Background
When I purchased Mollymawk, a Pan Oceanic 38 cutter, the previous owner had mentioned something about the engine alignment, but I thought that the vibration issues would disappear when I replaced the ancient Perkins 4108 with a Beta Marine 50. I had a reputable company do the bolt down and alignment as I wasn’t confident with that side of the installation. Surely my troubles would be over….
Many yachts are fitted with a standard R&D type flexible coupling. The problem is that it is flexible in the sense that it is a spring, and quite a strong one at that. This then has 2 metres of drive shaft and a large lump of brass propeller on the end. All this with only the cutlass bearing to keep it in place. Simple physics tells you that a mass on a spring will oscillate. Which is exactly what happens.
My experience was that I could feel strong vibrations at 2 bands of engine RPM. One around 1800, one around 2100. However, these would vary depending on conditions. When I turned the helm, it would reduce in amplitude, then increase as the rudder was straightened. Apart from being an annoyance, such things cause wear and damage over time. Something had to be done. I tried re-aligning the engine, having convinced myself that this was needed now that the new engine had settled on its mounts, but all this did was to shift the pattern of the vibration slightly. Something definitely had to be done!
The Solution
After some research, I spoke to Bruntons at the Southampton boat show. I wanted to hear what they had to say, and look at the device itself. The SigmaDrive is also a flexible coupling but in the sense that it can pivot freely rather than rely on bending; it is essentially a marinised constant velocity joint as found on a front wheel drive car. I liked the look of the build, and I asked for a quote. Subsequent ordering was straight forward; I gave details of the gearbox and drive shaft diameter and paid the money.
Fitting
One issue I found with the SigmaDrive is that it is shorter than my R&D type coupling. Due to the existing engine bearers when I fitted the Beta Marine engine, I had already had to bring the propeller shaft fully forward (though leaving clearance between the shaft anode and cutlass bearing for water ingress) and install a 1 inch spacer to the drive flange. Fitting the SigmaDrive meant adding an additional ¾ inch spacer. Having discussed this previously, Bruntons had supplied me with extra-long 80mm studs, but these were fractionally too short. However, they put me in touch with their suppliers who were able to send me some 100mm M10 studs of the correct grade of steel. Fitting is straightforward.
Results
I have carried out 2 test runs of several miles each and was delighted by the change in feel of the drive. Yes, there is still vibration: certainly some from the engine, and some from what feels like cavitation at very high engine speeds (above ¾ max RPM and what I would not want to run the engine at for any length of time); I suspect that a new cutlass bearing might help matters. But the overall impression is that the things are much happier in the engine bay, and there is a consistent smooth drive all the way up to (and beyond) ⅔ max RPM, an engine speed that generally feels about right for long engine passages and gives me over 6 kts. I checked the shaft clamping collet after the first run. The bolts are tight and there is no sign of movement
Ultimately the question has to be asked, is it worth it? Alongside everything else to do with yachts, it always seems a lot of money every time, and I had to spend an extra £100 on a spacer and longer studs. But in total, it is probably no more than the cost of a lift out and a new cutlass bearing. There is a lot of talk around the subject of yacht engines and propellers, but little thought seems to be given to the gearbox and drive, despite their importance. So my answer has to be a definite, “Yes, I would have no hesitations in fitting one to any yacht with a shaft drive”. It significantly reduces vibration and makes far more sense from an engineering point of view than a standard flexible coupling. I cannot comment on longevity, but the device is simple and has a relatively easy life compared to a car CV joint which has a long life span.
