Posts: 3508
Location: Elk River, Minnesota | Hiya,
Although many would disagree with me, I would say start by moving your current motor up one hole. Any time you can raise the motor a hole or two, you are gaining efficiency in running. Saves gas since there is less resistance in the water, and depending on your prop combination, you may notice very little loss in handling performance.
In almost all cases, the anti-ventilation plate is in it's best position when it is anywhere from 1" to 2" ABOVE the lowest point of the hull....reason: water immediately starts to fill the hole the boat made after it passes. If the anti-ventilation plate is in the water as you are at full speed, you are adding drag as well.
Any time you raise the motor, the trade off is a little less handling, which can be handled by a prop change. In most cases, going to a steel prop will increase bite on the water, tip cupping will further increase bite for lifting the bow and cornering, while cupping on the trailing edge will increase stern lift. That is why on bass boats you see very aggressive props with a bunch of cupping with a jack plate installed as well.
In your situation, going up from a 75 hp to a 90 hp will only give you about another 3 mph (roughly 10hp will be an increase of 1 - 2 mph) with a prop change and a height adjustment, you might be able to see up to 5 - 6 mph, but I would not expect much more. Your real gain will be in low end power. Hit the gas and you will feel it more in your seat.
I do have a question for you: What is the WOT range for that motor? If memory serves me correctly, that motor can turn up to 5500 RPM at it's maximum level. If that is the case, I would say you are OVER pitched by about 2 inches. One of the main indicators of being over pitched is lower RPM and the inability to trim up...there is so much downward push by the prop, the bow pops up quickly and the motor cannot keep it there. A lower pitch prop will lessen that effect, while getting you the extra RPM's needed to keep the bow up at higher trim levels.
So...my suggestion would be this: First, be sure your motor can reach it's maximum RPM's while lightly loaded (just you and your gear in the boat). If you cannot reach the max RPM's but are close, I would raise the motor one hole and re-test. If that does not get you there, go down in pitch. Get that max RPM.
I know when I was doing my testing, I found the best I could turn was a 17 pitch. As soon as I went to a 19 pitch, my motor could not turn it. I lost some 600 RPM's because it was just too much and was over what the motor could handle.
Steve
Steve
Edited by VMS 7/23/2011 8:34 AM
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