I have a 15' grumman 1981 w/ a merc 50 3cyl 81-83 is what i was told the plate says #5797770 any how there is a trim anode on the motor but is that enough and should i add more and where? (just got it)
In the absence of any other replies, whilst I'm not an expert re. outboards (or much else, for that matter) I would have thought that providing you lift the leg out of the water when it's not in use, i.e., at your mooring, you should be fine with just the trimtab anode.
Hopefully someone with more info than I regarding outboards will be along soon to confirm/refute this.
I dont know anything much about Mercury engines but 5797770 does check out as an American made 1981 2 stroke model.
As far as I can discover the trim tab is the only anode. Likewise for much later models.
If it were not for the trim tab also being the anode I'd suggest bolting one onto the AV plate but since that would be right next to the trim tab there's not a lot of point.