Rudder Shapes

[Brian Blazer]

We were just having a discussion on another board about rudder shape. There is a guy that is making Carbon Fiber rudder blades. These blades are .090 thick with no wedge. He is using a prop thrust water pickup to cool the engine.

So up for discussion....which is better, the wedge (60/90 speedmaster), or a blunt nosed .090 rudder with a thrust water pickup?

Some good points were made...most things that are built for speed are round (space shuttle, submarine, planes, bob-sleds).

Opinions? Thoughts?

Brian

 

[TimD]

interesting....

Has anyone tried one as a replacement for a wedge, and if so was there any noticable behavior differences?

I've only ever used wedge rudders.

 

[Mike G]

Didn't Brian Callahan from RCBOT do his theisis on this? If I remember the part I read right the wedge is better for even flow and no flutter.

 

[Don Ferrette]

"If I remember the part I read right the wedge is better for even flow and no flutter."

You remembered correctly about the flutter. I don't recall if it was captain PhD who said though......... <_< <_<

 

[Lamar Huff]

Here is a link to the article:

http://rcboat.com/527_3.pdf

Later,

Lamar

 

[Terry Keeley]

Ron VanWagnen told me he tryed thin, flat ss rudders a few years back and couldn't get the boat to track straight

 

[lohring]

The thin wedge rudder is surprisingly (at least to me) faster than a conventional, aluminum wedge. The secret is that it has to be strong enough. A non-heat treated steel rudder handled very poorly and obviously bent under load. A heat treated rudder worked very well.

Lohring Miller

 

[Geraghty]

Hey Terry,

All we ever used in the west was flat blade rudders out of SS sharpened to a razors edge.We also used prop thrust for cooling.

Blade rudders werealways 1-2 mph faster than wedge rudders on the radar,but...........they are a P.I.A. to make.

As long as the centerline of the pivot [ruddershaft]was designed to intersect the point where the leading edge and bottom of the rudder intersect,the rudder tracts straight as a string and never flutters.

 

[Preston_Hall]

You remembered correctly about the flutter. I don't recall if it was captain PhD who said though......... 

I read the article and saw the section where he said (basically) that water spray was an indication of waisted energy. Well I guess it's true. But before you start sleeping with your physics book, think about this......................

 

[Terry Keeley]

Hey Terry,All we ever used in the west was flat blade rudders out of SS sharpened to a razors edge.We also used prop thrust for cooling.

Blade rudders werealways 1-2 mph faster than wedge rudders on the radar,but...........they are a P.I.A. to make.

As long as the centerline of the pivot [ruddershaft]was designed to intersect the point where the leading edge and bottom of the rudder intersect,the rudder tracts straight as a string and never flutters.

Ya, I seem to remember running an old Hughey Hydro with a flat ss rudder, never seemed to have a problem. Maybe Ron didn't have the geometry right as you mentioned, possibly too much lead?

 

[Terry Keeley]

The thin wedge rudder is surprisingly (at least to me) faster than a conventional, aluminum wedge. The secret is that it has to be strong enough. A non-heat treated steel rudder handled very poorly and obviously bent under load. A heat treated rudder worked very well.
Lohring Miller

Was thinking about trying to make a very thin wedge rudder for my SAW set-up, how thin did you get the back of the blade? What kind of material and heat-treat did you use?

I'm cutting the water back so much now when running saw that I don't think I need much more than maybe an 0.050" hole down the back of the blade, might be interesting drilling that, maybe EDM? :blink: Maybe a 7075 blade might hold up?

 

[DaveMarles]

Terry , why not try a kitchen knife blade, its sharp, wedge shaped and heat treated.

Dave

 

[apache5056]

Wonder how mad the wifie would get if the steak knives come up missin'...??

 

[lohring]

Try a McMaster-Carr #3956A12 hammer knife. Without the handle it's nearly the perfect large rudder and can be ground down to smaller sizes. It's cheap and adds more than 1 or 2 mph in our tests.

Lohring Miller

 

[Terry Keeley]

Try a McMaster-Carr #3956A12 hammer knife. Without the handle it's nearly the perfect large rudder and can be ground down to smaller sizes. It's cheap and adds more than 1 or 2 mph in our tests.
Lohring Miller

Now that looks interesting, could you measure how wide it is at the back for me? What were you testing that gained 1-2 mph and what was the other rudder you had on it? Thanks for the info!

 

[apache5056]

3956A12.

Blades are carbon steel and extra heavy on top, so you can easily drive these knives in with a hammer. They're great for hard-to-cut materials, such as tires and similar rubber products. Blades are 4" Lg. x 1 1/4" Wd. and secured to the handle with rivets. Choose from a shock-absorbing leather handle and a wood handle.

HUMMMM!

 

[Paul Pachmayer]

Hello Guys,

On riggers I only use the Hughey flat ss rudders, on monos and sport hydros I like the Octura wedge or the Speedmaster wedge depending on the size of the boat. I've tried the blade rudders on monos and sport hydros and didn't get the control I was after.

Paul.

 

[lohring]

We were testing a straight line gas rigger at well over the current record. The first rudder was a standard aluminum unit with a water pickup. A later calculation indicated that the knife blade had around 1/3 the drag of the aluminum rudder due to the narrower wedge angle.

Lohring Miller

 

[Terry Keeley]

Thanks for the info! Any idea how thick it is at the TE?

 

[lohring]

Ours measured .088" out of the box, but you can run a lot thinner blade by cutting off the back edge or tapering the blade from the back. The limitation is loss of strength, and we haven't gotten it that thin yet.

Lohring Miller