Rudder Performance

Has anyone done any studies on the effect of the rudder on overall performance of the surfski ?

What the best shape and length to ensure good turning and surfing properties whilst keeping drag to a minimum ?

I know Epic have done some "real" design work using CAD, etc. They have the two rudders - the "de-tuned" rudder on the V10Sport and a larger, more effective rudder on their other skis.

The reason they use the "de-tuned" rudder on the sport is that a larger rudder tends to make the ski feel more tippy; when you put rudder on, there's a turning moment from the rudder that tips the boat.

It's interesting to compare rudders: when you look at the Epic rudder, it has a comparatively thick section with a rounded leading edge; it's very much like an aircraft wing. Most other rudders are becoming more and more narrow with as sharp a leading edge as possible.

I'm told that the rounded leading edge makes for more laminar (i.e. smoother) flow over the surface of the rudder and hence less drag and more effective control. It sounds logical - but it would be interesting to hear from someone in the field of fluid dynamics who could substantiate this.

Hi,
I will try to write down some things I learned about rudders in sailing, kayaking and powerboating.

It is true, that a laminar profile in laminar flow will have the lowest drag. Problem is, that most kayaks yaw and pump due to the paddling and wave action. This will destroy laminar flow. Laminar profiles in turbulent flow have a higher drag than turnbulent profiles.
Even if the surfski/kayak would go perfectly straight it is questionable if there can be laminar flow around the rudder because of its position quite far back. At some point along the hull, flow will go from laminar to turbulant and the turbulent boundary layar can be quite thick. This will be especially true for surfskis with venturi drains which will create a vortex and thus turbulance behind them.
During surfing conditions there could be laminar flow as the boats moves almost in a straight line but I am not sure if you do not reach speeds high enough to go turbulent again. High speed sailing and power boats assume turbulent flow because of the high speeds.
Laminar profiles are also very sensitive to any defects or deviations from the optimal profile. Small scratches or cracks along the edge will greatly reduce their effectivness and performance.
The other thing to consider is, that there are thick turbulent profiles also. These have higher drag at their design point than thinner profiles. One reason why they are used is that they have a wider range of angle of attack where they work well and have a pedictable and fairly constant drag. Thin profiles have a much narrower sector where the flow can follow the profile before it seperates. Flow seperation produces very high drag and very low turning efficiency.

The conclusion for my boats was to use a thin profile for any condition where I do not need large rudder movements (flat water, round topped waves, straight courses, little drafting). I use a thicker and larger profile for surfing, complex drafting situations, courses which require tight turns. I do not know if my rudders have turbulent or laminar profiles.
For the typical rudder sizes the difference in drag between a turbulent or laminar profile of similar thickness should be small. Any scratch or blemish in the boat or rudder will have a bigger influence. So keep your boat as clean and smooth as possible.

Just my 2 cents.

Rolf

I think Rob intended laminar flow to mean that there was no flow separation as in "not stalled". Laminar flow in fact means that the water flows over the rudder without a turbulent boundary layer and I am pretty sure that this is not practically sustainable in the case of a ski rudder (bear in mind that very few aircraft have laminar flow airfoils). The practical goal therefore is to design a rudder which tolerates a broad range of angles of attack without stalling and produces a maximum of lift (turning) with the least amount of induced drag and rolling effect on the ski. Fortunately for ski manufacturers rudders operate in such a ?non-optimal? environment that there is not a huge amount to be gained from extensive optimisation. There are also other practical considerations such having sufficient sweep to cast off kelp and other debris which require a degree of compromise.

In theory high aspect rudders (i.e. those which are long and thin) are the most efficient but in practise they are very prone to stalling and produce a lot of unwanted roll when deflected. They are also more prone to damage and tend to collect debris more easily. In theory the surfboard skeg type rudder as on the Mako6 are not very efficient but in practise they seem to offer a very practical and well liked solution.

Epic offers 2 rudders for the V10 ? a standard rudder and a surf rudder ? and these are most probably the best designed from a theoretical standpoint. They both have properly radiused leading edges which make them able to deflect further before stalling. The standard rudder is swept enough to shed debris and represent a good all-round practical compromise. The surf rudder has a theoretically more efficient plan form but is more prone to collecting debris and inducing role when deflected. I have and use both which just goes to show there is no optimum solution for all conditions.

Oh...you mean the rudder is a collection of compromises- like the boat, the paddle, the wife and so on. ;D

I try not to compromise on the paddle or the boat!

This is a complicated subject. I think that you should try to learn about "lift" of an airfoil--this is not as simple as it sounds.

In any case, the point of the sharper, thinner rudder designs is to have less drag. Unfortunately, they are more prone to stall out at lower angles of attack. It seems to me that the Mako6 rudder is thinner and sharper--and as a result is likely lower drag, with the penalty of stalling out more easily. I find that on flat water, the Mako low profile weedless rudder accomplishes sharp turns better than the full size rudder.

how about if the course is flatwater with wide 90 degree turns? We have many races here that are like that. In fact, one is a canal race with no turns- only course corrections!!!

Would you swap out a full size blade for something smaller?

Any gain in performance on flatwater?