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Old 03-05-2018, 04:41   #31
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

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The prop shaft will spin much faster when motoring at 6 knots than it will when sailing at any speed you boat could ever do.
I disagree,unless you can provide some type of proof
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Old 03-05-2018, 04:51   #32
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

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My original question was. "Why does Yanmar recommend leaving SD20 in neutral while sailing"? It may damage the gears / saildrive. How? The


My belief or opinion is that it may damage the clutch and or cones, whichever there is.
Two ways to make a clutch not slip, you can make it so that the springs are stiff and then you have to overpower them when shifting making for excessive force to be required.
Or you can make them so that the engine torque applies a force that tightens the clutch, this gives you easy shifting and the more torque the engine makes, the tighter the clutch.
If a clutch needs engine torque to compress the pack, then when Force is applied backwards, the clutch could easily slip.

Now this is my theory, based solely on my belief of how the system works, take it for what itís worth, Iím no Yanmar transmission expert.
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Old 03-05-2018, 04:52   #33
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

A way to determine if the clutch actually works that way would be to see if excessive force is required to disengage it when engine torque is applied, of course that would require disconnecting the cable and dong things manually, and is it worth it?
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Old 03-05-2018, 04:55   #34
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

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Originally Posted by Dulcesuenos View Post
I disagree,unless you can provide some type of proof


Due to slippage and drive train friction, he is likely correct.
Take a boat with a visible driveshaft and use an optical tach to see
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Old 03-05-2018, 04:58   #35
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

More thread creep: Should one assume that leaving the gear in neutral while docked is also what Yanmar would recommend? With current and all, it seems the same argument would apply.
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Old 03-05-2018, 05:39   #36
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

I doubt current would matter, but never thought of it, perhaps if the current was enough to turn the prop, it might cut down on growth a little?
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Old 03-05-2018, 06:09   #37
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

A couple of points of info....

These transmissions are of the constant mesh type. Meaning all the gears are engaged and spinning at all times when the shaft is turning. So lubrication isn't an issue with a free wheeling prop. Splash lube is occurring.

With reverse engaged while sailing the output shaft effectively becomes the input shaft in terms of forces in the trans. So all forces are applied in the opposite direction of design intent. This could possibly have some effect on things like thrust face surfaces or thrust washers and of course the bronze (monel?) cone if equipped.

For what it's worth, just replaced every bearing in my Kansai vdrive. 3500 hours, and the most worn looking bearings I've ever replaced. The gears, thrust washers/surfaces and cone looked practically brand new. Trans worked fine, just noisey.
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Old 03-05-2018, 06:35   #38
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

Assuming the system does lock down the clutch more when engine torque is applied (that is in the manual for my KBW20, Iím not making this up).
Then if the force is applied backward, would this do the same? I can see how it may.
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Old 03-05-2018, 07:23   #39
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

I've always placed my KM2a gearbox in reverse while sailing, as leaving it in neutral just creates noise due to prop free-wheeling. Disassembled the gearbox a few weeks ago and reversed the cone clutch due to excessive wear on the forward side of the clutch.

Now, I don't know how many hours are on the drivetrain as the hour meter didn't work when the boat was surveyed and purchased over a year ago but I have put nearly 500 hours on the drivetrain since then.

What I found upon disassembly was that the reverse side of the cone clutch only had minor wear when measured and compared to the Kanzaki specs listed in my manual. Ordered and received a new cone clutch and will be replacing it later today.

With that being said, and now fully understanding how the internals of the gearbox work, I'm not really seeing how placing the gearbox in reverse while underway really causes any undue amount of increased wear-and-tear on the cone clutch. I would say here though that if the shifting mechanism isn't aligned correctly, that leaving the gearbox in neutral and allowing the unloaded clutch to constantly rotate while the shaft is free-wheeling, may cause unwanted wear on the shifting tab and the slot between the cone's two clutch ends. This, over time, would result in the cone clutch to not fully engage when placed in either forward or reverse.

Drag from the prop being locked in reverse? Well of course that will occur, but I haven't noticed any major difference in speed with the gearbox in either neutral or reverse (fixed 3-blade prop).

Just my 2-cents worth speaking from real world experience FWIW.
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Old 03-05-2018, 15:50   #40
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

Continuing my rambling....

If I understand A64's question, I say yes it's the same.

Certainly, lock up is proportional to torque. When I've used reverse under sail it would be difficult to return to neutral.

I don't think the cone is really in jeopardy, though the shifter and all related parts will see more stress. Maybe consider the cone taper and gear taper similar to a keyless taper on a crank pin on a common rod flywheel. The more it's torqued the tighter it 'sticks'. Think of that pop when disassembled. That's what the shifter pawl has to do in a cone clutch.

Personally, I think motor sailing with the engine at low revs is what's bad on the cone. Less torque at low revs and the force on the sails subtracts from that lock up torque.

I'm OK with freewheeling, doesn't bother me. It's my very analog knot meter when I'm below while sailing. Lots of things don't bother me.

Shifter adjustment... critical... probably killed a few clutches over the years when incorrectly set. And if the output shaft has any new parts installed the shim set at the front must be recalculated to get the shifter adjustment in the ball park. It's a blind race, and must be measured 4-5 depth mic. All the others can be dry assembled and checked externally with dial gauge.
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Old 13-09-2019, 03:43   #41
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

It's all about the clutch cone they don't warrant when sailing in reverse.
The torque without the engine running is not enough to engage the cone properly into the bell, risking it slipping. Both parts will polish each other and you end up with a slipping gear and guess which one?
It is quite a stupid construction having a clutch cone engaging gears on friction in an oil bath! Unfortunately all sail drives and a lot of shaft gear boxes use them.
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Old 24-12-2019, 19:16   #42
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

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Originally Posted by Gerrit View Post
It's all about the clutch cone they don't warrant when sailing in reverse.
The torque without the engine running is not enough to engage the cone properly into the bell, risking it slipping. Both parts will polish each other and you end up with a slipping gear and guess which one?
It is quite a stupid construction having a clutch cone engaging gears on friction in an oil bath! Unfortunately all sail drives and a lot of shaft gear boxes use them.
Sd 20 has a dog clutch,,,
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Old 24-12-2019, 20:27   #43
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailin

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Originally Posted by Dulcesuenos View Post
Sd 20 has a dog clutch,,,
Many clutches are only lightly applied without engine torque applied, this makes moving the shift lever not so hard, but of course the clutch would slip under high power so many transmissions are made with helical cut gears that will apply a side load on the clutch pack and the more torque the engine applies, the harder the clutch’s disks are pressed together.

Now if that is the case then if anything force applied backwards in the system should loosen the clutch pack or at least keep it at min pressure, this would likely allow slippage to occur if the prop applied enough force, and slippage is of course wear.

Now I’m not certain that is why, but I’d bet lunch it is.
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Old 31-12-2019, 04:24   #44
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

Ok , I don't have a yanmar , but the principle of props , seals and gearbox is the same for all . Volvo Penta recommends the fix prop. to freewheel but occasionally run the engine ( free of load) to cool down the gearbox .
I've never found the need to do that as the limited generated heat by the freewheeling prop dissipates via the gearbox & engine block completely .
I have a Volvo shaft seal ( the black type) and it's been in there for 10 years now , not a drop . And I very rarely ' burp ' the seal . But do grease it once and awhile . Most likely I will preemptively replace it next year .
The freewheel shaft revolutions at let's say 6 kts sailing never come close to the revs. whilst motoring at the same speed , far from .
Blocking a 3 blade prop in reverse makes you loose at least 0.5kts ,.....at least ! Tried it , checked it !
There is an immense strain on the gearbox when putting the gear in reverse whilst sailing , wrong move .
Changing the gearbox oil doesn't harm although VP claims it to be unnecessary . But when the oil gets slightly dark I replace it no matter what .
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Old 01-01-2020, 19:01   #45
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Re: Why does Yanmar recommend leaving SD20 in neutral while sailing

The following is the extract from an email of our local Yanmar dealer in Auckland with regards to my 3YM30AE in my Xp38. A fairly late addition to the thread, but hope it helps someone anyway.
"You can leave your gear in reverse when sailing as you have a properly engineered folding prop not a fixed one as is common in Europe on basic boats.
Additionally you have a dog clutch not cone clutch."
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