Yanmar 230 with 1.5:1 transmissions Hatteras 34

[micky]

I found a pair of Yanmar 230hp engines with 1.5:1 transmissions. Ideally I would need 2:1 transmissions, but I can get the engines for around $10k. Right now the detroits are 170hp with 1.52:1 transmissions.

Would the Yanmars with the 1.5 work on my boat? Hatteras 34.

 

[Boatsb]

I'm not sure why you think you need 2:1 transmissions

Do the math first.

Shaft size and material , hull shape and weight. Engine rpm and torque. Run it through the calculations and see.

 

[jim rosenthal]

They would work, certainly. As to whether you need to change the gear ratio for 2:1, you would need a formal repowering analysis by a Yanmar consultant to tell you that. My guess is that the 1.5:1 gears would work fine. Are 1.5 gears the optimal ratio? Close if not perfect. Most Cummins repowers in 34 and 36 Hatteras convertibles use 1.5 or 1.75:1 gears. You would be in the ballpark.

 

[micky]

I ran the calculators on boatdiesel and with the 1.5:1 transmissions props suggested are 16-17 diameter, with the 2:1 props suggested 19-20 diameter.
Right now I'm running 19 x 19 propellers with the detroits.

 

[Boatsb]

Boat diesel is a start but you need a real TVA to determine the best setup.


The TVA will be far more complete than the prop calculator.

 

[davidwigler]

Seems backwards. I’m not an engineer but if 2:1 spins the prop faster it should require smaller props. I always heard slower and larger was more efficient if you have room between the shaft and hull.

 

[Boatsb]

It's a reduction gear to the shaft.

2:1 is 2 engine rotations to 1 shaft rotation

1.5:1 is 1.5 engine rotations to 1 shaft rotation

Slower turning props have higher torsional stresses. Which transfer to the shaft and deflection of the shafts and struts.

 

[jim rosenthal]

David, 2:1 will spin the props slower, not faster. Very few modern boats have direct 1:1 gears. Actually, especially on bigger boats the trend has been towards deeper reductions and slower shaft speeds, also larger wheels with more blade area, in tunnels or indents in the hull. Slower tip speed means less cavitation, supposedly, although the tip speed goes up when you go to bigger wheels. But overall, more blade area, more blades, bigger diameter and more pitch, all turning less quickly are the modern thinking on powering boats.

The situation is not so much changed when you look at repowering older boats like mine or the 34’. In those cases, the hulls don’t have tunnels and you are limited by the tip to hull clearance. Therefore, you tend to end up with shallower reductions, faster shaft speeds, and smaller wheels. Without reengineering the entire bottom of the boat, which isn’t at all cost effective, compromises have to be made.

I think he is well in the ballpark with a 1.5 gear, but he needs a proper and power analysis done by someone who knows these engines.