25 amps electric heater, 15 amps alternator.

[Tonyr]

I would like to run a standard 300 watt (12 volt) fan driven space heater (sold for RVs and trucks, normally) on a boat with a motor that generates some 15 amps spare when the engine is run with no electronics switched on.

The concept here is that we need short term heat when we get up on a cold morning, probably for betweeen 30 and 45 minutes only, and while at anchor only.

I don't want to go to the cost and inconvenience of installing a "proper" heater if I can do it another way, and using the generated 15 amps plus running down a large deep cycle battery for another 10 more to give 25 total sounds arithmetically credible. The battery is my present house battery, charged by a solar panel, and would be (temporarily) hitched up to the motor. When we want to up anchor, I would switch everything off, connect up the usual starter battery, and let the solar panel take as long as it wants to recharge the deep cycle one.

Would this work? Any practical electricians out there to suggest better ways to achieve what we are aiming for?

Thanks, Tony.

[paladin]

Tony...it should work...what is the amp hour capacity of your house/engine battery.....? it would be a situation perhaps of starting the engine first and then the heater.....what is the data plate reading for volts...amps.....

[Tom Lathrop]

300 watts is a very puny heater, assuming that you are talking about ordinary resistance heating. In New Brunswick, I can't see how you would hardly know it is on and it would zap your battery very quickly.

There must be several other heat sources that would be a better choice. Even an Aladin kerosene lamp probably puts out more BTU's. I have used one on mild evenings and mornings in a 25 footer but that was in the south.

[Tonyr]

300 watts is pretty puny, I agree, but its a small boat (24 foot), and not really cold outside! Our present method is to put a cast iron frypan (empty) on the alcohol cook stove (Origo). Works OK, but produces more H2O than we like. I was thinking that the combination of the two methods might work.

Probably a simple way to go might be to add another marine starter battery (thus producing a battery "bank") and be prepared to run one down while heating the cabin, while temporarily disconnecting the other.

Incidentally, did you get the private message I sent you some months ago about the T50 fuel consumption?

Tony.

[RonW]

Hey Tony, you should consider a dual purpose battery, one which was designed for starting and to be drawed down as in deep cycle.
A 70 lb battery with a 25 amp draw will last for about 3 hours till it's dead. You can use 2 batteries, hooked together in parralel to double the amperage-( positive to posite & negative to negative)
Or jump up to a 4d battery.
http://www.trojan-battery.com/Products/Marine.aspx
The battery charts show a draw down chart by amperage.
You also can also start up that 4 stroke and let it idle, it's a different animal then a 2 stroke. It will only put out a few amps at idle though.
It will probably take 5,000 r.p.m.'s before that alternator is putting out the actual 15 amps.

--What did you ever do on the pitch of the prop, how much did you go, and what effect did it have?

[Tonyr]

I eventually bought a 4*13.5*15 pitch composite propeller from Comprop. It transformed the efficiency! I also bought a digital weighing machine (bathroom scales) and measured my fuel consumption with +- .2 pounds accuracy over at least one hour at a series of constant revs runs.

To cut a long story short, I am able to get less than .5/gal (Imperial) an hour at displacement hull cruising speed (say 6 knots in my case), which means that I can get around 12 mpg. Allowing for weather, wind, waves, and normal accidents, this lets me plan a cruise using 10 mpg for safety, and know that the figure will be realistic.

I have (as part of the fall post haulout tweaking) ordered a 12 gallon permanent tank, which with the two existing 6 gallon tanks will give me a nominal range of some 200 plus miles. If more is needed, I can get another 100 miles or so by taking along an extra two 5 gallon portable tanks.

We like the "shippy" feel of the boat with about 500 pounds ballast, and have played with its location to get the pitch trim and general boat behaviour satisfactory. I have also moved a heavy battery and the water tank forward.

All in all, an interesting first year as a power boat owner.

Tony.

[Bill Perkins]

I've wondered about using an electric heater too . My alternator puts out 25 amps . Let's say I get a heater rated at 20 amps . I won't be sleeping aboard ,so typically the engine will be running at cruising speed when the heater is on . It seems there would be allot of chemical activity in a battery being loaded with 20 amps while being charged with 25 amp . I envision it boiling like a soup pot ; is this a misconception ( I now gather that a useable heater will be drawing more than 20 amps )? My question is , would it significantly extend my batteries life if I switched on "both " when using the heater when underway? Then two batteries would be available to soak up all that electro-chemical activity . At rest or with the engine idling I'd switch to a single battery .

I'm also wondering if I should routinely switch both on when underway without the heater or other load .Would it extend battery life if both were available to accept the alternators charge ?

[Tonyr]

I don't see problems with a 20 amp draw against a 25 amp charge. The full current would not (I think) go "through" the battery, which has a quite high internal resistance, but would preferentially go directly to the heater circuit.

Comments from experts, please??

Tony.

[Tom Lathrop]

Yony,

I did get a message on 9/6 but there was no question there. Nevermind. The T50 seems to always work out to less that 2 gal/hr ever time I check it. That is always over a significant period of time and speeds so I really don't have a handle on specific usage at fixed speeds. It is way less than any of my friends get on any boat that is similar.

If we can assume that the battery is 12V and the draw is 25 amps, the resistance of the heater is 0.48 ohms. The internal resistance of the battery will be significant at this level of draw and the wiring certainly will. In other words a lot of you wattage is going to be consumed by the battery itself and the connecting wires. The actual output voltage at the battery is going to be significantly less than 12V and the wires subtract more.

In spite of the fact that heaters like this are advertised and sold, I still think it is not a good route to take. Better to set the battery in the cabin and hook the heater directly. At least you will be getting all the little heat available.