Let's Talk Props
Let's Talk Props
How convenient that I am just starting the next six week module of my marine tech certification classes. We're doing rigging and performance and our "project" is a 19' Grady dual console. We'll be stripping it bare of everything, gauges and all. Then we'll re-wire and re-power with a brand new Suzuki 150. Can't wait. First thing you need to tell the guy putting the engine in is how you use or will use the boat.
Colleges can offer full semester instruction on prop selection, prop technology and the who's, what's, why's, and wherefore's. Your new motor is going to have certain specifications as far as what rpm it should be spinning at full throttle. So after you properly break in the engine, you'll want to take it out, get it up to full throttle and see how fast you're going in speed and in rpm. The ETEC 90 will probably want to run at 5500 rpm give or take a 100 rpm here and there. You have a small center console so it probably won't take much to get it up on plane. If you're flying at like 35-40 mph at 5500 rpm, that is probably where you want to keep it. I wouldn't want to take a small cc any faster.
Props are weird things. 15 pitch means that one full revolution of the prop should push your boat 15" in the water. Every prop also has a correction as to how far it actually pushes. Sometimes pitch sizes are trial and error. You may be setting up a certain boat and you're sure the pitch should be some small number. Then you find out that its like a 22 pitch that makes it run nicely with a four blade prop.
1" of pitch = approx 100 rpm difference.
Each blade is worth approx 1" of pitch.
Prop diameter is worth approx 1" of pitch.
I could go on for hours. Nobody wants to read my babble. We haven't even touched on the angle of the blades, stainless vs. aluminum, the cut of the blade, the angle of the blade in relation to the prop shaft. Prop conversations among boat makers and marine mechanics is like a bunch of lawyers at a convention. You can debate the topic a hundred different ways and nobody is more right or wrong than the next guy.