Which rope cutter ?

[Boo2]

Hi,

I'm fitting a new engine and shaft to my UFO 34 and want to fit a rope cutter. The question is just which one ? The boat has a fairly normal yacht layout with a P-bracket supporting the aft end of the prop shaft and I heard that the scissors type are not ideal for this setup because of the shock loads on the bracket. Can anyone say whether this is true or not ? If so then I will need a disc type, are they all the same or is one better than the others ?

Many thanks,

Boo2

 

[john_morris_uk]

We've got one of these:

http://www.ropestripper.com

I've never heard of there being a problem with fitting them to a P bracket, ours is and its been fine for years.

 

[Tranona]

No problem fitting to a P bracket. The fastenings are designed to fail before there is any damage to the P bracket. In practice it will cut nets, ropes, plastic bags etc almost without you realising it. Have your shaft made to allow for the extra distance required aft of the P bracket as this makes installation easier and cheaper. All the dimensions are on the website John gives.

 

[pvb]

Ambassador Stripper - excellent bit of kit.

 

[Parsonsheath]

Ambassador Stripper - excellent bit of kit.

+1

 

[Three kings]

+1

+2

 

[Halo]

+2

=3

 

[Neil_Y]

To make sure you are fully informed before you decide, there are three distinct types of rope cutter for shaft drives. Scissor as you've mentioned, disc and shaver.
We supply the shaver type (not as well known in the leisure boat world as most of our work is commercial/Naval), the shaver is not as easily fitted especially if you have a folding/feathering prop but for fixed bladed props it's been chosen by a number of builders and commercial users who have done hours of evaluation and most have experience with the other types.
If you want an independent view talk to a fitter/boat yard who has fitted and seen what happens in use to all three type.

 

[[10753]]

I've got one of these

http://haroldhayles.co.uk/spurs-home

Fantastic kit and the service for replacement anodes and bearings has always been superb

 

[vyv_cox]

I disagree with the universal recommendation of the Stripper, although I believe they have made some arrangements to cope with the problem I suffered. Modern P-brackets have the least metal thickness they can get away with, especially in the bearing housing. The thickness is insufficient for the tapped holes for the bolts that hold the fixed part of the rope cutter to it. When a rope of fairly large thickness is cut there is a real risk that the bolt will pull out. See http://coxengineering.sharepoint.com/Pages/Pbracket.aspx for my first experience like this. I fitted the new P-bracket and then suffered a second failure in exactly the same way as the first. The ropes that caused the problems were 12 - 14 mm polypropylene and 16 mm braid on braid polyester.

I then fitted a simple disc type cutter, a Prop protector, following their successful testing in YM. I believe that Ambassador now make a collar of some sort to which the Stripper can be bolted on thin-walled P-brackets.

 

[Tranona]

I disagree with the universal recommendation of the Stripper, although I believe they have made some arrangements to cope with the problem I suffered. Modern P-brackets have the least metal thickness they can get away with, especially in the bearing housing. The thickness is insufficient for the tapped holes for the bolts that hold the fixed part of the rope cutter to it. When a rope of fairly large thickness is cut there is a real risk that the bolt will pull out. See http://coxengineering.sharepoint.com/Pages/Pbracket.aspx for my first experience like this. I fitted the new P-bracket and then suffered a second failure in exactly the same way as the first. The ropes that caused the problems were 12 - 14 mm polypropylene and 16 mm braid on braid polyester.

I then fitted a simple disc type cutter, a Prop protector, following their successful testing in YM. I believe that Ambassador now make a collar of some sort to which the Stripper can be bolted on thin-walled P-brackets.

Despite your experience, rope cutters have been fitted successfully on literally thousands of P brackets and worked. The clamp type is not suitable for P brackets as there is not usually enough tube to clamp to. It is used on plain stern tubes as fitted to Beneteaus for example or some early Westerlys that have very thin wall cutless housings.

BTW the OPs boat has a very substantial P bracket.

 

[vyv_cox]

Despite your experience, rope cutters have been fitted successfully on literally thousands of P brackets and worked. The clamp type is not suitable for P brackets as there is not usually enough tube to clamp to. It is used on plain stern tubes as fitted to Beneteaus for example or some early Westerlys that have very thin wall cutless housings.

BTW the OPs boat has a very substantial P bracket.

I know you are correct so far as the number fitted is concerned but there are certainly more failures than are widely advertised. Our ex-member from Latvia (Fuel?????) was one, he had exactly the same experience as me.

I did consider additionally drilling and tapping right through the brass liner of the cutless bearing but I thought this might bring more problems in its wake.

 

[Norman_E]

Sometimes the simple solutions are the best. I picked up a mooring riser, a thin rope which then pulled a much thicker rope into the prop, which stalled the engine. Restarting in reverse, then going forward allowed a simple disc type cutter to chop through the rope so that I could motor away. In fact I did about 35 nautical miles under engine and removed this tangle of it from the prop later.

View attachment 29837

 

[vyv_cox]

The YM and mobo article describing the tank tests found the discs to be virtually as effective as the cutters, the only failure to cut being electric wire. All the ropes and nets, etc., cut just as well.

 

[Neil_Y]

The practical test was seriously flawed (in my view) and for that reason we refused to be included in the test. Despite this we still have the RNLI and the Royal Navy removing scissor cutters to fit shaver. Christensen Yachts, Alicat workboats, Baltic work boats, Aqua star, Fleming yachts and Grand banks are all happy to fit the shaver cutters at the factory. The Hythe ferry, Port of London Pilot boats and many many fishing boats have also changed to shaver.

Talk to DK Collins in Jersey who have been fitting cutters of ALL types for a number of years to get an independent view on failures in use of the different types.


Why was the practical test flawed?
The practical test had a spinning prop in the tank with a large gap between P bracket and prop (as the test set up was made by Gator for testing scissor cutters)

The debris being tested was then fixed to the P bracket (this is never likely to happen in practice) the debris was then trailed into the prop, a blade eventually picked up the debris and wrapped it, importantly at this point the debris is not rotating with the shaft. It is also lying along the shaft line and pulled tight across the area of the cutter. The scissors will cut it and the disc will be spinning and pressing against it so cutting. In my view this test only served to show how you could use disc and scissor cutters to cut debris in the test tank.

In practice what happens?

The rotating prop catches the debris (not the P bracket), as a blade rotates it catches some debris and the length in front of the prop is wound onto the shaft, importantly it is now rotating with the shaft as it is effectively fixed to a prop blade. (This is what I wanted tested but this is likely to damage or break the test set up, as it can in boats with enough hp) The debris winds onto the shaft ever tighter and trying to fill the gap but it will not be lying along the shaft line, it will be perpendicular to the shaft if it is a pot being dragged off the bottom. When the debris gets to the cutters, with scissors this is like trying to cut with the line/rope in the same plane as the swing of the scissors, the debris is also likely to build up and apply pressure to the rotating blades from the rear and with the discs the debris is just pressed against the blade but not sliding past it.

Scissor cutters and disc cutters do cut debris in some situations, but in situations where the real damage can be done (long rope caught by prop winding between prop and P bracket) I think they may not be that effective.

I am clearly biased but I hope readers can see that I am trying to inform.

Look at the alternatives understand how they work and make an informed choice.

 

[Norman_E]

I can only say that the simple disc cutter that I fitted passed the only practical test it has had. It has a slightly serrated sharp edge. It cost a lot less than some of the more complicated options, and I have seen a couple of the scissors type with broken parts.

View attachment 29890