Gear box not engaging

Don S/V ILLusion · Jun 17, 2023

Damper plate noise? You can search for a description to see if that fits your sound

Hayden Watson · Jun 17, 2023

It could also be slipping clutch rings. This is an item that wears out on the Hurth HBW 50 gearbox used on the Universial engines (Kubota block) that were installed in many of the C30's. Many owner's have replaced the transmission but the HBW 50 can also be rebuilt.
Hurth HWB 10 Transmission Rebuild dissassembly part 1 A - YouTube

jssailem · Jun 18, 2023

For a good overview oof a marine transmision you might want to read this link.]

https://www.powerandmotoryacht.com/maintenance/everything-you-need-know-about-your-boats-transmission

Alex G said:
our gear box will not engage the propeller shaft in forward, the engine runs fine ,

but when we increase the RPM, the propeller shaft will not increase in speed .

oddly when we put the engine in idle and we shift to reverse and increase the REvs in reverse (there is a horrible noise from the gear box) ,

then we can go to idle again and then put it in forward and now the forward will work again

for a few minutes

Alex, welcome to the SBO forum.
Seems you packed a lot of frustration into your first post. I have no idea about your knowledge on transmissions, so I apologize if this is too elementary.

I like to approach the issue with a simple mind set. The marine transmission is not like a car. It is a basic system. Three options Neutral, Clockwise or counterclockwise rotation of the prop shaft. The prop is free to turn when in gear.

Do you have a power/shift gear lever combination? Single Throttle/Gear lever set up or a gear shift and a throttle lever set up?

I tried to isolate the various observations in order to help trouble shoot the problem.

1 & 2. The transmission needs to mesh with the forward gear from the neutral. You state it will not go into gear. There is no sounds mentioned (like grinding of gears, screeching etc).
The simplest malfunction would be the shifting linkage. You are not in gear. Otherwise the prop shaft would be spinning and the boat would be moving. So the linkage must not be moving the shift arm on the transmission into forward gear.

3. When shift into reverse and then apply power (increase the rpms from the engine) you hear a horrible noise. That sounds bad... Does the transmission go into reverse? Are you moving backwards or at least churning the water tied to the dock.
If not just bad noise... Then gain the linkage may be the cause. With out isolating the noise (transmission grinding or the damper plate "between the transmission and the engine") it is difficult to identify a cause for the noise.

4&5. You go through this trauma with bad noise and you get forward gear to engage. This again could be linkage. I am not sure what "Will work again... for a few minutes" means. You get the prop to turn and you can go zipping about the water, but with out action on your part the transmission steps to neutral after the allotted time? That would be strange. If the linkage is loose or not aligned, I guess the transmission could slip out of gear.

With out the engine running, I would have someone move the gear lever, while you observe the transmission linkage to see if it moves the shifting arm from forward to neutral and reverse.

Then let us know what you observe. If it is as simple as tightening the linkage you should be able to correct the problem with a screw driver, or identify the parts that are failed and replace the linkage that is not working.

Good luck look for the simple failures before you call the yard and tell them your transmission needs to be overhauled.

Hollyberry · Jun 18, 2023

Sounds like a damper plate disintegration.. hopefully that's it because that's the easiest and least expensive fix....

Hayden Watson · Jun 18, 2023

Alex G said:
thanks !! after watching the video it seems that the plates/rings inside the gear box are faulty, in particular the forward ones

That is the most common failure for this transission. It is ussually caused by sailing with the transmission in forward which causes the clutch plates to slip.

Hayden Watson · Jun 18, 2023

If you look at the schematics for the HBW 10 gearbox, the clamping pressure is created by a set of 3 steel balls that run in a ramped slot in the clutch body. When load is applied to the clutch in opposition to the rotation of the engine, the balls run up the groove and squeeze the friction plates tightly. When you sail in gear, the prop wants to turn in the same direction as the engine. This causes these balls to run down the groove which reduces the pressure and friction. This means that the clutch has magnitudes less friction when sailing in forward than when motoring. This causes the clutch friction plates to slip and wear out. Once they start to slip, the thrust washer also starts spinning which wears it out which increases the gap and you loose all ability to motor forward.
Understanding this mechanism is key to understanding why the transmission needs to be in the proper gear when sailing. For all fixed props neutral is preferred and if you must lock the shaft use reverse. For folding props and most feathering props, reverse to lock the prop. The one exception to this rule is the Autoprop which is what I have. When sailing, the Autoprop will freewheel in the reverse direction so if I put the transmission into reverse, I will cause the very problem I am trying to avoid. With Autoprop, you leave the transmission in forward to stop the reverse rotation of the freewheeling prop.

Hayden Watson · Jun 20, 2023

Alex G said:
does anybody know, to remove the gear box , does the boat need to be on dry land ?

Not usually necessary to be on the hard. My C30 has a Universal M25XP with a Hurth HBW50 gear box. To get the gearbox out, you unbolt the shaft coupling flange from the gearbox. Unload the rear engine mounts. Remove the shift linkage. Remove the bolts that hold the bellhousing to the rear of the engine.

After that it should slide out. Once the gearbox and bellhousing are removed, I can get to the bolts that connect the gearbox to the bellhousing which are inside the bellhousing. On some gearboxes, you can get at the box to bellhousing bolts from the outside which makes it even easier.

Hayden Watson · Jun 22, 2023

Alex G said:
Thanks !!!!!, A further question, when the gear box is engaged in either forward or reverse , is it locked to turn in the oppossite direction , ?, when in forward I could turn the propeller shaft forward by hand a,d the same in the reverse direction , meaning the gear box is not really engaging , unless I can turn the entire engine by hand

That is normal for a gearbox that has some hours on it.

If you read my post #10 I explain how the clutch packs work. It is sort like a ratchet in that the balls in the slotted groove allow the plates to have little pressure when you turn the shaft in the same direction as the engine but increase the pressure on the plates when rotated opposite to the engine. When the engine is turning, the water pressure on the prop is trying to slow down the shaft which is the same as turning in opposition to the engine.

My engineering logic leads me to believe that it is set up this way to provide for a degree of self-adjustment to counteract the normal wear of the thrust washers and friction plates. The problem of slippage in gear does not appear until the total wear in these items exceeds the depth of the slope in the groove. A freshly overhauled will require a fair amount of torque before it will slip but it will still slip at some point. That is why they say that if you sail with the transmission in gear, it must be in reverse gear (for every prop except for the Brunton AutoProp) to prevent transmission wear.

Here is a video that show how this works.

Ward H · Jul 21, 2023

I recently replaced my HBW-50 with a Twin Disc TMC-40. The motor needs to be raised 1/4" and the shaft coupling will need to go back a fraction of an inch. My motor already sat near the top of the engine mount bolts so I put 1/4" spacers under the engine mounts.
The other mod is the shift lever bracket. It is on the opposite side of the tranny, which reverses the action of the shifter. It's an easy fix. The shift lever on the tranny is pointing up as received. Loosen one bolt, turn it so it points down. Then make a new bracket to hold the shift cable in alignment. There is a bracket sold but it doesn't hold the cable in alignment with the tranny shift lever pointing down.
I made my bracket out of 1/4" G10 board using a small skill saw and belt sander.

My tranny rebuilder would only rebuild the HBW-50 with OEM parts. He's had too many returns with after market parts due to their poor quality. My research came across several postings from owners who went the rebuild route only to have it fail in 1 to 2 years so I chose going with a new tranny.

There is also the PRM 60 tranny which has the same specs as the TMC 40. I've read several threads of owners installing that tranny. A suggested supplier is in Everett WA. When I called and talked to him, he said he stopped selling them because of too many returns for odd noises. He is the one who pointed me to the TMC-40.

Here's a photo of the installed tranny with the bracket. Also the bracket template. I've also attached a PDF file with notes on the differences between the HBW-50 (ZF-5M) and the TMC-40.

Hayden Watson · Jul 21, 2023

Have you checked your HBW-50 to see if it can be rebuilt? If you have access to a small benchtop press, it is not a challenge to rebuild, and the parts are only a small fraction of the cost of replacement.

Gear box not engaging

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