Posts: 3480
Location: Elk River, Minnesota | Hiya,
Something is definitely NOT right with this set-up. Not sure of only one thing, though...what is the gear ratio for your lower unit?
I did a quick calculation with the numbers you listed and used a gear ratio of 1.81:1. this gave me theoretical speed of 55mph. This would mean you have a slip factor of 27% which is below where it should be for that boat. A slip factor of 12% to 15% is more typical...and in some cases, that slip factor can be as low as 8%.
I would agree that the lugging of the motor sounds right...and going down to a 19 pitch cupped steel is definitely the right move... By doing so, you should see an increase in roughly 150 to 200 RPM's per inch of pitch...thus going down 4 inches in pitch will give you anywhere from 600 to 800 more RPM's. That translates into much more power since the engine is turning faster as it compares to the pitch of the prop. Assuming a 700 RPM increase, you would be seeing a much better hole shot, and theoretically a speed of somewhere around 47mph. This is given a 10% slip factor.
Something is definitely not right. Assuming this boat is right around 2100 pounds, we should be seeing numbers right around that 46mph to 48mph given the RPM range is maxed at 5500. Generously, if we went with an increase of 900 RPMs with a 19 pitch prop, and a 10% slip factor, you would be seeing 49 mph.
I think a change in prop is in order. One that has a good amount of cup to it... I'd look at what treats has said with his recommendation. The tempest prop will be the most aggressive, which will lift the entire boat more out of the water, which will help quite a bit. I would also consider looking into the yamaha pro series of props as well. Definitely something with a vented hub to give you better hole shot.
I don't think there is anything wrong with the engine... I think it is just over-propped..
Definitely checking the fuel pump for issues is a good thing too...but by the numbers from the yamaha specs, that motor is to run as a max at 5500, with a gear ratio of 1.81:1, if something is wrong engine-wise, it could potentially show here as well.
Suggestion: Leak-down test on the powerhead as well, a good run of mercury powertune through the engine in spring, fresh plugs after that, and check that diaphram on the fuel pump. Can't hurt. If all of those come back decent, then start playing with props... I would think if something is wrong with the pump, it would show on your plugs... If something electrical is wrong (like a failing power pack) it would show on the plugs as well...
My gut says prop, but until you get in there and check, who knows...
steve
Edited by VMS 12/18/2008 8:26 AM
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