31.8 omg 25hp vs 35hp gearing quandary.

**RaisedByWolves** · 06-03-2020

I have two motors I’m tinkering with and I pulled down both lower units and found the 35 has much lower gearing than the 25.

The 35 is a 76 model and the 25 is an 84.

I want to use the 35 power head and what I’m wondering is which lower units gearing to use for the best top end speed.

Would the lower gearing of the 35 lower unit coupled with a larger more efficient prop be the way to go, or would the higher geared 25hp lower units gears with the smaller higher pitched prop be best.

This motor will be used on a 14’ aluminum semi vee.

**Txspeed121** · 06-05-2020

i am very intrested i wondered this myself.

**RaisedByWolves** · 06-05-2020

Yeah. I know from racing RC boats that the larger prop with less pitch will be more efficient and on a 16’ or larger boat glass boat would be the way to go due to the weight, but I feel the 14, tin boat might be right on the edge of going one way or the other.

And to clarify, I’m talking about standard props but discussion of choppers or other options is welcome also.

**Powerabout** · 06-05-2020

light and fast its always the tallest ratio.
that means maybe less acceleration and less load carrying ability but will always mean best top speed.

**RaisedByWolves** · 06-06-2020

Originally Posted by Powerabout

light and fast its always the tallest ratio.
that means maybe less acceleration and less load carrying ability but will always mean best top speed.

Yes, I can understand that as a general rule to go by.

The issue with these motors is that they are of the same displacement with only minor internal changes between the 35 and 25/20.

The carb and intake are the known differences, with port timing and one having points (35hp) and the other electronic ignition (25hp) being the other variables .

**DeanFHobart** · 06-06-2020

Hello,

Just as an suggestion I would just do some testing. I would call Ron Hill at Hill Marine... He is the owner of this web site and Hill Marine is on the top for their contact information. In the racing community the answer is always... Testing, Testing and then Test some more!!!

Good luck on your project.