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Rope Cutters

We are frequently asked as to the ability to fit rope cutters in front of Kiwiprop units. The first question is wether the installation is a shaft or Saildrive as the requirements differ considerably.


Some models simply attach to the shaft with a split coupling and being driven by the shaft - have no contact with the front face of the Kiwiprop unit.

Others - generally of the two cutter type - attach to the front face of the Kiwiprop unit and use it to drive one side the unit. This requires some form of attachment to the nose cone of the propeller for the moving half of the cutter.

Kiwiprop shaft type mounted units normally come with a faired shape whereby the composite nose cone reduces in diameter to meet the reduced diameter of the metal boss at the front face of the unit and offer a degree of streamlining to the unit.

Different models obviously have different drive requirements.  

We have supplied Kiwiprops machined to accept many hundreds of  Ambassador brand Kiwiprops - mainly on  Volvo Saildrives. The drive mechanism remains unchanged for shaft installations and involves machining the front face of the unit flush with the metal boss.

To ensure there is sufficient material to accept the cutter drive mechanism - we substitute a Saildrive style nose cone which has common internal dimensions as a shaft style nose cone - and then machine it flush with the front of the metal boss - then reduce the external diameter so that the external diameter is constant at a normal nose cone shaft diameter  of 2.700"  or  68.6 mm. The nose cone then in effect becomes parallel.

Two drive holes are then machined into this face to accept the protruding heads of the cap screws of the cutter. In the case of the Ambassador - the drive holes overlap the metal and composite sections so require tooling and the appropriate jigs to allow accurate milling of these two holes.

The modified Kiwiprop unit will then drive an Ambassador cutter.

Other brands will have different dimensions for the drive mechanism - but the only constraint will be the  Ø = 2.700"  is sufficient to accept the drive mechanism of the chosen cutter.


In the case of Saildrive units and referring only to Ambssador type units - two different methods are used.

In all cases the front face of the unit is machined flush with the front face of the metal boss, and then the drive holes to accept the cutter cap screw heads are machined into the metal and composite sections - the holes overlapping the join between the two.

In all cases the forward or fixed component of the cutter will need to be attached to the Saildrive unit by whatever method is specified by the cutter manufacturer.

VOLVO:  In the case of Volvo units - the Volvo part # collar that fits in front of the spline is removed and the rotating half of the cutter  fits directly into this space leaving the Kiwiprop or other propeller in the same position fore and aft.

This means the same nut can be used to attach the unit.

YANMAR:  In the case of Yanmar units the collar is also removed but installing the Ambassador cutter which is larger than the sleeve removed - has the effect of moving the propeller aft - which has the subsequent effect of reducing the cover available on the threaded section aft of the spline.

Spline threaded sections are only 23 mm long so the nut must cover at least ~ 20 mm of the thread.

Yanmar SD20's have smaller M16 x 2 threads on the shaft. All other models  SD31/40/50/60  use the larger M20 x 2.

The Kiwiprop solution for the SD20's  is to machine a " necked "  style nut that is longer than normal by the move aft and extends forward at a reduced diameter into the spline recess of the boss  generated by moving the propeller aft.

This also has the effect of reducing the length of spline covered by the boss of the propeller - so radial tolerances will be increased as the shorter the spline the larger the sideways movement on the boss - prior to the main attachment nut being tightened.

This is not an issue - the unit will remain firmly mounted over time but this type of mounting will have trade-offs that need to be recognised.

For the M20 x 2 Yanmars the internal Ø of the spline will be 25.00 mm. To reduce the Ø of a longer nut is not an option as the wall thickness remaining  (25 - 20)/2 = 2.50 mm is insufficient.

This requires that the internal splines at the aft end of the boss be removed and a new longer M20 x 2 nut machined. This approach delivers a non standard boss and does not address the concerns of higher tolerances from the shorter spline.

Yanmar SD60:

Yanmar have now released the SD60 - which will be the unit provided for all Saildrive installations over 30 hp where the rear face of the thrust collar, which on all previous models aligned ~ 2 mm aft of the rear face of the zinc, now aligns ~ 6 mm aft of the zinc. To fit an Ambassador cutter to a Yanmar SD60 Saildrive will require notifying Ambassador that you have an SD60 model with it's new requirements  both for the attachment of the static half to the rear face of the leg and for the moving half attached to the propeller unit.

Some newer Saildrives such as Twin Disc may have slightly different fittings to the above and each would have to be evaluated individually - but the same two basic approaches for Volvo and Yanmar style would still apply.

The photo below shows the boss being prepared for milling - the 6 x 6 mm Yanmar style laybrinth protrusion has not yet been machined off leaving a flush forward face. The tool in the center touching the cutter provides the set-off to ensure the correct radius of the upcoming 9.00 mm drive holes for an Ambassador cutter fitting.

Note how the mill straddles the metal / composite join so drilling is not an option - this can only be performed in a tool that holds a milling cutter securely.