I have a 1970 GlasPly 16' with a (by serial number) 1971 Mercury 650 4 Cylinder, it looks 100% original but the decals look like 1972 blue band and not red band, that has me kind of confused because I thought everything was red band until 1971. maybe mercury ran out of red decals?
Additionally - my boat came with a 12.5"x19" prop, I ran it because that's what came with the boat but I always noticed a performance problem. would only hit 3800RPM and 27mph MAX with 2 adults and 2 young children onboard. Also it would struggle to get on plane and keep it there. I have a 13"x17" prop that I'm going to try out, just curious if anyone has a recommendation on prop size. Looking for 30-35MPH 4800RPM-5300RPM MAX.
Maybe it was a very late '71 run and they switched to blue. What's the serial number, that'll tell a lot. There's also the possibility someone updated the stripes. Does the small "horseshoe" cover around the mid-section say "650-4" or something like that?
Even with a 2.3:1 gear ratio in the lower unit, that 19"-pitch probably is way too big for a 65hp. Prop it so your WOT throttle with only the driver in the boat is around 5300, or the top of the recommended rpm range. That way it'll do better when you have a load of passengers in the boat. These engines like to be revved, they Most Definitely do not like to be lugged.
See how much the rpm jumps up with the 17" prop. Then you can extrapolate what another 2" pitch reduction will do. You may end up having to go to a 13" pitch to get the proper WOT rpm.
You'll find that she's a lot Happier and Snappier with the correct prop, and will use much less fuel to boot.
2997339 was the beginning serial # for a 1971 650-4 and 3293184 was the beginning serial # for a 1972 650-3, Mercury's 3-cyl replacement for the 66 c.i. Inline Four. IMHO the 4-cyl is a far better motor than the 3-cyl ever was. At least the early Triples.
So maybe yours is a very very late '71 or even an "over run" '72, no telling. Quite rare in any case.
Far as propellers go, generally a higher-pitch, lower-diameter prop will be faster on a high-performance boat with a big outboard. Your 12x19 would work well on a ski boat with a big Inline Six, running in the 40's or so. Not so much on your 65hp.
A lower-pitched, larger-diameter prop will be more efficient. The larger-diameter blades may not allow the engine to spin up quite as fast, but a change in pitch will have more effect than the diameter change. So, a lower-pitched prop is just about always gonna have a larger diameter. Of course there are oddball sizes out there for different applications, but your issue is just simply being propped too high. On these inline Fours and Sixes, you're pretty much limited to 13" diameter, I don't think a much larger diameter will clear the L/U structure.
You will have to reduce pitch, regardless of what the diameter of the prop is, until you get into the recommended WOT rpm range with a light load. Period. The engine is gonna tell you what prop it needs, regardless of the propeller chart's recommendations. Providing of course the powerhead is in good shape and putting out rated hp.
I can't tell you how many props I tried on my ski boat in the 80's, trying to find the perfect prop for the best hole shot and good speed on my old ski boat. Once you find the right one, you'll know! The thing will come alive and be a pleasure to drive. Most outboards aren't happy at all with too much prop, it's like trying to drive around town in 5th gear, you won't get going very well when you do have to accelerate.
Try the 17"-pitch and see what it does...............ed
There is always the possibility over the years, that a 2:1 gear ratio lower unit was installed, when a 2.31:1 ratio was original.
Put the beast in Forward gear, then rotate the flywheel with a wrench (15/16" socket) until you see the propshaft has started to rotate.
Then,...Put a mark on your prop shaft and an aligned mark on the gear case. Then rotate the flywheel clockwise, and count how many revolutions you need to make with the flywheel for the marks to align again.
2 rotations makes the lower unit gear ratio 2:1
2 & about 1/3 rotations makes the lower unit a 2.31:1,...Which is preferred for the later 650-4 cylinder Merc.