There is a thing called K.I.S.S. - Yet I keep reading posts about spending $100 for this heater; $500 for that A/C & heat; and so forth, ad infinitum. I glad so many posters have so much extra money
that they can throw away or in this case - maybe - light with a match to try to keep warm.
All the boats I know about generally have either 30 amp shore power
or 50 amp shore power
. The smaller boats generally are always 30 amps. AND - the AC circuity is wired for 15 amp load branches or 20 amp load branches - that is, the max you can plug
in before the circuit breaker pops is 15 amps or 20 amps.
Assuming nothing else running in the boat which is unrealistic as you have lights, battery
heaters, etc. running you get maybe the maximum available for space heaters is 2 times 15 amps.
If you look at the label on the back of all the portable heating
units you will find that they rarely exceed 13 amps which equates to about 1500 watts of heating
. Using 3.41btu/hr/watt that adds up to 5115 BTU's. So the sellers like to throw BTU's at you as the numbers seem so much bigger.
If you go to Home Depot, Lowes or any discount home center you will find plenty of 1500 watt portable heaters for $10 to $15. These units put out exactly the same heat as all the $100 and higher cost units that are rated at 1500 watts.
So why do you want to spend 5 to 10 times the money
for the same amount of heat? Maybe the high priced units are prettier, or funny
shaped or whatever. Fact is they don't put out any more heat than the $10 unit. Maybe the high priced ones will last longer - but in this "Made in China" era I don't think so . . .
But - maybe they have better and/or safer mounting systems - again I don't think so as consumer law mandates the automatic shut-offs for tip over, etc.
So we go to Home Depot and by a handful of the $10 units and put them - one unit per electrical
branch circuit - in the main cabin
and one in the sleeping compartment. We run them both on low at night, or only one on high depending upon which cabin we are in. This keeps the shore power breaker from popping or cable overheating
when the water
heater or other unit automatically turns itself on or off.
I like the idea a poster had of running a separate 30 amp or 20 amp circuit from the shore power box to the boat strictly for electrical
heaters so that you are not stressing the normal circuits in the boat.
In order to get more than the limited heat available from electrical heaters you would need to go to a diesel
heater or other fuel
type heater which means installing vents and chimneys, etc. so that you are not allowing combustion gasses to get into the cabin. The available heat from these heaters is virtually unlimited as is the amount of money you can spend installing them.
I would suggest if you are going the electrical heat route
to spend the excess money you would spend on the "fancy-dancy" units on some form of insulation
to keep the heat in the cabin from escaping through the cabin top/roof, hatches, etc. Keep the heat from escaping and you don't need to keep adding so much all the time.
Maybe some form of insulated blanket or maybe a custom-shaped tarp with polyurethane panels
cut to shape laid over the exterior of the cabin-top. Take a look at the RV market and they have lots of stuff for temporary hatch insulation
pillows, etc. to help keep the heat in (and the cold out).