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Can Rudders be too thin?

Old 09-16-2007, 04:52 AM
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007clint
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Default Can Rudders be too thin?

Rudders are to deflect water for turning and stopping prop-walking; so my question is assuming you have the correct profile (including the correct depth) and assuming the material you use doesn't bend under the force it is placed - can a rudder ever be TOO THIN?

If the answer is no, why don't we see more titanium, hardened steels, or other exotic metals?
Old 09-16-2007, 06:13 AM
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wave waker
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Default RE: Can Rudders bee too thin?

ORIGINAL: 007clint

Rudders are to deflect water for turning and stopping prop-walking; so my question is assuming you have the correct profile (including the correct depth) and assuming the material you use doesn't bend under the force it is placed - can a rudder ever be TOO THIN?

If the answer is no, why don't we see more titanium, hardened steels, or other exotic metals?

since aluminum is the best bet material to use, cheaper than titanium, lighter than most of the material and has a very great ratio on weight to strength but it needs to be a lil' bit thicker to prevent from bending...rudders can be too thin if made by titanium or stainless.
Old 09-16-2007, 01:39 PM
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Default RE: Can Rudders bee too thin?

I will say no a rudder cant be to thin in the scenario you mention. I use a rudder that is .032 which I will guess is less than Half the thickness of any rudder I have personaly seen (this little rudder is on my .12 rigger). The only problem I have is hitting shore i have bent the mounting rod or broke it free from the rod ( but have never done any damage to the rudder itself). My rudder is a hueghey stainless rudder.


Paul


Here is a pic of the rudder I am speaking of, and the .032 is the thickest it gets.


I am having problems loading pics, as soon as I can I will.


Old 09-16-2007, 07:11 PM
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Sean Bowf
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Default RE: Can Rudders bee too thin?

I found that article I was talking about earlier Clint. Here is a cut and paste from it:

Now what shape should the rudder be for best performance? The current best of breed is the wedge rudder, but if you were to just look at the drag coefficients this would not be the correct choice. Elliptical shaped rudders with a 4:1 length to width ratio have a Cd of 0.32, bring that up to 8:1 and Cd drops to 0.29. Compare those numbers to a typical wedge rudder Cd of 0.8 to 1.0.

Now Lets assume a 40MPH boat one with a 1/16X1 elliptical and one with a 1/8X1 wedge rudder, using the below equation we would find a difference in drag of almost 2.5lbs!

So why are we using these "high drag" wedge rudders, well there are several scientifically based reasons:

- It generates a side load that counteracts the prop torque which allows the boat to track straight. It does this by keeping the water in contact with both sides of the wedge for its entire length which generates a side load which counteracts the prop walk effects.
- The shape propagates a low pressure area on the outside of the rudder even at very low angles of attack, this assists in turning, especially at high speeds.
- After the wedge ends the water separates very cleanly from the trailing edge which helps minimizes drag (keep those edges sharp)
- The wedge generates a water void behind its trailing edge. This void gets ventilated (filled) with air, air of course has a much lower drag coefficient than water due to the density.

If you were to use an elliptical rudder you might have to keep some turn on the rudder to counteract the prop torque and allow the boat to track straight. That slight turn in the rudder and the resultant change in frontal area (A) can generate far more drag than a wedge rudder.

Should you ever use a blade rudder? The answer is a resounding maybe! For a SAW boat it represents the lowest drag and since turning is not an important consideration a blade rudder is your best bet. For oval racing boats it depends on both the boat and the speed, its pretty common to see blade rudders on 6-8 cell class boats. In those classes the smaller props have lower prop walk characteristics and a blade rudder might be your best bet. The best way to find out is to try both and see what works for you. I find my N & O class boats do great with a blade, but my P and LSH really like a wedge.


Sean
Old 09-18-2007, 06:04 PM
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Sean Bowf
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Default RE: Can Rudders bee too thin?

Been thinking on this...mainly about the comment made on the elliptical rudder...and how that comment would apply to a thin bladed rudder. In short, if the elliptical rudder will cause more drag due to it having to be set off neutral to make the boat track straight... then I guess the question would be, where does the thin blade rudder need to be set to keep the boat tracking straight? If the blade rudder has to be set off to one side or the other of neutral, then would it cause more drag than a wedge shaped rudder that would make the boat track straight when set to neutral?

I don't know the answer to these questions...just throwing some food for thought out there...

But if you are using a thin blade rudder, and it is set to neutral and makes the boat track straight...and have no ill affect on turning, then I would say you have the right rudder. If the rudder has to be set off neutral to make the boat track straight, then the questions above come into play.

Sean
Old 09-18-2007, 09:47 PM
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007clint
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Default RE: Can Rudders bee too thin?

Sean - you think so much like me it is scary!

Everyone else think of this like a CLUE game. Sean has made a SUGGESTION, can anyone prove it wrong?
Old 09-19-2007, 06:26 AM
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Default RE: Can Rudders be too thin?

I will add this, Sean's copy paste article said this:

" After the wedge ends the water separates very cleanly from the trailing edge which helps minimizes drag (keep those edges sharp) "

This not only pertains to rudders but struts and turn fins also, you never want to taper the trailing edge of any of these surfaces.


I am using a blade rudder on my .12 and the boat tracks great, my .45 has a wedge rudder and tracks great also. I use the blade rudder on the .12 because no water cooling is needed and the .45 has water pick up in rudder (I don't know if a .45 rudder could be a blade not sure of the strength factor). Wish I could give better answers, the only thing that could be added would be that all leading edges should be sharp enough to cut with.


Paul
Old 09-19-2007, 06:51 AM
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Sean Bowf
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Default RE: Can Rudders be too thin?

ORIGINAL: gooycheese

This not only pertains to rudders but struts and turn fins also, you never want to taper the trailing edge of any of these surfaces.

Paul
I dont agree with this. If you look at the drag coeficients in the article between an elliptical rudder, and a wedge rudder, you will see that the elliptical one has much less drag. So thinking of the shape outside of a rudder (the strut)...the strut should have less drag if the back was tapered in a little. The strut will not be turned to make the boat track straight, so the comment the article makes about increased drag of an elliptical rudder due to having to turn the rudder a little to make the boat track straight would not apply to the strut... Just more food for thought...

Sean

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