Surfski Knowledge Base

Dan Mayhew
03/28/19 #24552

After the race last night the topic was raised of how the size of a rudder affects flat water speed. Does speed really vary if you go from a 7“ rudder to a 4” rudder on flat water? Is it worth it to swap out rudders for flat water racing?

Nicholas Cryder
03/28/19 #24553

Yes, it does. But it’s hard to quantify because it’s so athlete centric. If we’re talking drag, the numbers magnify as speed increases. So for the sake of flatwater racing, 6mph vs 8mph has two different differentials, and the reward is much higher the faster the speed. The drag is also much higher and therefor more pronounced and noticeable. The same is generally true of boats; very little difference between a V10 and V14 at 6mph. But it’s vastly different at 8mph. But more dynamic water, the more the handling characteristics and pilot skill become relevant to drag.

Reivers Dustin

03/28/19 #24554

good explanation. As you think this through, it's also true about wake drafting, your physical condition and the littlest features of your technique. Like compounding interest. One more reason I just so love this sport. We got layers to look at kids.

waterbornewarrior
03/28/19 #24555

If you had some sweet 4” and 7” rudders at your disposal, how would you make the determination? Asking for a friend…

Don

Charlie Brown

03/28/19 #24558

In technical terms, drag is proportional to the square of velocity. In subjective terms, stability, and therefore speed, is affected by all those other things mentioned above, including the rudder. It's why I like my V10 Sport. The boat may have a higher drag coefficient, but I can concentrate more on a powerful stroke than on staying upright.

Charlie

Pmarcus
03/28/19 #24560

So in Non technical Terms about boat speed and rudders…..I have had small and large rudders made by DK Rudders and Huki large and small rudders….and now on my Ozone Outrigger the swept back or surfing rudders….But my GPS doesnt usually lie, although I might…but I found to be pretty accurate about a .2 increase in speed when using a swept back or shorter flat water rudder. A little less turning capability but faster overall consistently!!

Peter

Paul Reavley
03/28/19 #24561

Rough calcs - If I assume Peter is going around 8 mph on Wednesday niters then .2 mph difference translates into about 40 seconds difference in his time on that course. Assuming the same rudder change as Peter those of us going 6 mph would see closer to a .1 mph speed difference but a similar time difference (approx. 40 seconds) in their Whatcom times (everything else being equal)

Steve Scoggins
03/28/19 #24562

Hey Peter I'll let you use this GPS next week so you can maintain Paul's expectations.

Steve

Paul Reavley

03/28/19 #24563

Steve,

Would you be willing to sell that GPS? And what is the +/- on accuracy?

Paul Reavley
03/28/19 #24564

The good news is if Peter is going slower than 8 mph then .2 mph speed difference represents a larger difference in his race time.

From the back of the pack it looks like everyone is going 8+

Steve Scoggins
03/28/19 #24565

I wouldn't sell or trade it for anything. It's there for me every time I need a boost in self confidence.

Steve

David Scherrer

03/28/19 #24566

For simplicity's sake, drag a 7 inch pencil through the water, then break it in half…..any difference?

Rick Lingbloom
03/28/19 #24567

Good point David! I think maybe I should just try no rudder at all! Pretty sure that then I would be able to keep up with … uh … oh never mind.
Rick L

Charlie Brown
03/29/19 #24571

I've been thinking (Yikes!). Maybe I should think twice (Yes!) But I'm going to post this anyway (Oh No!!) What we need is a good engineer who isn't rusty from exposure to water (Reivers, do you know where I can find one?). I had to dig out a very old textbook. The problem of drag might be estimated. Drag has two components, form drag and skin drag. Form drag is proportional to the projected area of the boat in the direction of flow. I estimated this at about 20 square inches for a boat submerged to 3 inches with a 4 inch rudder. Adding 3 inches of 1/2“ thick rudder adds abut 1.5 square inches to the projected area. That is an increase of 7.5%, which actually is a lot. However, drag coefficients indicate to me that skin drag is much larger and more important than form drag for a streamlined body like a boat and rudder. Skin drag is proportional to the wetted area of the boat. I estimate the wetted area of a boat and 4” rudder to be about 12 square feet. Adding 3“ to the rudder adds about 24 square inches, which is an increase of 1.4%. That may not seem like much, perhaps negligible, but at 8 mph (really?), that is 0.11 mph. This is (surprisingly to me!) consistent with Peter's observations.

Kevin Olney
03/29/19 #24573

I’ll be switching to my 4” rudder for tomorrow’s PNW-ORCA race.

https://www.webscorer.com/register?raceid=173999

Reivers Dustin

03/29/19 #24575

At one point I did get interested in the question of drag. Before we had Whatcompaddlers there was a group mostly dedicated to sprint kayakers. That sport has had serious research. One of the members of the group had done post-doc study on hull dynamics. He never said, but I suspect he was employed in large vessel research. He had access to a high level modeling system for form and skin drag. His somewhat intimidating discussion really boiled down to what we experience for ourselves. You want to paddle the tippiest boat you can properly handle in the conditions. The size of rudder is a factor here because this surface has a stabilizing effect. This regards stability, or axial inertia (tendency of the boat to keep from rolling). Note that in big stuff, you also want stronger 'turning moment' from your rudder (so the boat doesn't broach).

I rarely change rudders. I have the biggest rudder Jude makes (9-inch weedless). I had DK make me a “MONDO rudder” (note obscure reference to a character in “Blazing Saddles”). It is huge. But it's a hassle to swap rudders. In the Gorge a couple years ago I decided to put the big surfing rudder on to gain responsiveness. I scared myself so bad I can't get the urine smell out of my paddling shorts. It was So Different I could not manage the boat. Big lesson for little reivers: don't go muckin about with crap.

Almost everything you fiddle with is what they call second order effects. Just work on the motor. That's the real problem. And by the way, I'm an Electrical Engineer. So according to the other branches of engineering, I'm worthless at talking about anything you can physically see.