Daily Archives: August 27, 2010

Same Boat, Same Shadow, New Graphic!

Two days ago I posted an article here about wind shadow for Trudeau One. ONE is the first serious race boat from Trudeau Yachts since the legendary  J-Class, and ONE’s shadow engine is a truly powerful, tactical weapon. I think any serious sail racer owes this feature considerable thought and attention.
(But I’ve been wrong before.) 🙂

As I mentioned in the last article, I wasn’t too happy with the graphic displays  I came up with for shadow effects, and was still working on it. Today I went back and repeated all the measurements, trying to come up with a simpler graphic that might illustrate the effects but still be useful for race skippers.

Here’s that second attempt below; this time I ran around measuring the shadow intensity leeward of the ‘wind-blocking boat,‘ trying to define the borders of the “WS>20%” blanket zone.

The blue coordinates below show the points where WS=.20 (in other words, a ‘20% windshadow effect’). That new data matches rather nicely with what I posted two days ago.

Here’s the Bottom Line: A boat in the ‘orange bubble zone’ on the below graph will fall into a substantial shadow effect with a serious drop in wind impulse power. At the edges of the Orange bubble the effect is 20%… but central to the bubble and closer to the windward vessel the shadow strength goes to 100%.

In my hands, in this boat there is no noticeable turbulence effects on the Windward side of the shadow vessel; it’s all about Downwind Shadow, and that influence projects a strong, realistically-sized blanket on leeward boats within several mast-height distances from the ONE root prim.

The graphic below also shows the two features I emphasized previously:

  • There is a relatively small but very definite bias for stronger shadow effects aft of the shadowing boat. A boat approaching the windward shadow-caster from behind will get hit sooner with a heavier hammer than one on an opposing tack that passes from the front.
  • Since the shadow script radiates from the root prim, the effective area is shaped like a funnel… or actually a cartoon chat bubble :-). In other words, the Shadow Zone is very narrow immediately next to the boat, but then expands out fairly quickly.
  • A lee boat that that passes very close to the shadowing vessel  [2-4 min horizontal separation) therefore may only see a shadow effect for 5-6m of linear distance while passing. The shadow will be maximal for that short spurt, but if a crew has enough forward momentum, they might well be able to glide through that segment and find clean air on the other side.
  • What’s that line from Winston Churchill? “If you’re going through hell… JUST KEEP GOING.

It’s worth commenting briefly on three more points:

  • Watch out for sim edges. As Alain pointed out yesterday, shadow doesn’t transmit well over sim boundaries. You might play those boundaries  to tactical advantage.
  • For two boats traveling  perfectly parallel on a beam reach, the shadow effect is 100% with boat separations of 2.0 m – 14 m (beyond that, the shadow effect decays progressively with distance; it is minimal around 50m separation).
    This means that a boat passing leeward with 14 m separation sees the same maximum shadow as a boat passing with a scant 2.0 m separation.
  • There is, however, no advantage to giving the Windward boat extra room; the shadow effect will be just as bad a boat length away. I’m bringing this up here to illustrate that ‘funnel shape’ I talked about before.
    With a small 2.0-4.0 m interboat separation, the shadow effect on the passing boat has a very short geographic range. In contrast, if you “stay a safe 12 m away from the windward boat,” the actual intensity of shadow is the same, but the linear effective range while passing is far, far greater.
    Passing a Trudeau ONE on a course set 2m leeward to the shadowing boat is an interesting tactical option in my opinion… a gutsy crew might be able to pull that one off and grab the lead.
  • However, if you trying that same leeward maneuver with a larger 14 m interboat separation, the ‘course distance” you’ll spend under strong shadow is far greater. In my opinion, passing parallel and leeward at 14 m isn’t ‘tactics…‘ it’s suicide. 🙂
  • Hey! You try it!

    And please tell me if the new graphic makes more sense to show effective shadowing than the last one I posted!