1987 5.7 omc

[Nick_1987 5.7 OMC]

Hey everyone, just wondering what the RPM should be on my engine while at WOT. It’s a 1987 5.7 omc with a 1.41:1 outdrive. I have a 15 1/2x 15 right now and at 1/2 or 3/4 throttle I think I’m doing 4300rpm and 30mph. I have a stainless steel prop that is 14 1/4 x 21. I want to see if I can swap the 15 pitch for the 21 pitch and see what it will do. It’s for a 2550 Bayliner ciera sunbridge.

 

[Scott Danforth]

about 4500 RPM +/- 100 at wide open throttle with full load (that means with everyone on board, including women and children and the dogs, food, gear, etc.

sounds like you are propped right for the boat now.

a 21 pitch will be too large of a prop on a boat that big with that drive ratio

you always prop for WOT. if you lug the motor you kill the motor

to jump to a 21 pitch on your boat, you need different gear ratio or a lot more motor

so what makes you want to change props?

• because you have the 21p? (if so, sell the 21p and buy the prop you need)
• because you want to go faster? (if so, you need more motor or a different boat)
• because you want better fuel economy? (no chance, lugging the motor burns more fuel and just kills the motor)

 

[Nick_1987 5.7 OMC]

about 4500 RPM +/- 100 at wide open throttle with full load (that means with everyone on board, including women and children and the dogs, food, gear, etc.

sounds like you are propped right for the boat now.

a 21 pitch will be too large of a prop on a boat that big with that drive ratio

you always prop for WOT. if you lug the motor you kill the motor

to jump to a 21 pitch on your boat, you need different gear ratio or a lot more motor

so what makes you want to change props?

• because you have the 21p? (if so, sell the 21p and buy the prop you need)
• because you want to go faster? (if so, you need more motor or a different boat)
• because you want better fuel economy? (no chance, lugging the motor burns more fuel and just kills the motor)

Thanks for the reply Scott, would it hurt to try the 21 pitch for 1 trip ? Because with this 15pitch if i give it 100% gas I will reach 5000rpm in a heartbeat. I was hitting 4300ish at around 60% throttle +/-. And the gear ratio is actually 1.31:1 sorry

 

[Scott Danforth]

your gear ratio doesnt make sense. you should be about a 1.47:1

really simple, if you put the 21P on the boat and when you give it the gas and it sounds like marbles running around in a dryer. stop. turn around, get the 15P and change it at the ramp before you go any further.

most I would ever put on the boat would be 17P if you are hitting 5000 RPM. however prior to doing any of that, I would verify the tach is reading right.

 

[Nick_1987 5.7 OMC]

your gear ratio doesnt make sense. you should be about a 1.47:1

really simple, if you put the 21P on the boat and when you give it the gas and it sounds like marbles running around in a dryer. stop. turn around, get the 15P and change it at the ramp before you go any further.

most I would ever put on the boat would be 17P if you are hitting 5000 RPM. however prior to doing any of that, I would verify the tach is reading right.

Ok thanks Scott