If you are correct that the motor is turning 4,000 RPM, then a prop with less pitch is called for. I would not be happy at less than 5,000 RPM as that engine is only 13 + cu in and designed to rev on the high side, say 5200 to 6200 wide open.
A small tach would be a good investment for the OP.
As to the rated RPM, all the older OMC motors were rated at the 4,000-5,000 RPM range as you say. When they came out with the 9.9/15 powerhead, they went for the higher RPM range.
That engine was the latest design by OMC before going to four stroke, so it is the newest design two stroke. The old motor in that class was a 9 1/2 HP affectionately called the toilet bowl because of the shape of the motor.
His motor is rated at the prop.
I ran those motors from 1974 to 1990 and am very familiar with them. The reason his has the pin drive is that it is designed for higher thrust than a thru hub in reverse. It should run so close to the thru hub on top end that it is negligible on that boat. It uses an exhaust snout rather than a thru hub which does not steal much from the power on fishing boats as it adds very little drag. The thru hub aerates the prop in reverse whereas the pin drive deposits exhaust gasses on the tips of the prop. .
Take the prop off and see the factory casting number. It is silly not to do a real examination before saying you cannot see the ID number.
If you are worried about which position to put the pin, try different positions and see the effect.
A small tach would be a good investment for the OP.
As to the rated RPM, all the older OMC motors were rated at the 4,000-5,000 RPM range as you say. When they came out with the 9.9/15 powerhead, they went for the higher RPM range.
That engine was the latest design by OMC before going to four stroke, so it is the newest design two stroke. The old motor in that class was a 9 1/2 HP affectionately called the toilet bowl because of the shape of the motor.
His motor is rated at the prop.
I ran those motors from 1974 to 1990 and am very familiar with them. The reason his has the pin drive is that it is designed for higher thrust than a thru hub in reverse. It should run so close to the thru hub on top end that it is negligible on that boat. It uses an exhaust snout rather than a thru hub which does not steal much from the power on fishing boats as it adds very little drag. The thru hub aerates the prop in reverse whereas the pin drive deposits exhaust gasses on the tips of the prop. .
Take the prop off and see the factory casting number. It is silly not to do a real examination before saying you cannot see the ID number.
If you are worried about which position to put the pin, try different positions and see the effect.
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