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Transmission Gear Ratio and Prop Sizing


New Member
We have a rebuilt Perkins 4-108 and ZF-12-M transmission with the higher forward ratio of 2.63/1 in our 384. However, I believe the prop is same as original - 3 blade 16x11. It doesn't seem to get the boat to speed without maxing rpm from the Perkins. I believe the original trans was closer to 2:1, and most I have seen on this forum have the 2.1/1 version of the same ZF-12 transmission. Does anyone also have the 2.63/1 gear ratio? And did you change props to compensate? Any other recommendations to get better performance without have to get such high rpm on the perkins?


Terence Thatcher
My ZF 15 is 1.88 to 1. Seems to work well, with cruising speed at 2000 rpm. Not sure changing your prop will work. Anyway, if you add up the cost of a haulout and new higher pitched prop--even if that worked, which I could not predict--the cost of a new transmission with proper ratio isn't much more. Sorry you are facing this, especially after the expense of a rebuild. The transmission in my boat is easily removed and replaced while she is in the water.


James M. Cleary
On our 1978 382 we have the Yanmar 3QM30H engine with a KH18 Kanzaki transmission with a ratio of 2.03/1. With a three blade fixed 16X11 RH prop, we cruise at 2200 rpm doing 6.2 kts in calm conditions. Hope this helps your inquiry.



New Member
Thank you both for the comments! Previous owner put in the rebuilt Perkins and transmission. Trying to find out why they went with the higher gear ratio option - if there was some advantage, or maybe just opportunistic.


Dave Ahlers
Jason, attached is the ZF repair manual, it may help.
In speed reading it quickly tonight it appeared (at first glance) that there is a choice of output ratios in each gearbox. Find your box and see if you have the ability to select a more advantageous ratio. Let me be clear - I am not sure this is true