Freewheeling Prop

[Cisco 37c]

Hello,
I have a 1980 37’ cutter with a 30hp Yanmar and have been told that when under sail, to put the transmission in reverse to keep the prop from turning because it will wear out the transmission bearings. True or false?
Thanks for your help!
Cisco

[Dsanduril]

There's no yes or no answer for you, it depends on the transmission (and the prop). For most newer transmissions Yanmar recommends neutral (and staying in reverse may cause damage/void warranty) for a few older transmissions Yanmar provided the option of neutral or reverse for folding/feathering props, but only neutral for fixed props.

Yanmar Transmission While Sailing

[Cisco 37c]

Thank you for the reply. The engine and transmission are 1980 vintage and the 2 blade original prop was replaced with a fixed pitch 3 blade. I don’t have the transmission specs available at this time.

[billknny]

Originally Posted by Cisco 37c

Hello,
I have a 1980 37’ cutter with a 30hp Yanmar and have been told that when under sail, to put the transmission in reverse to keep the prop from turning because it will wear out the transmission bearings. True or false?
Thanks for your help!
Cisco

It sooooo depends. You need to confirm based on YOUR transmission. Some want to be in neutral, some in reverse, some in neutral OR reverse. In NO case to you want the shaft spinning with the transmission in gear. All this assuming a standard fixed blade prop, and a boat limited to standard displacement speeds.

If you have a feathering prop you'll almost always want to be in reverse right after you shut the engine down to force the blades to feather, then you can put it in neutral.

Then, as a extra complication, if you have an Autoprop, they tend to drive the shaft "backwards" so need to be in forward to stop rotation.

It's complicated, and you need to be sure you are talking to someone who has both the detailed knowledge of the requirements, and the EXACT details of YOUR drive train. Any other answers are as likely to be wrong as right.

[a64pilot]

Originally Posted by Dsanduril

There's no yes or no answer for you, it depends on the transmission (and the prop). For most newer transmissions Yanmar recommends neutral (and staying in reverse may cause damage/void warranty) for a few older transmissions Yanmar provided the option of neutral or reverse for folding/feathering props, but only neutral for fixed props.

Yanmar Transmission While Sailing

A little history in that. My 1987 KBW20 for example it was recommended to select reverse when sailing to prevent wear and noise in my owners manual dated 1987.
It wasn’t until decades later that faced with significant repair and warranty issues of the later model cone clutch transmissions that Yanmar decided that all transmissions need to be in neutral now when sailing.
For the old style mechanical clutch pack transmissions like my KBW-20 they aren’t saying there will be any damage, but are saying that it’s possible that the transmission may not go into neutral for engine starting, which would of course only mean that the engine starts in reverse.
They don’t explain why or how, but what it is is if your sailing at a good clip the torque provided by the stopped prop will act to hold the clutch plates together, the transmission is designed with helical cut gears that as torque is increased it forces the clutch plates together. This is done to prevent clutch slippage and allows weaker clutch Springs so that it’s not hard to move the shift lever, more torque produced, the tighter the clutch is pressed together.

There is no damage, the worst that can happen is if your moving along at a good clip and try to select neutral, it will be hard to do so, I guess on a really fast boat you may not be able to at all.
All that you need to do is head up into the wind, slow the boat down, that reduces the torque produced by the stopped prop and you can now easily select neutral.

I can only assume that they now say sail with all transmissions in neutral, possibly cause they don’t want to explain why it’s OK to do so with the old types, but not the new designs.

Personally I don’t understand why they dropped the clutch packs and went to cones, unless it’s less expensive?

I turn my motor off with the transmission in neutral, my Autoprop feathers in neutral under sail, it doesn’t matter where the transmission lever is or if the engine is running. stop rotation and the blades go to a pitch of least resistance.
It pretty much has to, the blades freely rotate, they aren’t connected to anything or each other.

You can see in this video the the blades sort of flop around, it’s a balance of centripetal force and induced flow that determines pitch, centripetal force forces the pitch flat, induced flow drives it to a higher pitch, these balance of course.
https://youtu.be/xdxY5Q7wXek

[Dsanduril]

Originally Posted by Cisco 37c

Thank you for the reply. The engine and transmission are 1980 vintage and the 2 blade original prop was replaced with a fixed pitch 3 blade. I don’t have the transmission specs available at this time.

Unfortunately that puts you right at the split between the Yanmar 3QM30 and 3GM30 engines (for 30HP). The QM was 1980 and earlier, the GM 1980 and later. The GM was frequently paired with the KM2P gear (cone clutch, for which Yanmar requires neutral), the QM was probably the older KH18 (multi-disc clutch - I haven't seen a Yanmar recommendation for that specific unit, but for their other multi-disc units they recommend neutral for a fixed-blade prop).

Worth trying to get the details of the drivetrain, but Yanmar's default is neutral in just about all cases (i.e. the risk of the clutch getting stuck while sailing in reverse is greater than the wear on the bearings).

[a64pilot]

However if you have a fixed pitch prop, it’s fastest to let it freewheel in neutral, a stopped boat prop is more drag and slows you down some.

[Wotname]

Originally Posted by Dsanduril

Unfortunately that puts you right at the split between the Yanmar 3QM30 and 3GM30 engines (for 30HP). The QM was 1980 and earlier, the GM 1980 and later. The GM was frequently paired with the KM2P gear (cone clutch, for which Yanmar requires neutral), the QM was probably the older KH18 (multi-disc clutch - I haven't seen a Yanmar recommendation for that specific unit, but for their other multi-disc units they recommend neutral for a fixed-blade prop).

Worth trying to get the details of the drivetrain, but Yanmar's default is neutral in just about all cases (i.e. the risk of the clutch getting stuck while sailing in reverse is greater than the wear on the bearings).

Yes, at 1980, there were two options - a 3QM30 or a 3GM. The 3GM30 was introduced in 1983.

If you post a picture of the top / front of the engine, it will become obvious to some of us on CF e.g. I have had 2QM20 and a 2GM20; the differences are obvious. Or refer to the service manuals. The manuals for GM series (and the QM series IIRC) are available in the CF manuals section.

[Cisco 37c]

Thank you everyone for the great replies. There is obviously a wealth of knowledge available here! I will be able to get on Cisco next week and will have all my records available along with model numbers, pictures and all. The engine is. 3QM30 but I can’t remember the xmsn data.
Sounds like neutral / freewheeling is the recommended gear for sailing but I will confirm.
Now for the real question Has anyone tried to run an alternator off the freewheeling prop shaft? Can I charge my batteries while under sail ? Seem to me that with the right pully combination, it is possible.......
Thanks again for all your help!!

[GarthGregory]

If you have the detail - The Mnfr. Plate on the Gearbox is the important one - contact the G/Box Mnfr direct - I don't think the Engine Mnfr. is that relevant , you may not get the right advice .

[Cisco 37c]

Thanks Garth, Been doing some research. The gearbox is a KH18 and the manual doesn't specify neutral or in-gear for sailing. However, the schematic shows the oil pump being driven from the input shaft. If you allow the prop to freewheel in neutral, the oil pump will not be pumping oil to the bearings. So, for the interim, I will sail in gear, not in neutral.
The FIX?? I ordered a MAX PROP EASY and will haul out next week for installation and fine tuning.

[Godshalk]

I have a Yanmar 48HP and Hurth V-drive transmission and just installed a new MaxProp, replacing a 25-year one that has gotten very floppy, so read the Owner's Manual. I was surprised to see that Max Prop recommends sailing in neutral but specifically NOT reverse - no explanation why and I am confused because I would have thought the prop would simply feather with no force on either prop or transmission. Hurth seems to have gone back and forth on this issue. I guess I should stop the engine, put it briefly in gear to feather, then to neutral?