We have a volvo saildrive and we use a zinc guppy on our sail drive bolts run out of the lazarette and over the side.
The guppy doesnt grow crud which we assume means it is working, but we've had the same zinc for 3 years. With that being said, we don't lose our sail drive zinc very quickly either.
03-09-2010 09:09 AM
I have the same setup (YM30 and SD20). I assume you are aware that there is a second zinc (pencil style) on the port side of the saildrive unit (inside the boat)?
As to how quickly they are used up, so far my experience matches yours. Last season was the first, and we pretty much went through the exterior zinc in about 7 months. FYI, you can get a split version that can be changed underwater. I haven't climbed down into the bilge to check how the pencil style zinc fared yet (on my list for spring commissioning).
Can't answer your original question though (sorry).
03-09-2010 12:04 AM
extra sacrificial zinc for saildrive
I use a galvanic isolator and have an extra zinc "fish" that is coupled to the the common ground on ye boate. no corrosion issues there.
i have yanmar ym30 with SD20 saildrive and flex-o-fold prop.
the saildrive appears to be electrically isolated from the common ground (which i assume is good to keep the aluminum leg from acting as a giant sacrificial anode for the rest of the boat).
i dont like how quickly the saildrive zinc corrodes, so i change it every 6-9 months.
does anyone think adding a second zinc "fish" coupled to the saildrive bolts from inside the hull and hung over the side would appreciably help anything/reduce rate of zinc usage? - i assume those bolts are electrically connected to the leg.