Ben
I produce the polars in Excel. Hulls are modeled in Delftship then exported to Michlet for drag analysis. (I use Flotilla for trim). I build relationships using various regressions from both Michlet and Delftship to estimate continuous drag data for different loads on the hull. This reduces the number of discrete displacements I analyse with Michlet.
The sails and foils are modeled in JavaFoil and I build regressions from discrete points to get continuous data. Mostly I am interested in finding the maximum lift coefficient that any particular rig can produce.
The windage is based on drag coefficients estimated from the shape. There is published data for various geometric shapes.
All the forces are then summed using the vector relationships. For each wind speed and true wind angle I determine the boat speed where boat drag just exceeds the sail drive through an iterative process. This particular proa model assumes there is no leeway and the water foils are adjusted to prevent leeway providing they produce enough force without stall. The sails are automatically depowered to prevent capsize if needed.
Essentially the model is all analytical with use of regression to fill the holes. If I want to narrow in on a particular condition I can redo the Michlet data for that condition to improve accuracy.
It is not too hard to convert to wrong way polars as I only need to swap the input data for the ww and lw hulls. I will need to alter the position of the mast and rudders but I already have allowance for this so I can play with different positions. There should be some fundamental difference because the shape of the two hulls is markedly different.
The Flotilla data I have used to check trim has not been used in the VPP. It is essentially a check that the hull will not bury. With the 18m proa I have actual observations up to 9 knots to date.
I do allow for planing in the VPP if the hulls are capable of planing. My analysis for the lift coefficient is based on empirical work from NACA. I also have Leo Lazauskas's Lifting Surface Program but the NACA formula aligns better with Savitsky and my own observations on the pedal boats. (Have now got top speed of 20kph on my latest pedal boat:
The biomechanical modeling I do for my interaction with the drive train on these boats is a good deal more complex than the VPP for a proa.
Rick
On 10/01/2012, at 3:08 AM, bjarthur123 wrote:
thanks mike. very clear.
this is the first i've heard of light-wind steering problems. do any of the visionarrys lose control like this? is there something different about the shape of the 18m hulls or rudders that can explain?
rick: i'm very interested in the wrong-way polars. what software do you use to make them?
neutral helm seems very slow to me, as the rudders then don't counteract leeway. with independent rudders, one can use the aft one to counter weather helm, and the forward for lee helm. either way shifts the sideslip drag from the low-aspect inefficient hull to a high-aspect foil.
ben
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
>
> Ben,
>
> The rounding-up issue, particularly on a rig with one sail, would stem
> from two different forces:
>
> 1) The weather helm caused by the COE of the rig being aft of the
> CLR created by the rudders/hulls.
>
> 2) The weather helm caused drag of the windward hull.
>
> In really light wind, particularly with rudders that aren't that deep,
> these two forces added together could require enough helm to nearly
> stall the boat. There just isn't enough flow over the foils to let them
> do their job without angles large enough to create a stall.
>
> But if you put the leeward hull to windward, the ww hull will try to
> twist the boat off the wind, creating a lee helm, which will counter the
> weather helm caused by the rig. As a result, the rudders could be
> almost at neutral, which could be quite significant at low speeds where
> flow over the rudders isn't that great.
>
> I never would have thought of this on my own, and when Rick first
> described the effect, it didn't make sense. But as Todd is fond of
> saying, experience is worth at least as much as theory.
>
> It looks like this is a nice way to balance things out at low
> speeds. //
>
> - Mike