You could also go with two minikeels set on the leeward bilge. This would help bring the COleeward resistance back a bit.
--- In harryproa@yahoogroups.com.au, Gardner Pomper <gardner@...> wrote:
>
> I am wondering if there is a way to predict the effect of a mini-keel. In my
> sidecar clone I am designing, my lw hull has excess bouyancy, so the draft
> will generally be around 6" when I am not really pressing the boat hard. I
> am willing to have up to a 2' draft, so I could potentially stick an 18"
> mini-keel under it, if it would help the rudders.
>
> I could go 2 ways with that mini-keel; I could have it a high aspect ratio,
> and have it only 6" wide, or I could go for a decent surface area, and have
> it 3 feet wide. The smaller one would have less drag, and would break off
> easier if hit, but would it actually help?
>
> - Gardner
>
> On Fri, Jun 18, 2010 at 4:40 AM, Arto Hakkarainen <ahakkara@...>wrote:
>
> >
> >
> > The balance, more managable size foils and balance are all issues that
> > support boards in addition to rudders. I guess we will hear more after this
> > summer how BD and ONO sail with added foils.
> >
> > Arto
> >
> > --- On *Fri, 6/18/10, JT <jtaylor412@...>* wrote:
> >
> >
> > From: JT <jtaylor412@...>
> >
> > Subject: [harryproa] Re: Rudder lift?
> > To: harryproa@yahoogroups.com.au
> > Date: Friday, June 18, 2010, 5:07 AM
> >
> >
> >
> > 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@yahoogroups.com.au<http://us.mc366.mail.yahoo.com/mc/compose?to=harryproa%40yahoogroups.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
> > >
> >
> >
> >
> >
>