Freewheeling Prop
Date: September 30, 1999
From: Paul C. Uhl endvr32@endeavourowners.com
As an FYI, there recently was a long discussion on a boating list-serv about whether it was better to let your prop "freewheel" (spin) when under sail versus keeping it in gear. This discussion covered several manufacturer's transmissions with their service and manufacturer reps adding comments as well. It appeared to be a unanamous agreement on NOT letting your prop freewheel--the reasoning being that 1) the transmission does not receive proper lubrication while the engine is not running, and 2) it causes premature wear to the transmission cone (clutch device). So what gear do you use? Forward? Reverse? Well that depends upon the transmission but from what I understand, the transmission on the Yanmar engine using the cone clutch device should always be left in REVERSE. Apparently leaving it in forward still allows the propeller to spin the shaft against the transmission. They went on to say that when you start the engine while sailing (remember your trans is in reverse) that you should also START it in reverse and then shift into forward (the water pressure on the prop makes it difficult to shift into neutral or forward while under sail). My recommendation would be for you to contact the appropriate service person, dealer, or manufacturer for your engine/trans type and find out.
Date: October 26, 1999
From: Joseph Palmer jopalmer@classicsailboat.com
(NOTE: We've been following this thread in the Tartan Listserve at Sailnet--we'll post updates as they're available.)
Does anyone have the thread on the prop free wheeling discussion we had earlier this fall. Im getting conflicting reports from Yanmars Mack Boring dist. regarding what was reported and then relayed to us. They are indicating the transmission should not be started in gear vs. the earlier indication it was better to start in gear rather than shift with engine shut off.
Date: September 10, 1999
From: Marc Auslander marc@watson.ibm.com
I think this depends on the transmission design. I only know the facts for the Yanmar cone transmission on my 2GM.
This transmission depends on torque to press two cones together. Shifting engages a spring which applies initial pressure, torque then forces the cones together. This all works properly in both forward and reverse then running the engine.
When locking the prop, it turns out that if you use forward, the torque of the prop trying to turn the engine is in the wrong direction, it loosens rather than tightens the cones. This can let the cones slip - with pretty disastrous damage to their faces. In reverse, the torque is in the right direction and forces the cones tighter together - so the don't slip.
By the way, I can see nothing in this design that could cause damage if you shifted when the engine was not running - as long as you don't shift into forward while moving. And nothing in my manual or the Mac Boring course indicated otherwise.
Date: October 29, 1999
From: Joseph Palmer jopalmer@classicsailboat.com
I'm getting a lot of feedback regarding the artical from Dennis Lawrence that appeared in the latest newsletter. In it Dennis told the story of how his Yanmar transmission went bad:
To free wheel or not?
After having to replace a transmission after only three seasons (Just out of warranty) in our Tartan 3500 I made sure to find out what I should and should not be doing do prevent this from happening again. After talking to Yanmar Reps I was told that the transmission should not be free wheeled and should be put in reverse while sailing. I was also told not to try to take it out of reverse when I was ready to stop sailing and start the engine. Yanmar said it was better to start the motor in gear than to take it out of gear without the engine running. I have been following these guidelines with out any problems. By the way I never figured out why the first transmission crapped out. After discussing the do's and don1t's with Yanmar I decided I was not doing anything wrong except taking it out of gear to start it after sailing. Dennis Lawrence, S/V Wind Rose,T3500 #31 The same can be said for all Hurth transmissions used on the Westerbeke and Universal diesels.
JP to all
Lets go back to starting in nuetral until we can confirm who at Yanmar made the no-shifting reccomendation. Since they are now contridicting this advise.
Date: October 29, 1999
From: dwendt@eunet.no
Freewheeling puts more wear on the output shaft bearing from additional revolutions, and more importantly, the output shaft bearing is not getting proper lubrication when the engine side of the gearbox is not turning. The whirling of the gears when the engine is running keeps a constant bath of oil on all of the internals. The gears are motionless when the engine is off...
Date: October 29, 1999
From: Jim Provos provos@sgi.com
I was reading in the Nigel Calder's "Marine Diesel Engines" book last night and I found some good information.
First of all, most sailboats use a transmission which has a planetary gear box. This is very similar to the planetary gears used in an automatic transmission in a car.
However, there are 2 variations of this, manually controlled and hydraulically controlled. In the manually controlled trans. the shift lever at the helm moves a mechanical mechanism in the trans. That engages a clutch pack in forward and a brake band in reverse. In the hydraulic transmission, the shifter merely moves a hydraulic valve in the transmission. This is how you shift an automatic transmission in your car.
From the diagrams in the book and from his description I see no reason what so ever why it would damage either of these types of transmissions by shifting them with the engine off. (Nigel also did not mention anything about this phenomena).
However, Nigel does go into detail about the problem of free wheeling the propeller while sailing. He said that allowing the manually controlled trans. to free wheel will not damage anything out right. It will just put more wear-and-tear on the trans. bearings, oil seal and also the stuffing box. Just by putting the transmission in gear will obviously stop this problem.
The hydraulic trans. is a different story. Since it needs hydraulic pressure to shift the transmission, moving the shift lever will have no effect. Even if the transmission was in gear before the engine was shut off, it will not be once the engine is stopped because there is no longer any hydraulic pressure. Again, its just like your auto trans. in you car. The pump also lubricates the gears and bearings. With no pressure, there is no lubrication. Nigel said the only way to stop the prop from turning is to put a brake on the output shaft.
Lastly, I think the transmissions in most sailboats with engines below ~40 hp are manually controlled.
Date: October 30, 1999
From: Paul Uhl endvr32@endeavourowners.com
I spoke with the head mechanic at our boat yeard today about the tranny issue. They are also a Yanmar authorized service center and it was his belief that the cone clutch or clutch dog type transmissions should be placed in reverse while underway. However, he didn't believe that starting the engine in reverse (or any gear for that matter) while underway was a good thing to do for several reasons. First it places a large load on the engine while it's trying to start up (also remember that it takes a few revolutions for the oil to lubricate the parts at startup). The faster you're going while starting in reverse, the greater the load. Secondly there are safety considerations for starting any type of engine with the transmission in gear as you immediately start to move.
I guess that's why most gas powered "vehicles" have park.
Date: October 30, 1999
From: Ken wood hmswood@email.msn.com
My observations that a large number of transmissions are Hurth. I have the service manual for Hurth and they are no planetary gears...I have also taken mine apart and can confirm that they have conventional gearing.
Date: November 3, 1999
From: George Gulden tartanyachts@hotmail.com
I have witnessed this same question regarding the free-wheeling prop versus the locked prop and how it effects boat speed on a different user group discussion list. The debate went on for months. Finally, some professor did a study and found the locked prop produces less drag than the free-wheeling prop. So there you have it.
Date: November 4, 1999
From: Brad Whitehurst bwhitehurst@fairchildcontrols.com
I don't know what study George was thinking of, but Practical Sailor published a summary of a study they sponsored at MIT a few years ago. The study results are available as a PostScript file on MIT's web at http://ptun.mit.edu/publications/pracsail.html though I couldn't get through today. I saved a copy of the PS file in an executable ZIP if anyone is interested. It is in the tartan-list FTP site as ftp://ftp.sailnet.com/lists/tartan/windrift/pracsail.exe
Be warned, you need either a Postscript viewer or printer to make sense of it. I have successfully converted it to an Acrobat .PDF file, which is fairly big, 472KB. I would however want to seek permission from the authors or MIT before publicly posting a converted version of their file, for courtesy's sake.
The gist of the report was that there are significant efficiency and drag differences between different fixed prop designs, not to mention, of course, folding and feathering props. Relative to freewheeling, they concluded, though test lab limitations prevented full testing of freewheeling props, that less drag would be created for any given fixed prop if it freewheeled rather than held fixed. Their analysis showed for a Michigan Wheel 3 blade prop, the fixed drag at 6 knots was about 40 lbs and the estimated drag for that prop freewheeling would be about 21 lbs.
Date: November 5, 1999
From: Joseph Palmer jopalmer@classicsailboat.com
George
Thanks for posting your response. So Im straight on all this, he did say it was ok to lock the trans in reverse when sailing? (i mention this because he begins by saying to keep it in nuetral when sailing with engine stopped) I think we can all concur that sailing with trans in reverse is whats reccomended here and that starting in gear is not and that shifting while engine stopped is ok and that the beer must be kept cold at all times or else the whole thing breaks down. go browns go osu, and have a good weekend everyone.
Date: November 8, 1999
From: Joseph Palmer jopalmer@classicsailboat.com
Dennis
Thanks for responding. The contact info is not that important anymore since I think we can all agree that:
1. While sailing prop should be locked in reverse
2. Mack boring is advising not to start in gear - currently - and probably rightfully so given the liability starting in gear may cause-3. Therefore we must start in neutral which requires us to shift into neutral from reverse, before starting the engine, even though that may or may not damage the gear case.
So I guess thats it- for now
Date: November 9, 1999
From: Rick Clarke rainprof@erols.com
Ref the drag of a prop, I have some experience with this - in aircraft. Yes, a freewheeling prop in the windstream has significant drag. It is attempting to make the engine rotate. That is why take-off performance calculations use a worst case scenario of a freewheeling or "windmilling" prop after an engine failure on takeoff. If you get the prop feathered during the failure, then you're actually improving the aircraft's performance over that of the required tests.
Now, that's in takeoff with no time to do or evaluate any other configuration. Once the aircraft is in flight, the windmilling prop does create drag. This is experienced every time one practices emergency landing patterns by closing the throttle and gliding with the prop windmilling. If one were able to feather the propellor one would glide MUCH further. In a prop that has controllable pitch, but not feathering capability, the glide is extended a great deal merely by bringing the prop to high pitch and reducing drag. Low pitch is more or less flat to the airstream - high pitch is the opposite.
If a pilot is unfortunate enuf to have an engine fail and the engine seizes rather than merely ends power output then there's big trouble. The prop is frozen. The drag is MUCH greater. A friend had this happen and when executing the 360 degree spiral down to an emergency field needed DOUBLE the altitude to make the turn that he ususally did when practicing with a windmilling propellor.
SO, in my opinion, this brings us right back to the problem of the wear on the transmission and I believe that's been answered. Of course I don't have this problem with my T34C because I put the gear shift into reverse with the 2-blade prop vertical and hiding behind the keel. No muss, no fuss, no wear, no drag.
Date: September 6, 2001
From: Bradd Wilson marina@portcolborne.com
I have no experience with the generator but a very bad experience with free-wheeling. On returning from the Bahamas to Miami in '97 we free-wheeled under sail at fairly high speed for 6 hours. When we put the engine in reverse to set the hook in No Name Harbor, there was no reverse. The fast prop spinning with considerable drag had thrown all of the fluid out of the gearbox. Luckily there was minimal wear and the damage was remedied with adjustments only, but had the duration been longer, the result could have been catastrophic. This was a Hurth transmission on a 4108 Perkins. Most manufacturers recomend locking the shaft in reverse or using a "brake'. I'd be wary of free-wheeling.
