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Lee Bow Effect

The “Lee bow effect”: How to think about True wind and current

True wind and currentMost people think that when sailing in current there is a “lee bow effect”; current “striking” the leeward side helps while from the windward side hinders performance. There is also the theory that current “messes up” true wind direction. This is caused by using earth and water references in the wrong context.

Let’s say you’re sailing along in a nice light 5 knot breeze. Everything is fine; your speed is what you expect, you can tack and jibe with no issues, your boatspeed is the same on both tacks and the polars agree with your boatspeed. The world is good.

You’re on port tack doing 4.5 knots just like the polars say you should, when you glance at the GPS and your world falls apart.

Your GPS is reporting a speed of 8.78 knots! Huh? You tack onto starboard. Yep, boatspeed is still 4.5 knots, and the GPS still says 8.78 knots. What’s going on! So you stop the boat. Sails-a-flapping in 5 knots, and boatspeed is zero; that piece of kelp is just sitting there next to you. And the GPS says you’re going 5 knots! What the ‘heck’!

And there’s more. If you had looked at GPS course (over ground, COG), you would have noticed that COG was 20° to the left of compass reading on port and 20° right on starboard. That GPS speed that knocked you for a loop is speed over ground (SOG).

OK, we have posed an unlikely scenario where the water is moving at 5 knots (we all call this current), and there is zero earth wind. That wind you feel is due to your boat being shoved into the still earth wind at 5 knots by the current. This may be an extreme case, but it illustrates how current can affect sailing. In this scenario, you can sail upwind at 4.5 knots in zero earth wind!

The point is that your boat moves relative to the water only when there is a speed difference between water and air (‘water wind’). The boat has no concept of current; after all it can’t read a GPS and there is nothing on the boat that feels the water moving relative to the land.

The performance perspective

In order to monitor boat performance, you need to monitor boatspeed and true wind relative to the water, because that’s the boat’s world. If your job is making the boat go fast, you should not be using anything with current in it.

From the performance perspective, the lee bow effect is wrong. The boat follows your polars properly relative to the water. Current doesn’t ‘hit the boat’ from one side or the other, it carries it along. The “lee bow effect” is the result of mixing ground and water references when thinking about boat performance.

The navigation perspective

Navigators look at how to best get around all those things attached to land like the course marks, laylines and obstructions. The actual navigational effect of current is where you do see lee bow effect. That 8.47 knots was your speed over ground (SOG) and it was real. It is the vector sum of your 4.5 knots boatspeed (through the water) and the 5 knots of current. Whenever you need to do calculations involving stuff on land (marks, laylines and headings for instance), you need to include current.

In summary

  • From the ground wind perspective (the reference for weather forecasts and GRIBs)
    • Ground wind direction and speed does not change with current
    • Polar performance changes with current
  • From the water wind perspective (the reference for polars)
    • Water wind direction and speed changes with current
    • Polar performance does not change with current

Rules for true wind with current

  • Use the water wind perspective for performance
    • Boatspeed is predictable and the same on both tacks (except for wind shear).
    • Polars will tell you whether you are performing up to snuff.
    • Current is irrelevant because the boat only knows about the water and not the land.
    • Wind direction changes if the current changes. Its just another wind shift, no different from any other wind change.
    • Anything referencing marks or the land must be adjusted for current. This includes headings and laylines.
  • Use the ground wind perspective for navigation and routing
    • Marks stay put.
    • Speed and heading will be different from polars on all points of sail
    • Boatspeed/heading needs to be corrected for current. Or you could just use COG/SOG.
    • Water wind needs to be converted to earth wind if you are using GRIBs for route planning.

Sailing performance is vital for winning races. In order to monitor performance, you need water wind.

Tactical decisions involving marks require ground wind and ground boatspeed and heading.

How current affects instrument choices

GPS based systems

  • Your “boatspeed” is land referenced, not water referenced.
  • Water speed is required to calculate current. Therefore GPS systems cannot provide current.
  • If you don’t know the current, you could be in unfavorable current and not know it.
  • Performance monitoring is difficult or impossible because your “boatspeed” varies with current.
  • Boatspeed will differ tack-to-tack, so getting into the groove quickly is hampered.

Wind information

  • GPS systems provide SOG, which is problematic for calculating water wind.
  • Water wind is required to obtain your polar speed. No water wind, no performance monitoring.
  • Wind shifts can produce more gain than performance monitoring (see Tacking). Water wind is needed to monitor wind shifts. No water wind, no rational tacking on shifts.

Ockam instruments

  • Provides the best water wind solution.
  • Earth wind output is simultaneously available.
  • Provides real-time current readout.
  • Laylines are corrected for current.
  • Live polar displays provide accurate performance monitoring.
  • Wind statistics provide information to help you decide whether you should tack on shifts (see Break-even tacking).

 

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