Posts: 3480
Location: Elk River, Minnesota | Hiya,
Much of the determination of the prop you will need is based on the gear ratio of the motor itself. Many older models of the mercury 50hp had a 1.85:1 gear ratio which meant you needed a lower pitched prop to get the boat up and moving. If the motor is the same year as the boat, the 12 or even an 11 pitch might be in order.
Now...there were some 50hp motors made in recent years that had a larger gear case which had a lower unit designed more for pontoon boats and larger heavier rigs or commercial use with a gear ratio somewhere around 2.33:1. It was called a bigfoot design which would handle the props that A-B-G was suggesting and were designed to push big loads. Those props are ones that would not only fit the bigfoot series in 40 and 50hp motors, they would also fit some larger motors in the 70 - 115 hp range as well.
If you have a prop now that takes something like a 10-1/2 x 13 prop, you will have the smaller standard gear case and the lower pitch props are what you would probably need to stay with, so the suggestion for the 12 I would say is pretty darn close without running the boat.
You will definitely know if the prop has too high of pitch if it takes a long time to get out of the water when you throttle up and when on plane and the motor sounds like it is really working hard. Pitch is the theoretical amount the boat would move forward with one turn of the prop. Overall it will move less than the pitch value as the propeller must "slip" a bit in the water in order to get the prop moving...otherwise, it would be like trying to turn the prop in a box of heavy wet sand.
In many of the smaller outboards, a tachometer was not very necessary. In your case, it might be helpful to determine correct prop pitch and diameter. Much of that depends on what you want out of the rig.
Steve
Edited by VMS 4/18/2012 12:17 PM
|
