Ben
The peak negative Cp on the lifting side of a NACA0012 at 5 degrees AoA is 2. So at 15kts the negative pressure produced in a non ventilating situation is 56kPa. So unless the rudder is more than 5.6m deep there will be air sucked all the way down the leading edge. The sheet of air will cover a good proportion of the lifting face for a 1m deep rudder so most of the lifting face is ineffective.
At 5kts the negative pressure is only 6kPa so a rudder more than 0.6m deep will be working effectively below that depth.
The degree of ventilation is strongly related to speed and there is also a relationship with depth of immersion.
If a rudder is fully ventilating it becomes the same as a planing surface with air on one side and water on the other. It is a water deflector rather than a lifting surface. It is still effective but efficiency is down on a non ventilating foil. That is why it needs to have a large area to do the job.
I only have thoughts on how the rudders would be mounted - no detail. The most likely would be to mount them in wells that go from hull bottom to deck top. The wells would be tapered outward at the top longitudinally and laterally so the wedge block that carries the shaft frees as soon as it lifts. The rudders are only 0.12m long and 0.400m deep. The wells would be made large enough for the rudders to drop through. The rudders are bi-directional and non-cambered with shaft on the middle of the section. If they did hit something that bent the shaft the upper trailing edge would only strike the wedge block not the hull.
The wells are not hull penetrations. They are integral with the hull the same as a centreboard case on a dingy or trailerable yacht.
I would not get fancy with the wedge block design until I had destroyed a few rudders, If that happened I might consider a wedge block with a cast polyurethane block inside a stiffer shell. This would allow some initial delfection under impact load but it might also cause vibration problems.
One key aspect here is that the rudders intended for non-ventilating operation will be a fraction of the size of a ventilating rudder. If you shift the function of lift production from the rudders to a non-ventilating centreboard then the rudders can be even smaller. The reason they are the size I have them is to get the required strength. Also my centreboard is 0.6m deep. It could be smaller if it were not for strength considerations.
Rick
On 15/05/2011, at 2:32 AM, bjarthur123 wrote:
rick,
i'm a bit confused about ventilation. my understanding is that it is essentially nonexistent at the small lifts normally used to counter leeway and to keep the boat on course. many many boats have transom hung rudders. must such rudders really be 3x as big?
thanks for all the other tidbits. flat-bottomed V-decked bows make a lot of sense, though i'd still like a way to move weight aft. the load path from the mast to the beams also makes sense, though that's less of an issue with a schooner. structural forces on a cantilevered rudder-- with you there too.
rob does indeed raise the front rudder, but only when off the wind on boats w/o a balestron rig. i'd be curious for more details on how your rudders retracted and dealt with collisions. no holes below the waterline is a big plus for me in rob's designs.
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 Willoughby
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