking.

I can attest to one thing loose props or misfitted props will brake shafts. My bank account will prove it. Tim

[SKYCHENEY]

Think about it. The more torque, the more force onto the taper. As you accelerate, the prop is pushed further onto the shaft and the fit gets even tighter to the point that it will no longer get any tighter.

My theory is that the key just holds the prop from spinning until it is fully seated. You may achieve that just by tightening the nut, but if you don't it will eventually be set completely when you run the boat.

The only other purpose for the key that I could see would be a safety for reverse (but isn't that what the nut is for?)

[Tim Powell]

Think about it. The more torque, the more force onto the taper. As you accelerate, the prop is pushed further onto the shaft and the fit gets even tighter to the point that it will no longer get any tighter.

My theory is that the key just holds the prop from spinning until it is fully seated. You may achieve that just by tightening the nut, but if you don't it will eventually be set completely when you run the boat.

The only other purpose for the key that I could see would be a safety for reverse (but isn't that what the nut is for?)

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

[SKYCHENEY]

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

I know. Theory is one thing, reality is another. My props are off getting tuned right now. When they come back, I will testing (bluing), lapping as necessary, and installing WITH keys. Why? Because that's the way I've always done it and I haven't had a problem.

[Tim Powell]

I know. Theory is one thing, reality is another. My props are off getting tuned right now. When they come back, I will testing (bluing), lapping as necessary, and installing WITH keys. Why? Because that's the way I've always done it and I haven't had a problem.

What is the saying do as i say not as i do

[bigbill]

gene, lapping takes some metal off of the prop(its softer than the shaft) any high spots will sand down, just like lapping a valve in an engine.
krush, i know it is not scientific, but the prop will have shiny spots and dull spots. when a good mating surface is achieved, the prop will be shiny all of the way around the bore.

[GJH]

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.

[34Hatt]

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

[saltshaker]

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.

Proper fit is the key and that can only happen if all the components are matched. When I had my new shafts made, they asked for the props and the couplings to ensure a good fit. You'll have to ask Tim but I'd suspect Jim has more of Tim's money now.

[rsmith]

The Earth is flat!!!! I guess if a lot of people keep repeating something it will become fact. This lapping makes a lot of $$ for the yards but what does it really do? These are low RPM shafts turning at roughly half the engine speed. The prop is boronze or nibral for the most part. When you crank the prop up on the taper the hub expands and conforms to the shaft. If their is a fit problem the stress is on the prop hub. If the prop hubs are cracking maybe. But the shaft failing because of unequal compression from a softer metal? 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.
Lapping valves has nothing to do with a static fit and is only for compression. The fact that their are pounding thousands of times a minute into a seat I dont see any similarity.
If fitment is such a big problem how about the other end of the shaft at the engine coupling. Heck that just slides on and is squeezed together with a couple bolts. If their was unequal compression anywhere it would be there.