"Michael Briggs" wrote in
:

Have plenty
of extra hp to run the genset, only need aprox 85 hp to run boat at
cruise speed.

It's not the power that's the problem, it's the FREQUENCY. A 4-pole
alternator needs to turn a CONSTANT 1800 RPM shaft speed to produce 60 Hz
AC power (1500 RPM is 50 Hz). Considering the pulley sizes for the
ratio, then engine would be turning less.

However, how are you going to run the big diesel at, say, 1800 RPM to get
60 Hz to run the motors on AC, Air conditioning compressors, fridge and
freezer compressors who all demand fairly stable frequency....and back
the boat in the slip...slowly...no changes in engine speed allowed, like
ALL BACK EMERGENCY OR WE'RE ALL GONNA CRASH!...???

Then there's the fuel at $4/gallon. The big diesel anchored out for the
night has to sit there and turn 1800 RPM at 30HP, all 7 litres of
displacement, 24/7 at anchor, or drifting or any time you don't need
propulsion.

Then there's maintenance. The big diesel has an hour lifespan limit at
some point. The faster you get there, running this stupid main engine
genset, the faster the engine shop will own your savings accounts to do
the overhaul. Most of the 9000 hours on the meter was pulling the genny
to drive the fridge in the galley and a few lights. This is insane!

That's why 65' boats all have one...or (gasp) maybe TWO...separately-
powered gensets with little engines puffing away at 2/3 of a gallon per
hour all night doing what you need.

Get a full genset. This main-engine-driven-AC-powerplant is economic
insanity.....

On Sat, 04 Mar 2006 23:44:13 -0500, Larry wrote:

Get a full genset. This main-engine-driven-AC-powerplant is economic
insanity.....

Not necessarily. If you already have one or more gensets for use at
anchor, and you spend a lot of time underway, it might make sense to
have a way to generate power from the main engines. The hydraulic
equipment can take care of speed regulation totally independent of
main engine speed, no reason to run it at 1800 RPM, all it's doing is
driving a hydraulic pump. It might be more cost effective however to
drive a large alternator from the main engine, either mechanically or
hydraulically, and use the DC output to power a large inverter. That
eliminates the complexity of hydraulic speed regulation.