Noticing black dust on my Alternator ...

When I replaced mine, it was original to the boat. I couldn't find a bearing kit for the circulating pump. Saw many for the raw water pump. Even if I found the bearings, it would've taken much longer to press out the shaft or find someone to do it, and I have a press. For the $100 or so for the new pump, it was worth it in the time savings. If, when my sherwood goes, I'll rebuild that. I think they go for $250+ new.

Well,

You have two 'water' pumps. One is the raw water pump which is mounted off to the side of the engine and the other is the circulating pump that is mounted on the engine in the center front. If the engine was in a car, the pump I'm talking about would be the water pump.

Go to www.skidim.com and look up circulating pump. It will show you a picture. When you pull off your belts and if the shaft on the circ pump wobbles from side to side and/or doesn't spin freely, that's the pump you would need to replace.

Or, possibly (not to go against the boat dr)another condition could cause the belt dust; I had it on mine but it was due to the circulating pump going. This caused the pulley on the circ pump shaft to wobble causing misalignment and belt dust.

With the belts off, wiggle the circ pump pulley. If it wobbles, you need a new one.

... pulley and bracket of my 1990 SN, FORD 351 (w/500 hours). I can depress the belt 3/8" (manual says 1/4"). No idea how old the belt is, it has the original PMC number on it and it looks to be in good serviceable condition.

Should I replace it ?

cheers - John