bertram31.com

Hello,
I have a pair of 6LY 315hp @ 3300RPM coupled to 220A 1:5/1 reduction ratio on my 31b project boat. During the mechanical revision of the boat, one of the transmissions is bad. I will replace it but have the opportunity to change the reduction ratio and invest on replacing the good one. My boat has the Cabrera Hns prop tunnels and after talking to Regino Cabrera, he indicated me that the best setting that he obtained was with 24” of diameter. I verified and have the clearance to accommodate that diameter. The shaft are 1.75” and are at 11 degrees. By using the basic prop calculator from boat diesel it provides an ideal prop of 18.2 x 23.8 in 4 blades with the 1.5:1 reduction ratio and 21.8 x 30.6 with a note of:”Decreasing the transmission ratio will improve prop efficiency”
If I add cup it changes to acceptable reduction ratio and change the numbers to 21.8 x 29.1. By placing 24” it provides 24x28.
The question for the knowledgeable is, what would be the ideal setting?
If I replace only the bad transmission now and later decide to change the ratio it will be more expensive down the road

Hopefully Bruce will see this, he might have an idea if what you want to do will get you to where you want to be. The Tunnels kind of change the game. I am not sure if anyone on the board as of right now have worked with tunnels on a 31.

Try and post the question on sbmar forum. Tony Athens (who owns seaboard marine) redid a 31 Bertram named "SHOOTER" a few years back. They installed tunnels (at the owners request) and I think they mated it with yanmars. The Yanmars were high HP tho like over 400 I think...

If you search it you might pick up some useful info

Here is a link for you to the Shooter project.

https://www.sbmar.com/repower-projects/ ... m-repower/