Alternator Installation
With almost no exceptions1, the alternator that our engine came with was specified and installed to just keep the starter battery charged and so will be totally inadequate to supply even a part of the electrical needs of a modern cruising boat.
But a well-specified and properly installed upgraded alternator can supply a substantial part of a cruising boat’s electrical needs by charging the batteries when the engine is being run anyway for propulsion, as well as providing backup for a separate generator and stepping in for the solar panels when the sun don’t shine—what’s not to like?
For example, on our McCurdy and Rhodes 56, typically better than half of our normal usage would be provided by the alternator while motoring in and out of anchorages and/or harbours.
Of course, in these days of ever-better and less-expensive solar panels, many cruisers will wonder if upgrading the engine alternator is even worth it?
The answer is a resounding yes on most-all cruising boats:
- Large solar arrays:
- Add weight and drag and therefore negatively affect both boat performance and stability to a much higher level than many believe.
- Often make sail handling difficult, and even dangerous.
- Make the boat more susceptible to storm damage.
- Wind generators are often even worse, and are noisy, too.
To be clear, I’m not suggesting solar arrays are a bad idea—though I do think wind is over for most of us—but I am saying that the most functional cruising boat electrical systems use a combination of sensibly sized solar, a good alternator, and, for boats with large electrical needs, a stand-alone generator.
So let’s look at what a real cruiser’s alternator is, and how to install it properly.
Hi John,
Lots of good tips here.
Regarding #14 on Airflow, I don’t like the idea of using the engine intake to suck hot air from around the alternator. If done well, it may cool the alternator just fine. The problem is that you are putting hot air into your engine which is less dense. From a combustion standpoint, you talk about massflow of air not volume flow of air as massflow determines how much fuel can be burned. Hot air will significantly reduce engine output at high rpm. There is a reason why engine builders talk about cold air intakes a ton. This not a purely academic thing, when I designed air compressors, one of our biggest issues was fighting intake pre-heating as it essentially lowered the output and could do it by 30% if we were not careful.
The engine is a big air pump so much of the cooling in a lot of engine rooms is actually due to the engine pulling hot air out. This isn’t great but if you add the extra heat of a big alternator, it gets to the point where you absolutely do not want to send all that heat through the engine.
Another thought on #7 on only having 1 alternator is that in most applications, the second alternator outputs extremely little. The actual energy to start an engine is not very much. Assuming you are still using a lead acid start battery, it should still be at an SOC in the high 90%’s after a start which means its ability to accept charge is very limited. So you might hang the parasitics of a 50A alternator on the engine all the time to only pull a few Ah off of it each start, you might as well send that over a DC to DC battery charger.
Eric
Some engines include factory provisions for second alternator mounts or even factory-installed second alternator options with integrated wiring harnesses, control panel annunciators, etc. How much of your negative opinion of second alternators remains in those cases?