Can’t get new prop onto shaft

[OrangeCrush]

Hello,

I got a new Variprop feathering propeller for my Shannon 38. I provided careful dimensions of my aperture and they were confident it would fit, but now that I have it I can’t get it on. Looks like I need about another inch of space where the rudder is. Dropping the rudder is really not an option for me right now. If I could turn the rudder beyond its natural range I might stand a chance to squeeze it on, but that means 3 separate steering systems would need to get disconnected (two wheels and an autopilot) and even then it’s not a sure thing. The prop shaft could be disconnected at the coupling, but it’s pretty short and I would have to torque the stuffing box to get it around the coupling to temporarily slide it into the hull (if that were my strategy). It sorta seems to me that taking the prop apart and putting it back together after the hub was on might be the only way, but it they don’t recommend taking it apart yourself and I’m not sure even that would buy me enough clearance.

Obviously I’m gonna speak to the manufacturer about this but by the time they’re open I will no longer be down at the boat (coronavirus has complicated my boat work schedule). Right now I’m pretty discouraged that I may have spend thousands on a prop I can’t use, but as usual I’m hoping someone on this forum might be able to tell me something I haven’t thought of. Any suggestions?

[Orion Jim]

I’m betting you’ve got a couple inches of glass and filler in that prop aperture in your rudder before you get to the actual rudder shaft. I would get out my die grinder and do a little exploratory surgery. It is easily repaired if you find the shaft is close to the surface.
It may just provide the wiggle room you need.

[Wotname]

Cut the rudder to suit and re-glass it
or
Pull the engine forward
or
Drop the rudder
or
Disassemble the prop
or
Try turning the rudder (with steering disconnected) and possibly only needing to reshape the edges of the rudder aperture.

No easy way out!

[Wotname]

And please do lap the new prop taper to the existing shaft taper. It only takes 15 to 20 minutes and you will know for sure that the tapers fully matches rather than hoping they do.

[OrangeCrush]

Spoke to the builder of my boat and he strongly advised against grinding the rudder, since there’s only about 1/2” of glass max before I would hit the stainless shaft which curves and runs down the front edge of the rudder.

He said that the way to go is to cut some of the tapered threading off the shaft. He said that’s what they do with MaxProp which is similar. Since it has a special long inset nut it may need less threading and no cotter pin.

The existing threading is about 1-3/8” long. I think that if I cut off the little cotter pin nub and about 3/8” of threading it would go on smoothly. That would leave me about an inch of threading for the long nut to grab onto. They included some threadlocker when the sent me the prop

Obviously I’m gonna talk to the makers of the prop before I cut anything, but I’m thinking this may end up being the best solution.

[OrangeCrush]

Originally Posted by Wotname

And please do lap the new prop taper to the existing shaft taper. It only takes 15 to 20 minutes and you will know for sure that the tapers fully matches rather than hoping they do.

Can you explain what this is? I’m unfamiliar with the concept of “lapping” a taper. (But it sounds like a good idea)

[Wotname]

You will want to have two threads (minimum) exposed once the nut is torqued up.

[OrangeCrush]

Originally Posted by Wotname

You will want to have two threads (minimum) exposed once the nut is torqued up.

It’s a special nut that won’t leave any threads exposed regardless. See pics:

[Wotname]

Originally Posted by OrangeCrush

Can you explain what this is? I’m unfamiliar with the concept of “lapping” a taper. (But it sounds like a good idea)

Simply put, the taper provides the friction needed between the shaft and the prop. The primary purpose of the key is to prevent the shaft from turning when you torque the nut, it's secondary purpose is a back up the friction provided by the taper.

The taper should have at least a 90% contact area but when when you fit a new prop to an existing shaft, that contact may be considerably less, even as low as 50%. Clearly there is no friction where there is no contact. While the machinists cutting the tapers of both the shaft and the prop try to get it correct, it is simply not possible to know if the tapers fully match.

Some dispute the need to check but anyone familiar with machining would always check the tapers before a final fit and lap as necessary.

The process is simple, use bearing blue (dyed grease) to see where the shaft taper engages fully with the prop taper. If you have >90%, you are good to go. If less, then use a grinding paste (valve grinding paste etc) between the prop and the shaft and rotate back and forward by hand until the contact area gets to 90%. It is quite quick as the prop hub is relatively soft.

For some night time reading review the links in the my post below taken form this thread.

You will also find opposing views in the thread so it's your choice


https://www.cruisersforum.com/forums...ff-225525.html

Originally Posted by Wotname

Hard to know if you are serious or if you are taking the mickey - I understand you are a Kiwi so you are probably taking the mickey.

For anyone who doesn't want to take my unfounded word about the inherent advantages of lapping prop when first fitting to an unknown shaft, work you way though these links from a variety of professionals.

Propeller Fitting Instructions
Lapping Propellers to Shaft | High Seas Yacht Service
https://stevedmarineconsulting.com/p...-installation/
https://www.havelockslipway.co.nz/services/props-shaft
https://www.ashleyriverboatworks.com...-to-the-shaft/
https://www.pressreader.com/usa/soun...81934542718996
https://www.passagemaker.com/technic...pellers-part-2
Marine propeller installation - boat prop instalation
https://henleyspropellers.com/products/shafts/
Keep your prop on - Ocean Navigator - January/February 2003

[Wotname]

Originally Posted by OrangeCrush

It’s a special nut that won’t leave any threads exposed regardless. See pics:Attachment 213696Attachment 213697Attachment 213698Attachment 213699

OK, got it! In this case I would be taking advice from the prop maker as to the minimum amount of thread needed (as you have already mentioned).

[OrangeCrush]

Spoke with the prop maker and he said the order of things he would do is:

1) cut off the cotter pin nub since that's totally useless here
2) remove the coupling from the front of the shaft and see if that buys me a little extra space to slide the shaft forward temporarily
3) cut off a couple threads worth of shaft if needed as long as I still have threading at least an inch behind the end of the taper.
4) cut 1/4"-1/2" off the front of the shaft as long as I still had 1" of shaft overhang behind the stern gear.

I think with numbers 1 and 2 I could hopefully get enough clearance. Hopefully won't have to do all 4.

5) disassembling the propeller and 6) grinding down the rudder would be the absolute last resorts.

[Rough Magic]

I would start by doing 2 and 4 before I cut the prop end off the shaft. If you ever need to put your old prop back on, you may be looking at a new shaft. Folding props, wonderful as they are, do fail on occasion. If you ever need to send it back for repair, you'd want to put the old prop on while you wait.

[Cheechako]

I would never cut a rudder for a prop install unless you just need a 1/4" grind or something..

You need to bite the bullet and get a prop that fits, they wont take it back?

or:
Pull the rudder enough to create space,
or
Pull the engine coupling to move the shaft forward temporarily.
or
Why not disassemble the prop? Would that even help?

As with all things "boat" this is not surprising. Measure and study everything before ordering.

[OrangeCrush]

Good point, Rough Magic... I'll start with #2 and see what my options are from there. I think even without the cotter pin nub I could securely put the old prop back on, but your point is well taken.

Agreed, Cheechako. I'm really not going to be grinding the rudder no matter what. Despite the issues I'm afraid this prop may fit about as well as any other folding prop. If you're interested you can see the long thought process that led me to this point by reading my forum history. Dropping the rudder will also be a huge ordeal on my boat, so I'm not really considering that. I think I only need about 3/4" more clearance which I'm feeling hopeful that I can achieve. Taking the prop apart would help, and they'll guide me through that if need be but it doesn't sound easy.

I appreciate all the support and I'll let you know how it goes!

[gonesail]

beautiful looking prop by the way. hope you get it on without too much trouble