Fasttr
Well-Known Member
I've posted about this before but I thought I'd try again with some actual performance numbers. The dealer delivered my boat with a 13.75 x 13 prop. Top Speed was 21 MPH at 6,200 RPM (at limiter). Since then I have tried a 13.75 x 15 and a 13.375 x 14. Here are the results...
Note that the spreadsheet works by me editing the slip % until the MPH matches what I experienced in real world lake time. For the most parts the tests were done under the same wind and wave conditions but certainly some environmental variation is probable. Obviously the 14 pitch prop appears to be the best choice since it performs better with a loaded boat and the engine RPM range falls between the recommended 5,000 and 6,000 RPM regardless of load. But is there really anything wrong with using the 15 pitch prop if it will not rev past 4400 rpm with a fully loaded boat? In a car I would equate that to lugging the engine which could be bad but I can't imagine the outboard engine working that hard at 4400 RPM. It would seem that the 15 pitch prop might even deliver better MPG since there would be lower revs/mph. I'm thinking I'll stay with the 14 pitch prop but what I'd really like is a 13.75 or even a 14 x 14 prop. Unfortunately I can't seem to find one to fit the Yamaha F90. Comments?
Note that the spreadsheet works by me editing the slip % until the MPH matches what I experienced in real world lake time. For the most parts the tests were done under the same wind and wave conditions but certainly some environmental variation is probable. Obviously the 14 pitch prop appears to be the best choice since it performs better with a loaded boat and the engine RPM range falls between the recommended 5,000 and 6,000 RPM regardless of load. But is there really anything wrong with using the 15 pitch prop if it will not rev past 4400 rpm with a fully loaded boat? In a car I would equate that to lugging the engine which could be bad but I can't imagine the outboard engine working that hard at 4400 RPM. It would seem that the 15 pitch prop might even deliver better MPG since there would be lower revs/mph. I'm thinking I'll stay with the 14 pitch prop but what I'd really like is a 13.75 or even a 14 x 14 prop. Unfortunately I can't seem to find one to fit the Yamaha F90. Comments?
