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Dan Corcoran (b393capt)

Isolation Transformers

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I read that Isolation transformers provide extra protection against galvanic corrosion, specifically AC induced corrosion caused by nearby boats, over and above galvanic isolators, but they seem to weigh 65lbs or more. A little to much for a 35 foot boat.

Is there a version of this that simply mounts at the dock, rather than gets installed on boats?

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  • Dan,

    An alternative is to not distribute the AC shore power directly, but only have a (modern power switching) charger connected to the shore power, and connect all on-board AC consumers to an inverter only.

  • I had not thought of that, tell me more. How to recognize from it's specs that a charger is capable of this, and doesn't connect DC ground to AC ground? Do combined charger/inverter do the same thing? Is there a good way to take an appliance running on the inverter unattended, like a dehumidifier, and have the inverter shut off its load if the charger looses power so that the battery is not drained?

  • 1) There is no such thing as DC ground, there is DC negative. Any charger connecting the two is not worthy of being on your boat. That said there will be crap out there that does this. I'd stick to a quality brand anyway, if not just for the fact that you don't want this stuff to catch fire and burn up your boat.

    2) I dunno, they will probably connect AC ground on the input side to AC ground on the output side. You don't want this. My incoming AC ground is not connected to anything on the boat other than the charger case. AC ground on the inverter is connected to the (metal) hull. In your case it would just be connected to the ground line in your AC distribution system.

    I use a separate Mastervolt charger and inverter. The inverter can be told (through a PC/Masterbus interface and a software program) to recognise certain alarms and take particular actions. I'd have to check but I am pretty sure you can have it switch off completely at a low voltage.

  • This brings me to something that I have been wondering about for some time. My ship, ex British navy, timber build, has a grounding strap going round the whole boat. Everything is connected to it. My batteries are connected to the propshaft through the engine. All the skin fittings are Aluminium Bronze, and appear as solid as the day they were made. The onl degradation is to brass screws and one propellor. That one is Manganese Bronze and severely degraded. I suspect that the prop was replaced (with a cheap one) following a collision with a submarine (I know the ship was in a collision). I have no shore power and use solar PV panels plus a generator for top up and power tools. I believe that my AC ground should be connected to the ship's ground, and that DC negative should ONLY be connected to the engines. Would people recommend that I completely disconnect the engines when they are not being used, to eliminate an possibility of a ground-loop between the engines ?
    As for isilation transformers, the size would depend on how much AC power you need. If it is only a minimal requirement and your shore power is 110V, you could use a regular transformer that has twin 110V primaries (very common in Europe).

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