Study hall for Gearboxes

[Alan Wheeler]

Hurth Marine Transmissions:
Hopefully this maybe of help to those with Hurth Gearboxes.
Hurth have two types of boxes. A model number prefix of HBW identifies the smaller box and the larger Hydraulic box has an HSW prefix.

The oil in Hurth marine boxes should be changed at the start of each season and/or at 250 hour intervals. If you are doing heavy work, then it is recommended that you change the oil every 100hrs. Change immediately if the oil is discoloured or if you have a prop wrap up in rope. Disclouration can be caused by overheating and by clutch slip due to overloading. The brown tinge to the oil and the burnt smell is fibre particles from the clutch discs.
The fluid to use in the ZF range is ATF (autotransmission fluid) Both the Dexron 11 and 111 is perfectly acceptable. Synthetic ATF’s like Redline and Amzoil etc are even better, as they halp reduce heat. It is not recommended that an additive be used.
Model Capacity in Quarts
HBW 40 (4M) .32
HBW 50 (5M), HBW 100 (10M) (nee 5) .44
HBW 125 (12M), HBW 150 (15M) (nee 10) .63
HBW 150A (15MA) .59
HBW 150V (15MIV) 1.06
HBW 220, HBW 250 (25M) (nee 20) .79
HBW 360 1.50
HBW 360A 1.60
HBW 450 1.90
HBW 630 2.10

All Hurth marine transmissions have damper plates. They are installed on the engine's flywheel in front of the gear. They resemble a small, spring loaded clutch.
It is only visible when you remove the transmission. All the damper plate springs should be in place, tight and not loose or rattling. It is good advice to change the plate whenever you have the transmission off the engine as it is then when it is easiest to get at. A failing or failed plate can cause rattles and thumbs and clunks and sometimes these can be felt through the boat while underway.

If you have issues with shifting into gear, first check and adjust your linkage. Linkage problems are very common on Hurth boxes. If the problem still exists, see if there is a difference in the difficulty in engaging forward versus reverse. If it is significantly more difficult to engage the box in forward as opposed to reverse, this indicates that the thrust washers are worn. The box will require service or replacement.
Small mechanical Hurth gear boxes do not have filters. One could be fitted as an after-market add on and would probably aid the longevity of the box. But a small amount of engineering is required and best done by a professional.