russoboat-
I'll give you some reasonable standard advice, and others will also pitch in. Some research may also be in order, to better appreciate the scope of the problem.
The really bad part of all of this is that your're in salt water for 7 months on end. Probably impossible to totally avoid some rust/tarnish/corrosion under those circumstances.
First of all, be sure that everything on the boat that is large enough to make a connection to is grounded electrically together and that your sacrificial zincs on your motor are in reasonable shape with no coatings such as paint or marine growths. This electrical continuity protects everthing connected together so that only the most active element in the group (zinc) gets reacted into the salt water and corrosion cycle, in theory.
Small screws and independent fittings may be to small to connect to this arrangement. Clean and polish (leaving a wax coating from the polish) all such metals. Small screws may be cleaned and then coated with a clear polyurethane paint finish to keep out salt. Do not use regular steel wool to polish as it will form galvanic cells with the stainless and ruin it's anti-corrosive properties. Problem areas must be re-polished as often as needed to protect them from the salt air and water. You just have to work at this!
You're fighting corrosion with the various salts in the sea and also rust from the air/water/oxygen attack forces. To stop the corrosion (really galvanic corrosion), you need the zincs. To stop the rusting and tarnishing, you need to cut off the water and oxygen from the metal, hence the cleaning and wax coating. The sea is a nasty critter, chemically, and you have to put up a worthy fight!
Another thing about corrosion: If you're using shore power, don't leave the power on, in fact, you should disconnect the power completely whenever possible. Most all professionally installed and legal shore power systems connect the shore power ground to the boat's 12 volt ground system. This can set up some ver large electrical forces because of the electrical charges involved from bad grounds and voltage leaks from nearby boats and electrical equipment such as pumps, lighting, etc.
These leaking charge sources turn your boat's entire collection of metal parts into one big electrode ready to be dissolved. You can plug in and turn on the shore power when you need the power to recharge batteries, run the heater, etc., but leave it plugged in curing moorage only at the risk of dissolving a lot of your precious boat. The zincs will want to go first, but the rest of the motor and the metal part of the boat that's in the water will follow. And if the electrical forces are great enough, the process won't wait for the zincs to go first. Even the wires inside the boat in the salt air will turn white with corrosion and turn into dust. Ever seen a solderless fitting covered with white powder with most or all of the metal gone? Can you spell c-o-r-r-o-s-i-o-n ?
Sorry to be such an alarmist about the shore power issue! I know a lot of big boat owners leave the shore power plugged in all the time. But it does take it's toll, and I wouldn't let it have any part of my C-Dory I could keep for myself.
Joe.
