Installation hints - Rope cutter type DISC on a shaft with folding prop?

[skippeer]

I have digested many discussion threads regarding benefits and risks with different rope cutter types to our sailboat with Flex-O-Fold prop on a shaft.
Now have I bought a Plastimo Twinfast rope cutter (disc model).

Plastimo recommend a distance of 6-10 mm between P-bracket and the rope cutter.
I have a 60 mm distance between P bracket and the prop, now am I considering the optimal location on the shaft so the disc can do it works best.
Is the distance between the prop and the rope cutter disc critical?

 

[Daydream believer]

Personally I would have the cutter as close to the P bracket as possible. If a rope got caught it could catch the P bracket then start to go round the shaft. If it got a couple of turns between cutter & P bracket before the cutter could do its job you are stuffed because it will act like a giant cotton reel . The free end of the rope will just flap round by the prop , Possibly jamming that as well.
If it was aft of the cutter it might not free the rope from the prop but at least you would not end up with a bent shaft. Hopefully the object would just slip off the blades of the folding prop if hooked when sailing. When motoring-- Who knows.
There are other things that you can catch,. IE. polythene, nets & weed with strong stalks. polythene & nets tend to be the worse- Or have been to me at least , both needing a dive overboard or a lift out. One wants to keep maximum polythene out of the gap between P bracket & cutter so that the shaft can keep turning against the friction it creates

 

[skippeer]

Last spring did I run out of time so now is the time finally to install the Plastimo Twinfast disc rope cutter on our 35 mm shaft.
When I refreshed my memory by reading this post, HR parts info, other installations shared on the net and the Swedish instruction is the picture very different how to do it.

Key is not to have the rope cutter close to the bearing due to the engine is standing on rubber feets so the shaft can move a bit when under force from the propeller.


Distance between stern cutless bearing and the disc:

• Swedish instruction recommends 6-10 mm distance
• HR Parts recommend 17,5 mm distance
• images on internet shows disc installed close the propeller hub

Anyone with experience of how to do the best installation?

 

[Tranona]

Your shaft looks like it has ben made to take an Ambassador Stripper. The normal length as it comes from the factory leaves approx 20mm between the cutless and the prop to allow water to flow in and out of the bearing and the fore and aft movement as you go in and out of gear.

The downside of using a disc cutter is that wherever you place it (and it does not really matter provided you leave the 20mm clear to the bearing) you will have vulnerable exposed shaft where rope can be caught. Unlike a scissors cutter which positively cuts any obstruction you will be relying on simple moving blade to cut the rope or net. Youi may wish to reconsider your choice of cutter given that your shaft is already made to take it.

 

[skippeer]

It is a VP stern cutless bearing.

I bought the disc rope cutter last year and the reason was that it looks simple to install and no/low risk of damages/forces from a scissor model.
Okay, I need to do the best of the situation.
The free space on the shaft is ca 60 mm, the disc require 18 mm so maybe it will be fine putting it in the middle of the free space,
20 mm to the bearing and 20 mm to the prop hub.

thx for helping me forward (next time I will pick the king of cutters).

 

[Tranona]

Don't worry about the forces of scissors type cutters, particularly with stern gear as robust as yours! Cutting ropes and nets etc around a fast turning shaft effectively is a violent event, and the design of the blades minimises the shock. Anyway, I would mount your disc right against the face of the propeller as it then stands some chance of cutting a rope that is caught round the prop when you are motoring. That is how they managed to get discs to work in the test that was carried out a few years ago.

 

[PaulRainbow]

I had a disc type cutter on my last boat, next to useless.

 

[Praxinoscope]

I'm not convinced that any rope cutters are particularly efficient, they all rely upon the rope conveniently positioning itself in the right orientation for the cutter to work.
I have no statistics to back me up, but I would guess from years of experience that rope cutters are on average effective only about 25% of the rope/ prop incidents. (Which I suppose is better than nothing).
I do have one on my present boat (a disc type) but still keep a very keen eye open for floating ropes, , usually unmarked on a lobster pot.
My previous boat a Long Keel Invicta just used to slide over them, but now I have a bilge keeler this ain't going to happen

 

[Fimacca]

I took the view that something was better than nothing, but in lockdown product choice was limited.
I had the same issue with space and a folding prop. |Even less space in fact for anything bigger like an ambassador .
I had to mount mine on the shaft as far back as I could......

 

[asteven221]

This YT test by YM might be of some use:-

 

[Tranona]

I'm not convinced that any rope cutters are particularly efficient, they all rely upon the rope conveniently positioning itself in the right orientation for the cutter to work.
I have no statistics to back me up, but I would guess from years of experience that rope cutters are on average effective only about 25% of the rope/ prop incidents. (Which I suppose is better than nothing).
I do have one on my present boat (a disc type) but still keep a very keen eye open for floating ropes, , usually unmarked on a lobster pot.
My previous boat a Long Keel Invicta just used to slide over them, but now I have a bilge keeler this ain't going to happen

Simply not true. While disc cutters are limited in their effectiveness, this is not so of scissors type cutters. Particularly effective on boats where propellers are exposed such as on a P bracket or a saildrive. However, from personal experience protected propellers in apertures behind long keels are especially vulnerable if they do pick up debris because it often balls up and can jam the rudder. Had an article published in PBO in the late 1980s describing exactly such an incident picking up a piece of net in the middle of the Channel which stopped the engine and partially limited rudder movement. Managed to sail back to IOW and went into Sandown bay which was shallow enough to go over the side to clear the obstruction. Fee for the article paid for the Stripper which gave good service for the next 30 years, often showing signs of having cut something, either by a drop in engine note or bits of debris popping up behind. All this with a puny little Yanmar 1GM.

 

[Tranona]

I took the view that something was better than nothing, but in lockdown product choice was limited.
I had the same issue with space and a folding prop. |Even less space in fact for anything bigger like an ambassador .
I had to mount mine on the shaft as far back as I could......

Not sure how lockdown affects choice - all the products are available mail order. If you have room to move your propeller back you can insert a spacer in the coupling which effectively increases the shaft length and makes space for the device.

 

[Fimacca]

Companies just did not have the products in stock when I enquired T ! Pandemic or brexit - who knows........
Simples........

 

[PaulRainbow]

Not sure how lockdown affects choice - all the products are available mail order. If you have room to move your propeller back you can insert a spacer in the coupling which effectively increases the shaft length and makes space for the device.

It has decimated the supply chain for a great many products. I have products on back order from several major companies. There are severe shortages of lots of things, solar panels, batteries, electronics etc

 

[Praxinoscope]

#11 Tranona, I bow to your more extensive knowledge, it's just many of the boats I have seen with rope wraps also have rope cutters.

 

[Tranona]

#11 Tranona, I bow to your more extensive knowledge, it's just many of the boats I have seen with rope wraps also have rope cutters.

Well, the critical question is what type of cutter?

There are of course situations where even the best will fail to cut, and equally many situations where the debris has been cleared without the skipper being aware.

Guess the proof of the product's value is that it has been on the market continuously for 30+ years and guess those prepared to pay the price perceive value!

 

[Daydream believer]

I have had an ambassador & now one of my own manufacture. I have had numerouse instances where I have had obstructions. the amabassador failed me once Off Cap Griz Nez because of a net. I had to stop the engine as the raw water impeller failed at the same time. When hauled out at Dover the net had been chopped over 100 times & one slash of a knife & it was off. Just bad luck that i had a pump failure at the same time.
The second time involved a really large piece of polythene & a diver pulled up large handfulls of it. I was able to motor at 2 kts into Boulogne once I sailed to the port. The cutter did separate the polythene into 2 halves & enable the prop to rotate, albeit slowly
Anyone who has sailed past the Channel Islands will be aware of the vast clumps of long stringy weed with strong roots. This can stop a prop. Often there is bits of net in the clumps. The Ambassador pattern is good for this as well as the usual lobster pot
I would not be without a cutter. Unfortunately I have a feathering prop, so any cut pieces still foul the blades rather than slide off but that is not the cutters fault.
I have often felt either the drop in revs for a few seconds or a sudden clonk as the cutter has worked