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Old 27-08-2009, 20:43   #16
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Drag...

Quote:
Originally Posted by instinctyp View Post
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.
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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Old 27-08-2009, 23:40   #17
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Just went through a fairly extensive search of the prior threads on this subject. The only on-the-water experimental results I could find reported here indicated that freewheeling was faster than locking. Didn't find anyone who reported faster locked based on actually checking both cases.

Well either I can't find it, or I was wrong. Maybe it was another forum. Anyway for now I change my tune. I also have not been able to find a thread on this forum where someone actually did the experiment and sailed faster with a locked prop.

John
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Old 28-08-2009, 00:27   #18
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Amazing...

So, a prop that spins freely is driven by the water flow over it's blades. This means power (energy actually) is transferred from that water flow to the turning of the prop. This means that the water flow is reduced, i.e. there's drag (you go slower). The more power is needed to turn the prop (like when you would break it by adding friction on the shaft), the more drag there is. This tops at a maximum when you fully stop the prop.

So, you don't need experiments to know that a freely spinning prop causes less drag than a locked one... just remember high school ;-)

People might get confused with airplanes stopping the prop when an engine is shutdown (they hopefully have multiple engines ;-) but they feather the blades in that case, just like a feathering prop under a sailboat: you must stop the prop from turning with a feathering prop because the mechanism might not work otherwise.

About that prop not slowing down the boat much.... tow a bucket and see it yourself. It slows a lot!

cheers,
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Old 28-08-2009, 01:01   #19
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Quote:
Originally Posted by cantxsailor View Post
As slow as my old boat is(named "Turtle" for a reason) I can't believe the few square inches of prop face is going to make much of a difference especially if it is free wheeling. .0003 of a knot isn't going to make or break my day. ...........m
Think you might be suprised at the drag a prop creates- even when rotating.
We've a pal who purchased a DuoGen which had wind and water props. On his Lagoon 40 Cat when he deployed the water prop sailing at 7 knots, his boatspeed dropped by 1 knot or more.
When he lifted it out - back up to speed.
And these props are not really big.

Nothing more convincing than actual practical experience to know I'd never stick with a fixed bladed prop on a sail boat.

JOHN
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Old 28-08-2009, 03:27   #20
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Thanx Nick & Main Sail !!!

It’s nice to see that the laws of physics (mechanics), as deftly explained by Nick, are borne out by experimentation, as detailed by Main Sail’s very elegant demonstration.

Since drag is directly proportional to velocity (squared*), a larger outboard will increase the difference between a locked & freewheeling prop. Conversely, a slower boat will exhibit less difference.
* Assuming turbulent flow & a larger Reynolds number.
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Old 28-08-2009, 07:43   #21
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The speed loss is biggest when going slowest. The drag is not big then, but the forces on sails are just not big enough to overcome - the drag eats up the driving force then. Say we might be talking of 1 knot of speed for a boat going .5 knot (while she could make 1.5).

With more speed there is more drag but at a point the driving force is big enough to overcome the locked prop, the colony of barnacles that sail with me and a man overboard dragged by his/her tether. Say a boat going 6 could go 6.5 then = only .5 loss and just 10% of her actual speed.

A fixed prop makes so much sense to a cruiser that there is really no general dispute possible. It is actually a case by case study.

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Old 28-08-2009, 08:03   #22
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Originally Posted by GordMay View Post
It’s nice to see that the laws of physics (mechanics), as deftly explained by Nick, are borne out by experimentation, as detailed by Main Sail’s very elegant demonstration.
Despite the MIT study, the University of Strathclyde Ocean Engineering white paper, the Yachting Monthly data and my own research, which all basically show the same thing, that a fixed caused more drag, there are still a few die hard types arguing the helicopter blade theory..
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Old 28-08-2009, 08:13   #23
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Then I am lucky to have a fixed 3 blade rather than helicopter rotor fixed to my boat's prop shaft.

And if 'they' do not care to look up their transmission manufacturer's recommendations first then I believe marine transmissions are dirt cheap on your side of the pond.

Cheers,
b.
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Old 28-08-2009, 08:56   #24
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Forgot to mention. The transmission is internally lubricated, I believe it's call a bath type or something simular. It does not require the engine to be running for the transmission gears to be lubricated.

Damon

Let's not lose sight of the forest here and step back for a moment - are you saying this is a hydraulic (clutch) transmission?

If it is indeed hydraulic, that would explain the inability to lock it via the linkage. Also, and more important, forget about the conflicting theories regarding prop drag and lock it with a shaft lock as hydraulic trannys should not be allowed to free-wheel.
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Old 28-08-2009, 10:32   #25
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I'm obviously not bright enough to figure this out.

If my actual maximum hull speed is 6kts and with the available wind I'm doing hull speed with the prop locked or unlocked how is either way going to make any difference to my ACTUAL speed. I can't go any faster.

Now wether it damages the trans is another question. Mine has been done various ways for 31yrs with no noticeable tranny problems.

I'm sure at lower wind speeds this may make a difference. But as I asked before, is, given the real world difference of .005kt, my world going to be much different either way.

I think theres too much brain power being devoted to an unimportant(to most cruisers) difference in speed.

I think the question was about possible damage from locking vs freewheeling.

For me the manual is pretty clear, leave it in neutral when the engine is not running.

The prop may or may not turn but it doesn't make any difference.......in the real world that is..............m
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Old 28-08-2009, 12:09   #26
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Well either I can't find it, or I was wrong. Maybe it was another forum. Anyway for now I change my tune. I also have not been able to find a thread on this forum where someone actually did the experiment and sailed faster with a locked prop.

John
The last time this came up I posted a link to this file of the MIT propeller drag test done in 1991.

http://www.catamaransite.com/files/propeller.pdf

The MIT study agrees with Maine Sail's results. As does my experience which is double the normal guys' as I have two yanmars and two props. There's a huge difference between freewheeling and locking the props. I always freewheel the props when sailing. Obviously, if you're at hull speed with locked props...well...this way you'll achieve it with lighter winds.

One caution. Yanmars are designed to freewheel. It states this in the engine manual. Many other engine manufacturers recommend the props be locked so this is engine dependent, not boat dependent. Having a healthy transmission is the first priority.
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Old 28-08-2009, 13:43   #27
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And when it turns it makes THE noise. OK perhaps if you sleep in the fore or have a bigger boat. Those who sleep aft (or in smaller boats) hate this noise.
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Old 28-08-2009, 14:47   #28
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The last time this came up I posted a link to this file of the MIT propeller drag test done in 1991.

http://www.catamaransite.com/files/propeller.pdf

The MIT study agrees with Maine Sail's results. As does my experience which is double the normal guys' as I have two yanmars and two props. There's a huge difference between freewheeling and locking the props. I always freewheel the props when sailing. Obviously, if you're at hull speed with locked props...well...this way you'll achieve it with lighter winds.

One caution. Yanmars are designed to freewheel. It states this in the engine manual. Many other engine manufacturers recommend the props be locked so this is engine dependent, not boat dependent. Having a healthy transmission is the first priority.

I'll start over. I have been saying that I read somewhere on CF that someone had done the experiment of locked and not locked and had gotten the result that locked was faster for him. So I have been saying do the experiment yourself, since different people have been getting different results. A previous poster said that he couldn't find the thread where someone had sailed faster with a locked prop. Well I can't find it now either, so I'll now have to change what I tell people, that anyone that has done the experiment and posted the results has been faster freewheeling.


And ya ya ya, before some else says it again, only applies to trannies designed for it.

Still happy the previous owner put a 3 blade Max prop on the boat, John
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Old 28-08-2009, 16:05   #29
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Originally Posted by cantxsailor View Post
I'm obviously not bright enough to figure this out.

If my actual maximum hull speed is 6kts and with the available wind I'm doing hull speed with the prop locked or unlocked how is either way going to make any difference to my ACTUAL speed. I can't go any faster.

Now wether it damages the trans is another question. Mine has been done various ways for 31yrs with no noticeable tranny problems.

I'm sure at lower wind speeds this may make a difference. But as I asked before, is, given the real world difference of .005kt, my world going to be much different either way.
Okay, I'll try to brighten it up for you ;-) At those lower windspeeds, when you go slower than hull speed, you keep insisting about some totally ridiculous difference of 0.005 knots. Now, read the test results from MaineSail again: the difference is much much more than that, up to a full knot for many boats as you can read in some recent posts in this thread.

So your problem is, that you have this wrong figure in your head and keep repeating that to yourself. Think 1 full knot and decide of you want that.

cheers,
Nick.
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Old 28-08-2009, 16:09   #30
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The only way to sail faster with a locked prop is when the extra turbulence makes a big fish angry enough to start pushing you ;-)

ciao!
Nick.
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