Cam, I'll leave all your other issues to Deejay, our Scott Bonnar guru, and only comment on the horsepower point.
Shaft horsepower, whether in relation to an internal combustion engine, an electric motor, or any other source of mechanical power, is a concept invented by James Watt, the steam engine pioneer. He measured the amount of work his pit pony could deliver when hauling an ore car up a slope, decided rather arbitrarily that a pit pony could do half as much work as a full sized horse, and concluded that the power of one horse was 33,000 ft lbs-force per minute. Subsequently a more convenient metric measurement of power was established and named after Watt. It turns out that 748 Watts equals one horsepower. So, your electric motor is rated at 374 Watts of output power. It will consume more electricity than that, due to losses. Note also that electric motors have a power factor that is quite a long way from unity, so multiplying input Volts by Amps will give you a much larger number than the shaft output power of the motor.
To be fair, the manufacturers of mower engines have traditionally used a test procedure for measuring horsepower produced that might be described as sympathetic to their desires for high advertised horsepower numbers. However while this might increase their power ratings by about a third, it does not enable a half horsepower electric motor to compete with a 2 horsepower internal combustion engine.
Your little half-horse motor has the same output as a small bench-type electric drill - a slightly beefier home-use bench drill would be 3/4 hp. Of course industrial drills are more powerful than that.
Reel-type mowers are more efficient and therefore use less power than rotary mowers, excluding the power used in propelling the heavy reel mower up slopes. Generally, 18-19" rotary mowers have from 3 to 5.5 hp, with the more modern ones closer to the top of that range. Small home-use reel mowers up to 17" made from say, 1970 onward usually have from 2 to 3.5 hp, but I can't recall a 2 hp one made later than the mid-1970s. Deejay will be much better informed than I am on that subject. (The archetype of 2 hp reel mowers was the fairly early SB45 that used the Briggs 60102 engine).
If your electric SB45 is cutting well-maintained grass on level ground, half a horsepower should be sufficient. If you hit some strongly-growing summer kikuyu, or head up a slope, I think the outcome would be rather different. However as long as it does the job for you, that old Crompton-Parkinson has the virtue of being the original motor SB fitted to your old, collectible mower.
Last edited by grumpy; 23/08/12 10:36 AM. Reason: Add clarification
