Prop trials on 75hp Yamaha

[wpschaef]

I think this info might help some others who are looking at upgrading their props. I have a new 2007 170CC with a 75 hp Yamaha 4stroke on it. The prop was a13 1/4 X17 aluminum. Doing fine, until I stopped listening to my depth alert false alarms and had a real one. Long story short I ding my aluminum prop up moderately. Still usable after a little filing. Not perfect but ok. My RPM in the beginning went up to 6000 which is the stated red line. After the prop "customization" the RPM went up high enough to engage the rev limiter so I kept it backed off untill I could get another prop.
I got convinced that increasing the prop pitch would be a beneficial thing to do both for performance and for RPM range targeting so that it would be at about 5500 at WOT, so I got a 13 1/8 X19 stainless. I don't know why the 1/8th difference but at this point it's moot because I didn't keep it. Here's why. To fit the prop I had to remove the anode and take the fin off it or the prop would hit. I was told this would not hurt anything and have little to no effect due to the hydraulic steering. (I will reserve judgement.) The boat got up on plane faster, got more top end 2-3 mph, kept plane down to 20mph, but......... RPM would only go to 5000 which is the very bottom of the top end range. Also we all enjoy the benefits of our plastic hulls stablility under way, but now I was fighting the helm all the time and I mean fighting. If the motor wasn't trimmed just perfect and was low it would steer starboard dramatically. If I had it too high it would actually go to port almost as dramatically. This was, to me, not acceptable.
I am going to the stock size, 13 1/4 X17 in stainless and see how it goes. I will of course post results. By the way, it is a Michigan wheel Apollo series and they require a hub kit and an included gear housing spacer for the inside edge of the prop body. It is a small plastic ring that is very tight and must be snapped on to the prop. They do break but I called the folks at Michigan Wheel customer service and they are sending me a new one free of charge.
Hope this is helpfull.