rudder that changes shape w/ angle of attack

Happy to see that we all agree...

I see that the old bears agree that, except for some special applications variable camber rudders are not worthy. For keels its a very different affair; a flap on a keel and you won't recognize the boat.

I agree totally with Mr Speer; when you make a rudder with a planer and sand bloc from a wood board the naca 0012 is good and tolerant enough. When you try to get the last % of a race boat, modified series 6 or wortman are good if made within the tolerances. Turbulators may help a lot, the simplest is a strip of wet sand paper #600 or 800 glued and fitted in a strategic place of the chord. Sanding the gel coat to have a dull smooth surface, grooves 1/10 mm deep, zig-zag strips...there are a lot of tricks for controlling the turbulence and destroy the laminar pocket.

About keels, flaps habe been highly successfull and are simple to fit. The solutions are numerous from the simple flap to multi foil with controlled slots. This is another story.

Another solution are the daggerboards. Mr Noir designed about 25 years ago a very intesting boat named Nuits Blanches. Inside ballast, 2 asymetrical daggerboards like Terrorist with controlled angulation (simple rubber blocs permiting a lateral angulation) and a isometric hull, I mean a hull that remains water lines symetric whatever the angle of heel. The perfs of the boat were astounding, unhapily the IOR jauge wouldn't permit such intelligence without an enormous penalty... The movements of the boat were very confortable and gentle.

This solution remain valuable if you're not stuck in a jauge, and would lend to a very good fast cruising boat able to beach in turquoise tropical warm waters (I live there...) All is in the design, and there is no need for costly high tech.

Personnally I'm more fond of multihulls which have greater potential. A well designed low tech multihull will be faster than a high tech monohull.

When I see the money spent in the America's Cup for boats that late Philip Weld (of Rogue Wave and Moxie fame designed by Newick) called floating wagons to get an hypothetical 1 % percent improvment I think it's money thrown away by too rich people in an indecent circus. 14 and 18 feet dinghies races are far funnier and cheaper to watch and race. At least the dinghies capsize et the races are dog fights. Thanks to God, the use of guns and knives is not allowed by the jauge.

I plan to open a thread about very slim powerboats (hence Ilan Voyager the tri powerboat designed by the genial Nigel Irens), more precisely a working mono engine trimaran for coastal dive trips or fishing. The solutions may apply for a poor's man fishing boat. Would you be interested by this thread?

Saludos from Mexico.

 

The most effective turbulator is probably zig-zag tape. This produces a three-dimensional disturbance that promotes transition much quicker, and with less drag, than a patch of distributed roughness like a roughly sanded region. A program like XFOIL can be very valuable in deciding where to put the turbulator. If it's too far forward, there will be a drag penalty due to excessive turbulent flow. If it's tripped too far aft, laminar separation will happen first, negating the whole point of the turbulator. But tripping laminar flow in water is probably not as important as it is in air. For whatever reason - whether it's higher ambient turbulence due to waves, or particulate matter in the water - transition seems to happen earlier in water than in air at the same Reynolds number.

I think keel flaps are widely misunderstood. They don't increase the lift on the keel - that's set by the sail rig. But they do allow one to position the low-drag range of the section to center it on the operating condition. This can cut the profile drag in half or more - it's like saving half the wetted area of the keel.

For example, in the figure below, the only difference between the P30012 (green line) and P30212 (magenta line) is the camber.

If the section were operating at a lift coefficient of, say, 0.4 - 0.5 going to weather, the symmetrical section would be operating outside the drag bucket while the cambered section would be operating in the middle of the drag bucket. Similarly, going downwind at near zero lift coefficient, the cambered section would have a drag penalty while the symmetrical one would not. A flap would allow the crew to move the drag bucket to keep the boat operating in the groove.

Notice how much less of a penalty there is for the traditional NACA 0012 if you're going to use a symmetrical section. The laminar flow sections are great when you're in the low-drag range, but they can be a dog if you're outside it.

A similar argument could be made for an articulated rudder that carried a significant amount of weather helm when beating. A flap slaved to the rudder deflection might reduce the profile drag, especially for a boat that also had a keel flap to reduce the leeway angle so the rudder deflection was the major contributor to its angle of attack.

Since the keel doesn't necessarily operate at high lift, I don't see the point in sloted sections for keels.

I'm definitely with you on sailing multihulls. The whole point of AC boat design is to create a class in which enormous sums of money are spent for the least amount of gain - if there's a genuine speed advantage, the TV coverage is boring. I'm amused that I can go out in my little F-24 cruising trimaran and hit speeds comparable to the big 70 ft AC yachts.

 

still on the drawing board

So the old bears agree... Obviously many others through history have had similar thoughts on how to make a better rudder, but mechanical difficulties and plain old physics still favor the well designed single piece rudder.

Thanks for the refs on software, articles, books, and authors. I've already located most of the the titles (even out of print) and while i wait for the new refs will do some homework with my existing library. I have a comfortable chair that overlooks St. Martin - today is Anguilla day (kinda like 4th of July) and since the national sport is sailing, the local class C boats are racing around the island. They are a lot of work to sail and great fun to race - 28 foot LOA, wood hull, all removeable ballast (set up and break down is wet, heavy, and takes a long time), and UNLIMITED sail plan/area. A bit different from the yacht regattas across the channel...

I am not scrapping the idea of skins - I, with some help from my friends, will experiment on dinghy rudders. If we find anything interesting i'll post here. I guess I'm stubborn like that - but then again, I think a faster Anguilla boat can be built...

Thanks again,
Kevin Barry

 

I know this isn't totally salient to this conversation, but a zillion years ago one of the original Herreshoff's designed an articulating rudder... it was a full keel boat with a counter stern and the trailing edge of the keel was composed of vertically hinges segments. Inside the segments was a "J" shaped bar which had the 'full turn' rudder shape bent into it. When you were going straight the "J" lay in the same plane as the keel, holding the rudder segments straight, then as you moved the tiller the "J" bar rotated; as the tail of the "J" rotated out to port or starboard, the rudder segments aligned themselves with the bend. I know I didn't explain this too well, (I saw a picture and instantly went 'aha!') but interesting that we're still talking about an idea that was first put into production probably 75 years ago!

 

If I remember well

If I remember well it's Nathanael Herreshoff who designed that and it was far more than 75 years, it would be maybe closer to 120 years. In the 1880's-1900's N. Herreshoff has been very creative in sailing boats; catamarans with joint ball connections beams (Amarillys), integral structure (Gloriana), isometric hulls, new flow lines, winches, etc.

The rudders were just behind the keel and it was an attemp to have a better control on the boat. I write from memory and maybe I'm wrong as N. Herreshof invented so many devices.

Boats with rudders attached to the keel are notoriously unstable reaching under spinnaker. They tend to rotate to upwind; it's very frightening as I have lived myself in my very young years on the terrible sailing boats made under the RORC jauge. Slow, heavy and dangerous.

 

I believe Halsey Herreshoff built a boat for himself with an articulated rudder. It had thee panels, if you will. A fixed 'skeg', a usual sort of trailing 'rudder' and a 'flap' in front of the skeg. The flap moved half the angle of the rudder, IIRC. The idea was dropped, probably because it was outlawed by racing rules, and I never saw a published report on performance. There were articles in Yachting, probably mid 1960's.

Despite the general understanding of the brochure terms "fin keel" and "full keel" there have been plenty of fin keel yachts with the attached rudders. 5.5's for example. The rudder attached to the short keel is too far forward.

Hunter built some boats with a 'dinghy garage' beneath the cockpit. This required moving the rudderpost forward. In the review section of a Hunter owner's web site, there were comments that these boats were 'squirrely' downwind. It's not just an attached keel problem.

 

Hi Tom,

Last summer I built a new rudder using the NACA0012 section for my old McGregor Venture 17. Being a fiberglass-kinda-guy I went nuts with the whole project but stopped short of making trailing edge sharp and left it a .062" wide flat.

I have noticed a definite improvement in overall performance. But considering the slab-sided, .5 inch radius leading AND trailing edge configuration of the original that wasn't too surprising.

What is surprsing is the boats new low speed turning characteristics. It took me some time, and a lot of headscratching, to realize that the rudder is now more effective if it wasn't thrown hard over when tacking. An initial bit of rudder deflection is all that is required to really get that 0012 section working. Anymore and the rudder seems to stall and actually slow down the rate of turn...

<< What most people don't appreciate is that at low Reynolds numbers, the NACA 4-digit sections can have significant amounts of laminar flow, and the problem isn't so much maintaining laminar flow as it is getting rid of it without triggering excessive separation. >>

Is this what I am experiencing....?

 

Could be. Depending on aspect ratio, stall angle may be around 15 deg or so.

Once the turn starts, you can proably feed in a lot more tiller - the local flow at the stern will be at an angle because of the turn rate. A steady, positive push should spin it around nicely!