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Old 06-27-2012, 11:42 PM   #21
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I just think kinky and dragy rudders aren't necessary or desirable. One just needs a big enough rudder and enough deflection.
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Old 07-02-2012, 06:31 AM   #22
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Our boat was designed with a 1 1/2 gear reduction, 27 inch prop..

By time we bought her a PO had replaced the box with a 3-1 unit, so we installed a 32 inch prop to make up some of the difference.

As the rudder is for a launch it was way undersized with such a huge prop.

Even with a "backing rudder" (part of the origional install) the handeling was not great.

A pair of 2x2 SS angle iron bolted thru the trailing edge as a diamond is a big help at docking speeds.

Since we normal cruise at SL 1 (7K) , there is no loss from drag.

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Old 07-03-2012, 09:38 PM   #23
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The 45 degree diamond configuration suggested will probably give you some sort of improvement, but here is a true "thistle" cross-section plotted from the Gerr article mentioned earlier. It's basically a modified airfoil with the fishtail on the trailing edge. Gerr also strongly recommends the use of end plates (on the top and bottom of the rudder) to help channel prop wash through the fishtail. The line through the middle on the pic is a scrap of 1/4 inch ply to represent the existing flat plate rudder (actually 5/16").

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Old 07-03-2012, 09:50 PM   #24
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The 45 degree diamond configuration suggested will probably give you some sort of improvement, but here is a true "thistle" cross-section plotted from the Gerr article mentioned earlier. It's basically a modified airfoil with the fishtail on the trailing edge. Gerr also strongly recommends the use of end plates (on the top and bottom of the rudder) to help channel prop wash through the fishtail. The line through the middle on the pic is a scrap of 1/4 inch ply to represent the existing flat plate rudder (actually 5/16").

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Having hundreds of hours of advanced aerodynamics I can really appreciate the thistle shape..owning a 6.5 knot trawler and I think the thistle shape for a rudder is a complete waste of energy...but I bet it looks really cool..

On most slow, highly manueverable tug boats I see all kinds of things like flanking rudders, kort nozzles, rotating drives...but never a thistle rudder....hmmmmm

Not saying they don't improve performance but I just can't believe it's even measurable...is it? any independent written comparisons?
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Old 07-05-2012, 10:31 AM   #25
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I'm not one of those "I made it so it must be good" people. Our trawler (Sundowner Sea Tug) was a pig at slow speed, and only vague in it's response to helm inputs. I first read about the thistle on this forum and T&T and did some further research. You may find the boat owners quote in this article enlightening; http://navigatorpublishing.com/ondig...ad_edition.pdf
(It's a long 9 MB PDF download so be patient).

There was another website, "Mystic Rudder" which has since been pulled down as I believe the owner sold the boat ("Moose"). I have zero hours in advanced aerodynamics, and don't really care what shape it takes, the thistle improved low speed handling on our boat without having to resort to the standard bow/stern thruster route. And it did so at a very reasonable cost. As far as the "cool factor" there is none: you can't see the damn thing once the boat is launched. There may be other citations on Google or Bing. I just went with those two.

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Old 07-05-2012, 10:45 AM   #26
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I'm not one of those "I made it so it must be good" people. Our trawler (Sundowner Sea Tug) was a pig at slow speed, and only vague in it's response to helm inputs. I first read about the thistle on this forum and T&T and did some further research. You may find the boat owners quote in this article enlightening; http://navigatorpublishing.com/ondig...ad_edition.pdf
(It's a long 9 MB PDF download so be patient).

There was another website, "Mystic Rudder" which has since been pulled down as I believe the owner sold the boat ("Moose"). I have zero hours in advanced aerodynamics, and don't really care what shape it takes, the thistle improved low speed handling on our boat without having to resort to the standard bow/stern thruster route. And it did so at a very reasonable cost. As far as the "cool factor" there is none: you can't see the damn thing once the boat is launched. There may be other citations on Google or Bing. I just went with those two.

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All I am saying is that maybe an inch or two more of sudder size would have done the same thing...

tweaking "shape" is bigger at over 15-20 knots hydrodynamically and pretty obvious from the Wright Flyer to supersonic flight....but at less than 10 knots I think size and shape are the same....so shape is more "technical" than actual...and as far as writers...they get paid to write articles wherther true or not...and without some independent testing..it's hard to say whether one (shape versus size) matters....
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Old 07-06-2012, 10:14 PM   #27
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So, I guess that the real question is this: Based on your background and experience in aerodynamics, how would YOU empirically test a modicication such as this one? Inquiring minds would like your input.

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Old 07-06-2012, 10:26 PM   #28
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So, I guess that the real question is this: Based on your background and experience in aerodynamics, how would YOU empirically test a modicication such as this one? Inquiring minds would like your input.

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Post 10 is where I asked if there was any hard data to be found...there's lots of tests that can be done...if I were a designer that wanted my designs to become popular...some expensive tank testing may be in order.

Me???? I wouldn't even bother as I'm pretty sure of my estimates....and having driven enough single screw boats...I know whether the boat has enough rudder for reasonable manuevering and whether a few more inches of area wold do the trick. It's high speed singles where I would turn to true professional design help if I was disatisfied with rudder performance..
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Old 07-07-2012, 06:40 AM   #29
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Simple single screw test is to back at low speed for 1/4 mile.

If this is easily done , the rudder system is probably fine.

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