I have a Faria outboard tach that I can connect to my motor, but I haven't mounted it yet. For some reason anything that requires me to drill holes in my boat takes a really long time to happen. I'd also like to think of some way to only power the tach when the outboard is running without needing to manually switch it on and off. The outboard doesn't have remote controls so there's no ignition key that would be a natural choice.
My boat is very similar to a J/24. Same LWL and draft and it's only 100 lbs heavier. About a 1.5' more freeboard and beam though. So the 8 HP motor is more than adequate to move the boat, even if the prop isn't the right fit. I could probably make do with a 4 or a 6, but the 8 is what the boat came with. And I like the electric start and alternator, which I don't think you can get on anyone's 4 HP and I don't think the 6 HP tohatsu has a real alternator. The two cylinder motor runs smoother too.
I guess the idea here is that less pitch means the motor will run at higher RPMs for a given throttle. So the goal is to get a prop with a small enough pitch to develop the max rated RPM at WOT, so you get the rated HP out of the engine. What's the issue with using a prop that has too little pitch? The motor will over rev? Or that one will lose top end speed? Is the speed loss possible for a sailboat or is that something only planing boat would have to worry about. I know if you calculate the pitch x the RPM at the prop you'll get a speed far faster than a displacement hull boat could possibly go.