Fixed vs. Free Wheeling Prop - Updated !
- Thread starter Maine Sail
- Start date
Great test!
What do you think about a 2 blade in a cut out, behind a full keel? My Bristol seems to sail 1/10th knot faster with the 2 blade locked vertically. I like the quiet of a non-spinning prop and shaft while sailing.
What do you think about a 2 blade in a cut out, behind a full keel? My Bristol seems to sail 1/10th knot faster with the 2 blade locked vertically. I like the quiet of a non-spinning prop and shaft while sailing.
Bummer!!!! My owners manual says to lock the prop by putting the transmission in reverse. This is to prevent transmission toubles.
Can't really say..
I too am a lock it in reverse guy. I suspect if your dead wood is big enough locking it vertically would be better but who knows. This test really surprised me...
I wonder how 30 pounds drag translates into speed loss. I guess the only why to know would be to tow a boat at a given speed and see how much force it took. Then reduce the towing force by 30 pounds and see how much you had to reduce speed to make it happen. That way the drag would subtract from the net pulling power of the sails.
Don't know..
For most cruisers six seconds a mile is a non issue..
That I have no idea on. I would probably rely on the PHRF handicap rules where a 3/sec mile handicap is applied to a fixed three blade prop. If I recall it is a 6 sec/mile spread between a feathering prop and a fixed three blade.
For most cruisers six seconds a mile is a non issue..
How much drag does a freewheeling prop get from the transmission? If the freewheeling prop spins more freely than the freewheeling prop attached to a transmission, then the test isn't going to be accurate...
Dog...
I know people not reading the thread is one of your pet peeves but heck you didn't even fully read the original post..
I matched the friction of the test jig to that of my own boat as closely as I could. BOTH start spinning between .8 & 1.2 knots using this exact prop..
Also this is from post #34:
Dog... Please re-read it, specifically item #7 in the original post.
I know people not reading the thread is one of your pet peeves but heck you didn't even fully read the original post.. I matched the friction of the test jig to that of my own boat as closely as I could. BOTH start spinning between .8 & 1.2 knots using this exact prop..
Also this is from post #34:
With my boat and PSS seal my prop will spin at under 1 knot. I compared the jig to my own boat using the digital scale. I pulled my prop with the digital scale and Gorilla tape from the outer tip of the blade and then compressed the friction bearings on the jig, with pipe clamps, until I matched the pressure it required to spin my prop while on the boat. Every boat has different friction depending upon transmission, cutlass condition and stuffing box. This is measured against my own boat as the baseline.
Again this is NOT for exact numbers, though I have tried to replicate my boat as closely as I can including the props depth below the surface of the water, I did this because I know someone from which ever side does not win will probably enter into a debate about water density at a certain depth
Speaking from the perspective of one who used to work at the drawing table next to a really delicious and only slightly older German lady who was designing propellers. I used to chat her up quite a bit and that was one of the first subjects that came to mind. Such is the extent of my propeller design education although a few things stuck in my mind other than the memory of her marvelous ... but, I digress.
I think what you are seeing is the effect of disk area ratio. The three blade wheel is more than half bronze when you look at the circle the water has to flow through. The blades are spaced closely enough that the flow of each blade effects the other. The water turned by the forward surface of one blade encounters the flow trying to follow around the aft surface of the one next to it. Flow in the inter blade spaces is slower (or faster, i.e. accelerated more if you think from the perspective of the dockside observer) than it would be at the same distance from the blade edge if just one blade was being dragged through the water. When you let the blade start turning, the rotary motion aligns the interblade space more with the flow and assists the water getting through the openings.
In the case of the higher aspect ratio, twin blade prop, there is very little interaction between the flow patterns around the blades. The change in velocity of the water in the interblade regions is not significantly different with the full prop than it would be if just one blade was being dragged through the water. I would expect very careful measurements to show slightly higher drag for the Campbell when free wheeling but probably not enough to measure with your set up.
All drag is a function of putting mass (air or water) into motion or slowing it down (depending on whether you are looking from the boat or the dock). Friction is just what transferrs the energy. The drag of the prop is a function of the changes in water velocity in the entire region around the prop and not just the blades. The effected area is actually larger than the prop disk just as the circulation field that is causing life extends far out from an airplane wing, 2 - 3 times the width of the wing up and down in some cases.
It gets even more complicated when you put the prop in an aperture as on a traditional hull. Now even the 3 or 4 blade prop may be in a region of flow that is already considerably effected by the hull. Letting the prop freewheel can alter the flow so that faster moving water is pulled down to the hull and rudder increasing drag. This is the prop configuration that was most common back when the conventional wisdom that a freewheeling prop has more drag was established. It was probably correct in most cases then.
You have to look at the entired installation and hull for conclusive answers. Your rig is close to that of the installation on a modern hull. However, rotating shafts at an angle to the flow have more drag than motionless ones. See "Fletner Rotors" via Google. To be sure of what is happening on your sailboat, you need to duplicate the shaft and shaft angle.
I think what you are seeing is the effect of disk area ratio. The three blade wheel is more than half bronze when you look at the circle the water has to flow through. The blades are spaced closely enough that the flow of each blade effects the other. The water turned by the forward surface of one blade encounters the flow trying to follow around the aft surface of the one next to it. Flow in the inter blade spaces is slower (or faster, i.e. accelerated more if you think from the perspective of the dockside observer) than it would be at the same distance from the blade edge if just one blade was being dragged through the water. When you let the blade start turning, the rotary motion aligns the interblade space more with the flow and assists the water getting through the openings.
In the case of the higher aspect ratio, twin blade prop, there is very little interaction between the flow patterns around the blades. The change in velocity of the water in the interblade regions is not significantly different with the full prop than it would be if just one blade was being dragged through the water. I would expect very careful measurements to show slightly higher drag for the Campbell when free wheeling but probably not enough to measure with your set up.
All drag is a function of putting mass (air or water) into motion or slowing it down (depending on whether you are looking from the boat or the dock). Friction is just what transferrs the energy. The drag of the prop is a function of the changes in water velocity in the entire region around the prop and not just the blades. The effected area is actually larger than the prop disk just as the circulation field that is causing life extends far out from an airplane wing, 2 - 3 times the width of the wing up and down in some cases.
It gets even more complicated when you put the prop in an aperture as on a traditional hull. Now even the 3 or 4 blade prop may be in a region of flow that is already considerably effected by the hull. Letting the prop freewheel can alter the flow so that faster moving water is pulled down to the hull and rudder increasing drag. This is the prop configuration that was most common back when the conventional wisdom that a freewheeling prop has more drag was established. It was probably correct in most cases then.
You have to look at the entired installation and hull for conclusive answers. Your rig is close to that of the installation on a modern hull. However, rotating shafts at an angle to the flow have more drag than motionless ones. See "Fletner Rotors" via Google. To be sure of what is happening on your sailboat, you need to duplicate the shaft and shaft angle.
I'm sure you did and am not surprised at all. What I meant is that you would need a different test set up to be sure of the results for my boat with it's very steep shaft angle and fairly long length of exposed shaft.
My transitional hull design has quite a bit of stuff ahead of the prop. Even a 3 blade wheel with one blade vertical in the wake "shadow" of shaft, skeg, and hull, might have less drag fixed than when rotating. The wake fraction effects could even be enough on a hull like mine that two blades up and close to the water being dragged along with the boat could change the results measurably.
A great and interesting experiment that I don't mean to belittle at all. It is probably more relavant to the majority of modern boats than to the sailboat fleet as a whole.
Happy (Orthodox) Easter, one and all!!!
Maine Sail, good morning...
Great job, but you know you're going to get nitpicked from now to doomsday!!!
This was beginning to be like waiting for the results from one of those stupid idol/dancing/house buying TV shows. At least I'll be able to sleep better tonight.
Paul
PS What did I win???
Maine Sail, good morning...
Great job, but you know you're going to get nitpicked from now to doomsday!!!
This was beginning to be like waiting for the results from one of those stupid idol/dancing/house buying TV shows. At least I'll be able to sleep better tonight.
Paul
PS What did I win???
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Relative Motion
The propeller will not be aware of whether it is being pushed or pulled through the water or being immmersed into a moving stream of water. So my suggestion for making this simpler is get rid of the boat and motor and simply insert your test set up into a running stream of a constant velocity such as a tidal basin at maximum flow rate, you could see what effect changing velocity has. Since you are in Maine where tides are very high you should be able to find a nice spot without all the hassle of waves and not running into things and trying to steer while working the setup.
Great test, just a suggestion if you wanted to try more conditions and other propeller types. Many will question me for ground effects and proximity to shore effects, but since you are looking for relative differences and not exact answers the comparison should still be valid within reason.
Nice test.
The propeller will not be aware of whether it is being pushed or pulled through the water or being immmersed into a moving stream of water. So my suggestion for making this simpler is get rid of the boat and motor and simply insert your test set up into a running stream of a constant velocity such as a tidal basin at maximum flow rate, you could see what effect changing velocity has. Since you are in Maine where tides are very high you should be able to find a nice spot without all the hassle of waves and not running into things and trying to steer while working the setup.
Great test, just a suggestion if you wanted to try more conditions and other propeller types. Many will question me for ground effects and proximity to shore effects, but since you are looking for relative differences and not exact answers the comparison should still be valid within reason.
Nice test.
Ditto - Great job Main Sail
It's good to know about all these things like shaft angle, etc., etc., being an influencing factor but bottom line that was a great experiment write-up.
A comment of the PHRF handicap allowances for fixed-blade props is I don't think the allowance is enough - again, a lot of factors, but all the good racers use low drag props for a good reason - they get better results than the fixed-blade allowance.
What I'd be interested in is the drag for a two-blade feathering prop aligned with the strut. I wonder if PYI could come up with an old Max-Prop one could use?
Oh, and maybe an editorial comment: I've always heard how bad Maine winters were but this project makes a believer out of me!
Good job!
It's good to know about all these things like shaft angle, etc., etc., being an influencing factor but bottom line that was a great experiment write-up.
A comment of the PHRF handicap allowances for fixed-blade props is I don't think the allowance is enough - again, a lot of factors, but all the good racers use low drag props for a good reason - they get better results than the fixed-blade allowance.
What I'd be interested in is the drag for a two-blade feathering prop aligned with the strut. I wonder if PYI could come up with an old Max-Prop one could use?
Oh, and maybe an editorial comment: I've always heard how bad Maine winters were but this project makes a believer out of me!
Good job!
Re: Ditto - Great job Main Sail
You could spend a lifetime refining this test and checking all of the variables and still come to the same conclusions just with greater precision. This test should put to rest the basic question.
You could spend a lifetime refining this test and checking all of the variables and still come to the same conclusions just with greater precision. This test should put to rest the basic question.
For those who might be interested here are hull and prop drag curves from Yachting Monthly's current (May) article. The full article may be purchased from them or, of course, one can buy the magazine.
The prop drag curve was obtained by fitting a 3 blade 16" x 11" prop to an outboard leg and towing a skiff with the outboard attached.
The prop drag is similar to that so admirably measured by Main and the yacht drag is a theoretical curve of a modern 32 footer from the UK's most respected hydrodynamics test facility.
At 5 knots, surprisingly, the fixed prop drag equals half the hull drag.
For the nerds among us (which I think must mean everyone who has posted on this topic to date - me included!!) one can select a boat speed e.g. 5 kt and note hull drag = 41 Kg, then add prop drag = 20 Kg making 61 Kg.
Now look where the hull drag red line crosses the 61 Kg horizontal line and read off what the speed with a feathered prop would have been = 5.7 Kt.
The prop drag curve was obtained by fitting a 3 blade 16" x 11" prop to an outboard leg and towing a skiff with the outboard attached.
The prop drag is similar to that so admirably measured by Main and the yacht drag is a theoretical curve of a modern 32 footer from the UK's most respected hydrodynamics test facility.
At 5 knots, surprisingly, the fixed prop drag equals half the hull drag.
For the nerds among us (which I think must mean everyone who has posted on this topic to date - me included!!) one can select a boat speed e.g. 5 kt and note hull drag = 41 Kg, then add prop drag = 20 Kg making 61 Kg.
Now look where the hull drag red line crosses the 61 Kg horizontal line and read off what the speed with a feathered prop would have been = 5.7 Kt.
Attachments
It's not quite as bad as it looks. A basic factor in prop selection is the fact that the water isn't flowing over the prop at the same speed the boat is moving through the water. The drag of the hull moves some of the previously motionless water forwards. This is a big component of the drag. The closer to the hull, the more the water is moved. From the reference point of the boat, this is a reduction in the speed of the flow, usually called Va, Speed of Advance, or Wake Fraction.
Reductions of 75% - 85% are typical. If that drag curve is for a cruising type 32 footer, say 80% so the drag point for a prop operating in open water should be taken at 4 knots. It's still a lot of drag.
Did that article say anything about 2 blade wheels? I'm very curious now. I'm beginning to think a feathering 2 blade would make a much more noticable difference on my boat than I would have thought.