I've have ancient cube heater that I bought when they first came out, probably early 90's. Pretty safe, doesn't get real hot on the surface anywhere, even in front. However, even the newest electric heaters are unlikely to provide more than 1500 watts/5000 btu's. For my boat, that works OK until the temperature gets into the low 40's or 30's, then it gets pretty cold. By comparison, my Espar diesel heater is 16,000 btu's. That'll keep the boat toasty in the 20's.

The electric heater isn't likely to blow the breaker at around 12-15 amps, but when combined with the water heater and battery charger, you may be getting closer.

Seems like a crazy question, at least viewed from the arctic up here. My boat is in Milwaukee. It's 20 degrees, blowing 15 to 20, and snowing. The boat is on the hard, behind barbed wire fence, and covered in shrink wrap.

The idea of getting AC power to the boat and keeping it above ambient temperature with a heater sounds almost like tropical luxury from where I'm sitting. Water without a layer of ice on top and sailing in general seems like a distant memory from a prior life.

I gotta move south.

[QUOTE=plaineolde;16662]The electric heater isn't likely to blow the breaker at around 12-15 amps, but when combined with the water heater and battery charger, you may be getting closer.[/QUOTE]

I run a 1500w heater in winter when it gets cold. It does trip the circuit breaker when the battery charger and hot water heater are on. Of course it only gets done to about 5C - so not cold enough for ice or snow.

Am thinking of a diesel heater so we can warm up while away from jetty.

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Speaking of cringing I cringe anytime I hear owners trying to run 1500W heaters on-board.....

Many boats of this vintage were poorly wired, as AC and DC is concerned, Catalina included. Most all of these boats use a single 15A circuit breaker to power every outlet on the boat. This means the last outlet in the chain is depending on all the connections before it to work properly. I have seen as many as NINE outlets on one 15A breaker, all daisy chained....:eek: This is one thing in a dry home that does not move, vibrate or hoover at 90% RH continuously. It is another ball of wax entirely when you do this on a boat.

The MAX I like to see on any 30A shore power cord is 80% or 24A but 70% or 21 amps is even safer. This is electrical code on the mainland, that breakers not be loaded to any more than 80% of the rating. We also can't forget the loads of a water heater, 1200W - 1500W, a battery charger and any other AC sources that need to be added to a 1500W heater.

A single 1500W heater on a 15A circuit can pull about 13.6A. We need to consider that many marinas often have low voltage when doing any of these calculations. To be safe I prefer to use 110V as opposed to 120V..

So 1500W / 110V = 13.6A. We can already see that a single heater, running on high, on a 15A circuit is beyond the 80% "safe loading" and is pushing 91% loading of the circuit.. 12A max on a 15A circuit....

Running a space heater, on high, on old wiring can be a recipe for burning down a marina, and I have not even added and water heater, battery charger, microwave, hair dryer or coffee maker. At least two of the images below were done in the winter and caused by electrics SPACE HEATERS and the high resistance circuits often found on boats......

I see LOTS of stuff melted down by overloading a boats AC system. Breakers do not always trip at "face value" due to trip delay curves.. Any resistance in the terminations or circuit, like we get on boats, can heat a wire beyond the melting point well before a breaker trips. This is how boat fires start. Boats in general are LOADED with high resistance connections due to the corrosive environment we are in and an utter lack of maintenance of these critical circuits.. We all talk about re-bedding, standing rigging, running rigging, fuel filters but when was the last time anyone re-terminated the AC & DC terminations and replaced the AC inlet, outlets or breakers..???? These are all maintenance items when it comes to boats yet I see 40 year old vessels with the same shore power inlet & terminations it left the factory with....

If you want to add a heater a second 30A service to the vessel with its own main breaker and branch circuit for more outlets can be the safest approach.. Alternatively make sure every connection is clean, tight and corrosion free and turn off all other AC loads when running the heater on high and ONLY do so on a 20A circuit with 12GA wire and 20A outlets. Also pug it into the FIRST outlet on the circuit to avoid passing high currents through the daisy chain.. Many add a second service to run air conditioning but you can, & ideally should, install them for boats that winter and heat with electric too.

Drawing too much current is dangerous ESPECIALLY WITH OLD WIRING, ON OLD BOATS!!! Remember please don't load circuits, including the 30A shore cord, to more than 80% (this comes from land based codes as well). Just a single electric heater on high and a water heater can put you over the safe loading of the shore cord..

On a boat I prefer to not see them loaded beyond 70% due to the potential for corrosion/resistance and the fact that we use a CRAPPY and "marginally safe" standard for our shore cords..

[IMG]http://www.pbase.com/mainecruising/image/147215196.jpg[/IMG]
[IMG]http://www.pbase.com/mainecruising/image/147215197.jpg[/IMG]
[IMG]http://www.pbase.com/mainecruising/image/144339208.jpg[/IMG]

If replacing a shore power cord there is only ONE power cord to buy and that is the Smart Plug system. Some insurance companies even give a discount if you install a Smart Plug cord set and inlet..

The NEMA/Marinco/Hubbel plug standard SUCKS in the marine environment and causes nothing but problems.. As a marine electrician I would not be sad to see them outright banned and replaced with the Smart Cord standard or an equivalent quality plug system designed for the marine environment not just "adapted" to "work".....

If replacing a shore power cord please do yourself a favor and spend your money wisely, on the Smart Plug, This way at least one end of the cord will be known to be good, the boat end.....

BTW safely loaded shore power cords should NOT be "warm to the touch"! If they are, you have problems that need to be addressed.. I hear this repeated as an okay thing all the time. It is NOT okay for the shore cord or any AC wire/cord to be warm to the touch..

Sorry for the rant but this stuff needs to be mentioned...

On my Mk II, for winter protection from freezing here in the Pacific Northwest, I have a 90 watt 'can' (the ones with a small fan in them) in each cabin, and they run continuously throughout the cold season. I have a 1500 watt West Marine heater with a thermostat in it which I keep on the S/S stove top, not on a flammable surface. I keep the thermostat set quite low, so that the heater only comes on if the indoor temp gets down to about 40F. To get that cold inside, the outside air temp would have to be in the mid-30's or colder.

Here in Portland, Oregon, we've had only a couple days this winter that were below freezing. I'm hoping we've seen the end of it for this season. Crocuses are already blooming and the daffodils, camellias and magnolias will be right behind.

I have a 1500W heater BUT I only ever run it on the 750W setting. It also has the switch to turn itself off if it capsizes. We only occasionally get below zero C but we get a lot of water, I typically run a dehumidifier year round also.

[QUOTE=Nimue;16672]I have a 1500W heater BUT I only ever run it on the 750W setting. It also has the switch to turn itself off if it capsizes. We only occasionally get below zero C but we get a lot of water, I typically run a dehumidifier year round also.[/QUOTE]

750W is a good practice it is the high setting where I see trouble......

To Mainsail's point; on my C30, I used 2 shore power cords, joined by a water tight connector. I found that the electrical connections had pretty much welded themselves together, were blackened and somewhat melted. This got my attention. On my C36, I keep a pretty close eye on the shore power contacts and clean them regularly with fine emory cloth to get the best, lowest resistance connection I can. A Smart Plug upgrade is due this spring. I'm installing an A/C system and refuse to depend on that old style connector.

On that topic, I see some boats have 2 shore power connectors to the boat, running to a Y-connector on the dock. Other than providing a dedicated line for the A/C, I don't really see the advantage to this, as you are still drawing from the same old style connector on the dock on a 30 amp circuit. True a fire would not be on the boat, which is a plus, but is there some other advantage I'm missing?

Hey what about a vented solid fuel heater. I had one in a 25' boat I had and used briquettes banked and shut the draft down and it burned all night.

I use a propane "gas buddy" today with a ceramic plate. Keep it under the table to slow the rising heat. Does a nice job on the cool damp days.

A 1500W heater in Australia only draws 6amps (1500/240). But on our 375, we have separate circuit breakers for the water heater, charger and the port and starboard outlets.
Once the cabin warms up, the heater is set to low, and the thermostat turns it on and off as required.

I suppose 5C (41F) is not that cold.

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I love the photos of connectors that have self destructed. Electrical failures of that nature are a positive feedback loop. A bit of oxidation creates resistance, resistance creates heat, heat creates oxidation, and the loop cascades into catastrophic failure in short order.

Warm connections are a clear indication, but the most dangerous conditions are hidden or buried connections that go in to failure.

One easy test is to watch voltage. Check the voltage at the plug where the heater is when it's off and when it's on. There will likely be a 5% drop in voltage. If you see a 10% drop or more, your circuit is not suited to the current and you need to back off. The problem could be distributed heating over a really long line (probably safe), or it could be heating concentrated at a faulty connector (really bad) as shown in the melt-down photos. Either way, the conservative approach is to make sure that your load does not impact the line voltage. Too much voltage drop is an indication of danger.

I wrote an article that's in the Technical - Maintenance library about replacing the boat's shore power connector. Just an FYI.

Yes, I will admit to being a northern Califonia weenie. The high thirties is very uncomfortable aboard. The cabin top sucks up the heat and feels like it radiates cold. This is not possible thermally but, that how it feels. The bare fiberglass is an effective heat sink.

My little West Marine heater does not keep up well , if input it on the floor in front of entry of the fore cabin it is more comfortable. Most times I keep it on The floor under the starboard setee. I have a fan near the port forward bulkhead, I find if I turn that on facing down pumping the heat back down that rises to the top of the cabin the main cabin stay more comfortable.

I have two of these:

[url]http://www.amazon.com/Vornado-VH110-Whole-Vortex-Heater/dp/B005JB1M16/re...

They work wonderfully. I run them at their *lowest* setting, which is 750 watts each. Unlike other heaters, though, you can run them continuously (fan always on) and not have the heating element run continuously (it cycles heat on and off).

These are slightly better model with a larger fan (I used to have one of these but ended up taking it elsewhere):

[url]http://www.amazon.com/Vornado-Vortex-Automatic-Climate-Control/dp/B000GL...