Wind shadow is a powerful tactical weapon in sailboat racing. A skipper that successfully maneuvers between the wind and an opponent can effectively ‘blanket’ the downwind boat by stealing the air.
In real life, the magnitude and extent of the shadow a boat can cast depends on a number of variables, but a shadow’s effective range is usually three-to-five times the mast height.
How does this translate in SL Sailing? What’s the shadow magnitude and how wide is the blanket? It’s hard to get accurate, practically useful numbers for different boats, so this past week I’ve been playing around with the 1.12 upgrade for Trudeau ONE, collecting data on the wind shadow effect. (Huge thanks to Bunnie Mills and Chaos Mandelbrot for their help with this).
I thought some race-aholics might want to see the numbers!
The image above shows one easy way to collect shadow data. Two boats catch racewind (in this case 9.0 m/s from compass North, no variance). They drop sails and sit stationary in the water, with the windward boat on a beam reach heading. The leeward, ‘shadowed’ boat then moves to a series of stationary downwind coordinates to measure the intensity of the shadow and check the resulting effect on wind speed.
Since all the measurements are for ‘stationary’ positions, there is no lag effect and the findings are easy to reproduce.
The HUDs for several Trudeau boats give numerical readouts for shadow intensity; they indicate the fractional WS effect from 0.00 to 1.00. On the charts posted here I’ll express those numbers as a percentage of the max Shadow effect. With the windsetter blowing at 9.0 m/s, the chart above shows that the boat’s wind is inversely related to the percentage of WS effect. With WS= 100%, the wind powering the boat is cut from 9.0 to 3.6 m/s, a 71% shadow-reduction.
That’s pretty consistent with other boats in the Trudeau fleet, going all the way back to when wind shadow was introduced to the Trudeau line in the TWENTY.
The curve above shows the drop-off of shadow intensity with increasing distance between two boats. To get these measurements, the boats were overlapped and the shadowed boat was progressively moved further leeward. The image above shows an example of the setup, with a separation distance of 50 meters.
The resulting curve shows that T-ONE throws a maximum intensity blanket that extends over one boat length leeward (14m). At 20m separation, the shadow effect drops to 75%; it then continues to decline with increasing distance but is still present at 50m.
These findings seem well-consistent with the real-life predictions for the boat, and are confirmed by the wind speed changes on the shadowed boat with increasing lee distance, as shown above.
I admit I haven’t looked at Trudeau Shadow very closely for many months… but the numbers I got this past week are pretty encouraging. Two years ago a very large fleet of beta testers played ‘sailboat sandbox‘ with Trudeau TWENTY, trying to come up with just-right shadow parameters. When the boat launched, I was convinced a windward skipper could use shadow to keep any lee challenger at bay… but only if they were experts at it and knew what they were doing.
Conversely, I was pretty sure an excellent leeward skipper had all the tools needed to trash the best laid plans of some overconfident, windward Shadow-Maven. 🙂
(HEY, just like REAL-LIFE!)
Trudeau ONE fits right in that tradition, but with far more intelligence and subtlety. 🙂
Passing Data
OKOKOK… I know the above numbers-and-graphs stuff is boring, and for most sailors it’s all probably pretty obvious, anyway. But…
I’ve been trying to think of a way to represent the racing-important data in a way that would provide the real numbers but might also make quick sense to a skipper who wants to know the best way to beat a shadowing boat. I’m trying to figure out a way to say “Please don’t believe me, look at the real numbers, come up with your own tactics.”
I’m still working on it, but here’s what I think.
The chart below is a 2D-grid. It shows red lines that indicate shadow measurements for a boat trying to pass a shadowing boat on a course set to leeward separations of 2, 7, 10, 20, 30, 40 and 50 meters. This is what happens in real life; you want to pass that boat; you need to know when the shadow kicks in, and how to beat it.
(Conversely, if you are the windward shadow-commander, you need to know the best way to put an iron lock on that upstart pretender trying to sneak up on you….)
The 'bow' of the red boat rectangle is facing Left above.
Take a look at the 20m line above. It gives you the percent WS effect as you approach on a parallel course 20m leeward of the shadow boat.
When you are 25m away from ‘overlap’ (roughly a 50° angle from the shadowing boat), an 8% wind shadow kicks in. 
Don’t kid yourself, 8% is a lot, and in the next second when you hit 45° on that 20m course, it’s going to double to 15%. You can read the numbers above, and see the distance you’re going to need to cover to get past that Windward hammer. You got the momentum for that big jump? 🙂
I’ve looked at Trudeau wind shadow for two years, but I learned a few new things this week by collecting the numbers above. Maybe I’m slow, but perchance you missed them too. 🙂 Here are the two big items that might help win a race:
- I’m not sure why, but it looks very consistent and very clear that Trudeau Shadow angles backwards given the setup I described above. A boat traveling a parallel, 20m leeward course will see 15% shadow when they are 20m aft of ‘mast abeam.’ When they are 20m ahead, they only get 3% shadow. Factor that in, someplace… 🙂
- Trudeau shadow radiates downwind as a cone, and it’s extremely narrow right next to the windward boat. With a minimal, 2m boat separation the shadow only affects the passing boat for 5 meters (it’s pretty much a 100% sledgehammer there, though)! A lee boat with mega-guts and deep-seated momentum should be able to cross its fingers, close its eyes and blast past by snuggling very close to the shadow-boat while holding its breath. (GRIN!! Good Luck on That, You Try it First! Let Me Know!)
OMG; that’s enough numbers for today! Go sailing!
Metaverse Sailing 