Originally Posted by trekker
My next question.......What are some of the things to consider(pro or con) when looking at saltwater boats, as compared to Fresh water?
For boats used in saltwater you will want a freshwater (aka coolant) cooled engine. As opposed to a raw water (aka the water the boat's sitting in) cooled engine.
This is one reason why the diesels in so many of our boats last so long. The coolant circulating in the engine is proper coolant, like your car. So it fights corrosion and does a bunch of other things, all of them good.
The raw water (saltwater) never gets anywhere near the inside of the engine at all.
A raw water cooled engine in saltwater (like most outboards) is susceptible to corrosion and gradual interior deterioration unless it is religiously flushed out with fresh water. Easy to do with an outboard, not so much with a marine diesel.
Since there is no airflow in the engine room a radiator's not going to do much good so heat exhangers are used instead. Raw water is taken in with a pump mounted on the engine and passed through the heat exhanger. The engine's coolant is also passed through the heat exchanger in its own set of tubes and the result is the raw water absorbs the excess heat from the coolant.
There are three basic methods used for a closed or fresh water (coolant) engine cooling system. One is to pump raw water through a heat exchanger as described and then dump the now-hot raw water into the exhaust where it can be blown out of the boat by the exhaust. This is the most common system used in recreational boats.
The second is with a so-called dry-stack exhaust system which still uses a heat exchanger but the water exiting the exchanger is simply dumped overboard, usually out of the side of the boat in the vicinity of the engine room. The exhaust has no water injected into it at all. Lots of commercial fishboats are like this with their dry exhaust outlets up above the boat.
The third way is called keel cooling. The nice thing about this is there is no raw water pump or internal heat exchanger system at all. The engine coolant is circulated through a multi-folded tube that is mounted on the exterior bottom of the hull, either in a recess for this purpose or in some other way so it is not easily susceptible to damage.
So it's still a heat exchanger but in this case the raw water is the water the boat is moving through, not raw water being pumped up into the engine room. The exhaust in this case can be a dry-stack or in some setups there is still a raw water pump but its sole purpose is to pump cooling water into the exhaust. This last seems a little self-defeating since the great thing about keel cooling is you don't need a raw water pump and plumbing but you've put them on anyway to cool the exhaust.
You also want to make sure the boat has a bonding system that is designed for saltwater. This has more to do with the type of anodes are mounted on the hull, rudder, etc. than with the internal bonding wiring itself but it's not a subject I know much about other than what kind of anodes our boat needs.