Rudder thickness - putting the cat amongst the pigeons.

OK - time for a little stirring.

For some time now, the IOM class, and from what I can see, the USOM class, have been moving to progressively thinner keel foils. There is a trade-off between turbulence when the keel angle of attack moves a small way either side of zero, and the low drag benefit of the thin foil. There seems to be a general concensus that low drag is worth more than the cost of turbulence at high angles of attack. Carbon fiber allows these classes to use thin foils that still have sufficient rigidity to suspend the keel bulb with minimal flex. I don’t know what the other development classes are doing - maybe someone else can comment.

However now this is happening with rudders too. For example, Anders Wallin has a design for the rudder for his Noux 2 IOM on his website with a very slender 5.7% section.

Playing devil’s advocate, I’d suggest that, as rudders spend more time at high angles of attack than keels, and are significantly smaller in wetted surface, a designer might put more emphasis of a rudder foil selection which will hold a smooth flow for as long as possible, than on reducing drag at a zero angle of attack.

How much of this trend to thin rudders is based on either research or observed benefits, and how much is it just “follow the trend” like any other fashion?


Muz - thanks a bunch … I thought the thread was about multihulls ! Thought someone had a new trimaran design called ?pigeon" - [grin]

:cool: :bigmouth: :sleep1:

Gee Dick - I never thought of that.

I was tri-ing to be serious for a change.

I’ve seen the Anders profile and wondered the same thing. I read somewhere a while ago that the rudder can generate a lot of lift. You would think that something really thin would stall out quickly. Maybe its the length that stops that. Re keel foils, you would have to ask when does the thickness (cord?) get so thin there is no lift.

I can remember when the class wanted foils to be a minimum of 10 mm thick. This was due to the perceived cost of carbon being to expense for those involved at the time.


Precisely Tony.

All the foil analysis I’ve read over the years, mostly for RC sailplanes, stops short of the slender sections we are now using. We’ve got drag bucket and lift information for slightly thicker sections at low Reynolds numbers, but I’d like to see something for the very thin sections.

I’m about to build a new rudder for my IOM, and am still very tempted to stick to an old NACA0008 foil. Even a NACA0010 foil is only 7mm thick at the top of the rudder (70mm chord). True - it would be as thick as the keel - but would stall later and provide good lift at the speeds we sail. With access to the technology I have, it’s highly unlikely I can make my mold to these sorts of tolerances anyway - but it is fun trying.

Gee, I wish I had a study grant and access to the Wolsfon Unit testing facility to actually investigate this stuff. Oh well - back to the day job.

(Lester - are you listening?)

Changing topics slightly - here is a link to the (most excellent) Nordic IOM site showing a new design with a high aspect rudder - in the style of an IACC or Mumm 36 for example.

Looks cool - but my take on the science is that such a high aspect rudder design would not be a benefit to a boat like an IOM.

Again - comments invited.

To be honest, I have more problems with the boat name than the rudder thickness or shape.
Although, my USOM has s similar shaped rudder (it? a Cobra, Wick Smith?s design), quite thin and it seems to work well.


Gio - I agree with you…


A: - hey there sailor “B” - how did the weekend IOM races go?

B: - Not worth a crap! My “Spincter” got caught in someone else’s rigging so I didn’t finish one heat… but in the second race, my “Spincter” was moving really fast!



…[To be honest, I have more problems with the boat name than the rudder thickness or shape.]

Yes - it is certainly not a name you would want to “Google” with your “Safe Search” adult filter turned off. :-0

Hi Murray

I wish too (smile)! In fact some students ran a couple of different IOM fins, rudders, and bulbs through the towing tank a few years back. Any drag differences were so small, they were lost in the experimental noise. I think this reflected more on the tank dynamometer and electronics, so I no longer offer student projects in this area.

My testing tank will be the local lake then, and a NACA0008 it will be.

I have a cunning plan for making an accurate male mold for the rudder (no CNC milling involved). The male mold will in turn be used to take off a female mold.

If my cunning plan works, I will share it with you all. If it fails miserably, you shall never hear about it again.