Alpha One gen 1 ratio question

[Lift12]

Hi. Looking for info to grow my knowledge as I learn this boat. This post isn't about solving a problem per se.

This has been my first season on a new to me boat. Twin 5.7 Alpha One gen 1 (1989). I replaced the stock 1.5 Upper gear case with a new SEI 1.47 on the Port drive.

Question is Do I understand gear ratios correctly? At 1000 RPM is the prop turning at 1500 with a 1.5 ratio and 1470 with the 1.47? Obviously I haven't accounted for the gearing in the lower gearcase so I understand that actual RPM of the prop may be more/less than my example, but is the difference still three hundredths of a revolution?

There are a host of anomalies with the Port engine vs Starboard; less fuel consumption, higher WOT for example, that I am working to understand or correct. I wonder if this difference in ratio may be contributing some.

I appreciate your thoughts.

 

[HT32BSX115]

At 1000 RPM is the prop turning at 1500 with a 1.5 ratio and 1470 with the 1.47? Obviously I haven't accounted for the gearing in the lower gearcase so I understand that actual RPM of the prop may be more/less than my example, but is the difference still three hundredths of a revolution?

Howdy.

Actually you have it backwards. At an engine speed of 1470RPM, the prop will be turning 1000 RPM.

Just about all sterndrives (except for certain racing type surface drives etc) the gear ratio produces a lesser prop RPM.

If you want to play with the numbers a bit, go here: https://www.go-fast.com/Prop_Slip_Calculator.htm


Cheers,


Rick

 

[Bondo]

Question is Do I understand gear ratios correctly? At 1000 RPM is the prop turning at 1500 with a 1.5 ratio and 1470 with the 1.47? Obviously I haven't accounted for the gearing in the lower gearcase so I understand that actual RPM of the prop may be more/less than my example, but is the difference still three hundredths of a revolution?

Ayuh,..... The over-all gear ratio, is the stated gear ratio, combined upper, 'n lower ratios,....

'n Ya, in the grand scheme of things, the differences in yer's is irrelevant,....

The differences 'tween yer twins, could be motor health, or prop condition, among other things,....

 

[Lift12]

Thank you both. Now that I understand that the ratio is Final Driverimary Mover it is clear that 3/100ths is insignificant.