Erude 1978 70hp low wot rpm

[northrivergeek]

"Hey all I have a 3cyl 1978 70

"Hey all I have a 3cyl 1978 70hp and 1978 fiberglass 15 foot tri hull hurst bass boat. The prop is not original and can read the exact size I can make out that it is a stainless steel 17 pitch , and it is heavily cupped, I have a hydrofoil , which did help with getting up on plane.. the rpm gauge and speedometer my not be working accuratly, but RPM says at wot aprox 2500 -3000 rpm, I am a big guy 350lb, 2 large deep cycle batteries, and 12 gal gas. Would going to a dif prop really help the top end speed to say 38-42 mph, compression on all 3 cyl is 119, new plugs, just rebuilt carbs, Its never done better than 28-30 since I got it would a dif prop make that much speed difference ??
What is considered good compression range on this engine condidering age of it ??
Thanx in advance
Sean

"

 

[searope]

"You will probably gain slight

"You will probably gain slightly more speed with a lower pitch. But at least your motor will feel better, with less load and smoke and carbon deposits. The optimum torque is in 500-5500rpm.
Speed calculation:
5500rpm / 2.5 (approx gear ratio)* 17" = 37400inch per minute = approx 35 mph.
You also have a slip(less than 10%)and you end up with 31 mph. That is with a 17" propeller if your motor was able to rev full speed.
You can calculate different pitches yourself, but higher speed requires generally more power.
I might have done some calculation errors, but I'm metric, so you better check for yourself.
Robert
Sweden"