Windmilling prop

[Tim Welsh]

Getting my GPS out this weekend

For the heck of it, I'm going to set sail and get to max speed for the wind thats out there and reset my gps average speed and sail for two miles with my trans in neutral, get my average. Then reset it and lock the prop in reverse and report back to you'all monday. Tim Welsh H34 AKA Cabo Wabo

 

[Dan Jonas]

Yes

Scott,A litle hard to explain without a picture, but. When the transmission is in forward think of the way the blades cut the water. The forward edge of the turning blade is in front of the back edge and the rear of the propeller blade is pushing the water towards the rear of the boat.Putting the transmission in neutral does not change the blade orientation. Now instead of the blade moving the water back, the water is flowing back over the front of the blade. This forces the blade to attempt to turn in the same direction as if it were engaged in a forward gear. A better metaphor would be that it is "windmilling".Draw a picture and it will make sense. Just keep in mind that in both instances the water is going the same direction in relation to the propeller, so therefore, the tendency to turn has to be the same. Conversely, to go into reverse, you have to turn the propeller in a direction to force the water to move forward relative to the position of the propeller. In that instance, you are using the front side of the blades to push the water forward.Dan Jonas (S/V Feije II)

 

[Rick]

One last question...

The issue of whether the prop creates more drag if it's 'windmilling' vs. locked is an interesting problem. Most of the archive discussions indicate that there is less drag when the prop is 'locked.' This, however, may be a bit confused with the clearly defined answer for airplane propellers: They create MUCH more drag when they are windmilling, but the primary reason for that is that they are 'feathered' (i.e. the blades are turned parallel to the wind) when they are locked... , or more accurately, they stop rotating when the blades are turned 'into the wind' [although some aircraft propellers have hydrodynamically actuated propeller brakes, that engage after the propeller is feathered, which prevents a slowly rotating propeller from turning]thus an aircraft propeller, when locked, or feathered, prsents a much smaller surface area to the apparent wind. So, the GPS experiment should yield interesting results, I suppose. Lots of folks in the sailing community insist that a 'windmilling' prop creates more drag, but, heck, empirical data will be welcome, especially if accompanied by a hazy, drunken narrative of some kind.Oh, wait. Maybe that's just me, with the hazy drunken narrative thing.

 

[Fred Ficarra]

I thought that party animal, Tim was a sure thing

for such a narrative.

 

[Tim Welsh]

P.S.

I'll also get an average of beers per knots.

 

[abe]

Prop locked in reverse, ok to start engine

Called Hunter about that issue when I first took delivery and was told do not force it into neutral, just start in reverse. This was for new H36, I assume it applies to other models.

 

[Dan Jonas]

Windmilling

Rick,Regarding the aircraft propellers, you have it partly correct. Multi-engine aircraft are outfitted with propellers that will "feather", single engine aircraft are not (absent very few exceptions). The simple reason is that after an engine failure, the alternatives in a single engine aircraft are limited, where in a multi-engine aircraft you may want to reduce drag to the minimum amount and try to continue to fly to some safe landing spot. Feathering produces the least significant drag.Advanced emergency management techniques in a single engine aircraft emergency (complete power failure) call for an attempt to stop the windmilling prop assuming time, conditions and benefits are appropriate. This can be accomplished by setting the propeller control to maximum pitch and/or slowing down. Stopping the propeller will extend the glide ratio (increase the options). In a stopped position the blades are not positioned parallel to the slipstream and if speed increased enough, I suppose they might start turning again since there is no "lock".I have some personal experience with this having experienced a throttle cable failure that resulted in a non powered landing on airport. The glide ratio with the propeller turning produced a very impressive approach angle.I suspect that a windmilling boat propeller will produce more drag than if it were not turning.Dan Jonas (S/V Feije II)

 

[chuckwayne]

starting in reversse

I'd be very careful starting in reverse-the torque from the starter could be enough to shear the (very)small brass key they used in the 356, and presumably the 36.

 

[Eric Lorgus]

Windmilling Propeller Like a Solid Disc

The way it was explained to me in flight training (long time ago), is that a windmilling unfeathered propeller creates almost as much parasite drag as a solid disc of the same diameter (as the prop). Intuitively, that doesn't make sense to me, but I've always heard that windmilling is worse than locked in place.

 

[Rick]

Drag vs. Lubrication

Actually, the drag issue doesn't really concern me all that much with the winds being what they generally are in SFO. But an excellent forum discussion from a while back included a diagram of a typical sailboat transmission, and mentions the fact that lubrication of the tranny while 'windmilling' might be insufficient in some cases. So, if ya can't windmill the darn thing, and ya can't get it out of reverse under sail, and ya can't start it in reverse, well, heck. I think I'll stick with 'nudging' it out of reverse as I hit the starter, and hope for the best.

 

[Fred Ficarra]

Or

Rick, how about saving your lunch money and buying a folder. Most of us would be better off with fewer lunches.

 

[Guest]

Yeah Fred....

Genius that I am, when I decided to replace my old 2 blade fixed prop with this new 3 bladed one, I decided that the substantial $$ for a folding prop would be better spent on a new (assymetrical) spinnaker. However, in SFO, I might use the chute once a year or so maybe, so I should have spent the bucks on a folding prop.All things considered, though, I think I have this windmilling thing resolved, mentally... I just needed some facts and thoughts from you guys to help me figure it out. Later - Rick

 

[Dan Jonas]

Or a feathering prop

I picked up a Max Prop at the recent boat show. Should have it on in the next few weeks. I agree with Rick that the real issue is lubrication. We already got rid of the brass key. That failed in the first year and I believe it was repalced with stainless steel. The mechanic said he did not understand why the softer key was used.Eric, I think your instructor was trying to emphasis a point. The drag of a windmilling prop is significant, but not equal to a full on disk that would throw out about 28 square feet of "flat plate" area on a typical 72" propeller. Your intuition is correct. Actually the drag increases as the speed of the propeller increases. Don't ask me to explain the mathematics.Dan Jonas (S/V Feije II)

 

[Fred Ficarra]

Dan,

I've got a MaxProp too. It was easier to spell 'folder'.

 

[Phil Herring]

folder

Funny, when you work with computers all day long folder has an entirely different meaning. Had me spinning for a moment.ph.

 

[Tim Welsh]

Results with GPS

Ok, back from the lake. Last night with a steady wind I sailed 1 nautical mile with the transmission in neutral and came up with 4.3 knot average speed and 1.3 beers in one nautical mile. I then locked the prop in reverse and reset the gps and came up with 4.6 knot average in the next mile and just 1 beer. Speed picked up but alcohol consumption didn't. Tim Welsh H34 AKA Cabo Wabo

 

[Tim Welsh]

no more discussion on this string?

nm?

 

[Fred Ficarra]

Sorry Tim, we forgot about you.

True! I meant to comment after your scientific, beer aided, test. Good job, and dually noted.

 

[Jim Smith]

Tim, good test but

would you say that if you did a third test and only had 1/2 beer that you might have hit 4.8 or so? Just teasing ;DIt sounds like putting it in reverse had less of a negative effect then allowing the prop to windmill. And is probably much better on the tranny and (a point that may have been missed during this discussion) perhaps less likely to snag a crab trap then a windmilling prop might be (just an observation for us Chesapeake Bay folks who have to do crab pot dodging when they are in a channel)Regards to all! -Jim-s/v Forever

 

[Terry Arnold]

Test results unaided by alcohol

In the July 15, 2000 issue of Practical sailor PS says "In 1993 we worked with graduate students and professors at MIT to test a number of propellers. One of the tests checked to see if allowing a prop to freewheel created less drag than locking the prop in place. The answer was that "substantial" savings in drag can be had freewheeling "across the whole range of speeds." This test assumed that both props were located in the open behind shaft struts not in apertures of full keels."