I'll continue the discussion of DC power systems begun last month . . .
On larger twin-engine boats, a tradition began in the seventies of having three battery banks, one to start each engine, plus one as a "ship's" or "house" battery. In addition, these boats often have a generator set, with yet another battery! It seems to me this is overkill, and wasteful. The reason I offer this conclusion is that modern engines, especially Diesels, start quickly, usually within fifteen seconds! So in the case just mentioned, we'd typically have two starting batteries working fifteen seconds per trip, plus another (ship's) battery that works all the time and carries all the rest of the load. Uniflites, especially, used this system with an isolator as discussed last time.
It is simple to re-arrange your battery wiring so that you have only one starting battery, and two batteries paralleled for more ship's battery power. For starting, in case you need more energy than just one starting battery is ably to supply, you can use a "parallel" switch or solenoid (often called a "boost" solenoid) that gangs together the starting and house batteries. I've done this, and added the continuous duty solenoid between the starting and ship's battery I mentioned in last week's column. It takes both engines running for this solenoid to close; until they both are running, all energy for the first engine to start goes into the starting battery. This means that as soon as both are running, the engines are charging fully both banks, immediately, until full.
(You should know that when batteries are paralleled as with this solenoid, the "bank" thus created is treated as one battery for charging purposes. At the same time, the physics of a lead-acid or gel-cell battery is such that if it one is not fully charged yet is parallel-connected to a somewhat-discharged battery, the lower one will come to full charge before the full one is over-charged!)
The total cost of this change is about $50, plus, perhaps, a new battery cable to the second engine. And if you have a genset, this arrangement is even more beneficial, since there is virtually no risk that you cannot start something to initiate charging.
If, as a Club member, you want to discuss this more, feel free to give me a call at 647-0288 or e-mail me at bycjoe@comcast.net. I'll be glad to give you background or fax you a sketched wiring diagram of the various alternatives.
Until next time, keep your batteries up, and safe and happy cruising!
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Last updated 8/27/97 by SCR