In gear or N when sailing?

I recall reading somewhere a story about insufficient lubrication (and therefore increased wearing) of the gears and/or bearings, when the prop is dragged in neutral. Do you guys know something about that?

 

You generate plenty of rotational wear on the shaft and cutlass bearing when free wheeling . Some transmissions are suitable for free wheeling and some shaft instalations are designed to promote free wheeling. You will see many sailing yachts with captive oil bath ,roller bearing, shaft logs and free wheeling prop shafts turning shaft belted alternators to generate dc electricity as they sail. As for this volvo model trans, Im not familiar.

 

There should be an anti-siphon valve installed on the hose between the raw water pump and the engine. This should be at a loop above the maximum water line. The draw back of a turning shaft is wear on the cutlass bearing and transmission. On a Volvopenta, the clutch cone will wear out if you leave it in forward. In neutral it should not, in theory wear. However, a minor misadjustment would make it drag. They should be locked in reverse, so the cone clutch locks in.

 

Yes some gearboxes do not properly lubricate unless the input shaft is running. These were the non oil bath types that have an oil pump driven from the input shaft.


Daiquiri this is your area:

Of significance to sailing craft:
The drag from the prop is an interesting one, if the prop bearings have almost no friction and the prop can be spun easily by hand and it keeps running for a while, then you may get less drag letting it freewheel but as soon as you have friction you are loading and extracting energy from the turbine and it produces more hydrodynamic drag than if it were locked.

On vessels with the prop behind a fin you get the lowest drag when locked with at least one blade inline with the keel.

Other benefits of locking a prop are wear and tear on seals and bearing surfaces and you don't wind rubbish onto the prop especially rubbish a powered prop will often shred such as thin plastic and around here kelp.

 

I had planned , when on passage, to alternate between engines if it is proven to save fuel.
Something like every 6 hours start and shut down opposite side.

I spoke to ZF about this and they said no problems whatsoever to freewheel a SF25 box with a boatspeed on one engine of around 8 knots

I do wonder about the affect on the stuffing box of a lack of forced water lubrication

 

Of course there is internal friction, but that is only a fraction of transmission losses in a gearbox under load. From just stirring a can of oil you won't get smoke.

The TwinDisc manual (http://www.twindisc.com/Downloads/Manuals/Operators_Manual.pdf) says that windmilling or backdriving is allowed as long as normal vessel speed is not exceeded. Only a few types that rely on pressurized oil for lubrication need additional care, like starting the engine every 8 hours for 5 minutes or adding an oil pump driven by the outgoing shaft.

If you are in the business of building sailboats it would be rather stupid to install gearboxes prone to damage from sailing, especially because most gearboxes in that HP range have no such limitations.

 

According to Nigel Calder there are two common ways to flood your engine. The most likely:

"On any engine that is below the waterline, both the water-injection line into the exhaust and the exhaust pipe itself create the potential for water to siphon back into the exhaust, fill it, and flow into the engine via open exhaust valves. The injection line must be looped at least 6 inches above the loaded waterline at all angles of heel, and preferably 12 inches, with a siphon break at the top of the loop."

Less likely:

"...the installation of a scoop-type water inlet facing forward ... Anytime the boat is moving more than a few knots, this generates pressure in the raw-water system. In normal circumstances the vanes in the rubber impeller raw-water pump will hold this pressure at bay when the engine is not running. But if the vanes get damaged, water will be driven up through the heat exchanger and into the exhaust, where it will build up and flood the engine."

He goes on to say how scoops are also a problem when mounted facing aft and shouldn't be used.

So to answer your question, apparently whether or not you are in gear is irrelevant.

 

This is not true for typical marine propellers. Drag is always lower, if you allow the propeller to spin. Friction will make the difference smaller, but the drag will never be more than for a locked propeller. Read this http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf

Probably only true for a 2-blade propeller with narrow blades.

 

The effect become clear when you tow your tender in motor down position. If you left the outboard in neutral, to free wheel, she will stay in the water, less force is generated .... This is assuming your not going so fast that lower unit drag tilts the outboard. When left in foward gear, locked, she tilts up rapidly, more drag . Off course an outboard has a very low friction prop shaft, so this anology might not be scientific.

Normally the reason for locking a propshaft is to avoid wear on the cutless bearing, stuffing box, coupling. Prop shaft Thrust is handled by a thrust bearing. most vessels dont have a thrust bearing to absorbe reverse thrust. Also free wheeling props are noisey.

Also to fold a MAX prop..you must never put the engine in reverse. Never. Simple turn the engine off while in forward gear. The transmission will generate enough momentary bite, friction, to slow the shaft and allow water force to closed the MAX.

 

This is absolutely true on a sailing catamaran (I would imagine a power cat too). In mild conditions, you get about 80% hull speed with one engine on as you do with two engines on. This means you loose a bit of speed, but you are using approximately half the diesel per hour. Not only does it save on your fuel usage, but also hours on the engine, oil and belt changes, etc...

It is very economical.

Also, from running a 50' monohull I used to own, I can attest to the general neutral/locked prop argument from direct stupidity.

I was sailing along at 7 knots or so in a fine breeze with the prop freewheeling in neutral. I remembered the argument about locked props so I nudged the transmission into gear to lock it. SLAM! Whoops! Probably wasn't good for the transmission to do that.

Anyway, the result was that my 7 knot boat speed under sail immediately dropped to 5.5 knots. When I got it back to neutral, speed went right back up to 7 knots. That, without a doubt, confirmed to me it is much more efficient to let a prop freewheel if your transmission is designed for it (as most are).

 

Thanks for linking that study, It's interesting and I'm sure they are correct that the tow tank validated the math model. I certainly learned something from it wrt the level of shaft braking and total prop drag in a free stream.

How applicable to a vessel ? I would really have liked to see some real world tests.

In reality you want to look at the flow lines of the hull and the prop streamlined flow lines are what you want and if you have a rudder (or a wing) close behind the prop acting as a splitter plate then you may get substantially less drag with the prop locked. It's never quite as simple as these papers might suggest if we look critically at exactly what it is they modeled and how applicable it is to the relevant vessel.

Also it's worth considering that a lot of ideal experiments performed to validate mathematical models in ideal flow go out the window in a seaway, any one of surge heave pitch yaw or leeway can make the results significantly different. Which is why real life verification is essential and even then it's going do be quite different for different designs.

Catbuilders observation is probably very similar to the papers conclusions, light displacement hulls with little wake. I notice no difference on my 45 foot 16 tonne sloop whether it's locked or not, which fits with their observation of the results being pretty close for a medium displacement vessel.

 

Why do you think rudder behind the propeller would make a big difference in this issue? A locked propeller will have a turbulent swirling wake, likely more swirling and turbulent than a free spinning one.

It all depends on conditions and the size of the propeller compared to the vessel. It is not easy to notice the drag of the propeller while sailing alone, especially on a big and heavy boat like yours. If you compare two identical boats on a beat, you will typically see 5-10% difference in VMG between a fixed 3 blade and a folding propeller. The free spinning propeller will be somewhere in between.

 

Actually on a sailing boat the only time I notice prop drag is when I can feel it on the rudder or when you are going to windward in a seaway. The prop drag slows the acceleration of the boat down in the wave transitions. Its the same with a wet sanded boat. Difficult to record the speed gain...obvious when accelerating thru wave transitions. The boat that can accelerate from 5.5 to 6 knots the fastest will always arrive first.

 

You also need to consider whichever way it falls that the drag from a relatively small prop on a sailboat isn't a big player in total drag figure. which is probably the main reason I notice no difference on my medium heavy boat.

 

I'm not saying it is the case, just that it may be. It's certainly an interesting subject. Hydrodynamics has a habit of being very hard to predict from a reductionist approach. (Like adding another free wheeling prop on the same shaft aft of the prop).