Bruntons SigmaDrive

Posted by Steve Chandler 19/02/2019 0 Comment(s)

Usually my musings and blogs are sail related, but I’ve encountered something that has implications for anything (almost) that floats and uses an inboard engine.



Bruntons have been involved in the marine industry for over a century.  While best known for superior propellers, involvement in the stern gear package is unavoidable.  One of the products from this is the SigmaDrive, the first real progress in couplings in a hundred years or more.


The coupling, a vital part of the drivetrain, is relatively simple.  Usually it consists of a cast-iron forged fitting including a flange, a machined keyway and an exact hole pattern drilled to fit specific transmissions. 


In addition, a thrust bearing is at times required, and a damper is preferred by some.  This “damper” is commonly known as a DriveSaver, a sacrificial fitting between the transmission flange and coupling flange designed to fail in a catastrophic propeller impact, instead of damaging the transmission.


The system is fairly simple, but there’s a few details that can have far-reaching results.  The most ubiquitous of these is vibration, primarily caused by misalignment.  Unless the shaft is perfectly aligned from the transmission output flange to the propeller nut, there’s going to be vibration, plain and simple.  Here’s the rub, though…regardless of how perfectly aligned your shaft and engine are during construction, all you need to do to throw that off is set that hull down into the water.  Oh wait, you meant to float this, too?


Sorry, my little stab at humor.  It’s true though, as soon as your boat leaves the hard and becomes a boat, the keel and hull flex some (or lots!), destroying that alignment.  Up until now, the solutions have been limited.  Sections of propeller shaft fed through bearings to adjust the alignment, slight offsets, there’s not a whole lot of solutions.


The result of this misalignment is vibration, noise and wear.  None of those are good but wear, caused by vibration due to misalignment, is the Maintenance Demon.  Wear on the transmission, shaft coupling, pillow and cutless bearings along with the strut bearing will all land you in the yard occasionally, on the hard with the shaft pulled and bearings being replaced at the least.  And the shaft, what about that?  If your bearings get to a point before maintenance, is your shaft galled and possibly out of true from spot heat?  No problem, though, they just give shafts away, right?


 Ok, didn’t mean to depress you.  We’ve accepted this system for over a century because there really wasn’t an option.  However, the first recent development in this field is readily available.



Bruntons in the UK manufactures the SigmaDrive, a coupling that pulls the industry into the 21st century in this field.  Readily available for all transmissions for motors 10-1,300 horsepower, it covers the entire field.  If your transmission’s modified, they can produce the specific flange, and can custom-produce couplings for engines over 1,300 hp as necessary.


Ductile iron tends to kind of hate the marine environment of water, constant damp and salt.  So Bruntons have manufactured the SigmaDrive from NiBral, the nickel-bronze-aluminum alloy that is the most appropriate thus far for any marine application.


The coupling is sealed, lubricated during production, and the resulting maintenance is…none!  Due to its design, most times your coupling can be replaced without putting the boat on the hard…unless it’s required to get your old coupling off where it has rusted and seized its way onto the shaft.  By the way, given the alloy, the SigmaDrive won’t rust or corrode, or significantly bind to the shaft.


Once installed, the coupling can accommodate up to a 6⁰ offset in alignment, 3⁰ in any direction.  Simply put, when you splash and your hull flexes, your shaft from the propeller nut to the engine will stay in alignment.  Your shaft will pass directly through the strut bearing, cutless, pillows etc. all the way to the transmission flange.


If you reduce or eliminate the wear on the bearings, your next yard period may be a cause for celebration, not a requirement for a second mortgage.  By eliminating noise and vibration, you could find a reason to take the boat out that doesn’t include shouting over shaft noise or monitoring what cup or glass is approaching the suicide point in the galley.  As a bonus, you may be able to forgo the regular maintenance on your bearings because they’re not wearing any more!  The NiBral construction, with no bearings or rubber seals, extends the life of the coupling as well as the entire drivetrain.


At getaprop, we try to stay up with new developments.  This is one of the biggest advancements for a simple but potentially expensive system in a long time.  We prefer to vet new items before they go on our website, and in this case testimonials have been overwhelming.  The response online has ranged from the “I just ordered this, hoping for the best” to “After 3+ years and a jillion sea miles, won’t trade it for anything!”.  We’re convinced, particularly with the comments regarding the customer service and end-user support by the manufacturer.


If you have a SigmaDrive, please share your experience on our FaceBook and Instagram links.  As always, you can order from the website, or give us a call at 360-299-8266.







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