<Jim wrote: Our experience with big alternators is that they are far less efficient than an AC generator.>

But, but, I think the back end of a genset is an alternator not a generator.
A generator produces DC current/voltage and alternator produces AC
current/voltage......so the term AC genset is a bit of a misnomer.

I have used 45-55% as the efficiency for alternators unless they are
multipole, 12 or more, permanent magnet type which can run up in the 70-75%
range. Polarpower makes a unit like this. It can make a very efficient DC g.
However, we choose the Electrodyne for it's no nonsense construction, bullet
proof reliability, high output at low rotor rpm and ease of mounting on a
J-180 mount. I doubt that any AC genset is any better and I really haven't
found any DC generators that will put out 5 kw and mount as we
needed...perhaps they are out there hiding on the top of someone's old
flathead V8 Ford ;-) We used them to drive aircraft landing lights a long,
long time ago doing something that I really can't talk about!

Given that we have both DC loads and AC loads underway and they amount to a
collective total of say 4-5 KW on average 24/7. Using the main engine
alternator to deliver the current that the inverters need to drive the AC
portion of the load, which is never static but could be 8KW or more one
moment and 1 kw or less the next, allows the house bank to source the loads
on the inverters and thereby alternator.
If we used the AC genset for the AC loads which are varying it would be
loaded too lightly one moment and then perhaps run at 70% and then back at
10% etc all day when we're underway. Not good for the genset and really
wastes fuel when it is lightly loaded due to pumping losses.

Running a large alternator on an engine that is already running a constant
governed 1250 rpm say then alternator load really is just the load as the
pumping loses and other engine efficiencies are already pretty well
accounted for by the shaft load on it.

As far as the efficiency of a lightly loaded genset vs. one of an alternator
that is properly regulated and strapped to an already turning engine I would
leave that to the engineers that like to calculate that sort of thing.

We have a very accurate fuel burn system and I can watch the gph vary as I
turn the alternator on and off. You hear the engine take the load as the
governor advances the rack; the fuel burn will go from something like 4 gph
to 4.2 gph and the alternator is right up at 175 amps @ 28 volts or 5kw. The
Yanmar 3TNE28 uses within a teaspoon of .5 GPH at 5 kw. This is harder to
measure as the load is not as constant. However this is inline with the
specs on our 10kw unit.

Sooooo, from my vantage point I'd have to say that we are way ahead using
the big alternator thru the inverters to run the loads than we would be to
fire up the 10kw to run the same load pattern of AC/DC loads via the genset.

If we were to pull out the DC loads then the genset would even be more
lightly loaded and the efficiency would drop even more....I think.

So we're back to having a boat that runs everything underway off the main
alternator via the house bank and inverters. When the main engine is off the
house bank runs the inverters for the 120 volt loads and also run the DC
loads. If the house bank gets to 24 volts for more than 15 minutes the
genset auto starts and runs till it times out at 30 minutes of absorption
mode. This cycle can repeat itself indefinitely and does. How often the
genset starts depends totally on the amp hours used to bring the house bank
down to the 24 volt autostart threshold.

When we get back underway all we do is start the main, the alternator kicks
in and the genset will never start again till we shut down the main and the
batteries fall once again to the 24 volt auto start threshold.

No mess, no amp hour meters to monitor, no nothing...it is rock solid and
works....

The genset is protected to auto-shutdown for as sundry reasons if it has a
problem. If it does so then the battery low voltage alarm will sound at 24
volt if the genset fails in its start sequence and one needs to take manual
action.....start the main and fix the genset issue. Once repaired you are
back online.

Shore power is also automatic. If the inverters see shore power it takes
precedence and as long as its there the genset will not start. If you have a
shorepower failure the inverters kick in and draw from the batteries till
that ubiquitous 24 volt threshold is reached and then the genset autostarts
and will run till the batteries are at absorption plus the 30 minutes and
stop...as soon as shore power is restored it has precedence and takes over
again.

We also have load shedding breaker panels that automatically shed loads to
keep the inverter Ac load under 30 amps though they are rated at 33 amps.
These panels shed load in the order that I have specified. I do keep working
on the priority on each as Nancy was losing here hairdryer while the
coffeepot was on :-(  Damn I thought coffee was more important ;-) So now
the coffeepot is shut down if the before the hair dryer if required! Again
takes the issue of load management and puts it into a little box so you can
forget it. Turn on everything in the boat if you wish and it will managed
the loads to no more than 30 amps on each inverter.  BTW, this system is
common in the RV world and is the source of ours.

This is really a total electrical system that has turned out to be very
robust and hands free. We seldom know if the genset is running as it's in a
sound shield in a very well insulated engine room with a water lift muffler
feeding a gensep system.

We have other things that we like to do with our time...like plan the next
leg, get the charts (paper and electronic in order) and enjoy our
surroundings. Fussing with the batteries, charging regime was something that
we left behind in our sailboat days, thankfully :-)

Dave & Nancy
Swan Song
Roughwater 58
Caribbean Cruise '07

Jim wrote: Our experience with big alternators is that they are far less
efficient than an AC generator.

Reply:

Jim, can you say a little more about whatever experience Nordhavn has with
the relative efficiency of gen sets vs. big alternators? Is there any
quantitative data you can share that outlines why "an alternator may take as
much as three times the power (fuel) to produce electricity than a
generator"?

Dave wrote: Shore power is also automatic. If the inverters see shore power
it takes precedence.

Reply:

Dave, what is your reasoning for having potentially erratic shore power run
direct to your inverter rather than to a (cheaper?) charger? Your excellent
explanation in recent posts for why you like to draw off your batteries
rather than a gen set (varying loads will always get enough juice from the
batteries) would seem to apply to oftentimes limited or unreliable shore
power. And wouldn't using a shore-to-charger-to-battery-to-inverter system
let you use European shore power without a lot of appliance conversions?

Bob Frenier
Chelsea, VT

Bob Frenier emailed..

"wouldn't using a shore-to-charger-to-battery-to-inverter system  let you use European shore power without a lot of appliance conversions"

This is a great question that I'd like to know the answer of.  Two or Three years ago we were looking at a Nordhavn 47 and one of the sales guys said that by putting a "X" as the power was coming into the boat it allowed them to take in various forms of power and condition it to whatever the boat was using.  He acted like this was something common that they were doing with the Nordhavn's at that point.  Does anyone know what he was talking about??  We are possibly looking at upgrading to a Krogen 48 that we may use to travel to various ports and it would be nice to not worry about my power source so much if need be.

John Ford
KK44 Feisty Lady
Annapolis City Marina