Re: 87 force
If you Google " 1987 Force 125 gear ratio"; half way down the page you'll see a PDF of a 85/125 owner manual you can open and print.
The standard prop listed is a 13 X 19 ( that's what came on the boat from Bayliner). Since the heaviest model 19 ft Capri outboard weighs 1800 lbs(dry), I can't see going below a 17 pitch unless the boat has gained weight and/or the motor has lost a good bit of it's get up and go.
Gear ratio is 1.73 for the 125. Rpm range is 4500-5500. 125 hp @ 5000 rpm. Champion UL-18V spark plug. 50 to 1 gas/oil ratio.
A working tachometer is necessary to prop any boat.