Dave,
Mounting items which exert flexing stress onto your alloy can be risky. I am referring to downriggers. The length of the arm coupled with the weight of the ball & the motion of the boat will, in time, damage the metal. Could I suggest that a piece of alloy plate be welded to your gunnel ,or, if your boat is painted or you do not want any welding done, mount the downrigger base onto a piece of starboard or the like. I would also suggest some form of backing plate under the gunnel if you go with the starboard.
As for isolating the SS fasteners, there are plenty of products out there but definitely use one of them. Another way to go may be monel rivets, which from memory, do not react with aluminium.
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Regards,
Pete in Brisbane
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