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AeroSouth Rudder Installed on my Zuma


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Very happy with the AeroSouth rudder I installed on my Zuma. The boat is more responsive and easier to sail!, very nice upgrade. I would say the weather helm has been reduced by at least 40-50%. I haven't sailed enough with the new rudder to say boat speed has improved measurably but my enjoyment sure has! The rudder is designed for a Sunfish but had no issues with installation on my Zuma. More info can be found here: https://aerosouth.net/sunfish-fs-rudder.

 

Aero South Rudder1.jpg

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Once again people confuse weather helm with a heavy helm.

Weather helm is a function of the relationship between the center of effort of the rig and the center of lateral resistance of the hull, centerboard and rudder. Most boats have some weather helm because it is desirable to have the rudder contribute to the side force.  Just as the relative size of the main and jib effect the balance of a boat, so does the relative area of the centerboard and rudder.  A larger rudder, or a more efficient rudder can reduce the tendency of a boat to round up by shifting the center of lateral resistance aft.

 

Heavy helm is a function of the relationship between the axis of rotation ( in this case the pintles), the hydrodynamic center of pressure of the rudder blade and the length of the tiller.  For a very rough idea, The center of pressure is about 25% of the way back from the leading edge at half depth.  The effort needed to turn the rudder or hold it in a position to maintain a constant heading is the ratio of that distance to the length of the tiller using the pintle as the fulcrum. It’s a simple lever.   

This  seems like pedantry.  But I have always thought that understanding was better than misunderstanding.

 The Sunfish rudder head design (which is shared with the Zuma and the Vanguard Picos) only permits a limited range of motion from up to down, so to keep the rudder blade clear of the sand as you pull the boat up and down the beach, meant that the rudder was always swept more than you would want. This, combined with the long footed mainsail, made the loads on the tiller higher than ideal.  Better foils improve any boat.

SHC

 

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It is interesting looking back in time. Heavy helm is a very common "issue" on all sorts of older classes. Big curvy rudders, blade going way aft. Snipe. Lightning. Thistle. 14. Various early canoes. All sorts of catboats and dories. (catboats obviously draft restricted). On the other hand the New Haven Sharpies sailed with *balanced* very long chord short span spades. There is a limit to how much heaviness. Also the boat needs to manoever to meet its utility...

Now we might ask why?

Perhaps there is a lot more to it.

For one thing, the old timers actually probably knew how to sail their boats. And so the heavy helm wasn't a problem because they balanced their rigs...

A light helm is great for responsive sailing but it is also twitchy.

Everything has tradeoffs. Personally I love a light helm. But what the heck? I like racing boats that require my body to be 100% not in the boat...

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"Perhaps there is a lot more to it"

Yes, to understand something of why things are as they are or were, it is necessary to understand as much of the physical and social context as possible.

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11 hours ago, fastyacht said:

the old timers actually probably knew how to sail their boats. And so the heavy helm wasn't a problem because they balanced their rigs...

The thing is our boats are much faster than their boats, even when its nominally the same class. "Balancing" the boat so that there is no side load on the rudder is slow. It is maximising the induced drag from the centreboard, and means the rudder is just dragged through the water creating frictional drag with no benefit. Because our foils are (in general) so much better shaped than their foils everything underwater has seen major improvements in efficiency, and everything has to change to match. 

Weather helm ought to mean that the helm has to be pulled well to weather to keep the boat in a straight line. When you've experienced it then it is horrible, because it feels as if you are creating huge amounts of drag with all that effort, which of course you are. But if the rudder is tugging at your hand but the tiller is on the centre line then that isn't really weather helm at all. Trouble is from where our head is four 5 or 8 feet to windward most of us have little idea whether the rudder is on the centreline or not. Indeed, if the boat is "perfectly" balanced and making appreciable leeway the tiller may have to be [leeway angle] to the leeward side of the centreline to keep the boat of track. And if you have a boat with a gybing board (or trim tab I expect) then it gets even more complicated...

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Jim I concur. What I noticed as a kid was that in light air laser lee helm . This was slow. Heel vang to get helm made better. As wind increased weather helm so bad coildt tack properly. Hauling on cunno did the trick.

My ooint aboutvold timers is they also understood all this well befor Briggs was born haha.

But foil improvements it is kind of mind vlowing how crude the old ways were. And yet 19th centy Herreshoff and that Scotish guy too, they drew beautiful aerofoil section keels

 Look at Reliance. Built befor Wright abros.

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Thanks to the great explanations here, I learned a few things more.  To be honest, reducing weather helm on our "FS" rudder blade shown here was not the primary design objective.  Reducing its drag, weight and cost were more important to us.  You can read about the thinking and design process that went into it at this link. (AeroSouth FS Technology).  While designed for the Sunfish, it should fit most "Clonefish" and other small dinghies that use the same attachment / cheek flange, like the Zuma.   A rudder blade can not be seen isolation from the hull, board and free surface, of course.  The planform shape was chosen to reduce losses from the free surface and the surface-piercing, why it is relatively thin and narrow at the waterline.  Hundreds of small iterations on the shape were made in CFD tools to minimize induced and skin friction drag.   Some laminar flow is achieved, but this too was a compromise when selecting a cross-section.  Then there are the variances in manufacturing, which is never perfect if prices are to be kept affordable.  We make everything here in North Carolina and test all ideas on the nice lake behind our shop.

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Years ago some Thistle class people including my former boss did some analysis and testing for laminar flow centerboard sections. They worked better but only in smooth water.

Then while I was building them we got someone at OSU (I think) to run our Thistle rudders in the wind tunnel to show rudder hum. There was an article produced by my company's owner all about it. Of course the basic problem is usually attributed to the karman frequency matching a structural resonance. The idea of "lopping off" the trailing edge doesn't always cure that.

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  • 3 weeks later...

I'd be interested in that article on rudder hum.  I do not notice it on our "FS" rudder blade, however it is present on our new "Sabre" daggerboard at higher speeds.  Its sound amplitude varies directly with speed, the frequency is somewhat constant.  It's actually kind of cool, as it occurs about the same time the boat starts planing and really takes off. The advantage over class legal boats is very obvious at speeds above 10 kts or so, but it performs well at lower speeds due to overall lower drag and weight.  We keep the head of the board close to the upper limit of the trunk to avoid chatter of a loose board in the trunk, so the sound is likely coming from the wingtip vortex at the bottom of our highly tapered board.   I doubt this vortex has expanded enough downstream to impact the lower tip of the rudder, but its influence is surely there.   Base on our CFD predictions it is significantly smaller than the vortex generated by the class legal daggerboard, which has very little taper.

On 6/26/2021 at 1:07 PM, fastyacht said:

Years ago some Thistle class people including my former boss did some analysis and testing for laminar flow centerboard sections. They worked better but only in smooth water.

Then while I was building them we got someone at OSU (I think) to run our Thistle rudders in the wind tunnel to show rudder hum. There was an article produced by my company's owner all about it. Of course the basic problem is usually attributed to the karman frequency matching a structural resonance. The idea of "lopping off" the trailing edge doesn't always cure that.

 

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