Rudder position: What is ideal for its primary purpose?

Hi guys,

what is the primary purpose of a surfski rudder?

On a sailboat, you have a rudder far aft, and a keel somewhere in the middle. The keel is meant to keep direction, the rudder is meant to change direction, put simply.

On a surfski, the rudder is meant to change your direction, you might think. My take is that a surfski rudder is rather meant to keep your direction, and specifically to prevent you from broaching when on a wave.

Therefore it is not so relevant whether your ski can turn on a dime on flat water. It is more important that it keeps you on track when all sorts of lateral powers are pushing at your stern and bow. Of course it is nice when the boat turns well when on a wavecrest. But most of the time you will rather want to stay your course instead of changing it, right?

Why do I want to discuss this? I design boats from time to time, have them built or build them myself, and then outfit them with a rudder. But I am always asking myself what the ideal position of the rudder (distance to the stern) might be. On 640 elite skis, this is at about 80 to 90 cm, but it differs a lot among different models. Do skis with rudders farther aft tend more to broaching while turning better on the flat?

I think you are correct about the directional purpose of of the rudder. In sea kayaking it's often said that the rudder should not be used for steering. Many kayaks aren't even equipped with one but rather a fixed skeg. Using rudder for steering is frowned upon and one should use corrective strokes and edging. I personally think they take it a bit too far and there is nothing wrong with gentle steering input when cruising.
Of course turning a 20 foot ski with strokes only is a bigger chore than with a typically 17 feet long sea kayak.

I used to paddle a 14 feet sea kayak without a rudder or skeg. Weather cocking was a real problem. Even a small wind gust would turn the boat. With a ski, even a short 17 feet V7, its rarely an issue. Even without pressure on the pedals these boats keep pretty straight. And they don't turn on a dime with rudder alone. To make a sharp turn you need to lean the boat and brace heavily on one side.

An interesting topic and one I've been thinking about recently. Specifically in terms of modifying a "beginner" ski by moving the rudder forward. Such skis typically have rudders that are further aft compared to more advanced skis. I wonder if this would make the ski more directionally stable in big messy downwinders. It would be quite a big job and very expensive as I have none of the skills required to do it myself.
I hope I'm not hijacking the thread but I'd be interested to hear what experienced paddlers, designers and composite experts think about the idea.

Two thoughts. 

I am not a sailor, but isnt a sailboat keel to keep the boat from 1) being blown sidewise and 2) tipping over too far?

regarding rudders i will just point out that all boats are faster with rudder steering, even when going in a straight line. If not , K1 sprint boats, which go in a straight line in flat water fighting for 100ths of a second, would not use them. 

Hi Atlas,

my post was also motivated by the same problem you described. I tested a relatively flat beginner ski in small but steep wind chop and found it terribly difficult to prevent it from broaching. Then I saw the rudder was very far aft. As I liked the boat otherwise, I asked the manufacturer to build one for me that has its rudder 25 cm more in front, which he agreed to. According to experience with my own designs, this improves directional stability a lot. But I would like to understand why, and, more importantly, what the tradeoff is, as there is no such thing as a free lunch.

@ Zach: well, fitting a leaded keel to your elite ski to prevent capsizing might be worth a thought haha. But joking aside, if you think about it, broaching is nothing but a side-sliding movement of the boat. This happens to be the stern, because the sharp bow is buried in the trough, so the aft part of the boat has to swing around the bow more or less, which entails losing your direction. The question then is where to place your rudder to best prevent this. It seems to me that a rudder very far aft is not optimal, but I would like to confirm this, and understand why.

Impala - I agree a ski with a rudder too far aft tends to broach, especially in steep wind waves. I had an old custom kayaks synergy that did that. I assumed that was because as the ski tipped down a runner the rudder would come out of the water. I thought about trying to move the rudder forward but knew that was a bigger repair job than I wanted to tackle. 

My experience with the old Fenn XT is that the original sharkfin rudder, that is placed near the tail, would simply come out of the water routinely on the crest of a wave. The ski broached constantly. A 10" elliptical rudder pretty much eliminated the problem.
Paddling the Fenn Elite I had with the same sharkfin rudder placed much farther forward, broaching never seemed to be an issue. The elite performed beautifully. 
My understanding is the Elite performed even better with an upgraded rudder though.
Advantage of sharkfin is it not damaged as readily as deeper less raked rudders.

The problem is (a bit) that we are discussing something that we feel, but have trouble to observe, as the rudder is just out of our sight while paddling. I assume that even if the rudder does not 'stick out' behind the wavecrest, it is not very effective when working on the top of the wave where the water is less 'solid' (bubbly, and easier to push aside as there is less surrounding water). The irony is that this hits you all the harder once you try not to slide down the face of the wave, but to stay on top as recommended. That will not work with a rudder too far aft.