Drag effect of freewheeling fixed prop

[Maine Sail]

There have been so many discussions on the subject and the research quoted as absolute proof. Unfortunately, the prop tested was a three bladed motor boat type prop which is not the usual first choice for a sailing boat. There are a number of variables in prop design which may prove or disprove the conclusions regarding the quoted research, particularly if combined with the variables of hull design. My own suspicion is that the differences observed will change substantially with changes in the area of the blades. A narrow two bladed sailing prop may well offer less drag if locked than if spinning when the slip magifies its cross sectional area - and if it can be stopped in line with a keel or deadwood it could prove almost invisible. Data based on a variety of blade designs would at least allow extrapolation within first order of magnitude.

Yes, my Flexofold likes to be stopped in gear to fold it.

Rob.

And there's your proof. Our two blade Flex-O-Fold will not fold unless I lock it in reverse. The drag of the spinning blades is not enough to even cause it to fold. This is a prop that will gravity fold on dry land with the blades vertical. The split second the FOF is locked in reverse SLAM the blades fold shut.

I am actually the guy who did the referenced experiments. I still have a 100% open invitation for any "doubters" to come witness this first hand with some caveats:

1- You pay for my time at my hourly rate
2- You publicly eat crow on You Tube as a doubter...

I have tested a Michigan Wheel sailor prop in the same pitch and diameter as the prop in the video, that came off a factory produced sailboat. It still caused more drag when locked...

On our old Cape Dory 27 we had a two blade prop and marked the shaft for vertical to lock it behind the deadwood. If placed back in neutral, despite being hidden behind the deadwood, it began spinning again....

There will be some point at which a locked prop causes equal drag to spinning but those are not normal pitches which we see used on sailboats. For the "typical" sailboat prop they cause more drag when locked. But you don't need to just take my word for it the University of Strathclyde Ocean Engineering Department, Dave Gerr (Westlawn), MIT and a UK magazine all found nearly the same results I did..

Again I offer a 100% open invitation to any doubters to come spend a day with me dragging props through the water..... I have this confidence because I have spent countless hours doing this trying to get to the bottom of where or how this steadfast belief came about...

 

[lw395]

And there's your proof. Our two blade Flex-O-Fold will not fold unless I lock it in reverse. The drag of the spinning blades is not enough to even cause it to fold. This is a prop that will gravity fold on dry land with the blades vertical.......

As you point out, there is not much friction resisting folding the prop.
What makes the prop stay un-folded is the centrifugal force of it rotating.
This is enough to overcome the thrust when reversing, given a few more revs.

Your post is unfortunately, another rubbish proof of something that happens to be generally true.

 

[Maine Sail]

As you point out, there is not much friction resisting folding the prop.
What makes the prop stay un-folded is the centrifugal force of it rotating.
This is enough to overcome the thrust when reversing, given a few more revs.

Your post is unfortunately, another rubbish proof of something that happens to be generally true.

Except that the "centrifugal force" does not always open my FOF prop enough to even slow the boat, with a "few more revs", and the boat continues to move full ahead. Reverse based on centrifugal force is one of the real weak points of this prop. Crash stops, won't even go there compared to our old two blade "sailor" prop....

I very often need to go to max RPM, 3K engine RPM in reverse or 1350 shaft RPM,, to get the blades to open and stop the boat. Our FOF prop continues to spin in forward at just 2 - 2.5 knots of sailing speed and the shaft is spinning quite slowly, I've watched it and it is nowhere near the 1350 RPM I often need to get the prop open..

The centrifugal force at 2 - 2.5 knots under sail is a lot less than max RPM in reverse. My point is the "drag" of the spinning prop is not even enough to cause it to fold or overcome a very low RPM / centrifugal force to keep it open.

 

[lw395]

Never had an actual FOF prop, just a couple of vaguely similar folding props on racing boats.Once it is open in reverse, I found I could motor quite slowly in reverse, without the prop folding up. I suppose the drag of motoring a 40ft racing yacht in reverse is not huge at 3knots or so, but it's only the centrifugal force keeping the prop open.
I suspect there is a bit of friction, the centrifugal force really kicks on only when the blades move away from the axis. F=mr.omega-squared and all that. When folded, r is very small.

I think the drag of any fixed prop, either locked or spinning, is not really something we should have on a sailing boat. That's purely my opinion, based on my taste in boats.
I'm only arguing with your proving methods, not your conclusions.
The old logic of locked two blade props was probably right in the days on long keel racing boats, not saildrives.