Really need some advice on how to proceed with setting up the motor/prop on the new Bentz Craft…
Backstory: The 1999 has a different transom, and with 90 2 stroke and a 13 3/8" x 19P Michigan Ballistic wheel will run 61 with just me in it. I know what the hull/motor will do.
New boat: 1994, same motor, but previous owner raised motor up all the way on the transom and put on a 15P prop. Boat would run 32 mph at 5400RPM but was plowing the bow and the prop would blow out if trimmed up. I lowered the motor and it improved dramatically.
I then installed a 13.25" x 19P Michigan Rapture that I had laying around, and the boat ran 38 MPH but would only turn the motor 4000RPM.
I installed a 4" manual jack plate to make fine tuning the motor height easier on me with the poling platform, and get some more setback. With jack plate bottomed out, motor was about 1.25" low.
Got it raised to where anti-cavitation plate is just barely getting wet, and the boat runs 42MPH but the motor is still only turning 4400 RPM, and doesn’t want to get out of the hole very well.
I would like to get the motor to turn about 5500 RPM or so at WOT, but can’t decide what to do. Do I try a different prop? Different diameter? Different pitch? Different cup on the prop?
Suggestions from the pros would be greatly appreciated. I don’t need the boat to run 60, but I know it should run 50+ at least.
Also of note, with the motor where it is sitting now, I can trim the motor all of the way out of the water until I have a 10 foot tall rooster tail and only the stern in the water, only only turn the prop 5000 RPM…at 44MPH
all the 90 2 strokes I know of have a 2:1 or higher gear ratio
something in the information you have provided isn’t accurate
with a 2:1 gear ration, a 19 pitch prop, and only 10% slip, (average is more like 13%), to reach 61mph, you would have to turn that prop over 7500RPMS
something isn’t adding up
if you run the numbers through the prop calculator…
2:1 ratio, 5500rpms, 19 pitch prop, 12% slip, what do you get ?
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43.5 mph
I ran the other numbers you provided
19 pitch prop, 2:1 gear ratio, 4000 rpms, and 38MPH, you have negative 5% slip, which is mathematically impossible, so something isn’t accurate
here is link to the prop calculator http://www.rbbi.com/folders/prop/propcalc.htm
this may help work through you questions, but its only as good as the info you give it.
Where are you getting your speed numbers? Have you verified that the tach is reading correctly?
61mph is insane in a 16’ boat. My old 18’ cc Hydrasport would run a verified 54mph and it was barely in the water. I wouldn’t have wanted it to go any faster.
Prop calculator does not count for motor setback, prop type and design, motor height, etc…It’s a good tool for a normal boat I guess, but it doesn’t work in my case.
“Prop calculator does not count for motor setback, prop type and design, motor height, etc…It’s a good tool for a normal boat I guess, but it doesn’t work in my case.”
hate to be the negative guy here, but none of that matters to the prop calculator.
those things will influence rpms, therefore speed, but they have no impact on the prop calculator
the prop calculator CAN NOT be wrong, its simply a mathematical equation.
every time the prop makes one revolution, you go forward the number of inches per the prop pitch (minus slip)
your prop may not be what is stamped on it
your gear ration may not be what you think
your tach may be off, or your speed isn’t accurate, but the MATH isn’t wrong
“Prop calculator does not count for motor setback, prop type and design, motor height, etc…It’s a good tool for a normal boat I guess, but it doesn’t work in my case.”
hate to be the negative guy here, but none of that matters to the prop calculator.
those things will influence rpms, therefore speed, but they have no impact on the prop calculator
the prop calculator CAN NOT be wrong, its simply a mathematical equation.
every time the prop makes one revolution, you go forward the number of inches per the prop pitch (minus slip)
your prop may not be what is stamped on it
your gear ration may not be what you think
your tach may be off, or your speed isn’t accurate, but the MATH isn’t wrong
I started to say something as well… but sometimes people get a little excited.The fact is, the prop goes 19" per revolution , without the boat/motor attached. The slip factor tells you how efficient the boat/ motor set up is.