Looking for Z-Drive Propulsion Explanation

OK, so starting off with apologies if this has already been covered, but I can’t find a thread looking through the forums.

I’ve never sailed with Z-Drive propulsion systems and I’m looking for a good explanation as to their operation, especially how they rotate. I’ve worked quite a few years with Azipods and I understand those pretty well I’d say, but I’m just wondering how the mechanical shafts of a Z-Drive move in order to allow the rotation of the propeller?

I’ve tried Google and nothing I’m finding is answering the questions I’m looking for and the text books I have don’t cover Z-Drives that well.

If anyone can point me in the direction of a website with a great explanation or a book I can find that I can read up on them about, it would be greatly appreciated. Or if any of the Engineers on here who’ve worked with rotating Z-Drives wouldn’t mind lending me a bit of their time, I’d be indebted.

Thanks in advance!

There’s a cutaway view of one here: https://www.schottel.de/fileadmin/data/pdf/eng/SRP_EN_0913_web.pdf

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If you know azipods, you know a lot more about Z-drives than most US mariners.

Most USCG questions are out of the 1950’s. You will have quite a few questions on Steam plants. The electrical will have more up to date questions, but still be behind the times. I really doubt that you will find any z-drive questions on a QMED exam, if on any USCG exam.

Have you gotten Lapware or Capt Joe’s practice questions? Once you try the questions I think you’ll see that the QMED exams are ridiculously easy.

You’ll need to learn some American terminology, especially outdated terminology that no one has used in years. You’ll need to learn how to look things up in the CFRs. A lot of the questions can be answered by looking them up, but one can often answer them without looking them up. You’ll need to learn the USCG style of exam questions. A lot of the questions are badly written; that will take some getting use to. Questions usually have four or 5 multiple choice answers. 2 or 3 of them are easy to immediately discard. 2 of them may be close or both correct, but you have to choose the one that USCG thinks is most correct. Questions that require a calculation will have a wrong answer that matches a common error that people make. There will be trick questions that test American English reading comprehension, not the technical matter that the question is about.

There will be a page or two of diagrams that some questions refer you to to find the answer. It’s worth being familiar with those diagrams.


I KNOW ITS HARD TO BELIEVE, BUT UNTIL RECENTLY THE USCG PUBLISHED ALL THE QUESTIONS AND ANSWERS FOR EVERYONE TO SEE. I doubt that the have found time to add more than a few questions that have not been published b

You only need to get a 70% on the exam. You can afford to get a lot of them wrong. I think that a guy with your background would probably pass easily without studying at all. But I understand, you are type of guy that never does anything without being prepared. I suggest that you focus on the practice questions.


Thanks for the heads up on the questions tugsailor. And the vote of confidence!

To be honest the question about the Z-Drives was more out of professional interest than anything else. Saw a couple of tugs operating in Callao yesterday and was interested in how the Z-Drive allows for the rotation with mechanical drives. Must be a slip clutch or something similar I’m guessing.

I noted down the DINOCETF TCDPULPE (I added the last section for Engineering) so just need to learn it off heart! Have been using the website for looking through the CFRs and as long as I remember the mnemonic they seem fairly easy to navigate. Do you just get the books to search up answers or are they digital?

Are there many calculations questions in the QMED credentials exams?

Will look into getting Lapware or Capt. Joe’s for the practice questions because I’ve only got the practice exams from NMC so far. Would diagrams be included in the practice questions?

Thanks again for all the advice and tips. Truly appreciated you taken the time to answer all my questions and queries!

so what’s to know…there is an input shaft, some bevel gears, a vertical shaft to the hub and another set of bevel gears driving the final shaft out to the prop. all of this in a cast or fabricated case which looks a bit like the lower half of an outboard motor. the whole shooting match at the top rides on a bearing and has a big ring gear to turn it. not much goes wrong with them except seals can fail at the worst possible time.

end of lesson

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I don’t know, but I imagine that there must be some calculations for QMED. Probably pretty simple like Ohm’s Law. Reviewing the questions will tell you.

There are several sources of the publicly released USCG questions. As long as you have internet access and a few dollars Lapware is the best. They will generate custom batches of questions for you based upon the specific modules in your approval letter, if they don’t already have practice exams for those modules. In the meantime, just pick the questions for third engineer and start working through the practice tests.

The diagrams are included with Lapware, Capt. Joe’s, Seasources, and others.


Hey c.captain,
I get the actual drive aspect of it, the thing I was wondering the most was how the rotation of the “pod” or bottom drive is allowed for? I mean, you have the input shaft into the gearbox with the bevel gears, the down shaft into the bottom gearbox transmitting to the output shaft with the propeller on it. The bottom is rotated by the ring gear and the steering gear motors, but then is it like the differential on a car/truck rear axle that allows one wheel to turn faster than the other? Or is the rotation allowed for in another manner?
Just a curiosity I can up with having never sailed with them before.

Looks a good book but looking for more the mechanics behind the drives and not how to use them for navigation.
As an Engineer, I couldn’t tell you the first thing about navigating a boat!

During rotation the extra motion is taken up by the propeller going faster or slower depending on direction of rotation.

Huh. So for the time it takes for the lower section to rotate, the propeller speed is altered? Really didn’t think it would be that simple! Having said that, can imagine that in the time it takes for the lower section to rotate, you wouldn’t notice any difference in the output speed of the propeller and because nothing has changed at the input, you wouldn’t see any change in engine/e-motor load either.
I know that with the Azipods I sailed on in the past, you could make the full rotation somewhere in the region of 45-60 seconds with all four hydraulic motors activated and about 1/3 more than that with only two running. We also had the possibility of running only one hydraulic motor during sailing but wasn’t a viable option really, just an emergency take me home deal.
Thanks for the clarification dbeierl!

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You are overthinking it.

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Steamer, more than likely. Honestly just didn’t think it would be as simple as to let the propeller speed increase or decrease based on the rotation of the unit.
Guess we can file this one under the old adage of “no such thing as a daft question”.

The propeller would rotate plus or minus 1/2 turn each time the azimuth changed 180 degrees.

If you want to really overthink the subject, convert that 1/2 revolution to rpm change over the time required to rotate the assembly 180 degrees. Calculate it for continuous non-stop azimuth rotation for a full minute if you are looking for a whole number.

Hmmm… In the cutaway there is no obvious drive torque mitigation (such as counter rotating shafts). Do you really just brute force it? That must place enormous strain on the steering gear…

This is great thread. I’ve quadrupled my z drive knowledge.

In the cutaway I pointed to it would be less as the bottom gear is a reduction gear.

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Indeed, but then from what @Steamer is saying, it wouldn’t be that difficult to calculate it based off the gear ratios.

I know zip about these Z-Drives, but would assume that there would have to be a reduction gearing in place because you don’t want a propeller rpm of 500rpm (Sulzer ZAV40S) or 1800rpm (Cummins QSK60), but then would it make sense to anyone else to split the reduction between the two “gearboxes”? I.E. have one reduction between the input shaft and the top of the vertical transmission shaft and then another reduction between that and the output shaft?

@tugsailor, you’ve quadrupled my knowledge of the USCG Credential system with your threads for sure!

It is always done like that. I suspect that it is because the radial load on the pinion is roughly linear to reduction ratio, so you get away with lighter shafts and bearings if you split the reduction into stages.

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