ZF68 gearbox free spin when engine is off

[patrickcatellan]

Question regarding 2 x ZF68 gearboxes installed on a twin engine motorsailer. When engines are off there is no oil pressure in the gearboxes. When we move undersails only, is it okay for the gearboxes to spin freely due to the friction of the propeller in the water which makes the shaft spin? or is it necessary to install a brake on the propeller-shafts so as to stop the movement and block the rotation of the gearbox?

[markcouz]

It is preferred to have a system to prevent the prop from spinning (folding/feathering prop or shaft brake).

Most have a hydraulic pump on input shaft to create oil pressure for lubrication and operation, and have an oil cooler which relies on engine cooling circuit - none of these will work while freewheeling.

Freewheeling will result in unnecessary wear and may result in overtemp damage depending on boat speed and time spent Freewheeling etc.

(I am not a mechanic, the above is from my personal research).

[FPNC]

RE: Freewheeling / trailing .

This question comes up a lot.

Not all ZF gears area the same. Some of the larger ZF transmissions can be damaged very quickly if allowed to trail without the use of a trailing pump to maintain oil pressures while the output shaft is turning. These larger transmissions do not use all roller bearings and require lubricating pressure if they are to be trialed at all.

The smaller ZF gears that use ATF can be trailed with limitations.

The Memo (inserted below) was generated by the ZF USA Engineering department sometime around 2008. There may be an update that I am not aware of. This memo is much more detailed information than what you may find in the owners manual.


********
ZFI-M Engineering

Subject: Trailing Marine Transmissions, and Oil Recommendations

To whom it may concern:

ZF’s List of Lubricants for Marine Transmissions is known as TE-ML 04. It provides a general list of recommended lubricants for ZF Marine Transmissions. There are two types of oils used – they must not be mixed or used incorrectly. Be sure to consult the applicable owner’s manual and/or TE ML 04 Lubrication Recommendation List for Marine Transmissions for the type of oil permitted.

OIL SPECIFICATIONS:

The ZF 4-1M through ZF 110 Family use a Dexron Three or Four, automatic transmission oil per TE ML 04. ZF 220 Family and larger will use straight weight motor/engine oils per API service classifications CD/CE/CF/SE/SF/SG, per TE ML 04. SAE 30 motor oil is recommended for sump oil temperatures in the range from approximately 90 to 176 degrees F. SAE 40 is recommended for steady state temperatures ranging from 150 to 195 degrees F. TE ML 04 may be down loaded from www.zf-marine.com. Selected MIL specification oils may be used, and the oil list should be checked for these. Multi-grade and/or synthetic motor oils are permitted with certain restrictions for these families of oils. Mobil Oil Corp. Mobilgard 1-SHC is a synthetic
oil that is published as a straight SAE 40 weight oil but exhibits some of the properties of a multi-grade oil. This is synthetic brand and type of oil is permitted for use with the ZF 220 and larger Families, with some Product Family restrictions – see TE ML 04 for details. No friction reducing compounds or additives are permitted such as those containing sulfur, chlorine, and/or phosphorous, since these compounds may aggressively attack the sintered bronze clutch plates, and reduce the torque capacity of the clutches.

TRAILING:

Trailing marine transmissions is possible as long as the recommendations for the specific product are followed, for the frequency and duration of the trailing mode anticipated. The owner’s manual for the specific model of transmission should be consulted. However, a conservative method that will work well with all ZF Models equipped with hydraulically actuated clutches, but without a trailing pump is described below:

1. Emergency: Unit can be trailed up to 24 hours. Sump temperature should not exceed 176 degrees F. If a longer
period is expected, due to an engine or transmission breakdown, additional motor oil may be temporarily added to
the marine transmission up to the point where oil is emitted from either the breather or filler tube. Be sure to
remove excess oil prior to start-up of the engine – this is imperative!

2. Infrequent/Occasional: Unit can be trailed from 6 to 12 hours. Sump temperature should not exceed 176 degrees F.

3. Planned/Frequent: Unit can be trailed from 3 to 6 hours. Sump temperature must not exceed 176 degrees F.

4. Shaft Locks: A shaft lock or shaft brake may be optionally installed, in which case if it is employed, then there is no limit. The shaft lock must have a safety device to prevent engine start up until the shaft lock is unlocked and secured in a safe position. Failure to employ a safety interlock, may result in severe damage to the boat and or propulsion equipment, and can cause severe injury or death for anyone nearby if the engine is started, and the transmission placed in gear without the shaft lock being removed.
For multiple engine boats employing trailing on either an occasional or frequent basis, we recommend alternating propulsion mode between the engines. This will keep the hours between the two engines close, and will also ensure proper lubrication of parts internal to the transmission. For single engine boats, the engine may be started, and the transmission left in neutral if that is important. Run the transmission in neutral for approximately 5 minutes to re-lubricate clutch plates and bearings normally pressure lubricated from internal rifle boring of shafts. By following our recommendations in our owner’s manuals, good product service life can be attained, and customers/users will be satisfied with the performance of the product.

CAUTION! Trailing without heeding factory recommended procedures, may result in increased wear rates of load carrying components, and in extreme cases can even lead to bearing and or clutch plate damage. If for some unusual/unforeseen reason, our recommended procedures cannot be followed, then have the transmission inspected prior to the next engine
start-up, and look for signs of burnt oil, or unusual neutral clutch plate drag with the engine off. The output shaft of the transmission should be able to be “easily” rotated, with the possible exception of some large reduction ratio work boat transmissions, where the inertia of the system and the prop shaft friction torque may be unusually high. If the output shaft does not rotate, then this may be indicative of damaged internal components of the transmission.

[Khaos13]

Keep in mind that a freewheeling propeller causes the same drag as a solid disk of the same diameter. Same hold true in airplanes that have "lost" the engine. a freewheeling propeller will shorten the airplanes glide ratio considerably.

[rslifkin]

Originally Posted by Khaos13

Keep in mind that a freewheeling propeller causes the same drag as a solid disk of the same diameter. Same hold true in airplanes that have "lost" the engine. a freewheeling propeller will shorten the airplanes glide ratio considerably.

This is not true for a typical boat prop. A freewheeling prop causes drag, but less than a locked prop in most cases. Airplane props are a different story, especially when talking about constant speed props that would normally be feathered after an engine shutdown (as feathered is the lowest drag state for a boat or airplane prop). The airplane prop isn't de-clutched from an engine like a boat prop so it can never truly freewheel, which changes the drag picture drastically.

[Khaos13]

rslifkin, you are right of course; I didn't think that through. The fact remains that the prop creates enough drag to actually keep the engine turning and that energy comes at a cost, ie; drag. I'll admit, I've researched this since reading your post and was surprised by what I found. Tests have shown that the freewheeling prop does indeed cause less drag than a stopped prop. I even read that some engine manufacturers REQUIRE the prop to freewheel. I wish someone could explain THAT to me.

[rslifkin]

Originally Posted by Khaos13

rslifkin, you are right of course; I didn't think that through. The fact remains that the prop creates enough drag to actually keep the engine turning and that energy comes at a cost, ie; drag. I'll admit, I've researched this since reading your post and was surprised by what I found. Tests have shown that the freewheeling prop does indeed cause less drag than a stopped prop. I even read that some engine manufacturers REQUIRE the prop to freewheel. I wish someone could explain THAT to me.

I think the freewheeling requirement is on some mechanical cone clutch transmissions where the clutches may slip when placed in gear with the engine stopped and the prop trying to spin the shaft (which would be opposite the normal direction of power flow). You could safely stop the shaft with a shaft brake with the trans in neutral though.

[Khaos13]

Interesting... but still somewhat confusing. No wonder there is so much debate. I'll sail with my Hurth HBW 100 in gear. I've seen a jib sheet on a Morgan 41 go thru the block and get wound up in the prop due to the freewheeling nature of the BW Velvet Drive transmission. We'd been trolling a line and caught a few bonitos; we cleaned them in the cockpit. They are a bloody fish. We washed the cockpit down with buckets of seawater. We were 1/2 way between Anegada and Virgin Gorda (about 12 nm) when this occurred. This was the day tropical storm Andrew passed us as a "depression" on its way to wreak havoc on Florida. With the jib sheet in the prop, sheeted in as tight as that prop could make it, we hove to. OK, broadside to some 6 footers, we're stabile. Guess who had to dive into that rough, bloody water to try and free the sheet so I didn't have to cut it? Stop the prop.... just my $.02.

[Uncle Bob]

Originally Posted by Khaos13

Interesting... but still somewhat confusing. No wonder there is so much debate. I'll sail with my Hurth HBW 100 in gear. I've seen a jib sheet on a Morgan 41 go thru the block and get wound up in the prop due to the freewheeling nature of the BW Velvet Drive transmission. We'd been trolling a line and caught a few bonitos; we cleaned them in the cockpit. They are a bloody fish. We washed the cockpit down with buckets of seawater. We were 1/2 way between Anegada and Virgin Gorda (about 12 nm) when this occurred. This was the day tropical storm Andrew passed us as a "depression" on its way to wreak havoc on Florida. With the jib sheet in the prop, sheeted in as tight as that prop could make it, we hove to. OK, broadside to some 6 footers, we're stabile. Guess who had to dive into that rough, bloody water to try and free the sheet so I didn't have to cut it? Stop the prop.... just my $.02.

I don't believe that there is in fact any "debate", merely those that don't or won't understand. There has been significant testing done to determine that dragging a locked prop is somewhat akin to towing a bucket on a rope behind the yacht.

[GordMay]

ZF OPERATING INSTRUCTIONS, including ZF/ZFE 68 A, ZF/ZFE 68, & ZF/ZFE 68 IV
➥ https://www.solediesel.com/datos_pub...25A68IV_EN.pdf