Propeller rope cutters

[boatmike]

Need to fit something to Appela. She is a twin screw Aqua-Star 33 with 3 bladed props driven by TAMD 41A diesels. I think a previous owner has had Ambassador "Strippers" on before but taken them off, perhaps for his new boat. Cost of new strippers is nearly £500 per shaft. A similar piece of kit is "Gators" which are about £340. Then there is the "Quickcutter" at about the same price as Gators by the look of it.
I have discounted knife types as being better than nothing but not as good as these 3.

Question then.

I think the "Strippers" have been the best sellers for a long time and I am sure they work but has anyone here fitted Gators or Quickcutters and can give any guide on their performance? I know RNLI have gone to the Quickcutter type and suspect that they have the advantage of being more robust in extreme cases and have no bearings to wear. I do have a lathe and machining capability so the work to fit Quickcutter isn't a problem although it's significantly more work than Strippers or Gators would be.

What experience is there out there?

 

[Powersalt]

trol for a thread about a month ago on this very subject.

I have quick cutters and they are great with no loss of speed. Only draw back was the high cost of installation

 

[Bandit]

I am about to fit Quic Cutters to my Aquastar 48 in place of stripper and i am hoping for a better water flow and efficiency.

 

[gregship]

rope cutters

Having just purchased the ambassador stripper because i have a volvo prop i have to now modify the prop in order to accomodate the cutter.. it seems that this is common because in the fitting instructions they inform you what to modify. easy mod but annoying and oh dont forget you need a modified anode

 

[boatmike]

I am about to fit Quic Cutters to my Aquastar 48 in place of stripper and i am hoping for a better water flow and efficiency.

Interesting..... Do I detect from this and powersalts reply that the turbulence and loss of power with Stripper is significant? I can see that Quickcutter would have little effect on flow. Is this why RNLI have changed do you think?

 

[Tranona]

Having just purchased the ambassador stripper because i have a volvo prop i have to now modify the prop in order to accomodate the cutter.. it seems that this is common because in the fitting instructions they inform you what to modify. easy mod but annoying and oh dont forget you need a modified anode

That only applies to saildrives - not relevant for the OP

 

[Bandit]

Boatmike

It looks probable that the Quic Kutter creates much less turbulence, I intend to fit mine at 6 O'clock ie on the bottom of the P Bracket.

 

[boatmike]

Boatmike

It looks probable that the Quic Kutter creates much less turbulence, I intend to fit mine at 6 O'clock ie on the bottom of the P Bracket.

You have to be right that it will indeed create less turbulence. I also think it is less likely to fail and snap the standing part off the P bracket if it hits something really nasty like chain. What concerns me is, will this type of cutter deal equally well with nets, bags etc as it does with rope? I suspect that it won't be as aggressive. Anyone got any evidence to the contrary? If it's as good in every case as a stripper then why did the manufacturers decline to enter it in the magazine test?

 

[Jim@sea]

I think that on a twin engined boat I would not bother. Off Holyhead once I had rope round my Port shaft but I got back on the other engine. Certainly I would not want to spend £1000 on twin shafts.

 

[Neil_Y]

You have to be right that it will indeed create less turbulence. I also think it is less likely to fail and snap the standing part off the P bracket if it hits something really nasty like chain. What concerns me is, will this type of cutter deal equally well with nets, bags etc as it does with rope? I suspect that it won't be as aggressive. Anyone got any evidence to the contrary? If it's as good in every case as a stripper then why did the manufacturers decline to enter it in the magazine test?

Here we go...we did enter it right at the start of the planned test.

The test carried out for IPC magazines was planned for 6 or 7 Months, the testers were provided wth fitting details of quicKutters (they work best if they are closer to the prop as less debris is wound before cutting) and our advised best fitting position with drawings as with any customer we supply. I wanted them to report on how they are fitted and what is involved.

The testers didn't design a new test rig, they simply got hold of what was the old Gator test tank. This had a large gap from the shaft bearing to the prop of 55mm it also had a bearing carrier much larger than the prop hub diameter. So it didn't resemble a normal boat set up.
It made fitting of the scissor cutters easy with the large gap. We provided fitting details with drawings of how we wanted the cutter fitted as below.

The testers were unable to sort this, we even found them local engineers who could do the work for them. This is how we would have fitted a cutter to a boat with these dimensions.

Come the test, I drove up to Benson on thames to see if there was any way we could sort the fitting, and witnessed the tests they carried out.

In my view the test methodology simply showed how you could cut rope with scissor and discs cutters, they did not test what happened if debris was caught by a prop blade and was being wrapped onto the shaft. The difference between scissors/discs and quicKutter is that the quicKutter cuts debris that has been caught by the prop, it doesn't cut rope that is flowing past the prop.

If you look closely at the test videos you will see that the debris is fixed to the P bracket, this means that the debris is not revolving with the shaft it is fixed and also as it wraps it is drawn tight across the scissor or disc region. In practice when debris is caught it is first caught by a blade and is thus rotating with the shaft, being wound in as if on a winch, it will also commonly be being pulled up from the sea bed and if the boat is going forwards it will be angled aft of the prop. It will not be lying along the shaft line, unless caught by the P bracket which I think is unlikely.

Had they tested a quicKutter with the same methodology it probably wouldn't have worked very well as this is not what it is design to do. You need to catch debris with the prop and let it wind in one or two turns, it will then shred/shave it gradually with no shock loads. I suspect if they had done this with their test rig they would have broken the three scissor cutters as the debris winds up behind the rotating blades.They weren't willing to test this so we declined for both installation reasons and the test not reflecting what happens in practice.

I hope that helps, at this stage we had already gone through years of tests and approval with the RNLI (Tamar) The Royal Navy, Christensen Yachts and many commercial operators with naval architects and engineers evaluating the various options. The RNLI have used these cutters since 2004 and continue to remove scissor cutters to instal quicKutters.

 

[boatmike]

Hi Neil,
Thanks for that. I like to think that as a Chartered Mechanical Engineer I look at these things in terms of logic and what you have said makes a great deal of sense. I think the biggest risk is indeed rope and I can see that as you say, the problem generally arises when the rotating prop blades pick up a rope that is either floating or anchored to the bottom in some way, not hooked on the P bracket and then fed through as the test simulated. I think I am convinced! what you didn't say is how it deals with polythene sacks or netting though. My thoughts are that probably the net, being rope, will be cut if it winds around the shaft which indeed it will tend to do but a polythene bag or lump or netting can possibly become fouled on the prop without winding around the shaft in which case I don't see any of the scissor types doing any better.
Anyway Neil, I am going down to Appela later today and will measure up and give you a call on Monday. Thanks for explaining the test conundrum, hopefully others will be as convinced as I!

 

[boatmike]

I think that on a twin engined boat I would not bother. Off Holyhead once I had rope round my Port shaft but I got back on the other engine. Certainly I would not want to spend £1000 on twin shafts.

While I respect your opinion Jim and it's of course your choice it's often the case that a rope picked up with one prop will indeed flail around and foul the other. You have to be unlucky of course but there are many cases where this has in fact happened. In my case when bringing Appela back from Ipswich to Portsmouth I was fine when at sea but in the shallow water approaching the boatyard I picked up an old submerged mooring around both shafts. Luckily I had already arrived and the boatyard hauled me out but if this had happened at sea I would have been boogered!

 

[Bandit]

Ref Jim@ Sea mid way on a 70 60 mile run with rope cutters I picked up a tarpaulin on both shafts which stopped both engines.

Eventually by judicious use of adhead and astern on the gear box I freed starb enegine and limped in.

A Friend of mile with a fairline 47 with no cutters picked up an unmarked floating mooring line and ended up anchored to the bottom by one shaft in the Small Russel off Guernsey, he had to be cut free by divers.

Give me effective cutters every time.

Also dont forget on strippers and spurs to replace the plastic bearings on a regular basis say yearly, thats how i wore mine out in 3 years forgetting to.