Spin the bucket? (Reverse for Sailing?)

[Emerald]

OK,

a call for opinions backed by data if possible - this should be fun, as there are never any opinons offered up here.....

Here's the scoop. Emerald has a 3 bladed bucket for a propeller. I couldn't care less at this point about the virtues of feathering/folding/2 versus 3 blade, but what I do want to nail down is it better to let your 3 bladed bucket spin freely as you sail along, or is it better to lock it down and stop the spin? Of course, if someone wants to take pity on my bucket scenario and send me a nice Max prop for the cost of shipping....


.

 

[valentor]

This is strictly speculation, but it would seem that after the initial 'spin-up' the prop would offer little drag once the boat is moving.

If the prop were locked, it would constantly be trying to spin and would produce considerable drag.

It should be fairly easy to test. If you sail along with the prop locked and note your average speed. Disengage the clutch and continue to monitor speed and note the difference.

Please let us know the result.

My E-34 has a 2-blade feathering prop and once it is properly oriented, and the clutch is locked, I have measured boat speed increases of more than 10% in very light conditions.


-Steve

 

[ted_reshetiloff]

I have been advised by my Universal manual not to let the prop free wheel as it can damage the transmission. Something about the gears not getting tranny fluid where they should if the engine is not running, but I dont recall the details. If your engine's tranny shares the crankcase oil like many do this may not be an issue, but I would check with the engine manuafacturer first.

 

[Glyn Judson]

Prop drag revisited.

David and all, It's not the first time this has been discussed here and the opinions ran from one pont of view to the other. What I seem to remember is that someone used a helocopter as an example of comparative drag. A spinning blade will offer more air resistance and will allow the chopper to make an emergency landing. A nonrotating blade will offer less wind resistance and the chopper will drop like a rock. Let the games begin, what say others? Glyn Judson, E31 hull #55, Marina del Rey, CA

 

[Loren Beach]

Lock 'em, Danno...

Several years ago this was discussed on the YachtList. What I remember is that the locked blade has the least drag. A separate issue is that some types of tranmissions do not lube their gears when the engine is off and also should not be left to turn with the prop under sail.

I always remember the closing argument to the effect that you can "auto gyro" an engineless copter down with some hope of survival if the blades can rotate, but a locked blade will let it fall quickly and kill you for sure.

Having a Universal diesel with a Hurth trans, I follow the instruction in the manual to lock it in "reverse" position when sailing.

Loren

 

[Frank Langer]

Loren (and others),
I have a universal 16 hp (model 5416), hurth transmission and 2-blade folding prop. The manuals indicate that it is ok to leave the gears in neutral while sailing. Any idea why the difference, given that we all have universal/hurth combinations? Is it the folding prop, ie. very limited resistance/no turning? What if the prop remained stuck open? Is there any disadvantage to locking it in reverse, to avoid a potential problem?
Thanks,
Frank.

 

[Chris Miller]

you'd hear/feel a turning prop if it was stuck open. Pop the tranny in reverse to get the prop to fold- then it doesn't matter.

IIRC- spinning fixed prop develops the same drag properties of a solid disk? maybe I had too many rum+tonics the night somebody told me that...

 

[Frank Langer]

Thanks, Chris.
Frank

 

[Emerald]

I had a hunch this topic had some twists in it.

I have a Yanmar transmission, and Yanmar's preference is neutral or reverse. Either way, I can lock or spin without fear of hurting the transmission.

I find I can twist my head in to a knot reasoning this one out - freewheeling prop moves because of force on blades from boats movement through the water- resistance to propeller spinning is merely friction in portion of drive train and efficiency loss of propeller design - energy used is from relative water movement created by boat moving - so drag free wheeling should just be friction in drive train plus efficiency loss compared to trying to hold the propeller still which takes a lot of friction transfered from holding object - your hand/transmission to hull resulting in greater drag on hull than letting it spin, so let it go and spin, but then there's the helicopter thing, and check out the bottom of this page under the section, TECHNISCHE UNIVERSITAT TEST, 1997, second footnote on the chart for Drag, specifically reading, "Drag of P increased to 420 N (94.5 lbs) if locked."


http://www.flexofold.com/test.htm


which goes back to let it spin - does the helicopter pilot put a lot of pitch on the blade to make it really inefficient while spinning and hence slow down the drop of the aircraft verus not spinning and just dropping like a brick?


...ugh!



.

 

[rwthomas1]

I know a bit about helicopters as I have built and try to fly model radio control heli's. Autorotation is applying negative pitch to cause the blades to spin from the air rushing by as the heli falls. This still happens pretty fast and the heli falls, albeit under some control. The idea is to use the energy stored in the rotating mass of the rotorblades to stop the heli from smacking into the ground when it reaches it. As the heli falls, negative pitch is applied and the rotor spins up the pilot looks for a good spot to put the craft down. When very close to the ground the pilot feeds in full positive pitch to stop the heli before it strikes the ground. The act of feeding in positive pitch very quickly stops the spinning rotor blades but not before that energy has been used to produce just enough lift to land. Timing is critical. Going positive pitch too soon, as in too high, and you run out of lift before you get to the ground. You freefall the rest of the way..... Going positive pitch too late results in not using enough or all the energy and hitting the ground way too fast. Not good either.

The spinning blades do in fact create far more drag than stopped blades so I am inclined to believe that a locked prop has less drag. RT

 

[gareth harris]

Reversing the pitch is a good way to look at it, since the prop is essentially running at negative lift when spinning in drag.

A fixed prop will have a huge parasitic drag.
A spinning prop will have a much smaller parasitic drag, but a high lift vector on the prop due to the spinning, most of which will act as drag on the boat.

Which is bigger is not necessarily intuitive (issues relating to lift rarely are), and would need to be tested, I suspect that a spinning prop would be worse, at least in most cases.

On another note, the helicopter analogy in general is not a good one. Scale models aside, they do not use reverse pitch, but rather are designed so that at a medium flight speed, with minimum pitch, the forward combined with downward motion will create lift characteristics that lead to a steady, controlled descent (the induced lift must move forward of neutral, driving the blades). Approaching the ground, the pilot has to try to stop the forward motion at the same time as touching down, in the last few seconds cushioning the landing by increasing blade pitch. A helicopter can not autorotate in a hover, due to vortices around the blade tips.
When the blades slow down for either mechanical or aerodynamic reasons, helicopter pilots refer to the flight characteristics 'going ceramic'.

 

[ted_reshetiloff]

You guys scare me and I've only had half the rum tonics Chris has.

 

[Chris Miller]

Oh Yeah...

I forgot that Ted was present at a typical "Racer-dude Raft-up" that we have...
My liver is still trying to recover from summer...
:jollyroge :unsure:

 

[rwthomas1]

On another note, the helicopter analogy in general is not a good one. Scale models aside, they do not use reverse pitch, but rather are designed so that at a medium flight speed, with minimum pitch, the forward combined with downward motion will create lift characteristics that lead to a steady, controlled descent (the induced lift must move forward of neutral, driving the blades). Approaching the ground, the pilot has to try to stop the forward motion at the same time as touching down, in the last few seconds cushioning the landing by increasing blade pitch. A helicopter can not autorotate in a hover, due to vortices around the blade tips.
When the blades slow down for either mechanical or aerodynamic reasons, helicopter pilots refer to the flight characteristics 'going ceramic'.

Interesting, my models are far from scale with power-to-weight ratios impossible in real helis. The models do in fact have negative pitch which will allow you to hover inverted, and other cool tricks. Again, obviously not directly applicable to the real thing. Sorry to hijack the thread! RT

 

[gareth harris]

The models do in fact have negative pitch which will allow you to hover inverted, and other cool tricks. Sorry to hijack the thread! RT

I'll join you in the thread hijack. Those machines do sound like a lot of fun. A friend of mine showed me his model helicopter when I was a student, but was loathed to let anyone else fly it, due, I now understand, to the ease with which they can be broken. He did a lot of things that no real aircraft could do, and clearly had real skill. My budget at the time did not allow me to get my own. He also distanced himself from me when I came out with the idea of mounting a camcorder inside it - he noticed me eyeing the windows of the girls dorm as I thought of it.

 

[bwb]

"setting" the prop

I don't know much about fluid dynamics or about 3-bladed props, but on a couple of boats I race on, the skipper has us "set the prop" while we're tuning up before the race. The idea is to get the fixed 2-bladed prop oriented vertically so that drag is minimized. This is accomplished by sending one crew member into the engine compartment to manually turn the shaft until a preset mark is facing up, and then the helmsman throws the gearshift into reverse to lock it. Some say that you can get 1%-5% more boatspeed in light conditions by getting the prop out of the way. In a close race, every 10th of a knot counts.

 

[Kim Schoedel]

In gear or not?

I always leave the trans in gear while under sail for fear on the shaft turning the trans. Someone on the dock told me that being as though I have the Max Prop self feathering 2 blade, I do not need to leave it in gear. Any thoughts?

 

[Cory B]

IIRC, MaxProp recommends you shut down the engine WHILE IN GEAR with the boat going 5+ kts for the prop to feather correctly. And I've noticed on our engine at least that changing the gear when the engine is off does nothing, it doesn't actually change until the engine is running. With our old A4 we could actually change gears while the engine was off.

 

[Shadowfax]

I don't know which transmission you have, but my manual for the M 25XP states that you are to place the transmission in reverse while sailing. I've been told that the reason for this is to keep the prop from spinning, which actually causes less drag and if the prop is spinning it could cause problems in the transmission because parts of it are moving and there is no lubrication. Both ideas sound reasonable to me so I always sail with the trans in reverse and have never had a problem

YMMV

 

[u079721]

I agree with Cory. Depending on the viscosity of the grease within the Max Prop and the temperature of the water, the prop may not easily feather if the shaft is freewheeling. So either shut the engine down while in gear, or go into reverse while the boat is still moving quickly to give the prop something to feather against. After a few seconds it then won't matter if you leave the tranny in gear or not. Of course to check whether your prop has feathered you can do one of the above tricks, then put it into neutral and go below and check to see whether the shaft is freewheeling. If the shaft is moving then your prop isn't feathering correctly (as would happen if the grease were too stiff).