DUUUUUG said:
Would someone mind explaining what a zinc does?
I assume it is corrosion related since people seem to agree that zincs are only required if the boat is always/stored in the water.
DUUUUG,
First, zincs are only really required when the boat is kept in saltwater. The reason for this is that salt water conducts electrical current very well due to the dissolved materials suspended in it. Fresh water does not conduct electrical current as well, therefore anodes are not required or can be of a different material than zinc such as magnesium.
What most people refer to as "zincs" are also called sacrificial anodes. Sacrificial is the key word.
Metals are classified by the "galvanic scale" (sometimes called the nobility scale). Simply put this means how quickly they are dissolved by electrical current in a conductive environment such as salt water.
Here is the scale;
At the top are magnesium and zinc, which are relatively easily attacked by the current, nearer the bottom is 304 Stainless Steel (that's what Trim Tabs are made out of) which is pretty resistant to it.
But left on its own stainless will be attacked by the current in saltwater and can be damaged. This damage can be accelerated by "stray current" in the water. Stray current can come from the boat if a piece of electrical equipment is improperly grounded, from a neighboring boat, or an improperly wired dock.
So, to protect the stainless (or other metal such as the aluminum in an outdrive or outboard lower unit) sacrificial anodes of a material more prone (or less noble) are attached to it. If there is a good electrical connection between the sacrificial anode and the metal it is protecting the anode will attract the current and be attacked first (sacrificed). As long as there enough mass in the anode and the electrical connection remains intact it will dissolve and not the more noble metal it is attached to.
This is a simplified explanation, but I think it should help you understand the idea.
As I mentioned in an earlier post , if the boat is not kept in saltwater there is usually no reason for zinc anodes to be fitted on the trim tabs. Some boats kept in fresh water will fit aluminum or magnesium anodes to the Trim Tabs as protection, but in my experience it amounts to just extra insurance.
I did not put zincs on my boat's Trim tabs since it was kept on a trailer and at most spent a couple of days in salt water. But I never kept it at a dock with other boats or with electrical service. Damage to the Trim Tabs can happen if the boat is exposed to stray current from a nearby boat or dock in a very short time depending on its severity (in some cases only a few days). In this instance zinc anodes would protect them.
I hope this is helpful,
Tom
Bennett Marine
