What prop size and What can I expect?

[Singing Boat Guy]

I have a 1978 90 hp Mercury with a 17 pitch on it right now. My top speed is about 34mph in my 1979 16 foot Glasply. What is my max prop pitch that I can use on this motor? What can I expect from a 19 pitch?

 

[Silvertip]

Re: What prop size and What can I expect?

You can put any pitch you want on the motor. But selecting pitch involves more than just grabbing a number. You said your top speed is "about" 34 mph. When it comes to prop selection "about" is not a valid number. Also, without a maximum rpm number at wide open throttle and an accurate wide open throttle speed, we can only guess what you need. For all we know, you already have the best prop. About all we can tell you with the data you provided is a 19P prop (same style as you have) will drop rpms by about 400 RPM and speed may or may not increase. In fact it might decrease because the engine cannot spin that much pitch and reach its maximum "recommended" wide open throttle rpm.

 

[jimmbo]

Re: What prop size and What can I expect?

I have a 1978 90 hp Mercury with a 17 pitch on it right now. My top speed is about 34mph in my 1979 16 foot Glasply. What can I expect from a 19 pitch?

Slower accelleration, lower WOT rpm, and roughly the same top speed. Eventually a melted piston from lugging the engine. Rpm range for your engine is 4500 - 5000. Just about every boat I've ever seen in the 15 - 16 ft range with a 90hp merc runs a 17

Some people here will tell you to raise the engine higher on the transom to reduce drsg and gain rpm. Not really an option with your engine, unless you like drilling new holes, and are good at filling the old ones.