ZF Hurth transmission slipping

Oct 27, 2018
7
Catalina 30 San Francisco
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

.
Jul 29, 2018
503
O'Day 35 Buzzards Bay
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.
 

Ward H

.
Nov 7, 2011
3,129
Catalina 30 Mk II Barnegat, NJ
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.
 
Feb 8, 2014
1,300
Columbia 36 Muskegon
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.
 
Jan 27, 2008
3,008
ODay 35 Beaufort, NC
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