Will a 100 amp alt require more horsepower to operate than a 70 amp? Are there there any disadvantages to using a one wire (self exciting) alt?

Thank You
Rob

Yes , But I can't tell you how much. Basically, think of it as a torque converter and the electricity is transmission fluid. Alternators go into an electrical torque stall, which puts a load on the engine and drive belt . A 100 amp alternator may need to be revved higher than the 70 amp unit to produce adequate output. It may not have enough output at a prolonged idle. I feel this would be especially true of a one wire alternator that does not have the same field winding excitation as a 3 wire unit. Do you really need the 100 amp unit ? You could however swap pulleys to spin it up faster if you do alot of idling. BBB

Only when a full 100amps is required. Otherwise there is no difference. A boat should only need a very small alternator, say 30a.

What cord said, except I think the standard merc is around 60amps.

I use marinized one wire on mine. Mine are 60 amp and have no problem keeping up with the load. I also have custom pulleys to slow them down. I have no charging under 1500 rpm. I have a large electrical load I use electric water pumps on my motors along with other items. If a one wire goes bad ,theyre easy to repair or replace at a fraction of the cost of the merc junk. Brian