One horsepower equals 746 watts.  A perfectly efficient alternator would require one horsepower of mechanical input to produce 746 watts, but since there are friction and eddy current losses it takes
more than one hp to produce 746 watts.  Dividing 746 by the 13.8 volts that most regulators are set up for, one perfectly converted hp would equal about 54 amps.  So a 55 amp alternator that is actually
putting out 55 amps will reduce the engine's output by something more than one hp.  But as the charge is restored to the battery, the current goes down until it the battery is fully charged and the alternator supplies only the minimal current needed to run the instrumentation and spark a gas engine.  At that point the engine is able to put out its rated power.  If you now turn on the stereo/GPS/depth sounder/cabin fan etc, the alternator must supply the current for those consumers, so the hp at the shaft will go down. The advantage of a larger alternator is that it recharges the battery faster (or a bigger one in the same time).

One horsepower equals 746 watts.  A perfectly efficient alternator would require one horsepower of mechanical input to produce 746 watts, but since there are friction and eddy current losses it takes
more than one hp to produce 746 watts.  Dividing 746 by the 13.8 volts that most regulators are set up for, one perfectly converted hp would equal about 54 amps.  So a 55 amp alternator that is actually
putting out 55 amps will reduce the engine's output by something more than one hp.  But as the charge is restored to the battery, the current goes down until it the battery is fully charged and the alternator supplies only the minimal current needed to run the instrumentation and spark a gas engine.  At that point the engine is able to put out its rated power.  If you now turn on the stereo/GPS/depth sounder/cabin fan etc, the alternator must supply the current for those consumers, so the hp at the shaft will go down. The advantage of a larger alternator is that it recharges the battery faster (or a bigger one in the same time).

Just wanted to insert a change I made to my alternator circuit that may be of value to others.

When I installed the alternator, I put a 50 amp meter in the output leg to measure the charging current.  I was surprised to find that my 50 amp alternator rarely charged the batteries above 10 amps for more than a few minutes.  Then it would decrease to 5 or 8 amps depending on the number of
amp-hours I had drawn out of the battery on the previous day's sail.  I normally use 10 amp-hours at night.

At 5 amps it would take 2 hours to put the 10 amp-hours back into the batteries.  I did not like listening to the engine, so I found a way to get more current out of the alternator.

I ran an extra line from the +12V side of the battery through a 20 ohm 50 watt rheostat directly into the alternator's field windings.  That's where the voltage regulator also connects to the alternator.  Then I could adjust the rheostat and crank up the current to 10 amps.  I don't think it made much
difference to the batteries, since they didn't bubble (hydrogen gas) or get warm.  Instead, of charging them at 60 watts for 2 hours, I was able to charge them at 120 watts in half the time.