Rope Cutters & Boat Speed

[Amp1ng]

Hi all,

Furthering the investigation as to why I have such poor boat speed under engine I have another question this time focusing on the rope cutter I presently have fitted.

This is a scissor type cutter with double fixed blade clamped onto shaft and the non fixed blade with its peg located in a recess on the P bracket mounted plate. Whilst investigating the complete transmission for vibration (which includes Volvo MS2 gearbox spline mod) I am refurbing the rope cutter which needs replacement plastic bearings. When searching through the various makes and models to try and determine which one I have I noticed a recent magazine test on the different types of rope cutter. Reading through I spotted one of the stated drawbacks of the scissor types being loss in boat speed under engine, quoted at 1 - 2 knots. Is this really true?? Does anybody have experience of before and after speeds having fitted one of these type of cutters?

If correct this might explain why I struggle to get more than 4.5 knots from a Volvo Penta 2002 in a Fulmar on a calm day on fresh antifoul. I was expecting 6 knots at cruising engine speeds (2200 RPM).

Any suggestions on boat speed most welcome. Prop is a 15" by 11" LH fixed 2 blade, the exact gearbox reduction ration I'm not sure of, will check tonight but roughly 2:1.

Thanks

 

[sailorman]

Hi all,

Furthering the investigation as to why I have such poor boat speed under engine I have another question this time focusing on the rope cutter I presently have fitted.

This is a scissor type cutter with double fixed blade clamped onto shaft and the non fixed blade with its peg located in a recess on the P bracket mounted plate. Whilst investigating the complete transmission for vibration (which includes Volvo MS2 gearbox spline mod) I am refurbing the rope cutter which needs replacement plastic bearings. When searching through the various makes and models to try and determine which one I have I noticed a recent magazine test on the different types of rope cutter. Reading through I spotted one of the stated drawbacks of the scissor types being loss in boat speed under engine, quoted at 1 - 2 knots. Is this really true?? Does anybody have experience of before and after speeds having fitted one of these type of cutters?

If correct this might explain why I struggle to get more than 4.5 knots from a Volvo Penta 2002 in a Fulmar on a calm day on fresh antifoul. I was expecting 6 knots at cruising engine speeds (2200 RPM).

Any suggestions on boat speed most welcome. Prop is a 15" by 11" LH fixed 2 blade, the exact gearbox reduction ration I'm not sure of, will check tonight but roughly 2:1.

Thanks

i have an ambassador fitted & can turn the shaft by hand even on the coldest mornings.
i have a 32m/m s/s shaft with a conventional stuffing box with a P bracket & MaxProp
i do not see how the cutter can bind-up to cause that effect

 

[vyv_cox]

I have never heard of the speed reduction you mention and this was certainly not true on my boat. I detected no reduction in performance whatsoever when I installed a Stripper.

When the bearings have worn too far it is evident by a knocking noise as the rotating blade hits the fixed blade. Until then it would be unlikely that any seizure could take place.

 

[Tranona]

It is unrealistic to expect to get 6knots at 2200. You need to do some timed runs in flat water noting speed and revs. Ideally do it at slack water so there is no tide to worry about or be able to allow for tide (and wind perhaps) by doing runs in opposite directions. Using a measured distance is helpful, but reasonably accurate results can be obtained from GPS readings. If your prop is correct, you should be able to get around 6.5 knots at or near maximum rated revs, and a cruising type speed of 5.3 at around 2300. If you cannot get similar results to this then your prop needs looking at, and you can check the size using the Propcalc programme www.castlemarine.co.uk This of course assumes that your engine is in good nick and is producing its rated power.

The cutter will make no difference to the speed. Just make sure when you refit it there is the correct amount of clearance of the fixed cutter into the block - the dimensions are in the fitting instructions. If it is too far in then it can bottom when you are in forward, so wearing the bearings and eventually the faces of the cutter. Also check that the blades are correctly located in relation to the prop blades.

Hope this helps

 

[Amp1ng]

Binding

"i do not see how the cutter can bind-up to cause that effect "

The proposed cause for speed reduction was not friction but interference in water flow past the prop. A link to the article is included below. look on the first page and first paragraph under the "How They Work" section for Scissor types


http://www.prop-protector.co.uk/rope-cutters-1.pdf

 

[Tranona]

No, as others have noted if properly installed with the cutter blades positioned correctly they will have no impact on speed.

 

[Twister_Ken]

Given that the cutter blades are probably about 2% of the area of the prop blades, it's difficult to see how it can make a measurable difference.

 

[jwilson]

Hi all,

Furthering the investigation as to why I have such poor boat speed under engine I have another question this time focusing on the rope cutter I presently have fitted.

This is a scissor type cutter with double fixed blade clamped onto shaft and the non fixed blade with its peg located in a recess on the P bracket mounted plate. Whilst investigating the complete transmission for vibration (which includes Volvo MS2 gearbox spline mod) I am refurbing the rope cutter which needs replacement plastic bearings. When searching through the various makes and models to try and determine which one I have I noticed a recent magazine test on the different types of rope cutter. Reading through I spotted one of the stated drawbacks of the scissor types being loss in boat speed under engine, quoted at 1 - 2 knots. Is this really true?? Does anybody have experience of before and after speeds having fitted one of these type of cutters?

If correct this might explain why I struggle to get more than 4.5 knots from a Volvo Penta 2002 in a Fulmar on a calm day on fresh antifoul. I was expecting 6 knots at cruising engine speeds (2200 RPM).

Any suggestions on boat speed most welcome. Prop is a 15" by 11" LH fixed 2 blade, the exact gearbox reduction ration I'm not sure of, will check tonight but roughly 2:1.

Thanks

It's not any friction in the cutter blades that would cause loss of speed, but slight disturbance of the water flow over the blades. Effect should be minimal. I suspect that 6 knots at 2200 rpm in a Fulmar with 18 hp is optimistic: I'd have thought it would be achievable though at highish revs - 2200 is pretty low and diesels like hard work. From memory the 2002 is rated to rev to 3500 approx.

 

[Neil_Y]

No independant test data but many reports (on faster vessels) of loss of speed. Scissor and disc cutters offer a relatively large surface area to the water flow just in front of the prop. It's bound to have an impact on the props efficiency when compared to undisturbed flow.

One issue for the RNLI was speed when looking at re proping and changing the rope cutters from scissor to shaver on trent and severn boats.

You might see an effect on a small yacht prop but not as much as if you are running fast props at 20+ knts.

 

[prv]

No, as others have noted if properly installed with the cutter blades positioned correctly they will have no impact on speed.

That's a point - is there a particular rotational position that the cutter blades should be mounted relative to the propellor blades? I took my cutter apart on the beach (long story) and probably didn't put it back the same.

I too am chasing a mysterious speed reduction.

Pete

 

[Stork_III]

That's a point - is there a particular rotational position that the cutter blades should be mounted relative to the propellor blades? I took my cutter apart on the beach (long story) and probably didn't put it back the same.

I too am chasing a mysterious speed reduction.

Pete

On my boat saildrive volvo 3B folding prop, fixed cutter align with leg, prop cutters(3) align with gap between blades (open) so behind ears of blades when folded. With fixed prop cutters behind blades.

 

[BlueSkyNick]

to suggest that the rope cutter can reduce speed by 1-2 knots sounds like rowlocks to me.

As is happens, I stripped mine down this weekend and am about to order a service kit from Ambassador as at http://www.ropestripper.com/servicepacks.php

I think a lot of people fit these things and forget about them - its not that simple.

 

[Neil_Y]

It is certainly evident in many boats at the 25-50knot range and these guys look for every extra knot when they are paying ,000's in fuel and want speed and more speed.

Christensen yachts changed from scissor cutters for that reason, however yachts at 6-7 knots I can't think you'd notice, but they won't improve the performance.

We've got logged data (engine load and speed) from just running the shafts in tubes that show increased performance, same boat same props. All that does is stop the layer of water next to the shaft from spiraling as it heads towards the prop. This means that in practice water hits the P bracket leg straight rather than at a slight angle creating turbulence to one side. This was at 30 knts + with two 1000hp engines.

 

[john_q]

I had an Ambassador rope cutter and when you buy it you specify if you have a 2 or 3 bladed prop so you can line up the rotating cutting blade in line with the prop blades. I did not notice any speed issues after fitting.

 

[Tranona]

That's a point - is there a particular rotational position that the cutter blades should be mounted relative to the propellor blades? I took my cutter apart on the beach (long story) and probably didn't put it back the same.

I too am chasing a mysterious speed reduction.

Pete

The cutters should be positioned behind the blades. The fitting instructions are on the website www.ropestripper.com

 

[ithet]

I have a Moody 28 with VP2002 using a 15x12prop. So am probably a little lighter but lower hull speed (23.5ft lwl) than your Fulmar. The 2002 is rated at 3200rpm. I get about 5 knots at 2200rpm. On a good day - after launch with a clean bottom I can see max of 3100 and 6.5 knots. More typically she tops at 2800rpm doing 6 knots.

I think your 4.5knots at 2100 sounds reasonable for mid season condition.

Try this prop calc - http://www.vicprop.com/displacement_size.php
Think you will find that the MS2B ratio is 2.37:1. Try the calc at 2200 & 3200 rpm to get speeds.

BTW I have a disc type cutter.

 

[prv]

The cutters should be positioned behind the blades.

I presume you mean in front of . But thanks, I'll have a look if they're in the right place next time I'm at the yard.

Pete

 

[Tranona]

I presume you mean in front of . But thanks, I'll have a look if they're in the right place next time I'm at the yard.

Pete

Depends on whether you are looking from the back or the front! - But you will suss it once you look.

 

[prv]

Depends on whether you are looking from the back or the front!

The boat only has one front

Pete

 

[Neil_Y]

The boat only has one front

Pete

It is common for stern gear suppliers to refer to the back of the prop as the side facing the front of the boat which doesn't help at all! especially when you are having a technical discussion with an Italian engineer as I was yesterday.