Fixed vs. Free Wheeling Prop - Test Data - Page 2 - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 04-19-2009
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I like it, this is a real world test.

Just a question though, shouldn't you deduct the jig drag first before makeing assumptions?

25-12=13 and 50-12=38

So either 13/38 almost 1/3 less drag from fixed to free wheeling.

or

38/13 almost 300% increase in drag by not allowing the prop to free wheel.

Thanks for the insight.

Since you are kinna have the jig and all maybe you might try to a torque number with a stalled prop?
Maybe a one foot bar attached to the end of the shaft to your scale and the hinge locked.



Quote:
Originally Posted by Maine Sail View Post
No not necessarily. On this jig it related to 25 pounds and this jig showed roughly a 50% difference. Due to the angle of the rope and some other factors it is tough to calculate an accurate drag number.

I also load tested the jig alone, without a prop, at WOT and it had about 12+/- pounds of drag so you would need to subtract the 12 pounds from the 25 or 50 pound numbers to even get close. I think it is safer to say something like "nearly double" the drag difference rather than apply a number in pounds. I leave the .001's up to MIT...



Pretty close an not bad for a back yard hack who did not go to MIT...
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  #12  
Old 04-19-2009
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Quote:
Originally Posted by timebandit View Post
I like it, this is a real world test.

Just a question though, shouldn't you deduct the jig drag first before makeing assumptions?

25-12=13 and 50-12=38

So either 13/38 almost 1/3 less drag from fixed to free wheeling.
It's all in how you look at it I guess?

13 goes into 38, 2.92 times (38 divided by 13). So in actuality it has nearly three times (2.92) times more drag or 292% more drag, as a percent, when you subtract the jig.

1/3 more drag would be 13 X 1.33 = 17.29 pounds of drag not the 38 pounds it actually had after subtracting the jigs drag. Any way I look at this it is 2.92 times more drag after subtracting the jig.

Perhaps I'm just very bad a math..

Quote:
Originally Posted by timebandit View Post
or

38/13 almost 300% increase in drag by not allowing the prop to free wheel.

Thanks for the insight.
Yes nearly 300% more drag in fixed mode vs. freewheeling..
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Last edited by Maine Sail; 04-19-2009 at 02:00 PM.
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Old 06-16-2009
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Thanks for putting so much effort into testing this ages-old question. The only problem I see with your rig is that the prop is not forced to stay in the same plane- it is allowed to pitch up and down. This should be significantly affecting your results.
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Old 06-17-2009
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Transmission wear

is the other cited concern, besides speed.
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Old 06-17-2009
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Originally Posted by SteamboatWilly View Post
Thanks for putting so much effort into testing this ages-old question. The only problem I see with your rig is that the prop is not forced to stay in the same plane- it is allowed to pitch up and down. This should be significantly affecting your results.
Actually it is not allowed to pitch up and down the throw on the analog scale is only 1/8" or less. Once moving the prop stays in plane and was set to mimic my own boats prop shaft angle. I also gave it more line to make it vertical in the water, as opposed to matching my own shaft on my boat, and there was no noticeable difference what so ever between vertical and a slight angle like my boat...
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Old 06-17-2009
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Wow, Maine Sail, you do the most interesting boat things . Thanks!

I wonder: Do you still have one of those Martec folding props that gave you trouble hanging around? The really serious racers actually mark their prop shafts so they can align the props so neither blade is on the bottom--the theory being that one blade being on the bottom will fall down, under sail, and create drag. In my mind: Since that blade is hinged, anyway, any drag it would create would be truly insignificant. Secondly: Sailboats heel. They heel at different angles. Which way is "down?"

Still, it would be interesting to see hard data on the differences between a Martek with the blades oriented up-and-down, vs. sideways.

Jim
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Old 06-17-2009
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Wow Maine, That must have been some good music to come up with this rig. Great job, Great data. And it matches theory. If the water has to work hard to flow around the prop than the prop must create drag. If the prop can rotate the water flows easier around it hence less drag. In the limit ie if the shaft were frictionless there would be very little drag with the prop freewheeling.

I've always thought the reason for not allowing freewheeling was transmission wear and noise. I don't like the noise of the shaft rotating.

I have a Martec two blade folder and use reverse when sailing. I don't check the aligmnet as if the blade falls easy due to gravity it would take very little effort (drag) by the water to fold it back and you would hear the prop opening and closing under sail

Well done test.

John
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Old 01-26-2010
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Wow, this is great data.

But I think there is a question yet unanswered: How significant is the drag of the prop compared to the drag on the hull?

I'm sure there must be good data somewhere on the drag of typical sailboat hull shapes and lengths.

I played with this the other day in light air. I have a 33' Newport. At a speed of about 2 kts it seemed that it slowed our speed by about 0.1 kt by putting the transmission in gear vs. free-wheeling. I only played with it two or three times. Hardly statistically significant data.

I also had the boat serviced last weekend and discovered the transmission dipstick was missing from the plug! Has anyone heard of a dipstick falling off the threaded plug? Is there room in the casing for a loose dipstick? I'm planning on contacting the local Hurth dealer to get a new plug (with dipstick) and asking his opinion on the forward, reverse, neutral issue.

Thanks for all the information. I love this forum.

Rich
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Old 01-26-2010
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Quote:
Originally Posted by elcap373 View Post
.....I also had the boat serviced last weekend and discovered the transmission dipstick was missing from the plug! Has anyone heard of a dipstick falling off the threaded plug? Is there room in the casing for a loose dipstick? ...
Not hard to imagine the dipstick tube portion coming adrift from the plug.. Sounds like it's OK sitting there for the moment. I wonder, though,if another dipstick will be able to be inserted if the old one's still there... A pencil magnet might fish it out, though it may be difficult with all the other metal around....

Try a dentist's mirror and a flashlight to see if you can spot the old dipstick through the hole.. There should be a small vent hole near the top of the dipstick tube.. maybe you can fish it out with a bent wire.
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