Rick and Rob,
I'm wondering about your thoughts on how much ventilation was happening, and its significance, when Rare Bird was heading out of Moreton Bay at 16 knots.
Cheers,
Col
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Ben
> I think Rob is lifting the leading rudder. If so then the hull is
> doing most of the leeway prevention. So the figures I gave for the
> hull preventing leeway are valid. Even if the rudders can be offset
> then they will still not be as good as the centreboard because they
> are ventilating.
>
> Any foil that is ventilating does not work as efficiently as a non-
> ventilating foil. The latter needs to be about 3 times the area to
> do the job to produce the same lift. If they are generating lift
> then they will be ventilating - there will be a sheet of air down the
> suction side.
>
> Structurally the forces involved with a centreboard are lower than a
> cantilevered rudder mounted well above the water. The downside is
> that on large boats the centreboard case will impinge on
> accommodation although with hard chine hulls it could be offset from
> the centreline.
>
> If the centreboard is preventing leeway it will have same lateral
> force as the healing force on the sail plus any side windage from the
> hulls. A pair of ventilating rudders will need to be massive to
> achieve this. So even in a small boat the moment to be countered by
> the rudder bearings is not a lot less than what the rotating mast
> bearings have to handle. Overall I would be surprised if two leeway
> preventing rudders with bearings mounted on cross beam will be
> lighter than a centrecase inside a tower as I have shown. The load
> path between the wing bearing and centreboard case is more direct
> than mast bearing to beam rudders.
>
> Rick
> On 14/05/2011, at 4:14 AM, bjarthur123 wrote:
>
> > rick, could you please compare here more quantitatively the case of
> > two large rudders vs. one big leeboard and a small rudder? you give
> > numbers for the latter, but not the former.
> >
> > in rob's typical designs they ARE tweaked independently to prevent
> > leeway. at least when going upwind. yes, they break the surface of
> > the water and so could ventilate, but wouldn't necessarily do so
> > unless the AOA were large. so yes they need to be large to keep the
> > AOA small, which would create more drag. but how much more?
> > specific numbers would be great.
> >
> > the tradeoff is that two big beam-mounted rudders are lighter (b/c
> > the hull doesn't need reinforcing etc). lighter boat is faster (and
> > cheaper). but does the lighter weight compensate for them being
> > less hydrodynamically efficient? hard to tell without concrete
> > numbers.
> >
> > rob-- do you have specific numbers on how much lighter?
> >
> > ben
> >
> > > The biggest benefit comes from the advantage of the high L/D
> > possible
> > > with a centreboard under the hull compared with the much lower L/D
> > > that a slender hull can produce.
> > >
> > > For example taking the case of the design I have shown in 15kt wind
> > > at 60 degrees true the boat is capable of flying the ww hull but,
> > > assuming the wing is backed off to keep the ww hull on the water,
> > > forces are:
> > > Wing thrust is 626N
> > > Wing healing force 1255N
> > > Centreboard drag to balance this is 43N so L/D for the board is
> > > almost 30
> > >
> > > In the case of two large rudders, unless the twin rudders are
> > > cambered or they are set up to be independently tweaked to balance
> > > leeway the hull becomes the leeway preventer. I expect leeway will
> > > be as high as 6 degrees for a shallow long slender hull even with
> > its
> > > hard chine. In this case the L/D is 10 so the extra hull drag
> > > associated with resisting leeway is 125N. This is 3 times higher
> > > than the cambered centreboard and an extra 13% drag on the 626N
> > total
> > > drag.
> > >
> > > The case for tweaking the twin rudders independently does not help
> > > much if they are ventilating because they have to be very large and
> > > draggy to generate the required lift.
> > >
> > > In practice the above may not be as significant as the more stable
> > > control provided by a non-ventilating centre board compared with
> > > ventilated rudders with varying degree of immersion.
> >
> >
>
> Rick Willoughby
> rickwill@...
> 03 9796 2415
> 0419 104 821
>