Rudder Size and Stability

Does the size of the rudder have an overall effect on ski stability? For example; in side chop conditions will a 9-10 inch rudder make a ski more stable than say a 4-5 inch rudder? Or, does rudder size have no effect on stability at all?

There's a great discussion here on rudders:
All you wanted to know about rudders but were afraid to ask

Some of the answer depends on the turning moment of the rudder - a longer rudder generally has a larger turning moment than a short one and this would tend to spin the ski.

When you kick the rudder pedal, the rudder applies a turning moment to the ski, which, in an extreme case will spin the ski and tip you out!

So a larger rudder can actually add to the tippiness of the ski. But it also depends on the rudder shape... Check out the article!

Robin,

Thanks for the reply. I've read the rudder article in the past and it is informative but it doesn't really address a general affect on ski stability. I don't really have any concerns going into the wind, quartering, or running down wind. In side chop conditions with little to no rudder input will a deeper/larger rudder provide any increase in stability (or the feeling of stability)? Just wondering if anyone has experimented with swapping rudders for comparison in side chop conditions.

Steve

I don't think the rudder will add to your feeling of stability - I've thought about this alot and have used several different rudders on my V12 and last year on my Uno. A skeg, though, is different and will add stability. But, a rudder will flop left and right and unless locked in position, won't add to stability. Having said that, some rudders will make the ski feel tippier if they have a central or rear positioned post and roll the ski considerably with turns.

The most predictable feeling rudder to me is a swept back one with a forward post position. Also, you can probably add a bit of stability to the ski by using a larger weedguard placed just in front of the rudder. Erik

I have experimented with epic's universal rudder and their surf rudder on my V10S and found the universal rudder to be a little more stable because of the weaker rolling force. I use my boat in the great lakes (northern Georgian Bay) and wind driven side chop is a common condition. I think the universal rudder feels more stable in the side chop because any slight adjustments you make are more subtle and upset the boat less. The surf rudder feels much more reponsive but I do occasionally hit the surf rudder when I try to sneak over a shoal. The universal rudder gives me a extra couple of centimeters of clearance.

I like the surf rudder the best because it is so positive but the universal feels "more forgiving" in choppy wind driven lake junk.

I have owned every rudder from Huki, Think, Stellar, and Epic, Fenn, and Van Dusen(mohican). I have found the bigger the rudder the more stable the ski. Currently I have a 4inch, 5.5 inch that I cut down from an 8 inch and an eight inch rudder with no leading edge and one that is more typical of the standard rudders. I also found that the smaller the rudder with less drag that the difference is noticable in terms of speed from approximately 5-10 seconds per mile in the same ski with varying lengths of rudders. Wesley(surfskiracing.com)

Thanks for the replies. Since I only have the one rudder, a 5", it sounds like it may be worth while to purchase a second larger one ,around the 8" size range, to experiment with.

just to add to the debate i conducted an unintential experiment in rudder size/shape versus stability. i paddle a V12 and last week broke the epileptical off the boat in flat conditions after hitting a log. The boat became impossiable to steer and tippier. I replaced this rudder with a epic weed deflecting rudder and weed deflecter. i now feel i have new faster but tippier boat especially in cross wind/chop conditions. I believe the older longer rudder gave a greater surface area of rudder further from the hull of the ski therefore slowing its roll, the new rudder with deflector may have a similar surface area but its location close to the hull reduces its keel effect thus allowing the boat to roll faster. I will be buying another large rudder for rough conditions and hope the weed deflector will still assist in keeping the larger epileptical rudder weed free.

That is the exact kind of experiment/experience I was looking for. I've wondered if a deeper rudder would reduce the roll rate and affect on a ski. Sounds like it may.

I have a balance problem these days and notice stability a lot , i am definetly convinced my longer deeper rudder Does make MY ski more stable than a short one .(tried both)Lats

I wonder about the drag consequences of running a really big rudder.
Is it better to have a stable boat with a small rudder, or a quicker hull with a bigger rudder.
It would be nice to know the real effects on speed of each.

i concur with the view of wesley and others that a longer rudder increases stability and vice versa, having gone from the stock Think Legend sharkfin to a snapped-off version of said fin then to the Legend's longer surf fin. My (unscientific!) sense is that secondary stability is slightly better. Re speed, i would think it better to risk sacrificing a little speed if any for additional stability

Regarding the roll torque a deep rudder applies to a ski when turned:

I found this effect to be very pronounced on my Red7 (old Surf70) with its elliptical stock rudder. But after I got used to the effect, I often found myself instinctively using this force to mitigate the start of a capsize in choppy conditions, rather than using a bracing stroke.

So for example, when going straight in chop conditions, if I felt the ski tipping I'd just kick the rudder over the opposite way the capsize was going, and the rolling torque would correct things instantly. Yes, I know - it slows me down more than just getting a good proper stroke in. But my point is that you can use the rolling torque effect to dynamically correct an imminent huli.

When i replaced the rudder with a short 4" weedless rudder (nice one by DK Rudders!), the stability was slightly but noticeably degraded (torque steer aside). But of course, the boat feels slightly faster now.