I have talked with Rob some time ago, and did the calcs for his rudder foil, think it a NACA 0012. Got the Cd and Cl from Xfoil. GIven a 0.6 m2 area, 1.2 meter long and chord 0.5 meter.
Just for fun,.... 10knots results in range of 12-20kg of drag, (combined form drag and induced drag). With an angle of attack 0 to 12 degrees. Then the stuff hits the fan as the rudder goes past that and stalls. Drag quickly spikes to 500+kg past 16 degrees AOA.
While lift in the normal AOA range (unstalled) goes from 475 to 1300 kg.
So got the idea that rudder stock needs to be STRONG is understandable. AND also dealing with pins and stuff to handle kick up design is not so pretty and no little spring loaded detent is going to handle it.
Then the racers amongst the forum, say at 20 knots the drag is 40-80 Kg in the normal range for AOA. But run it into a stall at that speed and whoa, spikes to 2000 Kg. Turn that wheel hard over at speed and lift
starts at 1590 kg and runs up to 5000 kg.
Granted boats don't turn on a dime but if you fling that wheel/tiller over something is going to fly/break. Mostly handling the wheel/tiller will feel tender to slight adjustment when the HP's pick up speed and read that on some other older posts. That is a lot of rudder area, since it is doing double duty, leeway and steering management.
Non compressible fluids will really put on some load in a hurry. Did suggest to Rob to have steering limiters, but in my opinion band-aids the problem. Might be better to have keels or dagger boards on the lee hull and reduce the rudders some.
Anyway made lots of tables to mess with it and understand the loads for a pivot kick-up knuckle and the rudder bearings. Not real happy the end result. Keels could make is all better to downsize the rudder(s) and it's associated loads.
JT
--- In
harryproa@yahoogrou ps.com.au, Jesse Deupree <jdeupree@...> wrote:
>
>
> Generally a lurker here (just interested in proas) but I am an experienced sailor. Symmetrical sectioned rudders and foils in general generate lift simply by angle of attack (what you feel if you hold your hand flat out the window of a speeding car and then tilt it a little). Drag increases but lift increases more up to a given angle of attack, so it is beneficial.
>
> It took me years to understand that what I was taught was sideslip when sailing, and was taught it was bad, was actually a positive angle of attack by my keel/centerboard/ hull and was good. Same for a small amount of weather helm. The hull itself will also have a positive lift/drag ratio at small angles of attack, so generally when you are sailing you will want a
little "sideslip" and weather helm on. The degrees talked about are usually in the 2-4 degree range. Usually the optimal angle of attack for the boat will be a compromise between the best lift/drag angles of the hull and each foil.
>
> There has been a certain amount of experimentation with gybing boards and trim tabs on both keel/daggerboards and rudders- all attempts to create asymmetrical foils that will work on both tacks. Many modern racing monohulls that use a canting keel for righting moment use twin asymmetrical daggerboards for lift. Asymmetric daggerboards in the floats of multihulls are also common.
>
> There is no question that one benefit of the proa concept is that it could easily have asymmetrical foils, assuming they pivot around the leading edge when shunting. Shapes would have to be compromised for turning ability, but are much more easily built and used on boats that are asymmetric side to side than boats
that are asymmetric bow to stern.
>
> Jesse Deupree
> F-31 SORN
> Portland Maine
>