## How to calculate Wally

The Wally is a technique for gaining advantage of wind shifts when you can’t or shouldn’t tack **(read the overview)**. And yes, there are times and conditions where you shouldn’t tack **(see Break-even tacking)**.

In this exercise we will use the Ockam37 polar as an example. If you would like to follow along, you can download it from **here**.

### Vmc Angle

The Wally function uses the Ockam polar function VmcAngle which gives the angle of **maximum Vmc** at any true wind angle. Upwind and downwind targets are merely special cases of the VmcAngle at 0° and 180° true wind angle respectively.

As you can see from the diagram, for a 10° shift, the **fast-wally** will be the max Vmc at a true wind angle 10° greater than normal, while the **slow-wally** is at a true wind angle 10° less than normal.

### Calculating the Wally in 12 knots true

Now, using the Ockam37 at 12 knots we will calculate the Wallys.

- Find the normal target angle and speed
- VmcAngle(12 kt, 0°) is 39.3°. This is the normal upwind target angle in 12 knots.
- Boatspeed(12 kt, 39.3°) is 7.434 knots. This is the normal upwind target speed in 12 knots.
- Vmg is 7.434*Cos(39.3°) or 5.752 knots.

- Find the fast-wally (at +10° VmcAngle).
- VmcAngle(12 kt, +10°) is 31.1°. True wind angle is base angle + VmcAngle, or 41.1°.
- Boatspeed(12 kt, 41.1°) is 7.603 knots.
- Fast-wally is therefore (7.603 – 7.434) or +0.169 knots.
- If you weren’t wallying, your Vmg up the average wind would be 7.434*Cos(29.3°) or 6.483 knots.
- By walling, your Vmg up the average wind is 7.603*Cos(31.1°) or 6.510 knots; +0.027 knots. That doesn’t sound like much, but you are also
**lateral separation pumping**.

- Find the slow-wally (at -10° VmcAngle).
- VmcAngle(12 kt, -10°) is 39.6°. True wind angle is base angle + VmcAngle, or 29.6°.
- Boatspeed(12 kt, 29.6°) is 6.525 knots.
- Slow-wally is therefore (6.525 – 7.434) or -0.909 knots.
- If you weren’t wallying, your Vmg up the average wind would be 7.434*Cos(49.6°) or 4.818 knots.
- By walling, your Vmg up the average wind is 6.525*Cos(39.6°) or 5.028 knots; +0.210 knots (+4.3% or 35 seconds/mile). Yes. Slow-wally is very effective.
- For some boats, going a knot slow is a challenge. But, if you can handle it, you will be gaining a lot. And the
**Lateral Separation pumping**at -9.7° true wind angle difference will be quite rewarding. Otherwise pinch as much as you can get away with.

### The Wally plot

The complete Wally graph for the Ockam37 looks like this.

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