Worth and additional prop change or not?

[Yachtzee]

HI,
I wonder if I am in the best overall configuration possible.

Boat is elderly 16 ft by 6.5 ft beam aluminum semi v runabout with closed bow, plywood floor. I think that it is heavier than it appears to be.

Motor is 1969 merc 650 four cylinder (65 hp). Runs good, 130# compression.

First prop 13 3/4 x 17, mercury quick silver could bearly get up on plane

2nd prop 13 3/4 X15 WOT 4500 RPM max speed 26 mph

3rd prop 14 X 13 solas prop. Boat really jumps up on plane with this one. WOT 5000 RPM 30 MPH (via gps) 400 lbs of people 14 gals of gas. A bit faster with just one person.

Is it worth trying an 11 inch prop to gain a bit more speed I think WOT max rpm is less than 3400 rpm for this motor.

Thanks for any comments.

 

[steelespike]

Re: Worth and additional prop change or not?

Rpm rating for the 650 is 4,800 to 5,300.Before making another prop change you might weant to fine tune your setup.Raise the motor vertically to a point just before venting becomes a problem.If no height adjustment holes simply raise the motor and add 1/4 inch sticks to physically raise the whole motor.Just be sure clamps have a good grip and it pumps water.An inch could get you a couple of mph and some rpm.I don't think an 11 is the way to more speed a more sofisticated prop like a hustler turning point may help after you fine tune.The hustler may allow even more height.Don't forget to adjust trim
as well.Your speed ,30mph,is in the ball park though 35 is a possibility.

 

[Yachtzee]

Re: Worth and additional prop change or not?

Steelspike,
Thanks for the concise response, I see that I miss typed when I said less than 3400 rpm I meant less than 5400 RPM which lines up with what you said.
I will try raising the motor a bit to see if helps. I gained 1 or two mph by trimming the motor out form the 2nd tilt hole to the third, have not tried to trim it our any further, but the cav plate could be just a bit lower than the keel. maybe half an inch max.
Thanks

 

[steelespike]

Re: Worth and additional prop change or not?

Its not unusual to run with the plate an inch above the bottom of course it depends on the boat,motor combination.And the prop type etc.
Some folks will tolerate a little venting if they are gaining some performance.
On trimming some times if 3 notches isn't quite enough but 4 is too much you can buy or make wedges for between the mount bracket and transom.these allow a half notch gain or loss depending on which way they are installed.