ZF transmissions with different ratios fwd/rev

R

Ray Kautzky

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Joined
Jan 1, 2000
RO Number
1491
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13
During a recent failure of a ZF 63 IV tranny I found something very interesting that I hope will help me solve another problem. These trannies are designed to run full power in either fwd or rev. Since we all have counter rotating props one is always in fwd and one in rev when travelling fwd. The strange part is that the gear ratio between the 2 positions is slightly different. In my case the tranny is called a 2.5:1. The starboard one in fwd is actually 2.48:1 while the port is 2.53:1. At WOT (5100 on mine) that amounts to over 100rpm difference. I've always had an issue where my starboard engine doesn't quite make it to WOT (about 200rpm short) and also uses a bit more fuel. I have heard that if one prop is slightly tweaked (either add pitch to the faster one or remove pitch from the slower one) this can be equalized a bit. It would have to be a very small amount I'm guessing as you would only be doing one. I know that when we are cruising and the engines are synced I pull a bit to port so I think all of this make sense. I still need to find the other 100rpm. Will do a good tune up in the spring so maybe that will help.

Anyone else ever found this to be similar on their boats and have you been able to counter act by tweaking the prop? Would be interested in what changes you made. I have a 370 express running Volvo 420hp with 22 x 23 michigan wheel props (non cupped)
 
Your statement presumes that both engines are the same rotation. Also at 5100 rpm, the prop difference is 41 rpm.
 
Mine are Twin Disk 502 and they too run Cw/CCW full power.
One of them just runs on "reverse" all the time.
I'm heading for the boat in a few and will check the manuals
about the F/R gear ratios.
Strangely, logic would tell ratios must be the same F/R.
 
Yes engine rotation is the same. The ratio is stamped right on the plate on the transmissions. I always thought they would be identical. I realize it's not a huge difference but it is a difference. I would prefer to have the same thrust at the prop when the engines are synced.
 
Velvet Drive Vee drive units are slightly different right to left due to one being all gears and the other being chain. Not a big deal.
 
I have the same ZF ratio issue with mine.

We knew about it when we did the repower.

When we fine tuned the props, a slight bit of cup was added to one of them.

At displacement speeds, i match RPMs

when on plane I match the boost guages, as it was explained by my Yanmar/Repower expert.

This way the RPM's don't exactly match but the output does as the engines are both putting out the same load.

RWS
 
Oldhouseboater- I'm curious why the ratios would be slightly different on those VD v-drives due to chain vs gear linkage. Ignoring the reduction gears for the moment, wouldn't they use same size main gears on both chain and gear links , and if so wouldn't the output gear turn at the same rate relative to input gear on both types, no matter what size the link gear used was on the gear-type. So why would the reduction gear ration be different?
I know on my straight-drive Velvet Drives (72C's that cannot be used on both LH & RH engines, only one or the other as factory set up, and cannot be run full power either direcdtion) the ratio plates indicate they employ identical reductions for both LH & RH engines.

While equalizing the loads makes a lot of sense, I agree about the un-sync'd engine resonance question.

Ray- Since the stbd engine attains lower WOT RPM, but the port transmission employs a slightly greater reduction, wouldn't results be closer to equal if the transmissions or just reduction gears were swapped? I may easliy have that wrong but you might ponder that. Also, I probably wouldn't do much of anything else ( except scan the engines) before having the props scanned so you know exactly what effect any differences there might be causing.
Clearly these engines are gassers, so they are Volvo 8.1's right?
 
quote:
Originally posted by OLD HOUSEBOATER

RWS

How can you stand to run with the engines unsynked. I would go batty?

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=============================================

Synchronizing the 454's was sweet.

One was counterrotating. With th eTrojan side exhaust syncing them would produce a powerful and really cool sound. If one came out opf phase by th eslightest, you could easily hear it, like someone singing off key.

With the Yanmars, it seems the turbochargers mask the individual exhaust pors sounds, they kind of blend each cylionder together.

The only drawback is at idle. The 454's coming out of the side exhausts where you coyld hear each cylinder firing created a very unique note on the canal.

Her note is still unique, anyone can tell who she is just by the sound coupled with those waterline exhausts, but the 454's just had a sweeter sound at idle.

thrummthrummthrumm.

RWS
 
Your 72c units can be run under full power in the forward direction only. Direction capability to fit engine rotation is accomplished by rotating the hydraulic pump in the housing for the opposite rotation engine.

With Vee Drives you use standard rotation engines for both sides.

Ratio differences only occur in Vee Drive units.

Some examples of Velvet Drive Vee Drive ratios.

1.99:1 1.98:1

2.50:1 2.49:1

3.10:1 3.14:1

From a practical stand point these differences don't amount to a pimple on an ant's a$$. Only people who are anal even think about these slight discrepancys.
 
I did the reverse, in my new Walter V drives I had an idler gear installed so I could run two CCW engines. The Borg 72C 's are straight transmissions both still run in the same direction under power and feed the Walter V drives. Well unless I'm docking or spinning the boat with the throttles.
Bill
 
Interesting thread and I'm glad I read it all the way through. I'm contemplating repowering my old Silveton 34C from the original Chrysler 360s to GM 350s and I have power trains similar to what was discussed here: 71C Velvet Drives (1:1) and Walter V-drives (1.5:1). My old engines are counter-rotating. I've been told that if Walter rebuilds the V-drives, they can retrofit the idler gear so that both of the new engines will be the same rotation. That would greatly simplify the re-power project, although not make it any cheaper once the cost of the V-drive rebuilds is taken into consideration.

Bill
 
Sandy, This model of transmission is just designed that way. I don't believe there is anything to swap. It they could have made them identical they would have already done it and I wouldn't have this issue. I'm thinking maybe a 1/2" of pitch added to the port side. I don't want to do too much and then have to wait until the next haulout to change again. I'm heard cupping is also an option but these doesn't seem as easy to guage or calculate.

And yes enjoy the sound of both engines synced when cruising on plane across the lake. Anal or not. Currently at wot it's over 100rpm diffence.
 
quote:
Originally posted by Ray Kautzky

<SNIP>

Anyone else ever found this to be similar on their boats and have you been able to counter act by tweaking the prop? Would be interested in what changes you made. I have a 370 express running Volvo 420hp with 22 x 23 michigan wheel props (non cupped)

Click to expand...


I now find myself in the same exact spot.

We pulled the props and sent them to Propscan for some fine tuning.

Our ratios are 2.04:1 and 2.10:1

Propscan is recommending that we sync the engines, not the props (which was at best, a guesstimate)

Plus the synced engines sound much better with those wonderful Trojan built into hull muffler chambers and side exhausts !

Any info would be appreciated.

RWS
 
Over the years it is very common for the port and stb gears to have slightly different ratios. The goal is to match the load on each engine and have each one contribute doing the same amount of work.

RWS has it covered perfectly.

"when on plane I match the boost guages, as it was explained by my Yanmar/Repower expert.
This way the RPM's don't exactly match but the output does as the engines are both putting out the same load."

You can use the boost gages or fuel flow gages if they are calibrated well to balance the two engines very closely. For people who have boost gages and do this they have learned that a very small number of rpm's (like 20-30) can vary the boost (pt to stb) by a larger percentage and show how a little rpm change can shift the power loading greatly form one engine to the other.
If you have varied ratio trans it is not uncommon to get the prop pitches adjusted during a scan/adjust to make up for the unbalanced ratios.
Practically speaking I can barely notice the lack of 'synch' when the engine loads are matched and I have been doing this for over 20 season now with a few different boats. The boost gages are very sensitive and can allow you to make minor adjustments for loading in real time - and for those that also have pyrometers it will show you that balancing the engines for 'load' will reduce heat greatly in the previously burdened engine.

Hope this helps
 
Sync the engines and adjust prop for load. CANT STAND out of sync engines at cruise. Note: the difference in load, at cruise, amounts to diddly squat.
 
That information is inaccurate for diesels and the out of synch 'sound' on muffled turbo diesels are barely able to be heard.
These would be confirmed by my pyro and boost gages, the even fuel burn over many seasons and the Db meter I have utilized on the boat.

Hope this helps
 
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