RE: Block Heater
Here in BC, (Vancouver), we get ice on the water in the marina about one year in four. When we get that ice, it can be 1/8 in to 1/2 in thick. Outside temp at the coldest is rarely below -5 (deg C). I have had a boat in the water here every winter since 1977. In the early years I used a light bulb, 100watts. After selling the first boat, I acquired one with a hot water tank in the ER. Since then, leaving the hot water tank plugged in has kept that and the present ER plenty warm enough to prevent condensation and keep things dry and clean. The engine coolant is "anti-freeze" so can tolerate -10 or lower engine temp. The ambient temp is so the humid winter air doesn't condense and cause mildew and damp.
Some have block heaters, but for the purpose of getting the engine up to operating temp more quickly and bypassing the dense cloud of smoke some low injection pressure engines emit when cold. Your Lehman, if really old, may be such an engine. In the Marina where I keep my boat, old CAT 3208s were the worst for that phenomena, and only marginally better with block heaters.
If you are on metered Hydro, you will quickly see the downside in your bills. If on a flat rate, your marina is getting the downside.
Is there an up side? the only way to know is to try leaving the heater on, then don't and report whether there was any improvement in starting, or in getting past the cloud of smoke. The rest of us can only take wild guesses about any up side.