Different pitched props

[mratlndmrk]

On my new to me 380 the props are two different pitches....starboard is 22.5 and port is 23. At Sea Trial, the stbd engine turned up higher as would be expected. In going through the old paperwork from the prior owner I see that he had the props tuned in 2015 and the report shows that they were tuned to be exactly that. Is there any possible (logical) reason for that?

 

[juggernaut1]

The difference in pitch is due to the fact one transmission runs in "reverse" and the other in "forward" with slightly different ratios for each. To counteract the difference in ratios, the props are pitched .5 differently to obtain similar engine rpm. Same situation as on my 38.

 

[mratlndmrk]

Interesting....yet the starboard engine (lower pitch) ran at a higher rpm at WOT. I'll have to climb down to look at the ratios as it was not noted on my survey as them being different and my surveyor was surprised at the different pitches. Thanks for the reply.

The difference in pitch is due to the fact one transmission runs in "reverse" and the other in "forward" with slightly different ratios for each. To counteract the difference in ratios, the props are pitched .5 differently to obtain similar engine rpm. Same situation as on my 38.

 

[Tman]

That is an interesting setup (two different prop pitches) to me, but have heard of other boaters with differently pitched props between stbd and port. My understanding is that the overall gear reduction ratios are the same, however, the port engine transmission has an extra gear set to produce the counter clockwise rotation of the prop. On my prior 390DA, the gear ratios noted on my transmissions were both 1.6 I believe. My props were pitched identically and tuned, shafts trued, new cutlass bearing, and I had about a 1 GPH fuel flow delta between the two engines at WOT. Each engine hit the max 3065RPM. I gave up on chasing down or being curious about the 1 GPH or so fuel difference between engines at most cruise RPM's above >2300 RPM.

 

[fwebster]

There was one model of ZF transmission in one ratio where there was a noticable difference between forward and reverse final drive ratios. That transmissions was used in one or 2 Sea Ray models with gas engines and it was in the 34-38 ft DA's. That is the why and the reasoning behind SR providing props .5" different on Port and Stbd. No diesel powered boats had that ratio ZF transmission.

 

[mnm99]

Yes 2 different sizes with the ZF's. Here are my sheets for my 340. Normal to be .5 from the factory also. One runs forward one runs reverse. For some reason the ration is different I guess. I have 22 and 22.5's

 

[juggernaut1]

Interesting....yet the starboard engine (lower pitch) ran at a higher rpm at WOT. I'll have to climb down to look at the ratios as it was not noted on my survey as them being different and my surveyor was surprised at the different pitches. Thanks for the reply.

Have a look at the ratios online once you know which ZF model trans you have you will see the different ratios when comparing position "A" with position B". Mine is the ZF 63IV with 2.5 reduction. I also have the sheet from SeaRay showing the different pitch props.

 

[mratlndmrk]

I have the same gears (ZF 63IV) so I guess that's the reason. Been on the water my whole life but had never run into this before. At least I now know that there is a reason for it. Thank you to all who replied!

Have a look at the ratios online once you know which ZF model trans you have you will see the different ratios when comparing position "A" with position B". Mine is the ZF 63IV with 2.5 reduction. I also have the sheet from SeaRay showing the different pitch props.