Rope Cutter for a Saildrive

[DaveS]

Some months ago I responded to a thread on this with the following:

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I have Spurs fitted to my VP 120 saildrive. It was fitted to the boat when I bought it so I cannot really comment on initial ease of fitting, but it can certainly now be taken off and replaced very easily - which you do when replacing the leg anode. Other than one screw (see below) there are no fixings at all: the whole unit slides on to the splines before the propeller. Fixed and moving blades are permanently clamped together with plastic washers that allow free rotation without any noticeable play. The fixed blade is connected to a fork which sits either side of the trailing edge of the leg, preventing rotation while allowing for the bit of necessary axial prop shaft travel noted by other posters.

The only problem is that Volvo's standard anodes no longer fit straight out of the packet. You have to take about 10 mm (from memory) off the thickness to make room for the Spurs unit - but that's a 5 minute job if you have access to a lathe. You also have to drill a wee hole in the anode to accommodate the stainless wire which is run between an anode fixing screw and the screw on the fork.

Oh and yes, it cuts ropes very well indeed - especially nice new ones that you accidentally drop overboard!

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However...

Since then I have been thinking about getting a new propellor, and in the course of discussion on this it appears that Spurs do not (at least currently) make cutters for saildrives! So my previous post would appear to be erroneous. The description of my unit is accurate, however, and I would really like to find out what make it is in case I ever need spares. It has three moving and one fixed blade of apparently the same shape as Spurs. It is certainly not an Ambassador Stripper which (a) has quite differently shaped "serrated" blades, and (b) is driven from the prop by pegs rather than from the shaft splines. The boat was built in 1989 and I bought her in 1998 so, assuming it was new when fitted, the unit must be between 7 and 16 years old.

Any ideas? /forums/images/graemlins/confused.gif

 

[TigaWave]

If the blades are the same shape as Spurs then it must either be an illegal copy or one of theirs, as this is what is patented in their design.

Thats the reason why Gator and Stripper were able to come up with similar systems by changing the blade shape as I understand it.

And as far as I know the only rotating blade cutter for sail drives is the Stripper from Ambassador. Maybe its a spurs that was altered by a good engineer to fit?

 

[DaveS]

An adaption is a possibility I suppose. If so it has been extremely well done - it certainly doesn't look like a lash up.

An illegal copy? Again possible I suppose, and if Spurs don't make them for saildrives then there would seem to be a tempting gap in the market, but how would the copier sell them without advertising?

It has been suggested to me that Spur's American parent company might have formally made saildrive models, but if so why stop?

 

[steve_cronin]

There is one manufacturer who makes a "jaws" type for saildrives. I researched it a few months back but the "gen" is on the boat. Some searching on the net will bring it up.I'm sure.

I just tried and got:-

http://www.ropestripper.com/

The sharp discs are useless on saildrives as they rely upon the rope being stretched over their blades between a large hub and a relatively small shaft both rotating together with the "P" bracket providing a snatch to do the cutting . With a saildrive the lower unit is the same diameter as the prop hub and doesn't rotate so there is only a small displacement (the protrusion height of the blade - which can't be much if cavitation is to be avoided) and no snatch. Minor serrations help little.

Then of course there is the fact that a rope needs to already be entangled before it can even be started to be cut. With a Spurs or a Stripper, the rope is actively cut as soon as it reachs the prop. hub by a 50hp (or w.h.y.) power shearing machine. It doesn't rely upon being wound up tightly across an exposed (to bluntening by debris) half sharp disc.

Ever tried cutting through a piece of 3 strand polyprop rope with a Stanley knife, even one with a new blade? Shearing between two blades is far more effective.

Steve Cronin