How to tighten a prop nut.

[Ammonite]

Not sure if I'm picturing this correctly but I can't see how a 2m long extension will work. You'll be banging into the hull before the nut starts to turn and with that leverage will over torque the nut or break your boat hook. If you have scuba gear lock the shaft inside the boat as suggested and do up the nut firmly but not too tight with a regular length spanner bracing your other hand against the prop. If you get lucky or then back of the nut slightly the split pin should fit. And get your other half to hold the waterproof torch strapped to the end of your boathook if you are stuggling, or let it go on a lanyard when you need both hands. Then sort it properly when you reach your permanent berth

 

[Graham_Wright]

Not sure if I'm picturing this correctly but I can't see how a 2m long extension will work. You'll be banging into the hull before the nut starts to turn and with that leverage will over torque the nut or break your boat hook. If you have scuba gear lock the shaft inside the boat as suggested and do up the nut as tight as you can bracing you other hand against the prop. If you get lucky the split pin might even fit. Then motor gently and sort it properly when you reach your permanent berth

The torque at the nut depends on the length of the lever and the force applied to it. My intention is to measure what force is needed at the end of the "spanner" to create the necessary torque at the nut by using a torque wrench to calibrate. It is surprising how much bend there is in the extension which is 20mm diameter.

That has caused a memory of basic torque wrenches which indeed use bend in a flexible member against a scale. Thinks.

 

[Ammonite]

I get the bit about torque but what about the distance you need to just turn the nut one flat with that leverage before you bang into the hull and as you said you've got to take the bend into account. For now I don't think it's that critical. You did it properly before and look where you are now!

 

[Graham_Wright]

I get the bit about torque but what about the distance you need to just turn the nut one flat with that leverage before you bang into the hull and as you said you've got to take the bend into account. For now I don't think it's that critical. You did it properly before and look where you are now!

The boat, a Countess 33, has a high freeboard. She lies alongside a grassy bank at the the moment. I do not anticipate a restriction due to the angle you imply. The shore end could be dipped into the water if necessary. I will be below to "ratchet" the spanner (off, rotate and on).

 

[RichardS]

Please think before you post. It isn't as easy as you might imagine.

You ask how to tighten a prop nut.... And then mention later that it's underwater.

Welcome to comic cuts, as my Dad would say.

Richard

 

[Ammonite]

Good luck with that although I suspect you'd a more accurate torque figure by a feel after a bit of practice with a torque wrench and spanner although I guess it depends on what figure you are aiming for. Personally I just nip it up slot the pin back in and get on with your other jobs although I'm basing this on props where the torque required is pretty low ?

 

[Fr J Hackett]

I think you are over thinking this, just nip it up as tight as you can to get the pin in place move the boat and do the job again once you can dry out.

 

[Graham_Wright]

You ask how to tighten a prop nut.... And then mention later that it's underwater.

Welcome to comic cuts, as my Dad would say.

Richard

That's me!

 

[Graham_Wright]

Recommended torque is around 500 N/m. That's roughly 50kg on a one metre spanner. Mine is 2 metres. So 25 kg on the end should give the correct toque.

My torque wrench limits at 100 ft pounds. That's ~140 N/m. That needs a fair old pull versus the vice. Three and half times that would need a lot of Weetabix let alone underwater.

Calculation check please!

 

[Poignard]

I think you are over thinking this, just nip it up as tight as you can to get the pin in place move the boat and do the job again once you can dry out.

+1

... and the sooner you dry out, the better, because as sure as eggs is eggs this will fail again and next time it does you might be in a place where failure can cause you real problems.

 

[Graham_Wright]

+1

... and the sooner you dry out, the better, because as sure as eggs is eggs this will fail again and next time it does you might be in a place where failure can cause you real problems.

Why?

 

[scottie]

Why?

Because the taper will be damaged already by the propeller spinning

 

[James_Calvert]

Just a thought. Can you jam your spanner/socket/whatever you have on the nut... against rotation and torque it up from inside the boat?

 

[Poignard]

Why?

See post #32 from Scottie

 

[Graham_Wright]

Because the taper will be damaged already by the propeller spinning

Certainly a possibility. It was tight - very tight - originally. I will examine bore and taper. There is also the possibility of course, that the two tapers have matched profiles by spinning. I have already considered lapping them (see #14).

Out in the Severn Estuary, "lapping paste" comes free! In the canal the water is drinking water - gone through the Gloucester Folk and about to be re-used by the Bristol Folk.

 

[rogerthebodger]

Just a thought. Can you jam your spanner/socket/whatever you have on the nut... against rotation and torque it up from inside the boat?

That's one of my thoughts also

 

[AndrewB]

In fear of being castigated for awkwardness, I'll add a few constraints.

The prop fits only on a taper (no keyway).

No key holding the prop?? :confusion: Poignard is right, nipping up is a recipe for future trouble. The simple answer to your question is ... impossible.

 

[anoccasionalyachtsman]

With the information you've given your sums are right, but I'm a bit surprised by the 500Nm figure (It's about twice as tight as the tightest thing I've ever tightened). What's the thread or nut size?

 

[Ammonite]

With the information you've given your sums are right, but I'm a bit surprised by the 500Nm figure (It's about twice as tight as the tightest thing I've ever tightened). What's the thread or nut size?

It does seem incredibly high. How to you stop the on-board end of the shaft rotating without risk of damaging the hull unless you have room for a similarly long lever inside the boat? Even with plenty of packing that's a huge force. I guess there must be a way if that is the right figure but I much prefer my set up. Seems more like the type of setup you'd find on a trawler! ;-) Its about the same as a M22 lorry wheelnut. Have you had a chance to have a good look at the prop? I know nothing about this type of keyless setup but couldn't help wondering if overtightening it damaged the prop which is why it's now spinning on the shaft? Will be interested to know the outcome

 

[Iliade]

I'd lay odds that the earlier poster suggesting that you had a woodruff key which has split longitudinally is indeed the case. The chance of securing a prop against the several Nm developed by your engine using just a taper and nut, still less a castellated nut with split pin, is pretty small.

If you can find and replace the key, which will be well smeared over by the prop rotation, then the nut torque needed to limp somewhere for a lift out need not be that great.