Re: Beta 20 with Sea Prop 60 Saildrive
Obviously the engine is going to make the same amount of noise connected to a direct shaft as it will with a saildrive unit. However, noise is caused by vibration and a considerable amount of noise on board a small boat is the engine vibrating the hull, cabinetry and everything attached to it. Not much can be done about the amount of noise the engine makes. But much can be done about the associated vibration.
Softer motor mounts are the most efficient way of isolating engine vibration from the boat. However, with the precise alignment required with a direct shaft-to-prop setup, soft mounts would allow the engine too much movement. The cutlass bearing and stuffing box would suffer. I was contemplating a system called Aquadrive, basically a thrust bearing system that allows the engine to be mounted independent of the shaft. This would allow for the softer mounts without effecting the shaft. Unfortunately, in my application, there just wasn't enough room between the transmission and stuffing box to mount such a system. The only real option I had (to attain my goal of reducing vibration caused noise) was the saildrive. With the engine mounted directly to the saildrive unit the entire assembly could be mounted on very soft mounts, reducing the transmission of vibration to the hull. I can wiggle the engine/saildrive unit considerably by hand. The intense vibration of a diesel at speed is not felt through the cockpit floor as is common on many small boats. The rattling of stuff inside the boat is minimal.
Having had a few small boats over the years (everything from outboards to Atomic IVs to Volvo diesels) I would say the system I have in place is probably the quietest yet. It helps to insulate the engine room as well.
Yes, the saildrive has it's complexity and some concern about corrosion. However, having taken the appropriate precautions I have observed absolutely no sign of any corrosion after two years afloat.