Bjorn
You can check for cavitation by extracting the peak Cp for a given AoA then use that to calculate pressure - P= 1023/2 * Cp * V^2. If P exceeds 100kPa then it will cavitate. The 16 series NACA sections have the lowest Cp for a given Cl and blade thickness.
If the software you are using produces accurate results it will show if there is flow separation on the blade because this will be a factor in determining friction and lift. A surface piercing blade will sheet air on the low pressure side if there is flow separation. Once that happens the lift drops to about 30% of what it would be with attached flow.
For most speeds of interest on a HP, a surface piercing blade will behave as if end plated at the surface. So you can double the physical AR.. A surface piercing blade is actually more efficient than a blade under a round hull because the gap that opens up between the blade and hull allows vortex shedding. We could see them as surface eddies trailing behind both rudders. The aim of the top plate was to keep the gap closed till the rudder turned to about 20 degrees. They made steering more predictable and the loss of efficiency with the lower aspect was not as bad as it would have been without the top plate, not sure if the photo will post. Rudders have span of 600 and chord of 500. From memory, they are NACA015 sections so cavitation is a possibility below 20kts.
Flow separation is the most demanding limitation on a surface piercing blade. Cavitation of the blade should not be a problem below 20kts unless you have a thick section.
A HP balances out quite well as speed picks up and the load is taken off the windward hull. That reduces the tendency for the drag on the windward hull to produce weather helm so steering force is reduced. Steering response is very good above 8kts.
Rick
On 22 Mar 2019, at 5:15 am, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:In hindsight, I think simulating a rudder aspect ratio of 5:1 was maybe too much. A rudder might have 3:1? And if I've understood Rick right, the rudders are 1:1 or 2:1 on the 18m boat he's involved in. But then the rudders have endplates in one end (the hull) on that boat, so the effective span should be double that.Anyway, a lower aspect ratio would only further increase the induced drag, so the profile differences would be even smaller.Another thing to mentions is that I haven't taken cavitation or suction of air from the surface (like mentioned in the emails) into consideration, simply because I don't know how to.Lets me know what you think about this analysis!/Björn
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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