I am building a Cape George 38 sailboat. The large house bank is about 750
Amp/Hours at 24 volts. I also have a smaller 12 volt house battery and a 12
volt engine starting battery. I don't believe you need a very large vessel
to take advantage of a 24 volt system.

I have found the following items for 24 volt systems.

Large frame 140 amp secondary alternator
External regulator
4000 watt inverter / charger
Refrigeration and air conditioning system
Hydronic diesel furnace
Fans for heating and air conditioning systems
Radar
Chart plotter
All interior and exterior lighting
Lighting dimmers
Battery volt and amp meters
Anchor windlass
Halyard winch
Autopilot control system
Autopilot steering motor
Electric flush toilet
Sewage treatment system control
Bilge, pressure water, and fuel transfer pumps
Bilge and gray water pump controllers
Cabin fans
Bilge and exhaust fans
Tank gauges
Automatic self-cleaning raw water strainer
Water maker

Items only available in 12 volt, that I know of, are these:

Automotive style stereo
VHF radio
HF SSB radio
Propane control panel
Instrument system
Engine starting system for 51 HP Yanmar

Even the instrument system could be powered from the 24 volt autopilot
control system. It has an output for that.

Anything with a motor will run cooler, and last longer, on 24 volts as
opposed to 12 volts. Any item, at the same wattage, will use half the
current at 24 volts as at 12 volts. Since power loss in your wiring, and
heat in your motors, is based on the square of the current, the loss at 24
volts will be 1/4 the loss at 12 volts. You can take advantage of that by
having a much more efficient system with smaller wiring. And smaller wiring
is cheaper and lighter.

I have heard the auto makers are thinking about going to a nominal 48 volt
system. The higher the voltage, the greater the efficiency. This is why
industrial motors are 480 volt three-phase, high speed trains draw 12,500
volts or more from overhead lines, and long distance power transmission
lines run up to one million volts.

A down side is setting up a large 24 volt battery system. I have 12 Trojan
industrial cells in the bilge. Another good method is to use four of the
Rolls 6 volt batteries or three of their 8 volt batteries. A 24 volt system
with all the cells in series is going to be a lot easier to maintain than a
large 12 volt system with batteries in some combination of series and
parallel.

All Ancor marine wiring is rated for 600 volt service, so using it for 24
volts is no problem. Most 12 volt switches also have a 24 volt rating,
although it might be a few amps lower.

An interesting note: The Blue Sea brand DC light dimmers are rated by
current draw. So you can control twice the wattage at 24 volts as you can at
12 volts, with the same dimmer.

Rusty O'Shaughnessy
S/V Lorgadoir Eolais