How to tighten a prop nut.

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

The prop fits only on a taper (no keyway).
The shaft needs to be prevented from turning. The gearbox is a Hurth.
The nut is castellated and secured with a split pin.

Oh - and the boat is on the water.
 
I don't know if a Hurth gearbox can take the strain of tightening the nut, but in any case its not something I would want to impose on any gearbox. If you have the usual type of coupling with flanges on both shaft and gearbox I would separate them, push the shaft back a little bit and and bolt a length of steel to the shaft flange so that it can be held to prevent turning. You need to find out the correct torque for the nut, because without a keyway you risk problems if it is not tight enough. Most instructions for castellated nuts say something like "Tighten to specified torque, then back off to insert a new split pin". I don't agree with that because any backing off might loosen the taper fit. Instead I would first set the torque wrench a few pounds less than specified. For example if the torque is 150 ft lb I would set to 140 initially, then look to see where the nut is in relation to the hole. A guide line on the shaft end is useful. After that reset the torque wrench to 150 ft lb and turn until the split pin goes in. With a decent torque wrench you can measure the actual torque by adjusting the wrench up from 140 a pound at a time until it clicks. If you are within 5 per cent of the specified torque I would leave it at that.

All the above is fine on dry land, but in the water, I wish you luck.

One final point. With your setup a really good and accurate taper is vital, with both shaft and bore clean, fit the prop and tighten the nut gently by hand with a short spanner. With the shaft held try to turn the propellor. If you can do so you need to check the tapers because it should not take very much to prevent movement if the tapers are accurate. The torque required on the nut is to keep it secure, as much less should be needed to stop the propellor slipping.
 
As Norman says you are going to have to use the output flange to stop the shaft rotating you may be able to do that without separating the shaft from the gearbox output or not it depends on the configuration but a bar using two of the bolts then wedged against a block of wood ( not the hull) should suffice.
Now the under water bit hopefully you are not trying to do it with a snorkel but have breaking apparatus. Put on a lot of weights as this will give you some stability and help you in the water. Whilst all of what Norman says is correct needs must and I would advocate taking the prop off, clean the shaft, lightly grease with water resistant grease then nip up the nut and check where the hole is when it first shows signs of resistance if you can tighten a little further to bring it into line do so if not then back off until the split pin goes in.
If you are using a snorkel you have problems and it is going to take time but you must ensure you are negatively buoyant or working is going to be difficult.
This all begs the question as to why you suddenly have a problem, hopefully the shaft and taper are OK and have not been previously butchered.
 
Norman_E said:
I don't know if a Hurth gearbox can take the strain of tightening the nut, but in any case its not something I would want to impose on any gearbox. If you have the usual type of coupling with flanges on both shaft and gearbox I would separate them, push the shaft back a little bit and and bolt a length of steel to the shaft flange so that it can be held to prevent turning. You need to find out the correct torque for the nut, because without a keyway you risk problems if it is not tight enough. Most instructions for castellated nuts say something like "Tighten to specified torque, then back off to insert a new split pin". I don't agree with that because any backing off might loosen the taper fit. Instead I would first set the torque wrench a few pounds less than specified. For example if the torque is 150 ft lb I would set to 140 initially, then look to see where the nut is in relation to the hole. A guide line on the shaft end is useful. After that reset the torque wrench to 150 ft lb and turn until the split pin goes in. With a decent torque wrench you can measure the actual torque by adjusting the wrench up from 140 a pound at a time until it clicks. If you are within 5 per cent of the specified torque I would leave it at that.

All the above is fine on dry land, but in the water, I wish you luck.

One final point. With your setup a really good and accurate taper is vital, with both shaft and bore clean, fit the prop and tighten the nut gently by hand with a short spanner. With the shaft held try to turn the propellor. If you can do so you need to check the tapers because it should not take very much to prevent movement if the tapers are accurate. The torque required on the nut is to keep it secure, as much less should be needed to stop the propellor slipping.
Click to expand...

Some very sound thinking there!

I have tried to research advice from the Hurth information available without much success. There is a reference to its capability to withstand shock loads to avoid engine damage (snagged prop I suppose).

The suggestion to unbolt the flange is a good one (which I should have thought of). I have an aqua drive which is very accessible.

Wielding a torque wrench underwater is probably a no-no (due to Mr Newton).

My thoughts to date are to produce a ring spanner with a long arm that can be manoeuvred from the deck. (I have machined a nut driver and will weld a long Ø20 rod to it that will allow the boat hook shaft to slide over it. The boat is beamy so the length needs to be at least two metres. As it is in stainless, it needs to have provision for a line to be attached as it cannot be retrieved with a magnet). I also considered blocking the prop against the hull with a piece of wood. The trouble with that is that wood floats and will try to escape. I guess a line on it will allow it to be retrieved.

The problem I foresee is that the assistant on the "spanner" has to turn the nut to line up the split pin hole. It took half an hour to remove the pin surface diving!

When I first fitted the prop (on land), I gently slid it onto the shaft by hand. I then need a puller to remove it! I can vouch for the taper on the shaft as I produced it. I cannot say the same for the prop.

The spanner operative will be SWMBO. I will tighten the nut as far as I can with an open ended spanner. Return topside to get a subjective feel for the torque with the home made spanner and then go below again. One tap on the hull, give it a nudge. Two taps - pin engaged.

"A guide line on the shaft end is useful".

Yes indeed. Once the nut turns, the hole disappears. Under water angle grinder? Hacksaw.

I cannot see all that will be possible surface diving. The first dive with scuba had to be abandoned as the BCD valve stuck open. That has now been fixed. Fortunately, the vis is not too bad. Holding a torch requires another hand although I do have a head torch which is "weather proof".

Thanks for the advice.
 
If you're working in the water, have a spare nut, washer and spit pins :o

But why work in the water? There must be somewhere you can dry out unless it's a canal boat. Even then a lift will cost money, but the job will be so much quicker and less stressful, I'd have thought it would be worth it.
 
Fr J Hackett said:
As Norman says you are going to have to use the output flange to stop the shaft rotating you may be able to do that without separating the shaft from the gearbox output or not it depends on the configuration but a bar using two of the bolts then wedged against a block of wood ( not the hull) should suffice.
Now the under water bit hopefully you are not trying to do it with a snorkel but have breaking apparatus. Put on a lot of weights as this will give you some stability and help you in the water. Whilst all of what Norman says is correct needs must and I would advocate taking the prop off, clean the shaft, lightly grease with water resistant grease then nip up the nut and check where the hole is when it first shows signs of resistance if you can tighten a little further to bring it into line do so if not then back off until the split pin goes in.
If you are using a snorkel you have problems and it is going to take time but you must ensure you are negatively buoyant or working is going to be difficult.
This all begs the question as to why you suddenly have a problem, hopefully the shaft and taper are OK and have not been previously butchered.
Click to expand...

The posts have lost their sequence but, as in #10, it did break!

Lots of weight means you sink! Ankle weights were a boon (as the suit was not deep enough to lose its leg buoyancy). Weights without a bcd sound suicidal! Fortunately on the last free dive I could manually inflate the bcd by mouth and fine tune the buoyancy. Although the boat is alongside, the bottom is too far down to stand on which might have been a boon but the vis would have deteriorated from the stirred up mud.

I too do not understand why the prop should suddenly release. I could turn the nut around a quarter of a turn once the pin was removed and the prop spins freely. Maybe there was dirt and real torque has caused it to settle. Before launch, I could not turn the propeller by hand with the gearbox in neutral. (I believe that was due to the cutlass bearing settling down.) I don't believe I picked up any debris as the harbour is pretty clear (Sharpness).

I will remove the prop and examine it and the shaft to check for anything untoward.

I will have to use the scuba but need an assistant as I am too encumbered to remove the fins without assistance and cannot climb the ladder with them on.

(As an aside, escape from Sharpness into the Gloucester Sharpness Canal is through two bridges both of which have to opened. 24 hours notice is required and traffic has to be stopped. Jilling about waiting for the first bridge, the engine died. With a gentle breeze on the transom and me with arms spread wide, there was sufficient steerage way to transit both bridges albeit sedately!)
 
Stemar said:
If you're working in the water, have a spare nut, washer and spit pins :o

But why work in the water? There must be somewhere you can dry out unless it's a canal boat. Even then a lift will cost money, but the job will be so much quicker and less stressful, I'd have thought it would be worth it.
Click to expand...

My intention was/is to spend a while in the canal proving the wet systems (engine, heads etc) before nipping across the Bristol Channel to a permanent mooring. I have a four week licence (maximum without a BSS certificate). Due to leaks, I have already suffered the costs of eight crane hires (in/out, in/out, in and three mast step and unsteps) and the budget is bent!

Once in open water and allowed to hoist sail, drying out options are plentiful.
 
Not sure that any of the above is as important as ensuring the taper fitting the prop to the shaft is correct or you may find yourself doing it all over again but at least you would know how.
Ideally you would grind or lap in the prop .
Kelvin traditionally never used keys
 
OK I assume that the prop nut thread is right hand (tighten clockwise when viewed fro the prop end).

I would fit a spanner on to one of the nuts holding the coupling together and jam it up against the inside of the hull so when tightening the prop nut the spanner jams tighter to the hull.

On the outside you could tighten the prop nut with SCUBA gear with tank on deck or even a snorkel which has an exit valve at the mouth piece. This allows you to have an extension pipe to above surface.

If you then brace your feet against the underside of the hull you could tighten the nut quite tight with an extended spanner.

Just some thoughts
 
Rogershaw said:
OK I assume that the prop nut thread is right hand (tighten clockwise when viewed fro the prop end).

I would fit a spanner on to one of the nuts holding the coupling together and jam it up against the inside of the hull so when tightening the prop nut the spanner jams tighter to the hull.

On the outside you could tighten the prop nut with SCUBA gear with tank on deck or even a snorkel which has an exit valve at the mouth piece. This allows you to have an extension pipe to above surface.

If you then brace your feet against the underside of the hull you could tighten the nut quite tight with an extended spanner.

Just some thoughts
Click to expand...

Wow! Upside down! I am 79!
 
Graham_Wright said:
No - negative gravity! = buoyancy up against the hull.

Don't fancy that. I prefer to work vertically. Communication with "topside" is the real worry.
Click to expand...

Don't know your boat but could you do the old trick of heavy weights on the bow to lift the stern /prop out of the water then work from a dingy.

You in the sharpness dock. used to play along the river seven when I lived in the UK as my boat was moored for a time in Stourport.
 
i have done this in the water (with a keyway)

locking shaft - luckily i ahve a 65mm open spanner on board, that neatly fits over the bolts on the flexible coupling on the shaft, and torques against wooden pad on hull.

swimming was fun. and i didn't have to line up a split pin hole. large shifter, and torqued with feet against hull. wetsuit and weight belt, breathholding. took about an hour all in.

suggest you tighten nut, then motor slowly to a pier and drty out al least enough to line up the cotter pin hole etc. mind you, you have scuba kit, so may go better for you :-)

where is the boat?

ps i thought i had a keyless taper fit, but it was just that the woodruff key had split longitudinally...
 
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