First of all let's dispell the misinformation regarding just how much horsepower an alternator "pulls" from the engine. With NO losses one HP can theoretically deliver 62 Amps to a battery at 12.0 Volts. In the real world of belt losses, iron losses (from the stator and rotor of the alternator), diode losses, and copper losses one should figure a 50% efficiency for large frame alternators running fairly cool (less than 80 deg C). Small frame alternators are not as efficient, especially the "high output" ones which really push the flux density of the rotors in order to get the high output in the smallest volume. Small frame alternators get only around 40% efficiency at rated output.
So, in your case figure around a 2 HP engine load in order to get the 60A output at 12.0V. Your engine could easily drive a high output alternator yet you would want to install an "enable/disable" switch for emergency
full-ouput-to-propeller mode of operation or some means of limiting the alternator output when you need all that you can get for boat propulsion
Most "12V nominal" rated alternators are actually current
rated at or near 14V, not 12 so be famaliar with multiplying voltage times amperage in order to get a meaningful comparison in Watts when considering different voltages.
I am fundamentally opposed to using diode isolators in order to split an alternator's ouput and here's why. An alternator regulator
, whether external or internal, is capable of controlling only ONE output voltage period. If the regulator
is internal then you will never develop sufficient voltage to recover a deeply discharged battery. If the regulator is externally regulated then necessarily the regulation "sense" line would have to go to the house battery (the one potentially deeply discharged) in order to properly recover any lost
capacity with discharge/charge cycling.
If a diode isolator is used to separate the start-only start battery which is in very good condition then the fully charged start battery exhibits a very large capacitance and will not charge accept current
. It takes very little current through the diode isolator to charge that capacitor equivalent of the start battery and, as a result, the start battery can rise to as much as 16Volts while the house battery is at 14V or 14.4V (where you need to be in order to recover capacity). Yes, this differential is not the usually "assumed" 0.7V drop of one diode. That assumed number does not work
in this application. In fact the voltage drop of BOTH standard as well as Shottky diodes is between zero and 1 (or more) volts assuming direct current.
Now what we have in reality is BOTH direct current as well as a component of ac voltage variation on the alternator side of the diode isolator. The start battery charges its capacitor to the PEAK output voltage of the ac component that the alternator is capable of delivering. Meanwhile the house battery is stupidly happy pulling almost pure dc because it DOES charge accept current with not the same equivalent capacitance as the fully charged start battery.
About 18 years ago Powerline came up with the apparent bright idea of using two sets of internal diodes to give two isolated outputs from their high-output alternators. Their engineers failed to take into account that diode voltage difference, the equivalent circuit of the fully charged start battery, and the ac component of the alternator internal output. The result was a disaster to many people who literally cooked their start batteries
. Not everyone had this problem, especially those with "low quality" leaky start batteries
having constant loads.
One superior solution is to use a good quality battery combiner which can be programmed with voltages appropriate for your batteries.