prop vs rpm

[Quick3201]

OK, I read the Sticky. I'll provide all the info I have. 1974 Olympic 19' V hull, bow rider, 1974 Chrysler 120 HP outboard. New tach, set for 12 poles., 13x15 three blade alum prop. also have available a 13x17 prop. Running against the current in intercoastal waterway, WOY tach 3200 rpm water speedo 30 mph. engine "sounds" like high rpm. Running back with the current later in the day 3200 rpm, WOT 20 mph by water speedo. Three adults, 12 gal fuel, two batteries. New floor, so not waterloged. Don't know weight of boat, but ie seems prety heavy, could go to scales and subtract weight of trailer i guess. Jumps on plane good, top speed seems about right, I'm wondering if the tach is set wrong. Mfg. said to put setting on back to "5". 7000 rpm tach, adjustable for many # poles and cyl. Live out in the country, hard to find a competent boat shop with tank testing capabilities.

 

[steelespike]

Re: prop vs rpm

I count 13 poles on the illustrations. If you could beg/borrow a gps or a cell with gps it sure would help.
We could measure current.
Not sure of the gear ratio perhaps 1.73. If you pull the plugs and count the rotations of the flywheel for one rotation of the prop
that will help.Need to mark prop and flywheel to be accurate.
If the ratio is 1.73 and your speed is actually 30 at 10% slip your rpm would be about 4000.
We have to pin down the gear ratio and the rpm.
I feel a 15" is close to what we need.
At 3000 or 4000 rpm we can't make that up with a prop. when we are already at 15"
so we have to settle the numbers to try to be sure of the rpm and use gps speed to see if the motor is not up to par.
Does the boat respond to up trim? Are you using a whale tale or foil on the motor?

 

[Quick3201]

Re: prop vs rpm

I count 13 poles on the illustrations. If you could beg/borrow a gps or a cell with gps it sure would help.
We could measure current.
Not sure of the gear ratio perhaps 1.73. If you pull the plugs and count the rotations of the flywheel for one rotation of the prop
that will help.Need to mark prop and flywheel to be accurate.
If the ratio is 1.73 and your speed is actually 30 at 10% slip your rpm would be about 4000.
We have to pin down the gear ratio and the rpm.
I feel a 15" is close to what we need.
At 3000 or 4000 rpm we can't make that up with a prop. when we are already at 15"
so we have to settle the numbers to try to be sure of the rpm and use gps speed to see if the motor is not up to par.
Does the boat respond to up trim? Are you using a whale tale or foil on the motor?

Will try to get out to shop later this week and answer your questions. No, whale tail or foil. trim Ok