OK, I'm surfacing from the depth of the bilge to ask an actual boat-related question.
I need to replace a ball valve. It's the drain valve for my gravity-drain holding tank... something I rarely use, but don't want to be without. Normally, we get our boat pumped out, either in our home slip, or when we're in a harbor like Great Salt Pond in Block Island.... but when well offshore, we might drain the tank using the valve. Obviously, it's not often used.
The valve itself is a 2" stainless ball valve with female threads (2", or DN50) on both ends... the handle is marked 'MT Full Flow', but I haven't been able to find the manufacturer. The valve is screwed into a thru-hull which has a 90 degree bend, and is positioned just below the waterline.
When I bought the boat (new) in 2005, the valve was very stiff, to turn. All other thru-hulls are similarly protected, by the same type of valve, but in smaller sizes... and they're easy to open and close, but this particular one was not. Over the last 6 years, the valve got progressively harder to turn, and I had to resort to a short length of 1.5" pipe nipple to give me additional leverage.
Last fall, in trying to open the valve, the stem broke.... snapped right off, with the valve in the closed position. The stem itself was apparently made of brass or bronze.
So, the valve needs to be replaced. However, since it is protecting a thru-hull just below the water line, it would normally require a short-haul to do it.. costing around $400. A replacement valve could cost anywhere from $125-$300, depending on type and material.
Yesterday, I was able to remove the sanitary hose from the inside end of the valve, in order to drain it (since the volume of the hose doesn't get evacuated when the tank is pumped out).... not a pleasant task, I assure you, but someone had to do it. Once finished, I could feel the surface of the ball inside the valve... and it feels like it's severely corroded, which is perhaps the reason the valve failed.
I'm wondering, though, if it would be safe to do it in the water. I can easily reach the thru-hull from the outside, and could block it with a wooden plug temporarily. The seal might not be perfect, but I'm guessing it would be good enough to reduce the water flow to a trickle while the valve is unscrewed from the thru-hull, and a new one screwed on.
So, I have the following questions for the crowd:
1) What does everyone think of the safety implications of doing it while in the water?
2) When installing a new valve, would I use teflon tape on the threads, or some other compound?
3) Should I stick with a stainless ball valve... or switch to a Forespar marelon valve? The marelon valve would be cheaper, and I've had good luck with them on previous boats.
Any/all responses welcomed.