Ratios and propellers

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,924
I have a couple of lower units I'm selling and a guy needs one to fit his 88/85 short shaft.
My 85's are long shafts.
The 125's I have are both short shafts.
​IF?? he put the 125 short on his 85 short, the gear ratio is different, what would this do to the performance?
What kind of prop change would he need to do?

​I never understood(didn't try either) what would happen if I did this swap?
​I used to ask Frank :( , unfortunately that's not an option.

​THANKS Jerry
 

pnwboat

Rear Admiral
Joined
Oct 8, 2007
Messages
4,251
The 125's pre 1990 or so, usually came with 1.73:1 ratio lower units. The 1988 85's usually came with 2:1 ratio lower units. If you put the 1.73:1 125 lower unit on the 1988 85HP motor, it'll degrade the performance because the gearing will increase the load on the 85HP motor. A lot also depends on what type of hull (weight, length etc, etc) he's got. Hole shot and getting on plane with the 1.73:1 ratio will be worse than the 2:1 ratio with the same prop. I wouldn't be surprised if he would have to go to a 15 pitch or lower to get a decent hole shot and getting on plane, but this again depends on the boat/hull. Top end may not be as good.

I switched from a 1.73:1 lower unit to a 2:1 on my 125HP motor on my 1988 18ft Bayliner Cobra. The difference was significant. In my situation, I wouldn't even think about going back to the 1.73:1 lower unit because of the increase in overall performance.

I guess the only way you're really going to find out is to try it and see what you get....if that's even possible.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
You can get the rpms back by going down a few inches in pitch. It takes a bit more power to spin a prop faster, even if it is lower in pitch.
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
it will probably lower his RPM by 600 - i agree, go down 2" in pitch and it will be close. old lower unit 1.73 with 15 pitch was 39 mph 2.0 lower unit with 17 pitch will get me 38 mph. My hull is not fast so my results might be a tad off.

the 85's had a 3 stamped on the lower unit and the 125's had a 4 stamped on them. 3 = 2.0 ratio -- 4 = 1.73 ratio
 
Top