Propshaft "overhang "

[Tranona]

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

Ideally 10mm. The cutter is 30mm thick so this gives an overall gap of 40mm. Typically the gap on an original installation without cutter can vary from less than 10mm up to 30mm or even more (on a 1" shaft). Retrofitting the cutter usually requires a spacer between the flanges on the gearbox and prop, which comes in 10mm increments so you can get close to ideal. If the gap is more than 40mm then the extra space is between the cutter and the prop to get the striker plate mounted in the ideal position on the P bracket. Sometimes an R&D flexible coupling which is 30mm thick can be used as a spacer.

 

[PuffTheMagicDragon]

The required clearance for a rope cutter is 40mm on a 1" shaft

My mistake. I was under the impression that the rope cutter was something that the previous owner had fitted and that the present owner was now discarding.

 

[vyv_cox]

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.

From a rotordynamic design viewpoint the 1 x shaft diameter is always the rule of thumb. The required gap for rope cutters exceeds this but there is a price to pay, additional wear and increased vibration being some examples. The reduction in vibration and increased bearing life resulting from replacing my Stripper with the far narrower Prop protector was remarkable.

 

[SimonD]

My mistake. I was under the impression that the rope cutter was something that the previous owner had fitted and that the present owner was now discarding.

Just to clear that up: no, I'm keeping the rope cutter (Ambassador type). The proposed flexible coupling (from R&D) will replace the existing 20mm spacer and is half an inch thicker. To avoid increasing the 'overhang', whilst keeping the correct clearance for the stripper, I would need to shorten the shaft by half an inch. That would be easy if it wasn't for the flipping hole drilled through it.

The options seem to be: (1) drill another hole half an inch from the existing one or (2) don't bother putting the bolt back through the coupling. Option 1 sounds dodgy but then the weak spot would be in the coupling, so unlikely that the shaft would break. Option 2 seems OK as not all boat have a securing bolt through the shaft clamp (AFAIK). Any other ideas?

 

[vyv_cox]

Some sort of positive location of coupling to shaft is needed IMHO. No problem at all to drill a second hole as the first one will not be carrying any load. Another option might be a grub screw through the coupling into a dimple on the shaft but this is less positive than a through bolt.

 

[SimonD]

Some sort of positive location of coupling to shaft is needed IMHO. No problem at all to drill a second hole as the first one will not be carrying any load. Another option might be a grub screw through the coupling into a dimple on the shaft but this is less positive than a through bolt.

Thanks Vyv. I had hoped that a second hole would be OK considering where the first one will end up. Another problem solved!

 

[scottie]

Is the coupling the split type with clamping bolts?

 

[SimonD]

Is the coupling the split type with clamping bolts?

Yes

 

[Giulio MAN]

sorry if I rise
the question is: how long is the axis, from skeg (support axis) to the propeller?

 

[scottie]

The split coupling should be able to tighten enough to take the drive and a locating srew into a dimple should stop the shaft moving fore and aft
A bolt through the centre of the shaft is not good practice
With a solid coupling a taper pin through the shoulder is correct

 

[SimonD]

sorry if I rise
the question is: how long is the axis, from skeg (support axis) to the propeller?

It's about 50mm.

 

[Neil_Y]

I don't often disagree with Tranoma (if ever?) but builders and architects now work to a gap of shaft to shaft and a half. In practice you want to get the gap as small as possible. Increased gaps result in accelerated bearing wear and shaft flex.

Whilst on the gap subject some soft mounted shaft lines will see as much as 9.00mm movement fore and aft (total 18.00 this is the worst case we've seen) For water to exit the bearing a minimum of 3.00mm is advised (by us) so 9.00 + 3.00 = 12.00 as a min gap.

 

[SimonD]

The split coupling should be able to tighten enough to take the drive and a locating srew into a dimple should stop the shaft moving fore and aft
A bolt through the centre of the shaft is not good practice
With a solid coupling a taper pin through the shoulder is correct

It's not that I disagree (cos I don't ) but I find it hard to believe Moody got it wrong!

It's probably easier to have a machine shop shorten and drill a new hole than drill and tap the coupling. That's probably the deciding factor.

 

[Tranona]

I don't often disagree with Tranoma (if ever?) but builders and architects now work to a gap of shaft to shaft and a half. In practice you want to get the gap as small as possible. Increased gaps result in accelerated bearing wear and shaft flex.

Whilst on the gap subject some soft mounted shaft lines will see as much as 9.00mm movement fore and aft (total 18.00 this is the worst case we've seen) For water to exit the bearing a minimum of 3.00mm is advised (by us) so 9.00 + 3.00 = 12.00 as a min gap.

There is no standardised gap. For example some Beneteaus and Jeanneaus have as much as 30mm on 25 and 30mm shafts. However, as you say 10-15mm in these smaller shafts is more common, and all that is needed to cope with fore and aft movement and still have space for water to flow.