Prop shaft rope cutter

Graham_Wright

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I had in mind to make a simple disc with a sharp periphery to act as a rope cutter. It seems to me that anything that "slices" to cut could jam up and do more harm than good.

However, someone has beaten me to it.

Headed "Stainless Steel Rope Cutter" on eBay it seems a very worthwhile piece of kit.

Priced at £38.50 but in an unspecified grade of SS it seems a good deal. (316 as an option).

Nothing to do with me;- I missed this opportunity!
 

vyv_cox

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There are many variations on this basic design, some with wavy cutting edges, some with constant radius. In the YM tests they did almost as well as the Spurs and Stripper.

I changed from a Stripper to a Prop Protector after the screws attaching its fixed block were stripped out of the P-bracket on two occasions, first time cutting 14 mm polypropylene, second time 16 mm multiplait. The wall thickness of the bearing housing in a P-bracket is now so thin that it seems not to have enough strength for a Stripper to do its job.
 

rotrax

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There are many variations on this basic design, some with wavy cutting edges, some with constant radius. In the YM tests they did almost as well as the Spurs and Stripper.

I changed from a Stripper to a Prop Protector after the screws attaching its fixed block were stripped out of the P-bracket on two occasions, first time cutting 14 mm polypropylene, second time 16 mm multiplait. The wall thickness of the bearing housing in a P-bracket is now so thin that it seems not to have enough strength for a Stripper to do its job.

Hi, I had a stripper professionaly fitted by a well known Gosport firm. I had purchased the boat-a Gibsea 96- but was not aware that the P bracket had been changed for a composite one. The engineer drilled and tapped the holes for the fixed cutting blade in the composite and used 8mm s/s cap screws to mount it. You know what is coming next-picked up a bit of net and ripped the fixed blade out. Upon investigation Stripper sell a s/s holder for the fixed blade that slips over a composite P bracket and that has legs that go up the flat part of the bracket to retain it. It is quite expensive. I made one to the same pattern using a bit of s/s tube and strip for about £12.00 including the welding. Worked fine after that.The well known Gosport firm were not interested-said they did as I asked! If you get a s/s tube the same diameter as the P bracket,slot it to go each side of the vertical leg and make up two strip s/s legs for each side-four in total-you can have the fixed cutter and the legs welded to the tube. Slip it on, mark and drill through the legs and P bracket and use long 6mm bolts,washers and lock nuts to finish the job. Hope this helps.
 

ghostlymoron

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There are many variations on this basic design, some with wavy cutting edges, some with constant radius. In the YM tests they did almost as well as the Spurs and Stripper.
Is this true? When were the YM tests? I had thought that the only ones worth fitting were the scissors type which are awfully expensive.
 

Neil_Y

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Pretty comprehensive S**ling T*day test reprint is on Ambassador's site:



Andy


Not very comprehensive actually, the latest type of shaft rope cutter which is now used by the RNLI and the Royal Navy is the quicKutter, a shaver cutter (there are three distinct types of cutter disc, scissor and shaver)
These are also fitted as standard on Aqua Star and Christensen as well as many fishing boats and pilot boats. The Port of London and Plymouth use them. These have been around since 2004 when they were first fitted on the New Tamar lifeboats. They have proven to be robust (one damaged in 6 years from chain) and effective Sessa 52 at full chat wound up a large net, no damage to drives. Many fishing boats report frequent situations where the cutter works against a variety of debris and there has been significant praise in the fishing boat press.

The MBY test did not include the shaver cutter as the test rig was made for the Gator a scissor cutter, it had an extremely large gap between the prop and the bearing carrier, with the shaver we like to have a much smaller gap getting the cutting blade close to the prop but leaving enough room for a 12-15mm rope to wind on.

The effectiveness of the discs and scissors in the test I believe was effected by the way the test was carried out, I would have liked to have seen a rope attached/caught by a prop blade and hence revolving with the shaft and winding onto the gap aft of the disc or scissor as this is the type of situation that does the most damage and is also where the shaver is most effective.

There were several errors in the test with regard to the shaver installation, you do not need to remove the shaft or P bracket, fitting is simply a cutter (three bolts) onto the bearing carrier and a spool (three or four bolts) onto the prop. Ideally the gap between prop and bearing carrier should be around 35mm for a soft drive and 10-15mm for a rigid drive. Bearings are happy with a 3mm clearance.
 

Neil_Y

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Is this true? When were the YM tests? I had thought that the only ones worth fitting were the scissors type which are awfully expensive.

The Royal Navy and the RNLI have both removed scissor cutters to fit shaver cutters. RN Survey boats, Severn, Trent and Tamar class.
 

misterg

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Not very comprehensive actually, the latest type of shaft rope cutter which is now used by the RNLI and the Royal Navy is the quicKutter, a shaver cutter...

These are the ones you sell, right?:D

I'm sure they're very effective, but last time I looked (2 years ago??) fitting wasn't an easy diy option, as it needed access to a lathe to machine up the spool for each job and an indexing head & drill press to drill/tap the prop to match the spool. (Please correct me if I'm wrong).

While I'm here: I can't help wondering if a (any) shaver cutter might rip out of an insubstantial 'leisure' grade 'P' bracket in the same way as the scissor cutter blades have been known to.

I don't intend to knock the product, but it doesn't (didn't) seem like a bolt-on option that the other types are.

I agree it would have been very interesting to see it compared to the others in the test, though.

Andy
(No interest in anything mentioned, but have an Ambassador Stripper fitted to the boat)
 

robertj

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There are many variations on this basic design, some with wavy cutting edges, some with constant radius. In the YM tests they did almost as well as the Spurs and Stripper.

I changed from a Stripper to a Prop Protector after the screws attaching its fixed block were stripped out of the P-bracket on two occasions, first time cutting 14 mm polypropylene, second time 16 mm multiplait. The wall thickness of the bearing housing in a P-bracket is now so thin that it seems not to have enough strength for a Stripper to do its job.

I had the same problem with picking up a line. Ripped off stationary block of the spurs cutter.

Must be Sadlers vyv

bob
 

Neil_Y

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Tis true....we do supply said cutter.

But I feel it's in every bodies interest to know of all the options and also understand how the test didn't ever test a rope caught on a prop blade. This was from going to watch the test carried out. We offered all parts more than 6 months before the test but 1.the gap was huge (to accomodate scissor cutters) between prop and bearing carrier so teh shaft would have been shortened in practice and 2.the test methodology never tested getting a rope caught to a prop blade and hence winding onto the shaft with some force (this is the situation that does damage, and this is the situation where the shaver cutter comes in to its own)

With regards to fixings, the big difference between shaver and scissor is that a scissor has to cut completely anything that goes between the blades, which results in a huge and sudden load. This is Ok on most ropes but it only takes one fish hook (stainless trace and swivels) and you are asking quite a lot from a fast closing pair of scissors. The shaver loads build up pressure and are starting to cut through the debris as soon as the pressure starts to build up, so it is gradual (but still quick in real terms) Fish hooks will sometimes take a chunk of blade out but the cutting system remains intact. We have many long line tuna boats using quicKutter at the end of the season it is clear the blades have dealt with some hard material.

Fitting is more involved and does require a machine shop with a lathe and facilities to drill and tap the prop to fix spool. So you're right, it involves some engineering skill/equipment to fit (but not what MBY explained, we supplied them with drawings many months before proposed trial and it was left for me to find a local engineer to Benson (within a few days) who could do the work but not really DIY fit. once fitted it has no effect on performance on high speed boats and is usually there for many seasons with no maintenance as there are no moving parts.

As a result we have seen units on commercial boats for more than five years still working that have required no spare parts. Only one to my knowledge has ever been knocked off from the P bracket when a piece of chain was picked up, the three fixing bolts sheered outside of the bronze and could be removed threads were undamaged, two other cases have caused the bolt which holds the blade to sheer. In both cases there was no damage to the bearing carrier. A new blade and bolt were fitted and continue to work these were pot and net commercial boats.

It's a bit like the wooden wheel, it's the best thing out there if you haven't heard of or seen anyone test a dunlop inflatable tyre. But you do need a tool to fit it.
 
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I had in mind to make a simple disc with a sharp periphery to act as a rope cutter. It seems to me that anything that "slices" to cut could jam up and do more harm than good.

!

I think you are re-inventing the wheel here. Not only are there many designs of rope cutter but they have been tested many times in the magazines and been in use for lots of years. Thats not to say they cant be improved but the tests and practical use have always shown that the scissor type is more effective than the disc type you are mentioning.

I would add, as an ex steel maker, that 316 stainless steel or indeed any common austenitic is useless as a knife for serious use. Thats why cutlery when it was properly made in Sheffield rather than simply stamped out in Taiwan used to have martensitic blades and indeed why razor blades are made of martensitic not austenitic stainless. Same applies to rope cutters.
 

Neil_Y

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I think you are re-inventing the wheel here. Not only are there many designs of rope cutter

(three distinct types scissor/disc/shaver many variations of disc and three variations on the scissor one style of shaver)

but they have been tested many times in the magazines

(shaver type has never been tested by a magazine)

and been in use for lots of years. Thats not to say they cant be improved but the tests and practical use have always shown that the scissor type is more effective than the disc type you are mentioning.

(True when comparing disc to scissor but not necessarily when compared to shaver, the RNLI and the Royal Navy have over the past 5 years been removing scissor type and fitting shaver type)

I might get flamed as blatantly promoting but I hope my contribution is seen as giving the full picture rather than referencing to old information or incomplete tests and comparisons. Teetering on the edge but trying to be informative to let people make an informed decision or do their own research knowing all available facts.
 

vyv_cox

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Is this true? When were the YM tests? I had thought that the only ones worth fitting were the scissors type which are awfully expensive.

Something like two years ago. The video showing all the tests was on the website, possibly under the mobo banner. The single blade varieties cut everything that the scissors and spurs did, except for copper wire. I'm prepared to take a chance on that one.
 

vyv_cox

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I had the same problem with picking up a line. Ripped off stationary block of the spurs cutter.

Must be Sadlers vyv

bob

That may have been the case with the first failure but the replacement P-bracket was a universal one, as supplied to many builders. The wall thickness at the bearing housing looked pretty similar in each case. Photos on my website.
 
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