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Old 21-09-2010, 13:26   #76
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Circles

Well, this thread is on it's third or forth time around the fishbowl and the name calling is begining to escalate. I can't add anything new as my knotmeter is not functional and untill I have a reliable method for calculating speed through the water in stable conditions I will not be able to give my own experience. Some entities are going to focus on only one aspect of the equation and ignore the possibilty of any related cause and effect, and the many variables that exist outside their world, so applying logic will be ineffective in convincing them to open their minds. Is it possible that the experts were wrong? Absolutley. But they have published their belief so I would have to think they had reason to believe they were correct, and without exploring thier data I for one am not going to simply dismiss it as "Hokey Science" simply based on the experiences of a small sampling of individual experience under different conditions. If I have new information I will re enter the this conversation but untill then I think most of us have more productive things to do with our time than argue in circles.

Best wishes
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Old 21-09-2010, 15:11   #77
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fixed prop for sale

;-)))

So, quite many pieces of data show that locking = more drag.

I have started looking for a good and cheap thing to replace our 3 blade fixed (to be kept as a spare). Sure I can let it freewheel but noted the difference between freewheeling and a feathering prop is still huge.

Surprisingly, at Bukh (Germany) they seem to have very nicely priced racing Gori.

Now lotto numers PLS, anybody? PLS!

barnie
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Old 23-09-2010, 21:31   #78
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Originally Posted by barnakiel View Post
;-))) Sure I can let it freewheel but noted the difference between freewheeling and a feathering prop is still huge.
Agreed.... Yet feathering enabled props on a sailboat are a rarity at best outside of racing circles, as they are very expensive, and prone to fouling.

Main Sail, The Massachusetts Institute of Technology (MIT), University of Strathclyde Ocean Engineering, and Practical Boat Owner magazine, have all run independent tests which arrive at the identical conclusion. They have shown most of us a way to maximize what we already own.

Gentleman, if your transmissions allow it, ... Freewheel your props while sailing.
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Old 23-09-2010, 21:40   #79
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Agreed.... Yet feathering enabled props on a sailboat are a rarity at best outside of racing circles, as they are very expensive, and prone to fouling.

.
Yep, the only non-racing boats seen with feathering/folding props are the ones that care about their sailing performance... and they are not all that rare. Nor are all of them that expensive, nor do they get fouled any worse than fixed props. For long term cruisers, folks who do a lot of miles, the savings in fuel acheived by sailing in lighter airs (where the folks dragging fixed props are "forced" to motor) help to offset the additional cost.

I've put one on every cruising boat that I've owned!

Cheers,

Jim and Ann s/v Insatiable II lying Cairns, Qld, Oz
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Old 30-09-2010, 13:43   #80
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Most sailing cats I can think of are very shallow drafted and less wetted surface, so their drag effects will be different from a typical deep draft monohull. .....


I disagree. First, Let me say that not all Cats have rounded bottoms. This is a photo of my Catalac on the hard. I had to change the brightness and contrast in order to get a clear view of the bottom, but you get the idea. This hull design is not horribly different from many monohulls and I definitely benefit from the props free wheeling.

I think this entire line of conversation might be called a red herring. We are talking propeller drag here, not hull design. A propeller fixed in position and dragged through the water offers more resistance than a propeller free wheeling regardless of what fiberglass you have hung above it.

This is what all the reports mentioned in this thread clearly indicate. As a matter of fact, whether it's a real world test like what Main Sail did, or a scientific paper like the one from MIT, the one thing all the reports have in common is that they all agree on this one point.

It's only those reciting the old wives tales that seem to disagree. Main Sail's test is rather simple to duplicate. Or, just sail your boat with fixed props and then freewheel them and come on back here and let us know what you've discovered about your boat.

Short of performing one of the above, there really doesn't seem to any point in telling us that all these tests are flat wrong and your opinion is correct, is there?

Or am I missing something?
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Old 30-09-2010, 23:29   #81
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Thanks for the picture

I don't see many cats out of the water as there aren't many in the PNW. I can see your point relating to hull draft and position of the prop and rudder, although your hulls are much more narrow than a multihull there are two of them and two rudders, props etc. Would be a very intricate calculation for parasitic drag indeed.

"I think this entire line of conversation might be called a red herring. We are talking propeller drag here, not hull design."

I think you're incorrect in that assumption. You believe the effect of the spinning vs. fixed propeller is limited to itself where others are considering the possability of downstream effects bassed on what previous designers have stated.

"Short of performing one of the above, there really doesn't seem to any point in telling us that all these tests are flat wrong and your opinion is correct, is there?

Or am I missing something?"

You are missing something. No one is disputing the results of MainSail, or anyone elses functional test. They clearly show that a free wheeling prop in our pitch range has less drag than when fixed. That, for all practical purposes can taken as fact. What a small group of us are trying to discover is A) Why have naval architects in the past stated that it will still slow the boat, and B) Is there any validity to their claim. Members of this forum have given testimonials, with emphisis, that thier boats are slower when the prop is not alowed to free wheel, and with no reason to doubt them, one would have to accept that A and B are not conlcusive or all inclusive. There could be multiple reasons for this, either their data was far more applicable to earlier hull shapes fitting the raceing trends and rules in that time period, or, they were simply wrong for what ever reasons, but since most of these designers are no longer around, getting that information from the horses mouth would be difficult at best.

"It's only those reciting the old wives tales that seem to disagree."

See above.

"Or, just sail your boat with fixed props and then freewheel them and come on back here and let us know what you've discovered about your boat."

Did just that last Saturday. Wind was 15 to 20 Knts. and a little gusty. We had choppy conditions ranging from 1 to 3 ft as we were going to windward. I only had GPS to give boat speed over ground, which was not a stable means for indicating speed. Readings were never smooth enough to be reliable for any length of time so my result are far from conclusive on any side of the argument. Even averaging the speed over time was not stable enough to give creedance to fixed or free wheeling. On a beam reach, we were hitting 7.5 knts. intermittently regardless of prop state, but that's so close to hull speed that the induced drag of wave making resistance far overshodows the small difference the prop would make given the insensitive speed indication and the abundance of power from the sails. Next time I'm out and conditions are more conducive to accurate data I will log some numbers and post again. I just want to know the whole story.

Peace,
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