What is it?

I need some help from trawler detectives on this. Boat is an 04 Mainship model 400. Went aboard today for a general check. Electrical humming sound heard, coming from the area behind the guest room closet. Shutting off the DC Main breaker did not stop it. Shutting off the 120v shorepower feed breaker did not stop it. Finally stopped when the aft shorepower breaker was shut off. So... It is 120v and electrically hot more than 1/2 way up the boat's port side without even cutting in shorepower feed. I removed a portion of the back wall panel in the closet to get at the sound. The object within is 15 1/2 in. long, 12 1/2 in. wide and 10 1/2 in. high, with a 1 1/4 in. spacer to keep it elevated from its horizontal shelf, white with no markings on any visible surface. I've had this boat for four years and never heard the hum before. This could well be chalked up to the fact that I am obtuse, but in my defense this was a particularly quiet and gentle day on the water. Believing that a prudent skipper knows his boats systems, I ask for your input on this. What is it?

Inverter of some sort? Where do the wires go?

Possibly a isolation transformer? We have one on our boat that I can hear humming sometimes.

Russell Clifton said:

Possibly a isolation transformer? We have one on our boat that I can hear humming sometimes.

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My guess as well.

Mainships were built with Charles Isolators. They are big transformers inside. There should be two of them one for each leg. My 2006 has them but I'm not sure if the 2004's did or not.

What is their function?

GFC said:

What is their function?

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They are an induction transformer that keeps your boat isolated from the shore power circuit. When you plug into shore power you are connected by the power lines to every other boat on the circuit. So if someone has a dissimilar metal problem or a ground problem you in essence have the effects of that same problem. With an AC Isolator Transformer you are not connected in any way or form to the shore power. Your AC Isolator is. The shore power and all of it's potential problems goes into your Isolator and through a transformer that then creates clean power it goes to your boat.

As an example I have two one for each 120V leg. In two years at my marina and of course every other marina that I have visited I have only just changed out my Zincs on the outside of the boat. Several boat around me go through them at least twice or three times a year.

I won't have a boat with out them. They are really not that expensive when measured in boat bucks. About $900 for each of the two that I have at 40 AMPS each. They look like they would be easy to add to any boat that did not have them.

Your Sea Ray may. My friends 2005 50' Sundancer does, it came from SeaRay with them.

Marine: Isolation and Boosting Transformers

Thanks guys for your help on this. I think Vinny nailed it. I was talking with the local dock gurus at The Bean this morning, and they also seemed to think it was some sort of isolation transformer with a relay drawing perhaps one amp which causes the humming.

ColonyCove said:

Thanks guys for your help on this. I think Vinny nailed it. I was talking with the local dock gurus at The Bean this morning, and they also seemed to think it was some sort of isolation transformer with a relay drawing perhaps one amp which causes the humming.

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I doubt an isolation transformer would have a relay.

Somewhere, there has to be a label, perhaps where you can't see it. The other hint would be the wiring. What type of wire, the colors, and where do the wires go.

Transformers sometimes hum.

The hum is normal for a transformer. Some are nearly inaudible, some are noisy as the dickens although that usually means a problem yet they can go like that for years.

Hums can be amplified depending on how they are mounted. A transformer that would normally be considered quiet could cause an audible hum if it is mounted solidly to the structure. The structure becomes a conduit and an amplifier of the hum.

Check the mounting and see if there is room to use rubber mounts and straps. It must be well and securely mounted as they are heavy for their size, you don't want it coming loose.

C lectric said:

The hum is normal for a transformer. Some are nearly inaudible, some are noisy as the dickens although that usually means a problem yet they can go like that for years.

Hums can be amplified depending on how they are mounted. A transformer that would normally be considered quiet could cause an audible hum if it is mounted solidly to the structure. The structure becomes a conduit and an amplifier of the hum.

Check the mounting and see if there is room to use rubber mounts and straps. It must be well and securely mounted as they are heavy for their size, you don't want it coming loose.

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Thanks. The noise is not so objectionable that I'm going to struggle to attenuate it. It is mounted securely, and that's how it will stay. I was just interested in what the darn thing was.