transmission brand & reduction

[Bertramp]

Aside from my "45 Hatt" ... I have a 24 Topaz straight inboard center console. Engine is a cummins 6BT-210 hp. Bought boat as salvage and have basically put it back together (over budget, but that always seems to happen). In any event, engine runs well, but does not turn up to 2600rpm as it should (only 2000 rpm). I believe that prior owner used at slow speeds and to save fuel deliberately over-propped. Problem is ... transmission label is not there (I think it's a twin Disc 502, but not sure) and I can't get prop worked until I know the reduction.
Bottom line ... what is easiest way to figure out reduction on a tranny ?

 

[motoryacht lover]

Verify this with some of the experts but I think you can photo tach the harmonic balancer on the front of the engine and then shoot the shaft. If the engine is idling at 800 rpm and the shaft is turning 400 then you have a 2 to 1 reduction. Once again verify this with a mechanic but this should get you started.

 

[Bertramp]

Seems like mechanic finally located the plate on the transmission (underneath) ... apparently it is a Detroit Diesel tranny (didn't know that they made them). I wonder if it is actually Allison as DD ?
Anyway it's a 2:1 reduction and slowly but surely I'm getting this little boat together. Runs OK, but it should run better.

thanks for the help folks

 

[Fanfare]

Twin Disc put DD labels on them.

 

[sgharford]

I'll just throw this out there - replace your fuel filters. I just bought my 43DC in December, when we sea trialed her she only went to 2400 RPM and not 2650 RPM (had the bottom cleaned prior to sea trial). "Best Cummins Engine Surveyor" on Long Island told me to have my props recut. Well - I replaced all the fuel filters and engines now run at 2650 RPM WOT.

What really gave this away was when we brought her home in 6 foot seas - the tanks sloshed up so much crud that she only did 1600 RPM by time we got into home port. If you have manufacturer's OEM props on their, good change it may be your filters as well.