There are several ways to deal with 240 Volt power on a 110 Volt boat. The simplest is to buy a 220 Volt battery charger to charge your 12 Volt batteries, and draw your 110V from your inverter as usual.
The best solution is a 240V/120V step down isolation transformer. This isolates the boat electrically from the shore power and helps to reduce spikes in the power.
The disadvantage of an isolation transformer with primary and secondary windings is the physical size, weight and cost needed to provide 3000-5000 VA depending on your current requirements.
On Just Imagine, we used a small 3000VA autotransformer which is much lighter at about 40 lbs (20Kg) and only cost about $150. The auto transformer only uses a primary winding which is center tapped to feed off 120 Volts.(see diagram below).
The disadvantage of this is that the power on the boat is not electrically separated from the shore power. The dock power outlet wiring can vary from marina to marina. When connecting shore power to the transformer, you need to measure the voltage between the neutral and ground on the output of the transformer before connecting to the boat. This should be zero, or just a few volts. With the 2 prong connector, you have a 50/50 chance of getting it wrong when you will have 220V between neutral and ground. To correct this you simply need to remove and reverse the 2 pin (+ ground) plug on the input to the transformer, and then measure your voltages again.
On Just Imagine, the transformer went to the Prosine 2.0 Inverter charger, and then to the electrical panel on the boat. The Prosine would go into an alarm condition if I got it wrong, but I preferred to measure with a volt meter before connecting to the boat.
You can also buy little 110V US test plugs with lights that show green if you have it right.Howard and Jayne [ Just Imagine ] 28-Jan-2008