Rudder profile vs water and drag.....

[Mike Larson]

Has anybody tested and determined why, some rudders per manufacturer are needed "wedge" profile, ( wider blade, top, slimming to botttom) over the traditional "straight" ie; speedmaster, for example?... drag is drag, easily,of course.... most wedge stlye possibly on sport hydro's and why?.... Ive a little experimental .12 rigger with a current although to scale wedge rudder( retro from another hull), that ive ideas the rudder may be issued with at least an over amount of drag, and these boats need all the help they can get...... I'm prepared to thin it down, and remove the wedge style, as my 20 boats and up all have straight profile rudders..... granted I'm not looking for a saw boat, as you often hear of actual knife blade rudders, and custom tricks to eliminate drag... I guess, question is, also looking down on all I've seen also a wedge profile to split the water........ what happens without this wedge part in any rudder?, its a pressure physics thing or what?, ala knife blade thick?.... what say does changing a wide at the top ( why?, this part of the rudder is almost always not in the water), tapering down style rudder to just a plain straight common style do?.... im sure lift encounters in here also...... basically old school boats just had a brass stock brazed to a round stock, but thin was thin.... what effects are what, as surely alot of machinists have tested this?.... lol, i need more than a 30MPH .12 boat.... thanks Mike

 

[lohring]

The thing we found to make the biggest difference is rudder width and area. Going from a standard width aluminum rudder to a thinner, smaller steel rudder was good for 6 mph at 100 mph.

Lohring Miller

 

[David Santistevan]

Funny, I just posted in the Outboard Forum a very similar post. In doing some reading I discovered that appendage drag (the rudder, prop and strut) is equal to the square of the speed. Now to me this seem like a huge deal. I am very interested to hear what more experienced folks have to say here.

D

 

[Mark Stein]

rcboat.com - past articles has some interesting reading on the subject

see: Multiphase Flow on a Ventilated Marine Propeller and Rudder

 

[anthony_marquart]

I know that the wedge rudder, though it creates alot of drag, also helps keep the boat stable. I've seen guys go to a flat thin blade,.. and prop blast water pick up and all the sudden the boat doesn't run so well. tail hop and such..

 

[Terry Keeley]

I pay beaucoups of attention to anything that touches the water!

 

[Buckshot]

I've been thinking about rudder depth lately and wanting to try shorting one lake side a little at a time to see what's what. I wonder if there is a relationship to the turn fin depth that could be put to use or relation to prop depth? Assuming that we are talking oval running not SAW with high lifting/steep pitch ratio props.

Any input.

 

[dwilfong]

I found thew testing and advice that the rudder at the same length as the turn fin works great. also went with a wider front to back rudder at a shorter length. The water will see a longer ruder but wider dose no seem to matter. I had to turn down the rudder throw also with this wider rudder.

David

 

[Adam Piotrowski]

I have tried Both the wedge and the Flat rudders on my little Hydro's (were every little bit counts) and the wedge gives more stability and is more forgiving to poor set ups and novice drivers but does reduce top speed and acceleration. The Flat blade in turn is Less Forgiving but definitely Faster due to Less Drag, and had fewer issues with creating unwanted lift. I Believe the wedge rudders started with people wanting to put the water pickup In the rudder trying to reduce drag but don't Quote me on that.

With the flat rudders you definitely need to make sure the foot print of the hull is as perfect as possible, when it is Then you get your stability and the Speed.

Also as David stated Length and Width is also a big factor, but what the Perfect length or Width is will change from set up to set up, Person to Person(driving Style), Pond Conditions etc...

With both styles IMO the bottom edge should be Sharpened and the leading lower corner rounded and sharpened also to prevent any lift from the rudder.

 

[Mike Cathey]

What I've noticed most about the wedge type rudders is they tend to be self-centering. My first boat (which admitedly was a hunk of junk that you didn't dare take you eyes off for a second it was so squirrely) tracked much straighter with a wedge rudder than a blade/propblaster. If you use a blade type rudder it is less drag and faster. But if you have any slop in the servo or pushrods they flutter some and the boat will seem harder to drive.

I have, however thought, about going back to a blade and prop blaster for less drag and more speed since you can built so much stiffer steering systems with the parts available now.

Are there any blade type rudders out there that are a kick up design with and use a brass or nylon sheer screw? Kick up rudders and turnfins are mandatory in the club I race the most in.

 

[Mark Stein]

A wedge is only stable if the leading edge is at or behing the centerline of the hinge. the flat SHARP trailing edge on the wedge creates a void as it travels thru water simulating the flow around a full blade without the inherent drag of a flat or contoured full blade

 

[Charles Perdue]

Mark, what is the source of the information in the first sentence of your post?

A wedge is only stable if the leading edge is at or behind the centerline of the hinge.

Charles

 

[bsmtboy2]

this is a good point for john Finch to chime in here, he has forgotten more about rudders than we all will know.

 

[Charles Perdue]

A rudder is stable if the amount of area of the blade BEHIND the pivot is greater than the area that is in FRONT of the pivot.

The only rudder servos we had, way back when, were a lot weaker than they are now. To get the riggers to turn, we moved the pivot farther back on the rudder blade. It took less effort from the servos to turn the rudder blade and boat.

I still run at least an 1/8 inch or more of the blade in front of the pivot.

Charles

 

[John Beardslee]

A rudder is stable if the amount of area of the blade BEHIND the pivot is greater than the area that is in FRONT of the pivot.

The only rudder servos we had, way back when, were a lot weaker than they are now. To get the riggers to turn, we moved the pivot farther back on the rudder blade. It took less effort from the servos to turn the rudder blade and boat.

I still run at least an 1/8 inch or more of the blade in front of the pivot.

Charles

Charles did you ever go to far with the lead on the rudder? A noted airplane designer told us that you could go as far as 25 percent lead on a rudder, so we tried it on a 67 twin, we had been running a 1.0" wide blade and made it 1.25" man talk about rudder fludder!!!!!! It shook the whole rear of the boat at about 60 mph and took out the servo gears. Naturally we were at the Indy Unlimited and multi-engine race, testing on Thursday night before racing on Friday. Didn't take long to change out the rudder and learn that 25 percent in air is different than 25 percent in water!!!

John

 

[Charles Perdue]

John, about 3/16" is the most that I have ever tried. It worked OK. The 1/8 inch that I currently run on a 1 inch wide rudder works very good. It may just be in my mind (if I had one) but the initial response of the rudder beginning to turn the boat at the entrance bouy seems to be a little quicker. This is the same on the 20, 40 and 60 boats.

Charles

 

[John Finch]

I played with the forward loaded rudders and it didn't take but about a quarter inch if I remember correctly to start a mono quivering at 45 mph. I, like Charles, had to run some of the blade in front of the rudder when the biggest servo we had was 50 inch ounces. Since we all have high powered servos I just use the speedmaster setup with all the blade from the pin back. There is never a problem with that setup.