Locked Prop vs. Freewheel vs. Boat Speed

[Scott B]

I'm fairly sure I've looked this up before as is a popular question. I thought I had a fairly interesting discusion on the level, but you're trying to upstage me???


The Hunter dealer rep that ran through our boat told me to put the trans in reverse, I saw a lot of discusion on this on the forums and looked it up in the Yanmar manual, which said to put it in reverse. At least that's what I recall and the manual is on the boat and buried under 20 ft. of snow :cry: One of those things that you put in the back of your brain but forget the particulars. Maybe I'll get some info at the Toronto Boat Show this Sat.? or will have to wait for the spring thaw.

 

[Maine Sail]

Even with a non-rotating three bladed prop at least part of one blade will reside in the wake of the 'deadwood'.

What dead wood would there be on a fin keel boat?

 

[Ross]

Ross, I get the impression that you group feathering props in the same class as snake oil.
Mike

Mike , Not at all! They are very useful if you are racing and are trying to gain 2 or 3 seconds per mile on your competion. For me they are of no consequence. I don't race, I am not in a hurry.
People also manufacture and sell high performance cars and airplanes and there is a market for them. I don't buy bottled water, canned soup or folding props.

 

[Mike B]

Ross they also come in handy on a long trip. The 1/2 to 3/4 knot improvement they provide adds up when you're doing an all day run. I had one on our last boat and felt without question is not only cut drag but also did a remarkable job of driving the boat through rough water. I'll need to save a lot of pennies for one for our new boat though.
Mike

 

[rpwillia]

Pick Your Poison

In the manual for the Universal MX-25 engine it states you can let the prop freewheel as it will not harm the bearings. I understand that Yanmar needs to prop to be locked in reverse as it requires the engine to be running to lubricate the bearings. I have a 3-bladed prop and it seems like I lose 1/4 of a knot when I lock the shaft so a lot of times I let the prop freewheel. I did read somewhere that you were better off to lock the prop for speed, but that did not match my impression/results.

 

[Ross]

If we determine the blade area of the prop and the angle of the blades then we can rig an equivilant plate and drag that for an emperical test for the speed change resulting from dragging a prop versus a folding prop.

 

[Maine Sail]

If we determine the blade area of the prop and the angle of the blades then we can rig an equivilant plate and drag that for an emperical test for the speed change resulting from dragging a prop versus a folding prop.

What about just running a pin wheel though a bath tub while letting it spin freely and then lock the bale and do the same?? This is what my jig is hoping to accomplish actual in-water testing.

The general theories for years on locked/freewheel were based on aeronautical props. If I stick my arm out a car window at 5 knots I don't feel much if any resistance but if I stick my arm in the water at five knots from my dinghy it is much different. The viscosities are clearly different..

 

[Ross]

Mainesail, If you simply drop any manner of a prop into the water it will start to spin. Causing it to spin takes work. Work is measured in foot-pounds per second or horse power or kilowatts or any of a whole bunch of other units. Sails collect energy from the wind and convert it into sailboat motion even you knot meter wheel robs some of that power. Turning that prop requires power, not much but some.

 

[Maine Sail]

Turning that prop requires power, not much but some.

Are you saying locking it and dragging it through the water robs less power or more?

I understand that Yanmar needs to prop to be locked in reverse as it requires the engine to be running to lubricate the bearings.

Most Yanmar's in sailboats use Kanzaki transmissions and you can only use neutral. Some Yanmar's use a Hurth/ZF tranny and they can be reverse or neutral. Neither Kanzaki's nor Hurth/ZF gears should be left in forward gear when sailing. Kanzaki's are generally lubed by standard diesel grade oil and Hurth/ZF trannies generally use ATF.

There are a few trannies out there that should not spin freely due to lack of lube without the motor running. It's always best to check your individual manual for gear position while sailing..

 

[shorty]

Lock in Reverse (when I remember)

per instructions from Larry Berlin, Mack Boring. Yanmar 3GMF with a Kanzaki transmission

 

[Ross]

Are you saying locking it and dragging it through the water robs less power or more?




Most Yanmar's in sailboats use Kanzaki transmissions and you can use either reverse or neutral. Some Yanmar's use a Hurth/ZF tranny and they are also reverse or neutral. Neither Kanzaki's nor Hurth/ZF gears should be left in forward gear when sailing. Kanzaki's are generally lubed by standard diesel grade oil and Hurth/ZF trannies generally use ATF.

There are a few trannies out there that should not spin freely due to lack of lube without the motor running. It's always best to check your individual manual for gear position while sailing..

I'm damned it I know.

 

[kendall]

Mainesail, If you simply drop any manner of a prop into the water it will start to spin. Causing it to spin takes work. Work is measured in foot-pounds per second or horse power or kilowatts or any of a whole bunch of other units. Sails collect energy from the wind and convert it into sailboat motion even you knot meter wheel robs some of that power. Turning that prop requires power, not much but some.

Not spinning the prop means you deflect water around the blades, which also takes power.

There are so many possible hull configuration and prop diameter and pitch combinations That I rally don't think there is a one size fits all answer

The prop on a full keel with an aperture is going to have different drag characteristics than a fin keel, even with identical props. Simply because on the fin keel the water will have had more time to smooth out before it hits the prop,(would seem to indicate that prop drag would be a higher proportion of overall drag with the fin keel) With the full keel that water will still be turbulent, and more of the prop would be in the keels shadow.

Ken.

 

[Chuckbear]

The transmission in your boat will determine whether you can and should lock the prop with the transmission since doing it wrong can be very expensive. If you want to install a prop lock that is a different story. But unless your looking to race and worrying about every fraction of a knot it is not worth the bother. Our boat freewheeled for 17 years and tens of thousands of miles and we never paid it much attention and it had no affect on the transmission.

 

[Ross]

Intuitivly I am inclined to think that dragging a fixed prop requires more energy than one that is turning. There just seems to me that there is more slip with the free wheeling prop than with the fixed prop.
I think that the autogyro effect is different in that the autogyro has some control where the fixed blades would have the same resistance but would not lend themselves to control. The uncontrolled glide slope would be the same.

 

[Don12364]

Depends on the Prop

I have a computer program from a renowned marine power company to calculate required pitch and diameter for certain engine/boat combinations.
For some props it advises to allow it to freewheel for less drag and in other cases it recommends locking it. Unfortunately I cannot decompile the prog to see the criteria it works on.
Either way both cases appear valid so it depends on your prop.
Either way a non feathering or non folding prop is bad news, particularly in light winds.

 

[Maine Sail]

Either way a non feathering or non folding prop is bad news, particularly in light winds.

Do you have a fixed or folder/feather?

 

[Hermit Scott]

The argument that 'A spinning prop will transfer more energy to the 'vehicle' ' can't be definitely proven with out measuring total effect like speed. It does take work to spin the prop and transmission but it also takes work to turn all those little eddy currents in the turbulent flow behind a non spinning porp.

 

[RichH]

What dead wood would there be on a fin keel boat?

The zone of turbulence in the slipstream from the keel. Any foil (at any angle of attack greater than zero) operating in an (incompressible) fluid will produce a vortex that is parallel to and behind the foil - called a 'starting vortex'.

 

[Maine Sail]

The zone of turbulence in the slipstream from the keel. Any foil (at any angle of attack greater than zero) operating in an (incompressible) fluid will produce a vortex that is parallel to and behind the foil - called a 'starting vortex'.

Haw far aft does this travel before dissipating and not acting as a deadwood? My three blade is about 6 feet aft of the keel. When you say deadwood I'm picturing a prop in an aperture. Wouldn't a two blade locked vertical in an aperture see less turbulence & create less drag than one six feet behind a keel locked and not in a aperture?

 

[RichH]

Intuitivly I am inclined to think that dragging a fixed prop requires more energy than one that is turning. There just seems to me that there is more slip with the free wheeling prop than with the fixed prop.
I think that the autogyro effect is different in that the autogyro has some control where the fixed blades would have the same resistance but would not lend themselves to control. The uncontrolled glide slope would be the same.

Let me change your intuition.

The work of anything being dragged through a fluid is due to friction (viscous shear).
there are TWO components to this friction:

1. Profile drag or Cross sectional drag - the friction caused by the shape (think of a silloutte being dragged perpendicularly though the fluid). Friction cause by the 'shape' of the object. Profile drag will be the same if its spinning or not.

2. Parasitic drag - 'skin' friction caused by the fluid in contact with the surface of the object.

Profile drag is constant in such flow regimes and since there is vastly MORE fluid in contact with a spinning object/prop, the friction (skin friction) component from parasitic drag is greater; hence more 'work'. The autogyro doesnt crash (as fast) because it has MORE friction (parasitic drag) as it descends - does MORE work. Try an autogyro without a spinning rotor and see what happens!!!??! --- less work !!!! Same with a spinning boat prop - it does more work because the surface comes in contact with MORE water (which is a viscous fluid).