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Old 20-01-2017, 17:10   #1
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Sigma Drive

Has anyone fitted a Brunton's Sigma Drive?
Looking for opinions
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Old 20-01-2017, 20:39   #2
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Re: Sigma Drive

Yes, we just put on an SD100. We have less than a 50 hours on it, but it works fine. Their US rep, King Propulsion, was very helpful.

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Old 21-01-2017, 15:50   #3
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Re: Sigma Drive

By works fine, does it smooth things out, less vibration, noise?
Why did you fit it?
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Old 21-01-2017, 16:05   #4
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Re: Sigma Drive

Out boat previously had half an aquadrive - one CV joint and no thrust plate/bearing. Although the aquadrive lasted for 3700 engine hours, it wasnt designed to carry thrust. Under hard reverse, the spline in the center of the joint separated and we had no thrust. Beating the heck out of the aquadrive while removing it reseated the spline and so we pulled the transmission looking for the problem. Transmission couldve used new clutch plates, which were almost as much as new transmission, so we replaced tramsmission. Same drive ratio wasnt available, so we changed ratio (2.4 to 2.0).

Until I came across sigma drive, I thought I would have to fit a regular CV, a thrust bearing, and glass a thrust plate to the stringers. Sigma drive fit the application fine, but I had to have a custom bobbin fabricated to fill the space formerly occupied by the half-aquadrive.

The prop now spins 20% faster, but otherwise I note no difference in smoothness/vibration compared to aquadrive. I hope to experience greater longevity, though.


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Old 21-01-2017, 17:15   #5
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Sigma Drive

I have vibe issues I'm chasing. I believe most are due to excessive overhang that is being fixed this week I hope.
The Sigma drive looks to be a rzeppa joint, which has been around a long time and was what was used in the front axles of Military trucks forever and may still be.
Pretty sure I will fit one, it should allow the shaft to always run true without stressing the Cutlass or transmission.
I'd like a real thrust bearing with a CV shaft, but that is way more work and money that I want to spend.
Did you fit it? It looks like it just clamps on the shaft, seems to be a real easy install. Does it push the shaft back any or does it seat the shaft deep enough so that length remains the same?
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Old 21-01-2017, 18:14   #6
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Re: Sigma Drive

It seated the shaft 15mm deeper than the previous coupling on the back of the 1/2 aquadrive.

Im not familiar with rzeppa. Most marine CV setups I looked at, like aquadrive, are not rated to bear thrust (just rotation, with small angular offset).

Some say the transmission and engine mounts should not bear the thrust, and I understand their point, but in 3700+ hours it was the CV, not any other component that failed. (Although I replaced engine mounts at about 2500).

I understand that the overhang as measued by the distance from the cutlass or line cutter to the prop should be approximately one prop diameter.

The SigmaDrive was shorter than the previous half-aquadrive, so I had a choice of either putting in a spacer (bobbin) at the back of the transmission, or having a new, longer shaft made, customized to my variprofile, and shipped to Antigua, where the boat was. I opted for a spacer.

Rod at King Propulsion was helpful with getting the overhang right.

I can try to upload pics if they would help you.


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Old 22-01-2017, 07:09   #7
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Sigma Drive

That is OK, I have drawings etc that AB Marine sent me, it looks like maybe shaft length will remain the same going from a standard adapter.
Yes, nearly all marine drives have the motor mounts absorbing all of the thrust, that is not ideal in my opinion as the mounts have to be far stronger and therefore less compliant than if they only had to deal with torque and motor vibration, but everything is a compromise, just I want to get rid of my drivetrain vibe so that if I had to, full RPM and power are available.
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