Originally Posted by Marksman
In "Ted Brewer Explains Sailboat Design" he states that tank testing proved a free wheeling propeller produced more drag than a stationary one. The difference was slight but should not be ignored. Another test, while driving your car, stick a pinwheel out the window and feel the drag increase as it begines to spin. There is physical science behind this but I would be very poor at explaining it and I must get back to work.
I have no idea who did that testing or what he was referring to so can't comment on his statement.
There have been three studies SPECIFICALLY looking at sailboat props, one by MIT, one by the University of Strathclyde Ocean Engineering Department and one by Practical Boat Owner/Yachting Monthly where they towed actual boats through the water
and measured with a strain gauge, All three studies of SAILBOAT props all agree a locked prop causes more drag.
For those still in doubt feel free to watch an experiment
I conducted myself to finally try and put this question to bed
Prop Drag Video (LINK)
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 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.
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.
Because the jig is 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 come from the prop not being able to spin and spinning.
was always run up to wide open throttle to totally minimize any variability between locked, freewheeling and throttle response.
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.
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 was about 20-25 pounds including the test jigs strut. 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.
When 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 scale of things.
Freewheeling is little bit of a misnomer. The shaft was not actually allowed to freewheel with minimal to no friction. The friction bearings 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.
The results are quite clear, quite discernible and coincide with those of the MIT study, the University of Strathclyde study and some other prop drag tests like the one YM Magazine.
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..
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 to itself in both fixed and locked mode.
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. There is NO possible way that 2.5" nail caused a nearly 50% difference in drag or a 25 additional pounds of resistance.
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
. In light winds, and under hull speed
, with a fixed three blade Michigan Wheel, you will see less drag when freewheeling.
If we extrapolate the data and remove the test jig, 12 lbs. of drag, and use the high side numbers of both fixed and locked we have 38 pounds of drag in fixed mode (50 Lbs. - 12 = 38) and 13 pounds of drag (25 Lbs. - 12 = 13) in freewheeling. This is nearly THREE TIMES more drag. Again, we're not talking .001 fractions here we're talking nearly a three times difference in drag when this three blade prop was locked.
I'm sure there will still be doubters...
Despite all this I still lock my prop, unless I want to go faster..
P.S. Before you do either consult your gear
boxes owners manual..