Mastering the Art of Navigation: How to Use a Watch Slide Rule
In the world of navigation, precision and accuracy are paramount. Whether you're a seasoned pilot, a maritime enthusiast, or an outdoor adventurer, having the right tools at your disposal is essential for safe and successful journeys. One such tool is the slide rule, a versatile instrument found on some of our aviation watches designed for navigation. In this blog, we will explore how to use a rotary slide rule and unlock its potential for a variety of calculations.
What Is a Slide Rule?
A slide rule is a circular scale that either surrounds the watch face as a bezel or is located on the inner flange. It can be rotated in either direction, allowing you to perform various calculations related to time, speed, distance, and conversions. They are a valuable aid for performing quick calculations on the fly.
Understanding the Scale
Before you can make the most of a slide rule, it's essential to understand its scale. The slide rule typically consists of two circular scales, an outer and an inner scale, each marked with a series of numbers, usually from 1 to 60. The numbers on the outer scale typically represent time, while the inner scale may represent various units like miles, kilometres, or nautical miles.
Using a Slide Rule for Calculations
 Unit Conversion:
One of the most straightforward uses of a rotary slide rule is unit conversion. If you need to convert between different units, such as miles to kilometres or gallons to litres, the slide rule can be a handy tool. Here's how to do it:
 Identify the conversion factor you want to use (e.g., 1 mile = 1.609 kilometres).
 Rotate the bezel (or flange) so that the number on the outer scale representing the starting unit (in this case, miles) aligns with the number on the inner scale representing the conversion factor (in this case, 1.609)
 Now, read the number on the inner scale directly across from your desired value on the outer scale to get the converted result.
 Speed, Distance, and Time Calculations:
Slide rules are excellent tools for quick speed, distance, and time calculations. These calculations can be particularly useful for pilots and navigators. For example, if you want to determine how long it will take to travel a certain distance at a given speed, follow these steps:
 Align the speed you are travelling (in units per hour) on the outer scale with the distance you want to cover (in units) on the inner scale
 Read the estimated time of travel on the inner scale across from the desired distance on the outer scale.
 Fuel Consumption:
If you're a pilot or sailor concerned about fuel consumption, the rotary slide rule bezel can help you calculate it efficiently. You need to know your speed and fuel consumption rate. Here's how to calculate fuel consumption for a specific distance:
Understanding Fuel Consumption:
Fuel consumption refers to the amount of fuel consumed by a vehicle, aircraft, or vessel per unit of distance or time. It's typically measured in units like gallons per hour (GPH), litres per hour (LPH), or nautical miles per gallon (NM/G). To calculate fuel consumption, you need two crucial pieces of information:

Speed: This is the rate at which you are travelling and is usually measured in units per hour. For example, if you are flying in an aircraft, your speed might be expressed in nautical miles per hour (NM/H).
 Fuel Consumption Rate: This is the rate at which your vehicle, aircraft, or vessel consumes fuel, typically measured in units per hour as well.
Calculating Fuel Consumption with a Slide Rule Bezel:
Assuming you have the speed and fuel consumption rate, you can use a rotary slide rule bezel to calculate your fuel consumption for a specific distance or time. Here's how to do it in more detail:
 Align the Speed: Locate the speed value on the outer scale of the slide rule. If your speed is, for instance, 150 nautical miles per hour (NM/H), align the "15" on the outer scale with the "10" on the inner scale. This means that you are now working with a multiplier of 10 for your speed.
 Align the Fuel Consumption Rate: Find your fuel consumption rate on the inner scale. For this example, let's say your fuel consumption rate is 12 gallons per hour (GPH). Align the "12" on the inner scale with the "1" on the outer scale. This sets your fuel consumption rate as a multiplier of 1.
 Read the Fuel Consumption: Once you've aligned your speed and fuel consumption rate, you can read your fuel consumption for the desired time or distance. If you want to calculate your fuel consumption for 3 hours of travel, simply locate the "3" on the inner scale (aligned with the "1" on the outer scale) to read the result on the outer scale. In this example, it would be 30 gallons (GPH x 3 = 12 x 3 = 36 GPH).
 Reading for Distance: If you want to calculate fuel consumption for a specific distance, you can do that as well. Align the rotary bezel with your speed and fuel consumption rate as before. Then, use the inner scale to read the fuel consumption for a given distance. For instance, if you want to calculate fuel consumption for 300 nautical miles, align the "30" on the inner scale (aligned with the "1" on the outer scale) to read the result on the outer scale. In this case, it would be 360 gallons (GPH x 30 = 12 x 30 = 360 GPH).
 Multiplication and Division:
A slide rule can also be used for basic multiplication and division. To multiply or divide two numbers, align one number on the outer scale with the other number on the inner scale. The result can then be read on the inner or outer scale, depending on whether you're performing multiplication or division.
The slide rule is a versatile tool that can be incredibly useful in various navigational scenarios. Whether you're converting units, calculating time, speed, distance, or fuel consumption, this instrument can simplify complex calculations, allowing you to focus on your journey with confidence. As with any tool, practice makes perfect, so take the time to familiarise yourself with your slide rule and its capabilities.