I am thinking about adding a rudder to my tandem and have read a little about foil shaped rudder blades, but have not see one in person. Are these foil shaped blades symmetrical? shaped like this ()?
Do they really work better than a larger flat profile?
YES to all !!!
Considerably less drag going theoretically straight which is never really happening.
The real benefit is a much higher ‘stall angle’
Not just when actuating the blade to adjust boat’s path, but when deployed, just going through the water/ paddling… less drag.
Foil
Foil shaped rudders are the way to go. Drag is lower. You can fly an airplane with flat wings, but not well due to drag effects. Same with rudders.
About the NACA Foil…
– Last Updated: Nov-26-04 7:47 AM EST –
http://www.boat-links.com/foilfaq.html
"The main difference between a "thin flat plate" and a NACA foil is that the foil will generate lift, i.e. power, at a higher angle versus the boat's direction. Where a plain flat rudder will stall and simply slow you down somewhere around +/- 5 to 10 degrees from center, a NACA foil will be effective over maybe twice that range. Your boat's rudder will work better. Tacking will be easier. A little bit of weather helm translates into more sideways 'push' upwind."
Nice reference, thanks
Have often wondered what the mathematical definition of aspect ratio was in this context.
Mike
Yes to all questions w/ clarification.
They can be symmetrical, but that is not the most efficient shape. The most efficient shape is like the shape of an airplane wing, the top of the wing duplicated on both sides of the foil. The greatest thickness of the foil should 30% of the way back from the front edge. The most efficient shape for the front is totally blunt, but this is seldom done as people like it shapelier. The trailing edge should be like the top of a tear drop, virtually no drag. Also seldom completely done as it will sustain damage too easily.
So as you might have guessed by now you take all the ideals, add eye pleasing ascetics, pare it down to the practical and you have your ideally practical rudder. Now you can build it perfectly or try to find one already being built (not!).
Good luck.
Happy Paddl’n!
:^)
Mick
not yet
"Now you can build it perfectly or try to find one already being built (not!)."
Not YET Mick…not yet. 
Best advantage
is that it is QUIET compared to a flat plate.
Jim
Thanks for that but…
For a kayak or any other craft in air or water that travels forward the rudder MUST be symetrical.
A blunt rounded leading edge is not the end all way to go for a symetrical rudder, rather a parabolic one is prefered…
Even in the vast NACA or Goettingen foils selection, there are a few that are still THE preferred and most widely accepted shapes which balance lowest overall drag with a high stall angle. We are using one of these with our own foiled rudder and skeg systems.
There is no need for a foil to be brought to a knife edge as the final 5% or so is just not doing much but riding in a turbulent wake.
So why have it if it is only going to get messed up or possible cut something.
not so fast
i’m feeling feisty. a foil has to be pretty thick to produce a significant reduction in stall effect at kayak speeds. thick means more frontal area. more drag. i guess theres a place in ultimate performance where anything can be theoretically correct. and i agree ‘flat plate’ rudders and skegs should be faired on the leading and trailing edges. and naca foils may actually produce a measurable difference but i doubt it
Your thinking straight line flow over…
… the blade, and in reality that rarely if ever occurs. There is usually a pressure differential on opposite sides, even with small corrections during straight line paddling - but more so turning. With a flat blade, turbulence and subsequent drag increase greatly.
I used to have a link that showed how different foils react at different angles of attack (including a piece of plywood - which acts like your street sign aluminum blades). Don’t know if someone posted it or if I found it on my own…
yes i hear what you are saying
i agree theoretically. i also observe that faired plates can be very effective. lightning and snipe sailboats, among many, are known for speed and ability to sail high using faired plates. we are picking at nits. do you really think a faired plate vs a naca foil will show up on your gps? frontal area vs stall resistance? how often are we turning?
Yes … there is a difference
The Lightning and to an even higher degree Snipe folks out here are some of the most militantly technical sailors around. Ask any of them if they would switch to foils if the rules allowed… the answer would undoubtebly be yes.
Trying to keep it short,
Picture a flat plate travelling even dead straight which again is never happening anyway as a kayak or other human powered craft is paddled or pedaled through the water… now turn it ever so slightly (
Trying again…
– Last Updated: Nov-27-04 4:18 PM EST –
The Lightning and to an even higher degree Snipe folks out here are some of the most militantly technical sailors around. Ask any of them if they would switch to foils if the rules allowed.... the answer would undoubtebly be yes.
Trying to keep it short,
Picture a flat plate travelling even dead straight which again is never happening anyway as a kayak or other human powered craft is paddled or pedaled through the water.... now turn it ever so slightly (
Ugh..... not able to post rest of this. due to some technical thing I am ignorant to.. trying again.
Keep the plate going forward... the flow behind the leading edge breaks away immediately causing turbulence / increase drag.... this sucks both literally and figuratively.
Now do the same thing with a foil shape... flow easily follows contour of shape and does not break away 'till much later down the blade.
This increase in effciency can be translated into less square 'inchage' in the blade itself resulting in less skin friction and resistance to get the job done.
Yes this does show up on the GPS and Speedmate.... been staring at them for three years now trying to justify all the work involved in developing our system. Even on glassy 'oil slick' water.
Seat of pants observation. Paddle to and then stop at X speed and drop a plate type rudder into water and observe the coast down speed.....now try the same with a side to side, slop free precise foiled blade... Results both proven and theoretical cannot be ignored.