Gear ratio on mixed upper SX-M unit and lower duo prop

[Aenelson11]

I have a JC 266 Tritoon with a 5.7Gi-e and 1.51 SX-M upper unit. A mechanic at a local shop had put on a duoprop lower, but I’m sure he got the gear ratio that is needed wrong. At WOT I can maybe get to 4000 rpm. The boat used to run at a much higher rpm and at a higher top speed. I’ve tried different prop sets and none of them help at all.

I’m just trying to know what the proper total gear ratio should be for the duoprop setup on the JC 266 Tritoon should. Any help would be appreciated. They did sell the duoprop setup originally but I have researched and can’t find any ratio anyway.

Thank you!

 

[Scott Danforth]

upper gear box ratios are all the same for the drive. gear ratio is determined by the foot and is paired to the motor

gear ratio for a small V8 duo prop is 1.95:1
gear ratio for a big block V8 duo prop is 1.78:1
gear ratio for a V6 duo prop is 2.15:1

Which props do you have? on a 'toon, I would expect F3 or F4

 

[Aenelson11]

Pretty sure they are F4’s. So you think I need a 1.95;1 lower unit gear ratio?

 

[Scott Danforth]

Pretty sure they are F4’s. So you think I need a 1.95;1 lower unit gear ratio?

what ratio of lower do you have? remember, the upper is identical across all the drives, both SP and DP

If you have F4's and are a bit too deep or you may need to drop to F3's

make sure you are not chasing a motor problem.

 

[Aenelson11]

I’ve went one size down and got out of the hole quick and one size up and it took forever to get going. It’s been a while since I did that, but remember changing the props didn’t help the situation.

 

[Scott Danforth]

I’ve went one size down and got out of the hole quick and one size up and it took forever to get going. It’s been a while since I did that, but remember changing the props didn’t help the situation.

If you dropped down and it didn't change the RPM at WOT, I would look at your motor

 

[Aenelson11]

It definitely increased the RPM at WOT.

 

[Aenelson11]

The motor is almost brand new. (Obviously still could have an issue) Trying to figure out my lower unit gear ratio now. Have my mechanic taking the lower unit off to check it.

 

[Scott Danforth]

The motor is almost brand new. (Obviously still could have an issue) Trying to figure out my lower unit gear ratio now. Have my mechanic taking the lower unit off to check it.

You could have just turned the engine over 10 times and counted prop turns

 

[Aenelson11]

Yes. Would have been the the smarter move for sure. All of this didn’t dawn on me til I really started to think about what my issue may be.