Prop for Mercury 650

[pcgary54]

I have a 1975 15' Yar-craft Yankee with a 1975 Mercury 650 3 cyl. 2 stroke engine. The prop is a 4 blade 10 3/4D x 13 1/4P composite prop. I use the boat primarily for fishing, on a large lake (8mi W x 21mi L) I feel this prop is not quite enough prop for this boat/motor combination. The engine seems to hit max RPM at WOT, while the boat is still climbing out (approx. 8-10 mph) of the hole to get up on plane. Then when the boat does get on plane, it take a while to get up to a top speed of 26mph. Max RPM doesn't sound like it changes from climbing on plane (no tach available varify). This is the first boat I have owned that has a 65hp motor, but I would think that it would be faster (or at least as fast) than a 14' runabout with 35-40hp O/Bs that will normally do 30mph. Any help would be greatly appreciated.

 

[steelespike]

Re: Prop for Mercury 650

Without a tach we're pretty much out of luck.A Tinytach is about $47 and easy to install.Is your seed by gps?
There are 2 Yankees one is 700lbs the other 580.
Either way your motor should be able to make 30 or more.Your prop is one or two spots toward the high side of the prop chart.
Start by checking your setup.Is the antivent plate(just above the prop)visable on plane?Does the throttle open all the way at the carbs?Is the bottom clean and smooth?Good compression? Is the timing right?