electric heat is great if you can afford the electric bill. Hot water heat is less expensive to use, bu more expensive to install. A portable kerosene or propane heater can add a lot of heat at a reasonable price. A wood burning stove requires more attention, (you need to light it, keep feding it, etc. but it can be fueled with tree trimmings from your yard, or scraps cut off of projects.

With electric heat, it seems that any source you have will provide about 5200 btus for 1500 watts. If you use fifteen 100-watt light bulbs, or a fancy 1500 watt heater you get the same amount of BTUs. With light bulbs or infrared heaters you can focus the light to heat a given object rather than heat al the air in the room.

One of your pictures shows a large plastic dropcloth. If you drape that over your boat it will reduce the amount of area you need to heat, and it will trap the hot air. Putting an array of lightbulbs under the boat can raise the temperature of the boat even if the room temperature is cooler. Any hot air created by heating the bottom of the boat rises and is trapped by the plastic, helping to heat the top of the boat.

If you can find heater assemblies from old cars (the heater fans and the heater radiators, or heater cores) you might be able to tap the heat from a water-cooled engine, such as one used for a generator. Your cost here is the price of the burning gasoline or diesel fuel.

Or you could connect this to a solar collector system, and use a recirculating pump.

There are forced air propane heaters, and there are propane fired infrared heaters. The latter are less expensive, have no moving parts, and can direct the heat at the boat -- like lightbulbs. Some can attach directly to propane tanks.

There are propane and natural gas space heaters which are vented and which might suit your shop. Ther are also unvented versions.

Pellet stoves use a fan and forced air to burn commercially made compressed pellets made from wood scraps. The units tend to be well sealed and very neat and clean heat. Some can also burn corn. BTU output varies.

Check any old issue of Mother Earth News for thier ads for stoves and heating devices.

You have a LOT of options.

PGJ