Boat Wind Shadow Illustrated

This article illustrates boat wind shadow as modelled by the Windmaster networked wind system used for sailboats in Second Life.

In the following diagrams, the wind is coming from due west (from the left edge). The red lines form a grid of 10-metre squares to indicate scale. Each set of three images show the boat’s shadow when it is on a different point of sail – close-hauled, beam reach, and a dead run.

The shadow areas are mapped to a resolution of 1 metre, and each 1m square is represented by 6 pixels in the images. Although this results in a slightly blocky appearance, the Windmaster calculation engine actually uses floating point values for positioning – it can easily work to a resolution of 1mm.


Nemo with small sail

The boat is close-hauled. At this point of sail, the wind ‘sees’ a relatively small sail area, so the shadow is quite weak.


On a beam reach, the apparent sail area presented to the wind is larger. Note that the middle of the shadow area is a little darker – the shadow is now stronger.


On a dead run. The boat is now presenting the largest possible apparent sail area to the wind. The middle part of the shadow area is now quite dark. A boat which was directly downwind of this one, and less than about 8m away, would experience significant shadow.


Nemo with large sail

Close-hauled. The large sail on the Nemo creates a bigger shadow area than the small sail. The shadow extends nearly 25m downwind.


Beam Reach.


On a dead run.


Converted Rene Marine RM20

This shows the shadow model for a Rene Marine RM20 which has been converted to use Global Wind. The RM20 is a lot bigger than a Nemo, and the effect of this on the shadow area is clearly shown here, where it is close-hauled. The shadow area extends beyond 50m, and is also much wider at just over 20m.


On a broad reach. Again, the middle of the shadow area becomes darker as the apparent sail area increases.


On a dead run. There is now a very large heavy shadow area in the middle. A Nemo sailing downwind here would experience a severe loss of wind speed.










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