I notice far less vibration and maybe somewhat less noise. There isn't less noise inherently due to the coupling, it's just less noise due to less vibration from the shaft coupling. The benefits you see will depend on how good your shaft is aligned currently.
There is no key, which I think tends to scare people, but the connection is very, very strong. It uses a collar that has an ~1/8" cut machined through it so that it can clamp. If you zoom into the section view above you can see the cut in the collar. The collar is machined straight on the inside bore, and fits freely (but without slop) over the prop shaft. The outside of the collar has a precision taper machined into it that mates up with a taper on the inside of the body of the Sigmadrive. Six or so machine screws are used to slowly tighten a flange on the outside of the shaft collar into the Sigmadrive body, pulling the matching tapers together and applying a very strong clamping pressure on the shaft.
The beauty of this system is that it's not only strong, but it will come apart for maintenance a whole lot easier than a straight coupling that has seized itself in place. Like I mentioned earlier, when you want to disassemble, you remove the clamp screws and install them in tapped holes that are already machined into the collar. When you tighten the screws, they apply pressure to the body which pushes the taper apart. It's a really slick system.
I spoke to the engineer that designed the system when I bought mine. He did his PhD work on hydraulic shaft clamping mechanisms for us on ice breaking ships. The Sigmadrive works on the same principle, just using screws to bring the taper together instead of hydraulics. He also mentioned that design behind the bearing itself is taken from helicopter rotor connections.
The key in a straight shaft coupling is really just there for back up in case the fit between the bore and the shaft isn't great. The key connection will eventually destroy itself with forward/reverse if there's slop in the connection. It isn't meant to be the primary load path for transmitting torque.
We had a long discussion on this forum about this a couple months ago. Some people swear by their straight coupling and that's fine. My 2 cents is that they tend to work until they don't, then they become a major pain in the ass. They are hard to do major repairs on since the bore is customed machined to the shaft in a machine shop. If you're in a remote location, you need to have access to a machine shop that specializes in this kind of work and the shaft needs to come out of the boat for fitting. You might be able to cobble something together with a local machine shop and some precision measurements of the shaft with the shaft still on the boat, but it's just kind of sketchy. Sigmadrive doesn't rely on any precision fits to clamp.
ab-marine.com
