Changing reduction ratio VD 72C

[Jeff F]

Long story, but since many of us have extra time, here goes:

My Mainship 34 M1 is on its original transmission, which is a 72C 2.1 CR.

Since the PO swapped to a Cummins about 2400 hours ago it's been run LH instead of RH. I had seen the direction arrow but hadn't really twigged until I pulled the motor and transmission a couple of years ago. At that time I swapped motors from a 210 to a 250 hp but left the prop and transmission untouched apart from new seals and damper plate. The boat has a 20x24 prop which was about right for the 210 in light ship. I figured the 250 wouldn't mind that and would accommodate cruising trim. Wasn't really looking for more top speed.

Long story short, I think I've killed the reduction gear. I was getting gear noises late last season and there is a noticable fore and aft play at the coupling. No surprise, I guess.

In preparation for this event I acquired a 71C 1.5 locally about a year ago. My plan is to pull the old transmission, swap reduction gears, and swap props. I researched this when I repowered, and decided then that I'd be better off with a 1.5 reduction.

Any advice/concerns? I'll probably replace seals and gaskets as I did last time. I had the reduction gear off last time, and everything I read says they're interchangeable.

I'll introduce the prop question in a different thread.

Thanks!

 

[Pete Meisinger]

Captain, you lost me at "It's been run LH instead of RH" I didn't realize you could do that.

And then changing the reduction rate and swapping parts.. Big job way beyond me but it sounds like you have it well in hand.

Congratulations!

pete

 

[STB]

Hold up. Can you post the model numbers on each of the transmissions?

See the table attached for examples of CR2 model numbers.

I dont think you'd have made it too far with that much power in reverse. And, your boat might be turning or controls might be way off as the forward and reverse ratios can be different.

 

[Nomad Willy]

Call Mike at Harbor Marine in Everett Wa.

They will probably have whatever gears you want.
They are a BW gear specialist.

 

[SoWhat]

second gkesden. You need to ID the transmission model. I believe Cummins 210/250 has CCW rotation. Prop needs to be matched to whichever model transmission you have.
Presuming there won't be any issues changing gear ratios. You will need a much smaller prop.

 

[Jeff F]

It's 10-18-007. I'm assuming the original Perkins was a counter rotating model. Based on what I've heard this wasn't uncommon in these boats. The PO said he had swapped from a RH to LH prop during the conversion.

I'm not close to the donor transmission, but it's a 71 series 1.5.

Looking at it now I might just leave the transmission on and swap the reduction gear in place - it would be a lot less lifting.

 

[STB]

That transmission is a 72C (Not CR2) It should be about 2.1:1 in forward gear, regardless which way it turns. 2.31:1 in reverse gear.

It is not a counter rotating transmission. In forward gear, it'll turn with the same rotation as the engine, regardless of what that is. Opposite for reverse.

This whole reversing thing doesn't make sense to me. But, on a 72C it shouldn't be a problem, either way. It isnt running it in reverse gear, just spinning it the other way.

250hp diesel is too much for a 71C. Too much torque (.) You need a 72C for that. Or, you can detune and limit everything. Or, you can promise yourself you'll never use it...until that one time you are stuck in the mud...just for a..

Here's everything I've got on the 71 and 72 series, including HP-RPM curves:
-- https://drive.google.com/folderview?id=1PTuUfQRyBqaHGVztPRbFX43AoZqyguM_

 

[jleonard]

Having done a repower of a Mainship model 1 with a Cummins 270, I definitely agree that 1.5 to 1 is the correct ratio.
You may have to re-pitch the prop. I was running a 20 x 21 4 blade and it was right at 2630 at WOT. I don't think you'll be able to spin a 24 pitch prop to 2600.

Also a 71 series will die in short order a 250 hp diesel. You need a 72 or 73 series, or change to a ZF (or Hurth), or twin disc.
Per the manual a 71 series can handle up to 180 diesel hp.

 

[rslifkin]

You can spin a standard 72C either direction, just reverse the pump (otherwise it won't build fluid pressure and go into gear). I've got counter-rotating gassers with 2.57 reduction 72Cs behind them. One engine rotates CW, the other CCW. Prop on each side rotates the same as the engine, just the pump on one trans is reversed.

So the reduction gear failure likely was just age / power related. I've stripped the teeth off the stationary plate in both of mine (a few years apart). One reduction gear was a little rattly before the failure and had some gear damage, the other looked new inside and just needed a new stationary plate.

 

[Jeff F]

Thanks all. Here's a quote from an old manual I found:

'All Velvet Drive units except the 2.1O:l In-Line and
CR2 units may be used behind engines having either
rotation; however, the pump must be indexed for the
desired rotation. The reduction unit planetary carrier is different for opposite rotating 2.lO:l In-Line units and
early failure will occur on these units if they are driven

in the wrong direction.'

That's my case. The 2.1 came as either 10-18-007 or 10-18-008 depending on rotation. There is a big direction arrow on my reduction gear indicating CW rotation but it's been turning CCW since the original motor swap.

Going back to the 71c vs 72c, all my research leads me to believe that the reduction gear assemblies are common to both. My plan is to keep my existing transmission but swap out the reduction gears from 2.1 to 1.5 and change props. I don't think it matters that the 1.5 reduction gear is coming from a 71c. Am I wrong about this?

 

[rslifkin]

I'm pretty sure you're correct about the 71C and 72C using the same reduction units. You'll be able to swap that over without issues, I expect.

I didn't know about the 2.1:1 reduction unit being different from the others in terms of rotation tolerance. That's an odd quirk, probably related to the 2.1:1 being the only ratio (of the non-CR2 trans) that rotates the output opposite from the engine.

 

[Jeff F]

Having done a repower of a Mainship model 1 with a Cummins 270, I definitely agree that 1.5 to 1 is the correct ratio.
You may have to re-pitch the prop. I was running a 20 x 21 4 blade and it was right at 2630 at WOT. I don't think you'll be able to spin a 24 pitch prop to 2600.

Also a 71 series will die in short order a 250 hp diesel. You need a 72 or 73 series, or change to a ZF (or Hurth), or twin disc.
Per the manual a 71 series can handle up to 180 diesel hp.

Jay, I reviewed your accounts of repowering a few years ago and they helped me in many ways. Thanks for chiming in.

I'm actually considering a 19". The 20" really doesn't have enough tip clearance IMO.

 

[jleonard]

Jay, I reviewed your accounts of repowering a few years ago and they helped me in many ways. Thanks for chiming in.

I'm actually considering a 19". The 20" really doesn't have enough tip clearance IMO.

If you go to a 1.5 to 1 ratio you WILL get hull vibration. Easy to solve by fairing the keel ahead of the prop, and adding some gussets above the prop ahead of the rudder.
Picture of fairing below.

 

[Jeff F]

If you go to a 1.5 to 1 ratio you WILL get hull vibration. Easy to solve by fairing the keel ahead of the prop, and adding some gussets above the prop ahead of the rudder.
Picture of fairing below.

Thanks. Interesting to see that your shaft was longer than mine. I barely have room for a collar anode in front of the prop, so fairing may not be as effective in my case. Will play around a bit! Nice to have others to follow.

 

[FF]

Before spending big bucks check in Boats and Harbors with T-D rebuilders.

You may find a rebuilt Twin Disc with no core charge that is commercial grade .

 

[jleonard]

Thanks. Interesting to see that your shaft was longer than mine. I barely have room for a collar anode in front of the prop, so fairing may not be as effective in my case. Will play around a bit! Nice to have others to follow.

I had added a 1 inch thick solid steel spacer to give me room for the zinc.
The fairing job was an important step in prep for my repower that I was planning. I actually gained almost 250 engine RPM with the original Perkins 160 by adding the fairing.
That was huge in my opinion.

 

[FF]

"I actually gained almost 250 engine RPM with the original Perkins 160 by adding the fairing.
That was huge in my opinion."


Did you send a letter bomb to the designer or boat builder or both?

 

[jleonard]

"i actually gained almost 250 engine rpm with the original perkins 160 by adding the fairing.
That was huge in my opinion."


did you send a letter bomb to the designer or boat builder or both?

lol