Being slightly over-propped enough to burn up 2 transmissions in 400 hours?

[sgtPluck]

Hi,
I'm back in Ft Lauderdale FL trying to get to the bottom of my transmission problems that I've had with my ZF 10M / Hurth HBW100 transmissions.
I lost two transmissions in around 200 hours each. Two transmission service shops here in FT Lauderdale said that it is highly likely that the boat is over-propped.
When I had the second transmission put in I did do a sea trial, and although my memory of it is that I reached max rpm (3,000) I may be wrong - it may have been 2,9000 rpm, (my prop is a bit too fouled to redo a test today.)
I'm wondering if that is the case -that it might have been 100 rpm beneath max rpm - would that be enough to explain burning out the transmission in 200 hours? The shops here seem to think so, saying that the tolerances would be low with a fairly light transmission like this. But I'm wondering what you guys might think.

Another possibility for why I have had these failures, is that the transmission is a bit too light for my application. It turns out that if you look at the detailed specs for continuous duty for the ZF10, it is only rated for 19-24hp. A representative at ZF (and one at Foley who re-sell ZF gears) said they thought it was a light gear and intended for 'sailboat auxiliary' and not heavy use. I can't help but believe that my motoring the whole way down the ICW from New York might count as continuous duty/heavy use.
If that is indeed the case, I wonder would it still be advisable to down-pitch my prop a bit, making it potentially a little under-propped for the boat but lighter on the transmission?

[sailronin]

If you can reach within 100 RPM of max engine rated then you're not seriously overpropped. I would be more more inclined to investigate the 19-24 HP rating of the transmission. What is your HP? If much more than 24 HP would suggest investigating different transmission suitable for your rated output.

[perchance]

I agree with sailronin in that your prop is probably fine. I'm not a big fan of the zf 10 transmissions as they are fairly lightweight for the job intended. Without auxiliary cooling they they run too hot under continuous load and burn the oil which leads to clutch and bearing failure. In most cases they are hardly worth repairing as they are fairly cheap to replace.

[sgtPluck]

I'm really not sure if I'm reading the spec sheet right for the transmission. On the top of the sheet it says maximum input for continuous duty is 37hp at 5,000 rpm, but in the details it says max rated power for continuous duty is 18-24 hp. I'm really not sure if that refers to the power output or input.

http://www.tadiesels.com/transmissio...es/ZF_10_M.pdf

That aside, the ZF and Foley reps both indicated that it was meant for light duty, but that is just anecdotal evidence.

Either way, I've burned up two transmissions in 200 hours each of usage. Everything else with the installation checks out, alignment, shifting etc. I'd be happy to sacrifice a bit of boat speed if it meant not losing another transmission. I'm wondering is boat speed the only penalty of down-pitching my prop a bit.

[sgtPluck]

Well those are certainly the symptoms I have seen, burned oil and clutch failure.

My other alternative is to put a TMC 40 transmission in there instead, but that would require some re-working, and I'd hate to do that only to find that being over-propped was my problem all along.

[perchance]

Horse power in. Rather than reprop see if there any other gear ratios available in this model of transmission.

[David M]

You are clearly not severely over propped. Slightly yes, but probably by only an inch in pitch. This is clearly not the reason for your transmission problems.

It seems more likely your transmission is under rated for your engines horsepower.

[SkiprJohn]

Being slightly over propped does not burn up good transmissions. If you want to continue using your current transmission after another repair then under prop or cut down on your engine speed and continuous use. Otherwise find a different transmission that can handle the load. You didn't mention the horsepower rating of your engine.
kind regards,

[sgtPluck]

It's a 2008 Universal 25 XPB , the ZF 10m is the stock transmissions that comes with the engine. That's why it seems kind of crazy that this gear would be too light for the engine. Maybe it's not the gear, but my relatively heavy boat.

I think the cheapest thing for me to do, given that I have a brand new replacement ZF10m installed already, is to under prop (maybe modify my prop and take 1" off the pitch) and use the engine less & go slower. (I currently motor at 2000-2200)

What I'd really like to do is have the problem solved once and for all.

[David M]

No, a heavy boat nor being over propped does not add additional torque to a transmission. The energy applied to the transmission comes only from the engine. It is partially the amount of torque being applied to a transmission that determines whether or not it is overloaded. You are certainly not over revving the transmission which is where horsepower would start to be a factor...which is not relevant here.

It is either a junk transmission or it is underrated for the engines torque and horsepower or something else is going on like an alignment problem, improper instillation, improper lubricant, improper use or something similar. I am not saying it is necessarily improper use, just throwing all the possibilities out there.

[sgtPluck]

Believe me I've certainly wondered if I was the problem, but as far as I can tell I am not. On perhaps 2 occasions I have shifted from reverse to neutral only to accidentally follow through and throw it straight into forward, but if that is enough to kill a transmission then I am screwed.

So if I'm not going over 2400 rpms (I never do) then hp should not be an issue?

I don't think it is a junk transmission, unless both of them happened to be junk. Mind you, there was a user here who went through 5 ZF15s before switching make.

I've checked everything else, alignment, installation, lube. If there's something wrong it certainly isn't obvious.

I'm sorry to hear that there seems to be general agreement about the prop not being the issue, because that would be a cheap fix.

I've inquired about a PRM 80, it would require adjusting engine mounts and some other re-working, but I might do it.

[S/V Antares]

I had 2 zf10 failures due to over pitch. Then took 2 inches out and could easily reach 3000 rpm max rated but ran the engine at 2400 instead of lugging at 2100. and got the same boat speed. (About 6kn.) The trans temp dropped 50 deg. (Using a rayteck gun)

When I repowered to my current Beta I chose PRM120. I am done with small gears.

[sgtPluck]

Thanks Antares,
How far off the max 3,000 RPM were you before you re-pitched?

Re-pitching is the cheapest thing I could do for now. Putting the bigger PRM would require some re-working but I'm looking into it as an option.

[S/V Antares]

Sgt. Pluck, I was getting about 2900 but pushing it hard. With the re prop it jumped easily to over 3,000. The main benifit was better control in reverse. The first trans had a ratio just under 2/1 . I think it was 1.95./1. That gear was discontinued and the "2/1" you can get today is acctually 2.05/1. Just a little lower. All the ZF hurths should run a rt hand prop. A left hand prop will appear to work but uses the idler gear which will fail if used for any lenght of time.

[Boracay]

I put a ZF63 on my John Deere 4045D. Its rated for 142hp continuous at 2400rpm, the engine is rated about 85hp at 2500 so its a bit of overkill. Only a few dollars more so well worth the extra.

I did need to replace a copper washer as it leaked a bit of oil, but otherwise no worries. Smooth fast shifts without a clunk in the world.

I'm rather overproped as well. If I run my engine at its max. of 2300rpm (should be 2500) it sends loud messages of being unhappy so I usually throttle back to 1800. Only a half knot difference and much more pleasant.

I don't think that gearbox manufacturers have us cruisers in mind when they do their ratings. Cruising yachts are heavy and slow so the gearbox is going to be under load all the time. The oil is going to get hot.

Have you considered a ZF25M? Its rated at about double your hp and with luck could be a drop in replacement (please check before ordering...). Add an oil cooler, change the oil more often than the recommended intervals and you may not have to replace it for quite a while.