The last 10% of the job...

[Roger Long]

always takes the other 90% of the time.

I went down to give the prop a quick lap job since Maine Sail spotted some uneven wear patterns on the shaft when he was on the boat yesterday.

(Details at: http://forums.sbo.sailboatowners.com/showthread.php?t=119035)

It took a lot more lapping than it should have. By the time I had just about 3/4 of the rear of the taper showing surface honing, there was a step at the end of the shaft taper I could feel with my fingers. The forward part didn't even have the corrosion patina cleaned off and I could feel the prop wiggle on dry fit. The step would clearly prevent it sliding up the taper for final fit.

I called one prop shop that didn't return my call.

I called the first one everyone thinks of and described the problem including my check of the taper (it's straight, no sunlight visible under a steel straightedge). He said they would need the shaft as well as the prop and would probably need top make me a new shaft. Right.
I called their Salem main office and they said, "No problem, we'll run a reamer through it and can have it back to you in a week."

I remembered seeing props at Bracket machine when they were making my tang parts so I called and Steve said to bring it right out and he would run a reamer through it while I waited. I got out there and they can't find the 1" shaft taper reamer. I left the prop and he said that, if he couldn't find the reamer, he would just set it up on the CNC machine and do the same thing. They'll have it for me tomorrow so I should be able to make my July 5th launch date. That's the way you do it folks.

 

[kloudie1]

Getting closer to the prize ! Itsy bitsy things that can ruin a day if left undone..

 

[All U Get]

Hi Roger,
I stopped by the other day and tried to get an angle for a picture of the Strider and the Nina together. You were off somewhere so I just looked from A dock and Strider seemed ready to go. That would have been a good picture.
All U Get

 

[jibes138]

Very confusing description. If you have a step on the shaft you can feel with your finger how did a straight edge show no light? If the shaft has a step in it why would you run a reamer through the prop? The CNC machine should be able to trace the taper very accurately but depending on how it is held the bore may not be concentric to the prop, you really should balance the prop after machining it. a very small error in the centerline will cause a very large imbalance. Think of it this way, you will have a center of rotation that is offset from the center of gravity. Even if only a couple thousandths the prop CG will be orbiting the center of rotation.

 

[Roger Long]

Very confusing description.

Well here's what I ended up with (very exaggerated):

I never had this prop off except the first winter when the yard pulled it to replace the cutlass and put it right back on. I had it reconditioned and balanced this winter.

When lapping it, I was only seeing honing marks on the rear third for a considerable number of turns. Pushed on hard with a dry fit, it would wiggle slightly when the blade tips were pushed on.

By the time I got the surface patina removed for 75% of the hub length, the step had developed at the rear end where the hub overhangs the shaft so no lapping compound can act on it. The fit is now OK but the step will prevent the prop from moving forward on the taper when the nut is taken up.

They are just going to ream enough to take off the step at the rear. A little additional lapping and is should be good to go.

You're right that it probably should be balanced, yet again,but I'm not going to give a week or two of cruising. I'll roll the dice on a bit of vibration. The boat was very smooth until last summer and not bad then. This is probably the first attention the prop has had in decades.

Take home lesson: When removing a prop for reconditioning, check the fit then instead of just before you put it on under time pressure in the spring. A lot of yards just put props on without lapping. This one probably had a poor fit all those years but was just jammed on hard enough to work with a 20 hp engine.

 

[Roger Long]

Why it pays to do your own thinking.

I realized in the shower this morning that a possible explanation for my prop fit is that the shaft taper is slightly non-standard. Restoring standard taper would leave me right back where this fandango started. The boat and shaft were built in south Florida at the end of the 70's after all and just the pot smoke blowing into a machine shop from the general neighborhood could had addled a machinist's brain.

After some frantic thought, I came up with the idea that should have occurred to me when I was standing in the neighborhood machinist's shop two days ago with him explaining that he didn't have any prop tools. I suspect strongly that drawing the diagram for this thread clarified my thinking. Drawing things out always helps.

I called the machine shop as soon as they opened and told Steve I had an idea how to fix it faster and simpler than we had discussed and not risk screwing up the nice lapped section I already had. He said they had already figured out a way. They would machine a plain steel shaft into a dummy shaft taper and I could use that as a lapping tool to remove the step.

I said, "No, do it like this(long explanation since this was by cell phone)."

A straight, cylindrical bore of the diameter at the corner of the shoulder introduced by honing. It could be cut with a straight hone or in a lathe. Since this part of the hub overhangs the shaft, the taper is unnecessary. Making it straight means I could do extensive further lapping without a shoulder developing.

I stopped in the marina office earlier today and told them that I had a small prop problem but, "Bracket got right on it." and I would be able to launch Tuesday.

They said, "Right, we're still waiting for the shaft stuff we sent them. Good luck. We'll take you off the schedule."

I went out at 3:30 and picked up the prop. Total bill, 17 bucks. Once they got what I was talking about, they just stuck a flapper wheel in it and took off the step portion. Dang, I could have done that. It isn't perfect but, like I said, that part of the hub doesn't touch anything.

Remember from the post above, the "default" prop shop which I would have considered the only authority a few years ago and which many still do said that they would have to have both my prop and shaft and probably need to make me a new shaft. There is a lot of air between that couple of grand and couple of weeks outcome and the 17 bucks I spent.

The prop is on the boat.

Thinking: priceless.

 

[txtowman]

Just another reason to work through these problems yourself and do it yourself.

They wanted $300 to drop the the fuel tank on my pickup to change the fuel level sender.

Took me twice that, but the $300 can be used in a better way.(on the boat)