The Offshore Voyaging Reference Site

Q&A: Propeller Shaft Rope Cutters

Question: Do you have a rope cutter? I ask because we are thinking of fitting an Ambassador Stripper (stainless) to a new build alloy [aluminum] yacht with an alloy sterntube, and we wonder if it is possible to get the two to live happily together.

Answer: Some twenty years ago we picked up a ball of netting at sea on a passage from Bermuda. When we got to Nova Scotia, unaware of our new underwater decoration, we started the engine and put it in gear. The result was a bent shaft and strut that required having the boat hauled twice before we finally got things sorted out. As I recall, the total cost was in excess of US$5000, making the price of the cutters we installed the next year look like a very good deal.

That was on a previous boat. One of the first things we did in 1992 when we bought this Morgan’s Cloud was to install a set of Spurs cutters. Since then they have saved us from being disabled by a rope around the propeller at least a dozen times. Although even the Spurs could not clear the running gear when we picked up the whole cod-end of a net, some years ago .

Bottom line, there is a lot of rope and netting floating around the ocean and so we strongly recommend line cutters, particularly since getting disabled at the wrong moment, say entering a harbour situated on a lee shore, could even cost you your boat.

We have no experience with the Ambassador Stripper but we can tell you that any cutter you install on an aluminum boat should not compromise the isolation of the stainless steel shaft from the hull. The Spurs cutters are great in this regard since they have small plastic bumpers and plastic bearings that isolate the spinning part on the shaft from the fixed part that attaches to the strut or stern tube.

Does anyone have any experience with the Ambassador Stripper or any other thoughts on line cutters? If so, please leave a comment.

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I put Spurs on my Jeanneau 34 and upon mounting them I was surprised to find they were complete with their own set of tiny anodes that I made a mental note to inspect periodically…not something I had foreseen, but the apparatus looked so good and seemed so sturdy that I wasn’t unhappy…Unfortunately the anodes lasted only a few months but I wasn’t about to haul out just to replace them…then about a year later when I did haul out I was disappointed to see that half of the apparatus was totally missing rendering the apparatus totally useless. I don’t know how long this had been the case although I did succeed in replacing the missing part(s) after communicating with the mfr…So at the end of the day I found Spurs is one more regular maintenance item to schedule when my original perception was this would not be necessary…bummer!

Colin Speedie

We bought one of the simple French disc types with a serrated edge this spring, but sadly didn’t have time to fit it due to a chaotic haul out/relaunch schedule that we found ourselves stuck with. We had one on our previous boat for nearly ten years, and on the two occasions we needed it (that we know of) it did its job perfectly.

These are very basic, but have the advantage that they are easy to fit, relatively inexpensive and shouldn’t interfere with the isolation of the shaft.

Alan Teale

I comment as the person posing the original question to John.

Having discussed this with a number of folk, to whom I am very grateful for advice, I am inclined to go the way Colin suggests, i.e., to fit a simple disc cutter. In addition to the fact that this type does not introduce another stainless to alloy fitting problem, it also avoids the potential for damage to the sterntube due to the torsional loads that could be exerted on the fixed part of a scissor type (e.g., Spurs and Stripper) when faced with something very tough to cut.

For those who may be interested there is a reasonable overview of a selection of cutters in Yachting Monthly, April 2009.

Neil Young

In response to mounting on an Al bearing carrier we have one image here

This cutter (quicKutter) is very different in the way it operates from the disc or scissor types. It is also the one used by the UK RNLI and the Royal Navy.

We were asked to participate in the YM trial you mentioned but had to decline for two reasons. Their test rig was the one made for the gator cutter, a scissor cutter. This requires a large gap between the prop and the bearing carrier and the test methodology was designed to show how the cutters could cut and not what happens if debris is caught by a prop blade. I was there at the test.

To get these to cut rope they fixed one end of the rope to the P bracket so it would be dragged along the shaft close to the cutter, this also meant the debris was fixed relative to the rotating shaft. This could be the reason why the discs seemd to work so well.

The quicKutter cuts debris that is caught by the prop and hence rotating and winding on the shaft, this is after all what happens in practice if you look at the result of catching a line or rope. It winds in the gap until it welds into a solid lump or in worst case pulls the gearbox off the engine. The shaver cutter (quicKutter) is a fixed blade angled so that it shaves into the debris as a lathe tool would.

We have many fixed to al carriers and to date no damaged fixings, partly as there no shock loads when debris is cut.
You can see how it works better here.

Richard Elder

How much are spurs worth?
A couple of years ago an 82′ Italian Azimet motoryacht came into our shop. One V 12 CAT engine had started smoking badly & the factory mechanic changed the fuel injection system and put the engine back together with the boat still afloat. Before he completed the fuel system control re-connection the “professional” skipper helpfully tried to start the engine and it ran away uncontrollably to self destruction- connecting rods sticking through cylinder heads, oil all over the engine room etc. When we hauled the vessel we found a rope wrapped around the prop, the real cause of the performance problem. So at this point the bill for the spurs was at $235,000. But there is more! The boat was a wiring and design nightmare and leaked like a sieve through the decks and bulwarks. Having spent all his money on a new engine, the owner refused our suggestions to rewire the boat to proper standards and stop the water leaks. The day after we finished the engine rebuild the boat caught fire and burned to the ground. So how much are spurs worth? $2,000,000!

Ed Seling

I have tried both the Spurs and one I cannot name that was a plain SS disc ground to a razor edge. The Spurs worked but had lots of small parts difficult for a diver to work with and bearings and zincs that needed to be replaced. After losing 2 of the shaft connected bits ($80 ea.) I got rid of the things. The plain SS disc on the other hand was simplicity it self. The first time it was called into use (poly prop line) the edge simply folded over.

I now have a Shaft Razor made by Evolution Marine in Rockland ME. (Who would know better about this problem than a “Mainer”?:-)

So far it has been called into use only once but it did its job well and without stress to shaft or strut. I like the serrations on the front face in addition to the rim. They also machined a special shaft hole size for me at no additional charge. Good customer service.


Hi Ed,

The Shaft razor looks like a very nice product. As you say, the Spurs, while functional, have a substantial maintenance burden attached to them.


So John, I am qurious to know what you installed in the end – if anything.

Motor Boats Monthly did a test of various types and a nice video is located here:

Having looked into the various ropecutters the disc type appears the easiest ones to attach and maintain, but the knife like cutters the most efficient. However, I really dislike the idea of drilling into an aluminium the stern tube to attach a stainless item with stainless screws. Views and/or comments on this objection or related issues are as always appreicated.


Thanks, John.
I must confess that I, at a reasonably late point of the day, misread the question above as you were considering fitting an Ambasador type to Morgans Cloud. Thanks for clearing that up. If you find time it, would be great if you would you care to quickly walk me through the steps and materials you use in installing the Spurs cutter to avoid corrosion issue. (Mostly thinking about the fixed part attached to the stern tube)
– petter


Thanks John, that was nice and simple as well as pointing me to tefgel as an alternative to Duralac

Neil Young

So why would the Royal Navy and the RNLI remove spurs cutters to fit shaver cutters if the knife type are the most efficient? Christensen, Aqua Star, Alicats, baltic work boats and Austal also use shaver cutters.


Sorry John it was more in response to Petter “knife cutter smost efficient” but this was only in a test that compared disc and scissor types.

Withe regards to isolation we frequently attach stainless to Al P brackets, they are protected in the same way.
I have an article about cathodic protection I can e-mail if anyone is interested this is the part about quicKutters.
Isolating is not always the answer as they are all in a conducting electrolyte.
quicKutters are constructed of 316L stainless steel and therefore may be
regarded as cathodic, in much the same way that a stainless steel
propeller shaft is cathodic to a bronze propeller. The entire submerged
metal componentry of a vessel should in any event always be electrically
connected to sacricial zinc (or, in some cases magnesium) anodes which
should be strategically located, constantly monitored and replaced
quicKutters are cathodic to marine structural and propulsive metals that
comprise a vessel such as steel, aluminium, and bronze. Should a
quicKutter be fitted to a vessel, the additional mass of cathodic metal
componentry compared to pre-existing componentry such as stainless
rudder and prop shafts, bronze or stainless propellers, or brackets, is
relatively insignificant.


Neil, it would be great to receive a copy of the document you mentioned. You may use petterms – at – gmail – dot – com.

Eero Metso

Going to install a disc type rope cutter to 30mm propeller shaft. The problem is how much to leave free space between the disc and P bracket to ensure sufficient water flow through rubber bearing and effective cutting function of the disc. Depending where to look I have found a large scale of free space recommendations, starting from 6mm. The shaft has a direct coupling to 50 hp engine (on rubber cushions) and may move in a few millimeters on full throttle. How do you see an optimum free space between the disc and P bracket ?

Eero Metso

Thanks, John

Spurs says to their scissor minimum free space is 3/16″ (~5mm), Vetus says to their disc minimum is 1/4″ (~6mm). I think both suppose the shaft is not able to move forward, but has Scatra, Aquadrive, Python Drive or similar … where propulsion thrust is isolated from the gearbox and the engine. The manufacturer TOR of this product does not give figure for free space. Interesting is that Vetus and TOR look very similar, but their installation pictures show opposite directions of installation! Should toss coin ?

My preliminary guess was to leave ~10mm free space, but your 1/2″ (~13mm) would give more forward moving tolerance and probably should not affect on the cutting effect.

Eero Metso

Thanks, John

That’s a very good idea. Maybe I first check inside the vessel how many mm the shaft does move forward at full throttle. It is quite an easy check in this yacht.