Propshaft "overhang "

[SimonD]

When the previous owner fitted a rope cutter, a 20mm thick spacer was put in between the gearbox and propshaft flanges to create enough room for the cutter. I now want to fit a flexible coupling. This is half an inch thicker than the spacer. So, if I replace the spacer with the coupling, the shaft will move another half inch backwards. It's a 1 inch diameter shaft. Will this be a problem?

Shortening the shaft will be tricky as it has a bolt through it and the coupling. I don't want to have to drill a second hole through the shaft.

 

[pvb]

You'll need to re-position the ropecutter, otherwise there's no problem as long as there's space for the prop to turn.

 

[vyv_cox]

Rule of thumb is that the shaft unsupported length between the bearing and prop should not exceed the shaft diameter, so in your case one inch. If your propeller is a heavier one, e.g. a feathering one rather than a simple fixed two blade even that may be excessive. the consequences can be rapid bearing wear and shaft resonance.

 

[SimonD]

Thanks chaps. Vyv's rule of thumb has already been broken - the unsupported length is already about 2 inches and it seems to be ok. Maybe because it's a fixed two blade prop. In that case, an extra half inch seems to be fairly innocuous. Or am I just fooling myself?

 

[PuffTheMagicDragon]

I tend to agree with your last sentence. Sorry.

 

[SimonD]

Not surprised. I've done it before.

 

[PuffTheMagicDragon]

Not surprised. I've done it before.

We probably all have!

 

[michael_w]

Shaft shortening is an easy job for a machine shop.

 

[Old Troll]

Propshaft Overhang

Rule of thumb is that the shaft unsupported length between the bearing and prop should not exceed the shaft diameter, so in your case one inch. If your propeller is a heavier one, e.g. a feathering one rather than a simple fixed two blade even that may be excessive. the consequences can be rapid bearing wear and shaft resonance.

I have a 54mm shaft anode between the bearing and the prop shaft, with say 10mm each side for clearance. Shaft diametre is 25mm with a Two blade fixed prop. Could this be too much overhang and cause shaft resonance? Would the shaft anode have the same effect regarding electrolosyse etc if put on the shaft inside the boat?

 

[vyv_cox]

I have a 54mm shaft anode between the bearing and the prop shaft, with say 10mm each side for clearance. Shaft diametre is 25mm with a Two blade fixed prop. Could this be too much overhang and cause shaft resonance? Would the shaft anode have the same effect regarding electrolosyse etc if put on the shaft inside the boat?

I think you are saying that the anode is between the bearing and the propeller? In this case yes, the prop is largely unsupported and vibration/resonance is likely. I assume from your wording that the bearing is not in a P-bracket? In this case you would be better to have a hull anode to protect the prop/shaft galvanic couple, connected to the engine if the coupling is a solid one, or via a shaft brush if there is a flexible coupling.

With a P-bracket the shaft anode should be fairly close to the bearing with a clearance of 10 - 12 mm to allow water to flow through. Many people fit the bullet-shaped anodes with the flat face adjacent to the bearing. This is incorrect, they should be the other way to give a smooth entry for water through the bearing. Fitting the anode in the centre of the span between stern tube and bearing is bad practice, it places a heavy weight at the worst possible point and will often cause serious vibration problems.

 

[scottie]

I have a 54mm shaft anode between the bearing and the prop shaft, with say 10mm each side for clearance. Shaft diametre is 25mm with a Two blade fixed prop. Could this be too much overhang and cause shaft resonance? Would the shaft anode have the same effect regarding electrolosyse etc if put on the shaft inside the boat?

Can you get an anode to screw on to the thread on the end of the shaft

 

[Old Troll]

Propshaft Overhang

I think you are saying that the anode is between the bearing and the propeller? In this case yes, the prop is largely unsupported and vibration/resonance is likely. I assume from your wording that the bearing is not in a P-bracket? In this case you would be better to have a hull anode to protect the prop/shaft galvanic couple, connected to the engine if the coupling is a solid one, or via a shaft brush if there is a flexible coupling.

With a P-bracket the shaft anode should be fairly close to the bearing with a clearance of 10 - 12 mm to allow water to flow through. Many people fit the bullet-shaped anodes with the flat face adjacent to the bearing. This is incorrect, they should be the other way to give a smooth entry for water through the bearing. Fitting the anode in the centre of the span between stern tube and bearing is bad practice, it places a heavy weight at the worst possible point and will often cause serious vibration problems.

Thank you very much for the information regarding the likely prop shaft overhang and I shall look into changing to a hull or rudder anode, which should enable the shaft to be shortened. The yacht is a Vega so a relatively simple task to make the change. Thanks again for your valuable contributions.

 

[Old Troll]

Can you get an anode to screw on to the thread on the end of the shaft

I shall certainly investigate this option as I suspect that the original anode could have been attached in this manner.

 

[Tranona]

Thank you very much for the information regarding the likely prop shaft overhang and I shall look into changing to a hull or rudder anode, which should enable the shaft to be shortened. The yacht is a Vega so a relatively simple task to make the change. Thanks again for your valuable contributions.

An anode on the rudder won't do anything for the prop unless it is connected electrically. If you are not getting much erosion with a shaft anode you may find the type that fits on the prop nut sufficient. It needs a special nut and is not as big as a shaft or hull anode, but much easier to install than a hull anode.

 

[charles_reed]

Old Troll,
I use a 12mm wide, disc anode, about 40mm od on my 25mm shaft. It clamps on and saves a lot of erosion of the very expensive prop nose anode.
Two things to watch for:- 1. Make sure the holes on both halves are protected from reacting, I just dip them in AF and allow it to dry before fitting.
2. Inspect regularly for getting loose - it makes a terrifying noise whan it does.

 

[GrahamM376]

Rule of thumb is that the shaft unsupported length between the bearing and prop should not exceed the shaft diameter, so in your case one inch. If your propeller is a heavier one, e.g. a feathering one rather than a simple fixed two blade even that may be excessive. the consequences can be rapid bearing wear and shaft resonance.

Having fitted a fairly heavy Featherstream to 1.1/4" shaft, with a Stripper rope cutter and maybe a 15mm gap between that and the P bracket, I suspect you may well be right. Cutless bearings now only last 2 years before excess play. Shaft was new 4 years ago and no sign of anything out of alignment. I had also fitted shaft anode further forward than you suggest so will move it aft closer to P bracket.

 

[SimonD]

I'm persuaded that pushing the shaft back another half inch is not a good idea. But, if I take half an inch off the inboard end of the shaft, I'll need to drill another hole through the shaft (there's a bolt through the coupling and shaft). I suspect this is also not a good idea. Any thoughts?

 

[PuffTheMagicDragon]

Pardon me for being slow to understand.
You now have a 3/4" spacer and a 2" overhang. Your flexible coupling spacer is 1/2" thicker which means that you will therefore have an overhang of 2-1/2". Your shaft is 1" diameter. If you wish to reduce the overhang to the accepted distance of one diameter you will need to cut off 1-1/2" off the inboard end, not just 1/2".

This will probably mean that the existing bolt-hole will be totally removed and your new hole will be replacing what you now have. I fail to see the problem. Have I missed something?

 

[Tranona]

Pardon me for being slow to understand.
You now have a 3/4" spacer and a 2" overhang. Your flexible coupling spacer is 1/2" thicker which means that you will therefore have an overhang of 2-1/2". Your shaft is 1" diameter. If you wish to reduce the overhang to the accepted distance of one diameter you will need to cut off 1-1/2" off the inboard end, not just 1/2".

This will probably mean that the existing bolt-hole will be totally removed and your new hole will be replacing what you now have. I fail to see the problem. Have I missed something?

The 1* diameter is not the generally accepted overhang, it is 2*diameter. The required clearance for a rope cutter is 40mm on a 1" shaft and thousands of boats are in service with this gap. Cutting 1 1/2" off will mean the rope cutter will not fit.

Agree that 2 1/2" is not ideal but there are many boats with this type of clearance that do not have any problems. Ideally the shaft should be shortened to maintain the 40mm gap for the rope cutter, but the addition of a flexible coupling will go a long way in reducing the amount of movement on the shaft by isolating it from the engine.

 

[GrahamM376]

............. Ideally the shaft should be shortened to maintain the 40mm gap for the rope cutter......

But what size gap should there be between the rope cutter and P bracket to allow water flow through the cutless?