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Old 10-17-2015, 06:05 PM   #11
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Well I was cleaning the props up and had them sitting next to each other... Just noticed the pitches are substantially different between the two.
Does anyone know a good pitch or the proper prop for this boat (7.4L, Bravo 2s)? I assume make these two props spares...
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Old 10-19-2015, 12:59 PM   #12
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https://www.mercurymarine.com/en/us/...tor/#/step-one
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Old 10-19-2015, 03:22 PM   #13
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That only goes back to 2000, but I assume the 7.4 is still the same? Looks like it suggests a 17.25x25.
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Old 10-19-2015, 04:21 PM   #14
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That only goes back to 2000, but I assume the 7.4 is still the same? Looks like it suggests a 17.25x25.

Not sure what parameters you put into the calculator but I would think that 25 inch pitch is too much and 21 - 23 inches is more in-line for overall performance.
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Old 10-19-2015, 08:27 PM   #15
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Not sure what parameters you put into the calculator but I would think that 25 inch pitch is too much and 21 - 23 inches is more in-line for overall performance.

I put a 10,000 pound boat (because it says weight minus engines) with two 7.4Ls, Bravo 2s with 2.0 ratio, 180 gallons and an extra 1,000 pounds of stuff.
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Old 10-19-2015, 08:49 PM   #16
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I put a 10,000 pound boat (because it says weight minus engines) with two 7.4Ls, Bravo 2s with 2.0 ratio, 180 gallons and an extra 1,000 pounds of stuff.

I tried it put used length at 35 feet which I believe is the boats LOA and the 2.0 gear ratio and got 23 inch pitch. Not sure why you would not include the engine weight of 2 big blocks, strange.
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Old 10-19-2015, 10:09 PM   #17
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I tried it put used length at 35 feet which I believe is the boats LOA and the 2.0 gear ratio and got 23 inch pitch. Not sure why you would not include the engine weight of 2 big blocks, strange.

That's what that program tells you to do. I don't get how the preferred way to find your prop is to enter the prop pitch and engine rpm... But second best is using weight. There could be a vast weight difference in various boats 35 feet long...
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Old 10-20-2015, 01:58 AM   #18
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That's what that program tells you to do. I don't get how the preferred way to find your prop is to enter the prop pitch and engine rpm... But second best is using weight. There could be a vast weight difference in various boats 35 feet long...

I agree but a 25 inch pitch just does not sound right to me, hopefully some with this model and power will chime in.

You said that you used 1000 pounds for stuff but the input is for passangers and stuff, 4 passangers will be 600 to 800 lbs then add water and gear you are close to 2000 lbs.

BTY using weight can vary a fair amount as well. Weight and length go together to some degree along with beam and height. Also a full fuel tank is over 1000 lbs alone.
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Old 10-20-2015, 11:13 AM   #19
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So how do size and pitch both play into the performance of the prop? Does a large pitch and small diameter perform similar to a small pitch and large diameter? Or does one number affect ow speed/acceleration and the other affects cruising/top speed?
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Old 10-20-2015, 11:43 AM   #20
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Think of pitch as the amount of forward travel per revolution. A 21 inch pitch will move the boat forward 21 inches for one revolution, this is in a perfect world but due to efficiencies (slip) the actual distance is somewhat less. Increasing pitch puts more load on the engine and it has to work harder, too much pitch and the engine cannot achieve its ideal WOT. Diameter along with pitch determines the amount of blade area which is part of the equation for the thrust and load, the number of blades is another factor. Generally decreasing diameter and using a larger pitch does not perform well. For more info try googling boat propeller theory.
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