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Old 06-18-2016, 06:35 PM   #1
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Default Prop Sizing

Looking to get a second prop and use current as a spare. I have a 1999 Maxum 2100 SC, 5.0 L small block V8. Number on outdrive shows 1.62R, which I am assuming is the gear ratio? Current prop is a 3 blade. Want to say wide open on good water I am 35-38 MPH. Would say RPM around 4000. Honestly don't know pitch of current prop. Seems to perform well. Bought boat with current prop

Mostly just use for cruising 25-30 MPH. Some pulling kids on tube.
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Old 06-18-2016, 10:06 PM   #2
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Originally Posted by lifespan View Post
Looking to get a second prop and use current as a spare. I have a 1999 Maxum 2100 SC, 5.0 L small block V8. Number on outdrive shows 1.62R, which I am assuming is the gear ratio? Current prop is a 3 blade. Want to say wide open on good water I am 35-38 MPH. Would say RPM around 4000. Honestly don't know pitch of current prop. Seems to perform well. Bought boat with current prop

Mostly just use for cruising 25-30 MPH. Some pulling kids on tube.
Now this is a thread high jack, going from a 4600 to a 2100 no comparison.

The WOT should be 4600 - 4800 so you are over pitched.
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Old 06-20-2016, 01:43 PM   #3
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A bit of a guess, and assuming the engine is running correctly and is in good condition, that boat with that engine should see somewhere between 45 and 50 MPH at 4600-4800 RPM WOT if it is propped right.
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Old 06-20-2016, 02:23 PM   #4
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I always thought it was 4400 - 4800. 4600 -4800 is a tight target. Either way, 4000 is over propped for the weight. Maybe there are too many people or gear on the boat. Typically these boats are spec'd to run basically empty with 1/2 load of fuel and water (if there is additional tankage). Max RPM start to creep down as 'stuff' and people start getting loaded.

Run the boat at WOT, with as little trim tab as possible. Trim the drive out one click at a time until it cavitates, then bring it back in until it stops. Speed is irrelevant at this point, as is running with or against current or tide. Those factors only impact speed, not rpm.

Either weight needs to be lowered or the pitch needs to be lowered until you're in the 'sweet spot' for RPM. Prop based on the typical loads (people, gear, coolers) etc.

Avoid the thinking "I want to run with 8 poeple and perform well". Be honest with yourself and the expectations and reality of actually running with a large group of people.
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