Two Way Foils
Hello Proanauts,
I built a 16’ Wa’apa which has been paddled, but not yet sailed. My thoughts on rigs and foils are as scattered and varied as the many design concepts I’ve been drawing on this forum.
I was going to set it up as a tacking outrigger with a leeboard and a Dierking/Storer bungee and half a box rudder set up—like this http://proafile.com/forums/viewthread/167/
But I have too many questions about proa foils and rigs for that. So I’m going to set it up as a proa with bungee and box rudder/foils that are side hinged like James Brett (and many others) have tried—about 1/3 of the way back. The front foil would have a tiller lock. CLR would be adjusted by playing with both foils.
So here are some questions for the collective proa consciousness:
What should the cross sectional shape be? Symetrical in both directions? Flat on the lee side? How thick? How long? Is it just an arc? Is there a two way NACA section?
Folks who have tried GIS type rudders; is it really just a rectangular box with the foil in it? No form hugging shape? Any bearing materials?
Folks who have tried side hinged foils; what bad habits have you encountered? James Brett says his balance nicely.
Thanks for your help.
Chris
oh, two more things:
Spacing fore and aft; on madness the pivot points of the rudders are about 55% of the distance from the midpoint to each bow. The new Bieker/Brown proa has them closer to the bows. Thoughts? How close can they be, assuming that the CE of the rig is behind the midpoint of the Vaka.
Distance from the hull; I’ve drawn my prototype with about 30 degrees of travel before the back of the rudder hits the hull. That’s my seat of the pants guess about how much rudder can be used at speed without stalling. Thoughts?
....thanks
On the subject of rudder/foil placement, this sounds like a different animal than Madness which uses a daggerboard. It sounds like you don’t plan to. That makes rudder/foil placement less critical in your case, but putting them out more toward the ends of the vaka will give more authority to them. This plays into where you have your sail CE of course; you might end up with some weather helm if you have CE aft of the vaka’s midpoint, much like madness would have more weather helm if the daggerboard was moved toward the current bow.
On the Mi6 proa (http://www.users.on.net/~malcolmandjane/mi6.html) we placed the rudders at 50% midship to bow. Wider spacing is better for turning moment and hence the rudder area can be reduced, but if you go too far there can be issues with the forward rudder ventilating when pitching in waves.
I don’t think the section shape is too critical if you are reasonably generous with the rudder area to keep the loading low. We used a simple symmetrical circular arc section with a small (<2mm) radius on the leading and trailing edges. We used a symmetrical section because our boat was designed to sail with with the wind from either side, but for a boat sailing with the wind substantially from one side, an asymmetrical section (arc on one side, flat on the other) may be better. More complex sections such as Tom Spears’ section would only be required if you really want to optimise it for racing.
What should the cross sectional shape be? Symetrical in both directions? Flat on the lee side? How thick? How long? Is it just an arc? Is there a two way NACA section?
Folks who have tried GIS type rudders; is it really just a rectangular box with the foil in it? No form hugging shape? Any bearing materials?
Folks who have tried side hinged foils; what bad habits have you encountered? James Brett says his balance nicely.
Tom Speers proa sections are hard to argue with, they have worked well for several people. Rick Willoughby, on the Harryproa site, has made a reasoned case for a NACA 7 series foil, fwd section mirrored about its midpoint, it’s so close to an ellipse that it is what I’m using on the Bionic Broomstick (the ellipse that is).
Mik Storers GIS rudders are a special design that is slab sided for close to half its chord which works well in a rectangular case, particularly when the back end is open and several wraps of bungie act as kick up insurance. The Speer section rudders on P52 had cases molded over front half of section then held in place with bungies, a little stiff to lift and lower but they worked and popped up when necessary.
My biggest problem with side mounted rudders was picking up weed (and picking up weed…...) I never had trouble with ventilation but my rudders were canted forward about 5 degrees. Did have some ventilation thru the hole in the rudder for the pulley/lift line when the rudder was fully down and loaded up making a course correction.
Cheers,
Skip
thoughts on 2 way foils for proas .....
lift to drag is always better .... on an asymetrical section
seen by the fact of having already a lift at zero angle of attack !
so it seems right to > make the pressure side flat or nearly so
and the wind ward side having the most bend ....
>>> at the forward section > the center section being nearly flat
this would look like to be producing more drag on the trailing part
but tests have shown other wise >
most likely ... because at that stage of the flow
there is not much happening any ways on both sides of the section
seen at the central point of pressure at any foil !
being so far forward of the physical center
a generous rounding of the entrance > ist very important
because the flow has to make it around there well
to having it tightly adhere to the surface following !
or it will stall out at pressure spikes >
such as a breaker or gust hitting the boat .....
pure steering rudders having an entirely different function
so ... they would seem to be more of the regularly seen once !
turning 360°all around > at the side of the hull for protection .....
as seen on the >>> proadesign vector fin proas
the forward one >just an emergency device > generally running free >
where as .... the rear rudder just used for minor adjustments !
and at high speed down wind surfing > when a small ruder is effective enough !
because the main stearing is done by the boat it self
having the lateral area so far to wind ward >
where it litterally is hanging like a pendilum .......
adjusting itself surely !
finnaly .....
if you need volume on a asymetrical lateral area >
for more displacement > to float by itself ......
such as the vector fins and especially the hinged vector fins
then maybe a compromise of having a little bit of a curve
simular to the wind ward side of such !
for more technical information > see >