Transmission Gear Ratio and Prop Sizing

[_Jason_]

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?

 

[terry_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.

 

[jimcleary]

Jason
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.

Jim

 

[_Jason_]

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_a]

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