SigmaDrive Flexible Propeller Shaft Coupling


SigmaDrive is a flexible propeller shaft coupling that is effective noise and vibration suppression for most sailboats up to 1300 hp. SigmaDrives is unique as it is able to absorb thrust when installed as a standalone propeller coupling directly from the shaft to the output flange on the transmission. SigmaDrive solves issues due to imperfect engine alignment between the propeller shaft and the engine, compensating for up to 3 degrees in all directions.


Easy to fit

SigmaDrive is quick and easy to fit, usually without the need to haul out the vessel. The unit is compact and does not need any modifications to the propeller shaft as it uses a taper lock system to grip the shaft securely.


Elegant solution

SigmaDrive eliminates the need for costly precision alignments. SigmaDrive gives flexibility to the shaft line allowing the engine to operate at a maximum 3 degree offset (6 degree full sweep) without transferring any harmful side loads.

Build Quality

All metal construction

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SigmaDrive is manufactured using a sturdy marine grade Bronze housing and all metal interior.  The lack of rubber parts used by competing brands gives SigmaDrive and incredibly long and maintenance free life.


Noise & Vibration solved

SigmaDrive uses and internal rotating sphere to absorb a thrust load and a bearing system to absorb the torque load.  This gives SigmaDrive a unique ability to take thrust without the need for additional thrust bulkheads making it extremely compact.

Manufactured from a special bronze alloy, SigmaDrives require virtually no maintenance and containing no bearings or rubber in their construction. The knock on effect of using a SigmaDrive shaft coupling is that the forces typically generated from misalignment will not be transferred to the engine or transmission  extending their life significantly.

Unlike existing shaft couplings SigmaDrive does not use rubber to dampen the vibrations or take out misalignment. SigmaDrive shaft couplings are special constant velocity joints that are able to resist the propeller thrust without the need for thrust bearings in most applications. They are extremely compact and able to transmit higher torque than other propeller shaft couplings with similar dimensions. In addition, as they do not use rubber as a vibration suppression device, they are an exceptionally long wearing marine coupling, with minimal maintenance needed.

Why does a SigmaDrive work so well? Because SigmaDrive is a constant velocity joint which allows the propeller shaft to rotate without transmitting the engines movements and vibrations. Other attempts at solving these solutions rely on rubber couplings or Cardan units. The SigmaDrive requires none of these, is compact and easy to fit, usually without slipping the vessel or requiring modifications to the shaft line. It’ s a premium product manufactured from a special bronze alloy that is maintenance free and its internal components are hard wearing and long lived.

Bruntons sigmadrive low noise and vibration 600

Looking at the image on top of the schematic you can see the usual method of connecting an engine with a propeller. The three fixed points, consisting of the propeller mount, the mechanical seal and the engine / gearbox, are individually or collectively capable of creating the two potential causes of noise and vibration. Firstly, any imperfect alignment of the three points can create vibration and noise. Secondly those vibrations and the associated noise are then transmitted directly through to the hull of the vessel and thence throughout the boat.

SD100 sigmadrive section

The simple solution is to fit a SigmaDrive coupling in the place of the standard one. The whole system from gearbox to propeller instantly becomes much more flexible, with the coupling able to correct imperfect co-axial alignment between the prop shaft and the gearbox of as much as 6 degrees. As can be seen in the lower schematic with the SigmaDrive in place the reduction in the rigidity of the whole propulsion system means that the engine vibrations are significantly reduced or eliminated completely, and the prop-shaft can rotate naturally with no vibrations or noise to transmit to the hull.

Sigmadrive coupling vibration schematic


For most sailboat applications SigmaDrive is a simple effective solution for noise and vibration issues that are caused by misalignment. SigmaDrive reduces and even eliminates vibrations that originate from an imperfect alignment between the propeller shaft and the engine by adding flexibility to the shaft line. Thrust taken up on the engine from the propeller invariably changes the angle of the shaft line, particularly when the engine uses soft rubber mounts. Using a single SigmaDrive coupling will ensure the shaft is correctly aligned at all times, especially under high load. The result, reductions in many cases of around 50% in noise and vibration.

The SigmaDrive coupling is able to resist propeller thrust when installed as a standalone coupling directly from the shaft to the output flange on the engine’s transmission. The SigmaDrive coupling is manufactured from a special bronze alloy and is virtually maintenance free. The internal components are hard-wearing with a long service life. No rubber is used in the construction of the SigmaDrive and its unique design compensates for the movement of the engine when under load from the propeller shaft.

The flexible SigmaDrive coupling is a direct replacement for a hard coupling such as Buck Algonquin, or flexible couplings such as R&D and Polyflex. The coupling uses a keyless taper lock system to grip the shaft so no modifications are needed to install the unit.

SigmaDrive couplings are available for shaft torques of 185 ft-lbs to 4425 ft-lbs with a range of shaft diameters.

ModelMax. TorqueShaft TypeShaft Dia. (A)Min. shaft depthFlange Dia
SD55550 NmParallel ended V-Drive22-35mm, 1-1.5"A + 5mm4"
SD1001000 NmParallel ended V-Drive30-45mm, 1.5-1.75"A + 5mm5"
SD1501500 NmParallel ended V-Drive45-50mm, 1.75-2"A + 5mm5"
SD2252250 NmParallel ended V-Drive50-60mm, 2-2.25"A + 5mm5"
SD3003000 NmParallel ended V-Drive50-70mm, 2-2.5"A + 5mm5.75"
SD6006000 NmTapered with keyway70-90mmN/A7-8"

SigmaDrive removes the need for expensive precision alignment of the shaft line