Quote:
Originally Posted by instinctyp
Propshaft Spinning when Sailing. A Question.
With a fixed blade propellor (2 or 3 blade) what give the most drag when under sail. (a) A rotating propellor, or (b) Locked propellor.
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First let me say that if the prop on the
Kanzaki is spinning in reverse the shifter throw probably needs to be adjusted.
As for drag..
Over last winter there were a few discussion on other boards that lead to no hard conclusions on whether a fixed prop or a locked prop causes more drag. There have been two studies that have both concluded that a freewheeling prop causes less drag but these studies were done in test
tanks and some sailors argued that vortexes created within the
tanks throw off the results.
I don't like not knowing.
I spent a few late nights in the barn, over the winter, listening to good tunes and plugging away on this design. This jig told me what I needed to know. It was affixed off the side of my
dinghy and dragged through the
water ahead of the
motor, to avoid vortexes & whirligigs and what ever else,
and at a
depth similar to that of my own sailboats fixed prop.
I measured it both locked and freewheeling and I also measure the drag of the apparatus alone, 12 lbs., to subtract it from the actual drag of the props minus the test apparatus. I designed the bearings to have a similar resistance to the prop shaft on my own sail
boat so from that perspective all is quite comparable in terms of freewheeling.
The drag measurements were captured with a 50 Lb. analog scale (ditched the digital as analog showed better on video) and
GPS SOG so as to more accurately compare between the same prop in both fixed and freewheeling modes. The range of motion on the scale (movement of the hook) from 0-50 lbs. is about 1/8" so this did not affect any readings what so ever by changing the angle of the test jig in the
water..
The prop I used is a standard three blade fixed sailboat prop. It is made by Michigan
Wheel. So this post focuses on the Michigan
Wheel three blade prop which is perhaps the most common fixed prop used on sailboats in the US.
This is an age old argument, with a relatively easy test, yet surprisingly no one has done it, not even Practical Sailor..
The Test jig:
The Shaft Mechanism (the nail is the shaft lock):
The Drag Measurement Assembly:
The Hinge Mechanism:
The Digital 50 Lb. Scale that I decided not to use:
Michigan Wheel Data
The results of the Michigan Wheel MP prop were...well surprising to say the least. I want to clarify some points below so there is less confusion.
1) This test was only to determine if a standard Michigan Wheel three blade fixed prop causes more or less drag when towed through the ocean at a similar
depth to that of a sailboat, particularly my CS-36, and with a comparable shaft resistance to a sailboat (namely mine). It is not to give accurate numbers or data on how much drag the specific prop creates.
2) Drag is relative to the the drag jig I used. The drag jig alone, with no prop, created about 12 lbs. of drag in this configuration at WOT on my 4 hp Johnson
outboard.
3) Because the jig is the 100% the same in both fixed and freewheeling and the ONLY difference between fixed and freewheeling was a 2.5 inch roofing nail the only differences in drag comes from the prop not being able to spin and spinning.
4) The
motor was always run up to wide open throttle to totally minimize any throttle position variability between locked and freewheeling.
5) The pin point accuracy of the scale means little because it is only a control. The same scale was used for both fixed and freewheeling and it was only compared to itself in an A/B situation, fixed/freewheel.
6) The difference between fixed and freewheeling was LARGE so a pound or two here or there means very, very little. Average drag at WOT in freewheeling mode, including the strut, was about 20-25 pounds. Average drag in fixed mode, including the strut, was about 45-50 pounds. As you can see .001 differences in accuracy do not matter when trying to answer this question as related to this very, very popular sailboat prop.
For those worried about whirly gigs and vortexes and .0001 differences I spun the strut around, with the prop facing forward, and ahead of the struts "interference wake". I was surprised that I could not detect a discernible difference in load despite having to move the line a little higher on the strut. If there was a difference it was clearly less than one or two pounds and not noticeable in the big scheme of things.
7) Freewheeling is little bit of a misnomer. The shaft was not actually allowed to freewheel with minimal to no friction. The friction bearings I designed were tightened and adjusted to closely mimic the friction of my own sailboats shaft. This test was primarily for me and my own curiosity and then secondarily for the sailing community. This is why the depth of the prop in the water matches my CS-36T and the shaft friction was set to begin spinning at about .8 - 1.2 knots which is what it does on my own
boat.
8) The results for the Michigan three blade prop are quite clear, and quite discernible, and coincide with those of the MIT study, the University of Strathclyde study and other prop drag tests like the one in a the UK's Yachting Monthly magazine.
9) This
experiment & video below is about the prop used, a Michigan Wheel three blade "MP" prop. I make NO claims or suggestions about any other fixed type props including a two blade version of the Michigan Wheel MP. If someone wants to send me a two blade MP in a 1" shaft size I will be glad to test it too..
10) As far as I know this the ONLY video proof that clearly shows a fixed vs. freewheeling three blade sailboat prop being load tested and compared only to itself in both fixed and locked mode.
11) Before you get all fired up because you are a believer that fixed three blade props cause less drag, not more, PLEASE remember that the ONLY difference between the fixed and freewheeling modes was a 2.5" nail passing through both the jig and the 1" shaft to lock it in place. There is NO possible way that 2.5" nail caused a nearly 300% difference in drag or a 25 additional pounds of resistance.
12) I need a bigger motor! I was only able to attain a max speed of about 4.2 knots with the jig and prop in the water freewheeling and less in locked mode. I'd like to hit 6.5-7. Most sailors though are concerned about prop drag at less than
hull speed and the 4
knot range is less than
hull speed for most sailors. In light winds, and under
hull speed, with a fixed three blade Michigan Wheel, you will see less drag when freewheeling!
Results:
Jig Drag = 12 pounds with no prop mounted. Measured at WOT.
Locked Prop Drag = 45-50 pounds: 50-12=38 pounds of actual prop drag.
Freewheeling Prop Drag: 20-25 pounds: 25-12=13 pounds of actual prop drag when freewheeling.
Locked is 2.92 times more drag or a 292% increase in drag when you remove the jig from the equation!!!!!
As I said this is not even close so no need to worry about the .001's or a few ponds of drag here or their or even the rather "unscientific" method used..
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