Surfski Knowledge Base

Long Rudder for waves
Waterman Larry
Feb 7 2020 #25998

Hi All,

Our household paddles four different DK rudders all excellent. I went out yesterday on one of the biggest gorge downwinders in awhile and was wondering if a three foot long rudder would stabilize the surfski and allow turns to take place with lower rudder angle. Aside from structural issues on the boat, what would be the effect of having a high aspect ratio long rudder/keel on a skinny boat in the big stuff?

Please talk me out of it, but I might put together a long skinny rudder/keel.

Anyone try this yet? Should the chord be extra thin to diminish the radical rolling effect of hitting a pedal too hard?

Thanks,

Larry Goodson

John Rybczyk
Feb 7 #25999

You are trying to say that you want to try a rudder with a three foot long chord, correct? Not a rudder that extends three feet below your boat…right? Or do you actually want to try a rudder that extends three feet below your boat? Because if you're really going to try a rudder that extends three feet below your boat, then I want to watch! Think of the weeds!!!

Reivers Dustin
Feb 7 #26000

dude. your sailing genes are showing. Maybe put a weighted bulb on the bottom. The lever-arm would totally overpower any righting forces, so you could train your feet to keep the boat upright. Actually there is some of that going on anyway. I noticed when I put a very big surf rudder on I unhappily discover that my feet are doing more than I want to the rudder. These are reflexes trained on a specific rudder so making big changes is disturbing. “Small changes, Ellie” (movie: Contact).

On the other hand. DK has a crazy streak. He hides it well, but he does things with rudders that are mad science. Maybe your thoughts DK?

waterbornewarrior
Feb 7 #26001

Oh, man, I was hoping to stay mum, but RD called me out. I generally counsel paddlers that a rudder is a turning device, not a stability device. But rudder size definitely affects stability (roll), and small changes can have noticeable effects. People regularly go from a 7-8” stock rudder to a 4” flatwater rudder, and note the reduced stability. And it works the other way, too. Going from a 9“ to a 12” would probably be dramatic. Go too big and the roll will be too slow (and hard to correct). I’ve experienced this with some “contraptions.” An alternative to a bigger rudder would be a ventral fin. Install a fin box approximately below the seat. Then you can experiment with various fin sizes to achieve the desired affect. I’ve only tried pretty small ones.

Don

Larry Bussinger
Feb 7 #26002

For adding stability in the big stuff, I added a retractable skeg just behind the seat. (With the help of Sterling Kayaks). Its purpose is to slow the rolling just enough that I can respond. It kind of works like adding an inch or so to the width of the boat or adding a giant rudder with out having too much steering at high speeds. After using it, I found that it slowed things down enough that my technique improved and I don't use it as much. I can retract this one to 4-1/2 inches but rarely go below 3” anymore. I think it does slow you down some, but found that it is a lot warmer if you can stay on top of the boat.

I think a long, high aspect rudder at higher speeds would tend to cavitate.

See attached picture. The skeg is on both boats and is retracted by a sliding knob at the edge of the cockpit. The tape is to mark a intermediate position.

David Scherrer
Feb 7 #26003

I found generally speaking the deeper the ruddder the more laterial fast twitch you get. Seems anything over 9 inch becomes pretty noticeable. For a while I was blaming it on the boat, but no, if was the rudder….

Carter Johnson
Feb 10 #26036

Admit I did not read this thread in entirety. but the suspect the answer is simple

1) Email Surfski@… (DK rudders)
2) Tell him your objective
3) Get the best rudder on earth for your application
4) Surf better than you ever have before
5) Have Big smiles

Dennis Mowry
Feb 10 #26037

My suggestions:
( In addition to Carter's )
1) Choose a more stable boat rather than a rudder for stability.
1b) Skinny boat most of the time, fat boat when questionable.
2) Use the rudder for going straight.
3) Lean the boat to turn and use the rudder as an aid to turning.
4) Practice better technic.
5) Hire a coach.
6) Practice drills

Just suggestions,
:D'

Waterman Larry
Feb 10 #26038

Thanks All,

Dennis, I’m eager to hear more about the various effects of your retractable centerboard.

Larry Goodson

Dennis Mowry
Feb 10 #26039

Larry,
Check with Larry B on that one.
:D'

Larry Bussinger

Feb 10 #26045

The skeg slows the roll so that i can react more comfortably. It also seems to move the center of lateral resistance forward. i.e. the boat pivots around the skeg.

Or you can do what Dennis says. Buy a fatter boat. 🙂

Larry Bussinger

Waterman Larry

Feb 10 #26046

Thanks for the rudder, centerboard input. I’m going to do something. But I’m not sure what. Let’s discuss if you come to the Vortex or other Gorge races. By the way, the Gorge downwinders are storm sessions lately. Westerlies in the mid 20’s against big currents should be a welcome change to spring conditions this week.

Larry Goodson

Steve Scoggins
Feb 10 #26047

Yes! Please have Don make a 3 foot rudder. Try it. If it works, you’re an innovative hero, if it sucks, at least you’ll have a really big rudder for artistic purposes. Everyone including Don will likely try to talk you out of it. Don’t listen to them. Do it.

Steve.