ZF Hurth transmission slipping

[Garester]

I have a Hurth transmission on an M25XPB from 1990. I recently broke my strut and the prop hit the hull and broke. I got a new shaft, coupler, strut, prop, etc.
When I put it back together the ENTIRE shift/throttle assembly got a thorough re-do with all new parts. The tranny worked pretty well...at first. Then on the way home from the boatyard I noticed it wouldn't do any more than about 1500 RPM (my tach wasn't working at that time. I could make it rev higher but then the tranny would make a noise and lose traction and back down to the aforementioned 1500 or so. BTW, I changed the prop from a MARTEC ELLIPTIC RH13,14 to a fixed three-blade prop, which is 13 x 15, I got on EBAY. I know the higher pitch can cause issues, but the only issue I find is that I get full hull speed with only 2000 RPM. After totally tweaking the cables to work perfectly and trying the transmission without the shaft connected and without the cable connected it got full RPM's at the dock. I finally figured that it was slipping. I emptied it and added paint thinner, ran it for half an hour in all the different ranges and then drained it and added the required amount of ATF. It seemed to work perfectly AT THE DOCK. But once I took it out and sea-trialed it I got the same thing as it was doing before. It especially didn't like just giving it full throttle. Slowly raising the speed with the speed of the boat worked fine... mostly.
My question is: has anybody else had this problem and if so, were you able to remedy it or am I just doomed to buy a new tranny? I believe I've exhausted all the options at this point.

Thank you!

 

[dmax]

First, you are seriously over-propped if you reach maximum speed at 2000 rpm, you should be reaching it at around 3000 rpm and cruise in the 2100 - 2600 range. The prop is putting more strain on the transmission and not helping things. Get the prop straightened out and then see how the transmission reacts.

 

[Jim 5757]

What trans do you have there???

 

[Ward H]

Just for comparison my M25XP turns a 13-10 3 blade on my 93 C30. It reaches boat speed about 3100 rpm.
I’d try dmax’s suggestion before thinking of a new trans.

 

[Capt jgw]

All Hurth gears will eventually slip. It's a bad design and defective from the factory. If changing the prop doesn't help, consider swapping the Hurth with a New Age PRM 60D. It's a near drop-in replacement and will not slip. Cost is about $500 from ASAP Supplies in the UK, about the same or cheaper than the parts to rebuild the Hurth, which won't last anyway. The only difference is the PRM requires the engine to be raised 1/4" on it's mounts and the shift lever is on the opposite side and goes the opposite direction. Both easily corrected for.
Sounds like you're getting hull speed at about 66% of max speed, this should be more like 80-90% or 2400-2700 RPM. This can overstress not only the trans but the engine as well. And when motoring against a headwind for instance, you might not be able get that speed because the engine is only putting out about half it's rated power at that RPM. Max horsepower is usually at around 80-90% max RPM. With your setup your getting about the same performance as a 10-12HP engine that's properly propped, and burning more fuel to do it.

 

[jibes138]

As others mentioned your prop needs to be matched to your motor and boat. What is the maximum RPM rating on your motor? You need your prop to allow the motor to reach maximum RPM's. Fixed props can be adjusted by altering the pitch but only a couple hundred RPM. Not enough pitch and you can overspeed but that isn't your issue. Your trial needs to be underway to check for max RPM. Talk to a prop shop and have them do a calculation for you to give you a recommendation on the right prop for your boat. This should get you in the ballpark and they can adjust it from there. Or you can call Catalina and see what prop they used on the boat when it was new. Try this and see what you get : Prop-It-Now // Michigan Wheel

 

[Garester]

What trans do you have there???

I have the HBW50

 

[Garester]

First, you are seriously over-propped if you reach maximum speed at 2000 rpm, you should be reaching it at around 3000 rpm and cruise in the 2100 - 2600 range. The prop is putting more strain on the transmission and not helping things. Get the prop straightened out and then see how the transmission reacts.

Thank you for the very wise answer. Good information. It's been so long now that I don't even remember what was involved with that 2000 number. However, looking at the date of my original post I believe that was with the original prop which came with the boat. The guy who installed the new engine and prop in the 90s had everything done professionally and I am sure that the boat functioned properly for him as he was a racer and I have all the paperwork. He did things correctly. I think maybe the boat achieved a good speed but maybe above the 2000 number it would slip. I think. In 2019 the prop exploded, threw a blade and in the process took the strut and the shaft out with it. In the process of trying to replace the prop I went on eBay and accidentally bid on a random prop...then won unexpectedly. Oops. LOL. I took the prop to the prop/shaft guy who made my new shaft. We did the math and he true pitch & balanced the prop to the correct specs for engine/tranny/boat.
I have put a LOT of miles on this tranny since then. It was slipping when I bought it but worked well enough. It's just gotten worse over the years.
I think the prop is ok. Maybe we decided to "try it and see how it goes" and it was good enough for me. Who knows...Lol. I can't be trusted.

 

[Garester]

As others mentioned your prop needs to be matched to your motor and boat. What is the maximum RPM rating on your motor? You need your prop to allow the motor to reach maximum RPM's. Fixed props can be adjusted by altering the pitch but only a couple hundred RPM. Not enough pitch and you can overspeed but that isn't your issue. Your trial needs to be underway to check for max RPM. Talk to a prop shop and have them do a calculation for you to give you a recommendation on the right prop for your boat. This should get you in the ballpark and they can adjust it from there. Or you can call Catalina and see what prop they used on the boat when it was new. Try this and see what you get : Prop-It-Now // Michigan Wheel

Thanks Dave,
I appreciate you taking the time to respond. I believe we handled all that in 2019 when I replaced the prop.

 

[Garester]

All Hurth gears will eventually slip. It's a bad design and defective from the factory. If changing the prop doesn't help, consider swapping the Hurth with a New Age PRM 60D. It's a near drop-in replacement and will not slip. Cost is about $500 from ASAP Supplies in the UK, about the same or cheaper than the parts to rebuild the Hurth, which won't last anyway. The only difference is the PRM requires the engine to be raised 1/4" on it's mounts and the shift lever is on the opposite side and goes the opposite direction. Both easily corrected for.
Sounds like you're getting hull speed at about 66% of max speed, this should be more like 80-90% or 2400-2700 RPM. This can overstress not only the trans but the engine as well. And when motoring against a headwind for instance, you might not be able get that speed because the engine is only putting out about half it's rated power at that RPM. Max horsepower is usually at around 80-90% max RPM. With your setup your getting about the same performance as a 10-12HP engine that's properly propped, and burning more fuel to do it.

Thank you for the response and for the valuable info. You mean to tell me it's not as easy as just push-the-pedal-and-go?
;-)
I hadn't actually thought about those variables or that I may not be getting full horsepower. I think I neglected to mention that it slipped above 2000 rpm's rather than what I said. It does reach a good solid 6-8 knots at that rpm, but can maybe achieve a little more. I'm going to look into those PRM units. Although, I think I have and found them to be more pricey than $500. Of course this is now five years later...

 

[Hayden Watson]

Thank you for the response and for the valuable info. You mean to tell me it's not as easy as just push-the-pedal-and-go?
;-)
I hadn't actually thought about those variables or that I may not be getting full horsepower. I think I neglected to mention that it slipped above 2000 rpm's rather than what I said. It does reach a good solid 6-8 knots at that rpm, but can maybe achieve a little more. I'm going to look into those PRM units. Although, I think I have and found them to be more pricey than $500. Of course this is now five years later...

You can also rebuild the HBW 50 for about $500.

Hurth HWB 10 Transmission Rebuild dissassembly part 1 A - YouTube

 

[Garester]

You can also rebuild the HBW 50 for about $500.

Hurth HWB 10 Transmission Rebuild dissassembly part 1 A - YouTube

Thank you Hayden, have you done this before? do you know how intensive it is?

 

[Hayden Watson]

Thank you Hayden, have you done this before? do you know how intensive it is?

I have not done this transmission but have done other gearboxes. This type is by far the simplest. I was a farm diesel mechanic before going back to college to become an engineer.
The only specialty tools needed are a caliper and shop press. The press is the one that I and many others do not have in my personal shop, but I have always been able to find somebody that would let me do a quick press to get the job done.
They can be found new for as little as $100.

 

[Ward H]

I had to go with a new tranny to replace my HBW50 because my transmission guy, who used to do rebuilds for Mack Boring before they stopped doing rebuilds/repairs did not use aftermarket parts because they were poor quality. This was supported by my research showing many reports of rebuilt HBW50s failed again within a couple of years.
He only used OEM parts but the 8 week lead time turned into 16 weeks which have meant losing most of the sailing season.
I went with a Twin Disc TMMC 40. Much cheaper than a rebuild with OEM parts.