Rudder trouble

Hi I'm a new paddler more or less. This question might have been asked elsewhere in the forum and I apologize in advance if that's the case.
I've noticed it doesn't seem to matter what sort of ski it is custom built or brand name eg Fenn that at a certain point on a wave the rudder ceases to turn the ski no matter how hard I press the pedals. So I'm just wondering can anybody give me some clue as to why this should be?  is it just because as the ski slows down it's speed becomes the same as the wave and so would obviously cease to work or is there some other more scientific explanation.  ID be interested in anyone's thoughts.

Haha - this might be a good thread, lots of opinions here.

Not sure if you saw this article from some time back:
Rudders - All you wanted to know but were afraid to ask

There's a lot to it - the size of rudder, the shape, the airfoil shape, the length of the surfski hull, its rocker, etc, etc. 

Fun stuff though.  Some boats' handling is changed remarkably by changing rudders (the Fenn Mako6 back in the day was improved hugely by fitting a big elliptical surf rudder), some boats not so much.

"..is it just because as the ski slows down it's speed becomes the same as the wave and so would obviously cease to work..."

Yes, and, you'll notice, the harder you press the rudder (i.e. the higher the rudder angle), the more the ski slows down (the rudder becomes more of a "plow").  So, in essence, you are exacerbating the problem by trying to turn. 

Then, once the ski is going slower than the wave, the wave pushes your stern around and you broach.

So, a "trick" is to use gentle rudder pressure along with some body english and maybe a hard stroke on the opposite side.  

In fact, not for the faint of heart, but in a "need for a fast turn at the top of a wave" situation, I have found that leaning outboard on the opposite side of the turn and taking a really hard stroke can pivot the boat quite quickly (30 degrees I am guessing) - this is because the ski is still at speed, and, as you lean the boat the rudder goes back to being vertical and is fully engaged with the wave - but, screw it up and you will fall in.  This "technique" though is really no substitute for the "best" answer:

The best answer: as you get better, you begin to subconsciously read the waves and anticipate when you will need to turn and prep your rudder and paddle stroke in advance of when you need to turn.  I don't know if there is ever a good reason to push a rudder to the  stop or even close to it.
 
I still broach occasionally in the big stuff, but not nearly as much as when I was learning.  So, just stick with it, you'll get it.

Bill L

Yes, and, you'll notice, the harder you press the rudder (i.e. the higher the rudder angle), the more the ski slows down (the rudder becomes more of a "plow").  So, in essence, you are exacerbating the problem by trying to turn. 

Then, once the ski is going slower than the wave, the wave pushes your stern around and you broach.

Thanks for this. That is exactly what has been happening to me on the bigger waves.

I have a very large rudder - 9" long and 4" deep (DK "high Chord" 9"). Efven with this large surf rudder, a wave can overpower it at speed and steering still needs to be proactive, not reactive.

One thing Don mentioned is that many rudders have a sharp leading edge, which is detrimental to turning. This is because although a knife edge is hydrodynamic when it is parallel to flow, as soon as the flow becomes non-parallel (like when you turn the rudder at all), it causes unstable vortexes on the trailing side of the rudder (also called Stalling i believe). When the rudder stalls, its turning potential drastically drops.

For this reason, Don makes his rudder's leading edge somewhat blunt. Although this induces more drag when going straight, it allows a more gentle directional transition for the water as it passes around the back side of the rudder. This means you can have a higher angle of attack (rudder is less-parallel to the flow) but still not stall.

All rudders can be made to stall, but the leading edge sharpness/bluntness plays a substantial role from what I understand.

Also, think of screaming down a wave. Your bow is probably 2-4" deep in the water and most bows are fairly vertical for the first 2-4 feet. At speed, this is effectively a bow mounted rudder that your stern must overcome. Sometimes that is simply not possible.

Learning to anticipate where you need to be is part of the learning process. You'll get better at reading waves and begin to set yourself up for linking runs instead of chasing waves and stalling. This is a skill you're never truly done perfecting.

MCImes wrote: One thing Don mentioned is that many rudders have a sharp leading edge, which is detrimental to turning. This is because although a knife edge is hydrodynamic when it is parallel to flow, as soon as the flow becomes non-parallel (like when you turn the rudder at all), it causes unstable vortexes on the trailing side of the rudder (also called Stalling i believe). When the rudder stalls, its turning potential drastically drops.

I love my DK rudders.  Mark Raaphorst commented on the shape of the leading edge.  He called it cavitation.  He likes the DK shape and explained to me just how little a rudder has to turn to become ineffective.  Whatever it is, it works much better than the standard Epic Elliptical on both of my boats.  A lot more control, and more fun.  

One of the things Oscar recommends when a paddler is steering on a wave - and may be uncertain of just how much the pedals are affecting the behavior of the boat -  is to very gently steer the pedals back and forth.  For example, if you feel you are losing control, you are probably pressing too hard, so gently back off a bit and try again.   I do it all the time to remind myself just how responsive ski pedals are.  It takes only a tiny bit of pressure to alter the behavior of the boat all over the wave. 

I think the pedals/strings/tiller ratio is far too sensitive in all boats.  Manufacturers build in that crazy leverage to turn a ski at slow speed around cans.  It often results in the rudder moving too much when downwinding, IMO.  

It's not easy to learn to get on a wave and steer around.  Having a one size fits all footplate and bad leverage on the pedals makes it even harder.  A tall footplate helps if you have big feet.  If the balls of your feet are on the hinge of the pedal, that's a big drawback.  The soles of our feet can exert tremendous pressure.  Steering pedals only need fingertip finesse.