merc 650 prop decision

[evvy]

Hello! I have a 14' aluminum mirrocraft runabout (flat bottom soft keel style I think) with a mercury 650 on it. The boat is really light and small, but i want it to pull two skiers and go fast as possible. last year I upgraded from a 35 hp sea king and it still doesn't go as fast as I want. I got the merc for $225, so i have some room in the budget for a prop It would step right up on plane and hit top speed right away (like 3 seconds) but run out of legs. just me in the boat it was about 35 mph, four people in the boat was about 32, and with two people in the boat while pulling two on skis behind was like 28-30 mph. It was pulling two skiers pretty easily. I want high top speed with one or two people in the boat, but still want to be able to pull two skiers when I want. Current prop is 10.75 x 12, how much could the pitch go up without loosing the ability too pull two skiers? I saw the steepest prop available was 10 x 17. I was thinking that I could get that one and if it has trouble pulling people, i could put the current one back on for skiing, but that would be a bit of a pain in the a. Thanks if you could help! (Its a shame to waste a speedo that goes to 50 when the needle stops at 35!) Also, the transom is 20 inches with a cutout down to 15 inches for the original short shaft motor. The merc 650 is a 20 inch long shaft so it has a lot of drag there and scoops water into the boat a bit. I'm fixing that with a strait across (no cutout) transom board as well.

 

[steelespike]

Re: merc 650 prop decision

No doubt your transom is steeling 2 to 5 mph. You could use more pitch but selection would be more accurate with
more accurate info.
I think I would wait until you get the transom right and try to get setup so you can fine tune the height perhaps a makeshift
jack plate. For the health of your motor you need a tach,an induction tach like a Tinytach will work on any motor.easy to install,can be hand held or permanent.A gps for speed is important as well.
Do you know the year of your motor and perhaps the gear ratio?

 

[evvy]

Re: merc 650 prop decision

1975, don't know the gear ratio

 

[jestor68]

Re: merc 650 prop decision

I and the Mercury prop selector recommend a 15 pitch for your "speed" prop and keep the 12 for ski pulling.

The Solas Amita would be your best bang for the buck.

You are expecting one prop to be able to do it all. It doesn't work that way. The higher pitch for speed won't have good pulling power. The lower pitch for pulling power won't have good speed. That's just the way it is.

The fact that your speedo reads to 50 doesn't mean anything. My speedo reads to 80. Doesn't mean it'll do 80.

I can change a prop in about 1 minute. Not that big a deal.

 

[jestor68]

Re: merc 650 prop decision

1975, don't know the gear ratio

1.64. Be very careful about jury rigging that transom back to 20", or your Merc 650 could end up at the bottom of the lake.

 

[evvy]

Re: merc 650 prop decision

I realize that the speedo going up to 50 doesn't mean anything, and I kinda figured that I was going to end up changing props back and forth for best specific performance, witch isn't really a big deal. Thanks for the advice on pitch! Should I get a 10" diameter prop, or does that not make much of a difference? I see the Michigan wheel vortex is an inch smaller than the Solas at a slightly higher price.

I'm not going to be "jury rigging the transom". I am going to build a new transom board for it and not cut the notch out, so It will be much stronger than it is right now and the perfect height. I'll post some pictures to show you what I mean. I'm not stupid and I do not do stuff half-assed.

And just a thought, they should have a high and low range on engines, then with the flip of a leaver you could have good pulling power or higher speed without getting out of the boat

 

[evvy]

Re: merc 650 prop decision

The new transom board I'm making will go strait across, not drop down. Its still in winter storage, so she is rather dirty and full of stuff inside.

 

[jestor68]

Re: merc 650 prop decision

Since you are using a 12.75 diameter now, the 1/4" increase with the Solas won't make a difference.