Please explain this M42. Wouldn’t a shaft break allow single engine use? Also why does a stuffing box prevent single engine use?
In order to be able to use one engine only there are a few things you need to consider.
First is the oil pump in the gearbox. If that one does not function when the prop is freewheeling you have a problem. Of course you could just do it, but find out your gearbox will be damaged after prolonged periods of use.
The second is the gland itself. The outer part which is closest to the water will always be wet and when the boat is stationary the rest of the gland will also be wet. But when the boat is underway there is an under pressure in the area of the stuffing box, which means the inner part of the gland does not remain wet and may actually run completely dry, damaging the gland and over time causing heavier leakages than just the required drips. Of course this is not the case if you have to run the boat on one engine in case of emergency, but when you do this on a daily basis it does become a problem. The prop shaft generates heat while turning and thereby slowly 'burning' the gland, the water is required as a lubricant and cooling.
On my boat i had a water supply line running from the heat exchanger to the stuffing box, making sure the gland remained wet while the engine was running. Each engine had it's own water supply line. Of course I could make a system where copper pipes run through the bilge area, with switching valves etc, but I did not want to go that way. Only need to step on that pipe once and you could be in for a surprise.
So after some thought I decided to go for the PSS system, where I don't need to worry about the gland remaining wet. Since the engines were out of the boat, which meant the prop shaft was also out, this was the best time to change them. Otherwise I don't think I would have done it at this time. Taking out the boat, yard time, removing rudder, prop, shaft etc just to install a PSS is a bit over the top. Would have cost a fortune.
Now the total cost is about 2500 euro (around 2600 USD), which I will earn back in about 1300 liters of fuel, which is about 220 hours sailing on one engine. We do about 500 - 600 hours per year, so we earn it back in a couple of months.
I understand that our case is a specific case, won't apply to everyone and if I would have needed to take out the boat specifically for installing PSS seals it would have been around 8000 euro (or 8800 USD). Kind of ridiculous.