Why are RC boat rudders wedge-shaped?

[lt130th]

So much more drag than if the trailing edge were tapered. Just wondering. Is the drag intended as part of the design performance of the boat?

[Luck as a Constant]

For starters, it allows for the water pickup. Also strength


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[lt130th]

I know it needs the stock/width for the water pickups to be milled, but all it would take is extending the rudder a fraction for a tapered trailing edge to prevent drag from the laminar separation... For example, consider the airfoils on any airplane. No aircraft airfoil is shaped like a wedge of cheese because...well have you ever driven a light passenger vehicle behind a semi-truck trailer or large bus? The turbulent effect is alternating eddies because of the abrupt square trailing end of the truck trailer/bus. Same thing is happening to these rudders with a wide, flat back. Maybe its a negligible amount, or perhaps no one cares, haha.

[kfxguy]

Buy another rudder just like you have on one of your boats. Reshape it. Go make a couple passes with a gps with the squared off rudder then change to the one you modded and repeat. Then you may get your answer. I'm interested myself....

[NativePaul]

Have you seen the airfoils modern high speed aircraft use? Even going back as far as WW2 the P51 Mustang had a laminar flow airfoil with a much more wedge shaped first half of the wing than a traditional teardrop airfoil it was the only advantage it had over the Spitfire and did a fantastic job of equalising their performance.

Water is thicker than air over 800 times as thick and heavier too, as such it has more inertia and once it gets moving it does not want to change direction again, at low sppeds and airfoil section rudder would be best and indeed they are typically used on sailing boats, but at high speed the water will not close behind the leading edge fast enough to contact the rest of the rudder, which leaves you with high drag from a fatter leading edge and an ineffective rudder as the rest is in turbulent flow and not making proper contact with the water, whereas with a wedge the water is forced past it right from front to back and makes contact full length with the turbulence only starting behind it.

Flat plates also work but being thinner are much weaker if the same material is used and you cant hide a water pickup inside one.

See the rudders section of the fastelectricboats.co.uk website in my signature if you want more detail and diagrams etc

[gsbuickman]

Have you seen the airfoils modern high speed aircraft use? Even going back as far as WW2 the P51 Mustang had a laminar flow airfoil with a much more wedge shaped first half of the wing than a traditional teardrop airfoil it was the only advantage it had over the Spitfire and did a fantastic job of equalising their performance.

Water is thicker than air over 800 times as thick and heavier too, as such it has more inertia and once it gets moving it does not want to change direction again, at low sppeds and airfoil section rudder would be best and indeed they are typically used on sailing boats, but at high speed the water will not close behind the leading edge fast enough to contact the rest of the rudder, which leaves you with high drag from a fatter leading edge and an ineffective rudder as the rest is in turbulent flow and not making proper contact with the water, whereas with a wedge the water is forced past it right from front to back and makes contact full length with the turbulence only starting behind it.

Flat plates also work but being thinner are much weaker if the same material is used and you cant hide a water pickup inside one.

See the rudders section of the fastelectricboats.co.uk website in my signature if you want more detail and diagrams etc

+2 I couldn't have explained it any better

[Darin Jordan]

Here, guys... Read and learn.... The bottom line is: Wedge Rudders produce LESS drag...

http://www.fastelectricboats.co.uk/rudders.php

[keithbradley]

The wedge type rudder blade design offers a bit more stability over a double tapered/knife blade style. They've both been used, but the latter is typically a bit more twitchy.

[lt130th]

All seems logical. Thanks for the knowledge. Things like this which seem clearly & obviously not the optimum design, sometimes surprise you when you work out the data. Its a good thing we don't engineer things on 100% intuition, lol.

MP

[lt130th]

Have you seen the airfoils modern high speed aircraft use? Even going back as far as WW2 the P51 Mustang had a laminar flow airfoil with a much more wedge shaped first half of the wing than a traditional teardrop airfoil it was the only advantage it had over the Spitfire and did a fantastic job of equalising their performance.

Aah, now I'm in front of a real keyboard and not looking at a little cell phone screen. Yeah, NativePaul, I hear you. Cambered airfoils are what gave German dog-fighters superiority back in WWI. After WWII, aircraft airfoil had been so well documented and designed, just about any airfoil that can be imagined is cataloged in NACA form. Take a 2D cross-section of most modern supersonic wings and you'll find the diamond-shaped airfoil you pointed to. That's exactly what I would have expected a boat rudder to perform better with (for high speed). But I sort of ignored a key element of fluid dynamics (density)...specifically air vs water. There's one more thing I have been wondering lately. We sharpen props...why not rudder leading edges? And why don't I see more polished rudders? Again - negligible effects? I'll read the link that was shared here, too. My question may be answered there, already.

[NativePaul]

Race boats don't really need low speed manoeuvrability so we cut the rear part of the diamond off to save weight, doing the same to a high speed aircraft would mean a 50-60% reduction in wing area and necesitate 400mph landings.

I don't polish my rudders and turn fins for the same reason I don't polish my hull running surfaces or prop, to keep control of a semi stagnant boundary layer. I do however sharpen them and think most racers do here also, most of mine are much sharper than I would make a knife out of the same material (6061-7075t6 alloy), the exceptions are the rudders I have made from stainless knife blades in which case I just choose flat ground knifes without a bevelled cutting edge and do no further sharpening.

[jester_s1]

The trailing edges of airplane flights surfaces aren't usually knife edged either. For strength, there has to be some thickness in the control surfaces all the way back. A sharp, flat trailing edge of some thickness comes close to the drag efficiency of a knife edge one. What's really bad though is a rounded trailing edge since it encourages the airflow to stick to the control surface as it goes around. That creates a vacuum, which does contribute to drag.

[lt130th]

10-4, NativePaul. You're speaking my language. Great info here.

MP

[Fluid]

http://rudderbusters.com/Documents%5CBoatPropPaper.pdf

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[dasboata]

I ran flat SS 1/16 blades way back when in my SAW mono,,, the boat did not track straight with a flat blade