Disc type rope cutters. Any views?

[gravygraham]

Having downloaded the MBM article where they built themselves a tank with prop and viewing window to test a selection of cutter types, I'm thinking a pair of disc type will suit me. It seems they're among the cheapest but least effective (though only marginally), but are a piece of cake to fit. There's three plus points about these which caught my eye - price and fitting, as already mentioned, and supposedly no loss of speed.

Does anyone on the panel have any good or bad comments about these in relation to;
a) the other types available, and
b) not having anything fitted?

 

[Bandit]

Chocolate Tea Pot

I am a firm believer in rope cutters but they are there to do a job.

I have had spurs and stripper over the years and now i am fitting Quickutter as an effective low drag alternative.

 

[sharppractice]

We have disc type cutters worked well, near the needles on the way to weymouth from shamrock. chomped through a tangle of rope and stuff ,only saw it at the last moment heart stopping moment but all good.

 

[Ripster]

Got them on ours and three years in, no issues to date.

 

[Neil_Y]

I'm clearly biased, but I think you need the full picture.

The test rig used by MBM was built to test a scissor cutter (Gator) I believe.

The design had a large gap from the bearing carrier to the prop to allow for easy installation of the rotating blades of the scissor types. (the shaver cutter requires a small or no gap, mbm were unable to alter the rig for the shaver to be tested)

The test methodology involved fixing one end of the debris (to the P bracket support) and allowing a short 2m ish length to be dragged into the rotating prop. When it was wound around the shaft by the rotating blades it wasn't caught by a blade, it just got tight around the shaft but it was fixed as the front end was fixed. This meant that the debris was pulled tight across the disc and as it was fixed the disc was slicing, the debris was not rotating with the shaft as it it tightened.

We declined to take part with our shaver cutter as the test didn't represent what happens in practice and what a shaver cutter protects you from. The fitting options would not be how we fit the cutter in practice.

If you consider what happens if the debris is caught by a prop blade you will realise how meaningless this demo of cutting debris was. If the debris is fixed/caught at the prop the debris will wrap around the shaft but in this case the debris will be rotating with the shaft and wrapping around the shaft as if it were a rotating winch. This winding debris will fill the gap up to the cutter but will not be lying along the shaft and across the scissor or disc.

The shaver cutter comes into action when debris has wound on the shaft/spool and is pushing forwards into the fixed cutter blades which then shave through the debris.

Ask yourself why the RNLI, The Royal Navy, Aqua Star, Christensen yachts , Austal Image to name a but few chose a shaver instead of a disc or scissor.

 

[DAKA]

http://www.h4marine.com/QuicKutter01.htm


looks good to me

.

 

[PaulGooch]

<snip> If the debris is fixed/caught at the prop the debris will wrap around the shaft but in this case the debris will be rotating with the shaft and wrapping around the shaft as if it were a rotating winch. This winding debris will fill the gap up to the cutter but will not be lying along the shaft and across the scissor or disc.

I agree. I have a disc cutter, very close to the prop. I once forgot about the dinghy tied to the back of our boat (don't usually take the dinghy) and reversed over the rope. The rope wound around the prop blades and the shaft, between the prop and cutter. It pulled the dinghy tight against the bathing platform and if i hadn't have been idling and taken the boat out of gear, it would have most likely pulled the dinghy under the boat. The cutter made no impact on the rope at all, i went under the boat and unwound the rope, which is almost as good as new, except for some chafing where it rubbed the prop blades. This was just some 10mm braid.

 

[gravygraham]

Thanks folks. As usual, a mixed bag of responses, but something for me to ponder over all the same.

Can you moboers with the Gator, Stripper, and Spurs type comment on any increased drag and reduced speed?

I must admit, drilling and tapping into my P brackets fills me with dread.

http://www.h4marine.com/QuicKutter01.htm

looks good to me

I agree Daka, but are you able to comment on it, or indeed any other types?

 

[Ripster]

Thanks folks. As usual, a mixed bag of responses, but something for me to ponder over all the same.

Can you moboers with the Gator, Stripper, and Spurs type comment on any increased drag and reduced speed?

I must admit, drilling and tapping into my P brackets fills me with dread.



I agree Daka, but are you able to comment on it, or indeed any other types?

Dont forget you have to drill a small divot in the shaft to take the grub screw even for the disk type.

 

[Norman_E]

Next month's Yachting Monthly should have an article from me, with a photo of the tangle of rope that a plain disc cutter chopped through. Simple is often best, because there is nothing likely to break or go wrong with a simple disc cutter.

 

[gravygraham]

Dont forget you have to drill a small divot in the shaft to take the grub screw even for the disk type.

Yeah thanks - not got any probs with that though. Mullering an expensive bronze casting freaks me a bit though.

 

[Bandit]

Drilling into the shaft is poor engineering parctice and weakens the shaft.

With conventional ropecutters spurs and stripper I was told the rule of thamb for drag caused by the disturbance is a knot to 1.5 knot, a lot of HP and fuel!