The **Portland Speed-Time-Distance Calculator Slide Rule** is a traditional yet efficient tool for mariners, aviators, and navigators. It simplifies the calculation of speed, time, or distance based on two known variables. This tool works similarly to a mechanical slide rule and is ideal for rapid, accurate calculations without the need for electronic devices.

This guide will walk you through how to use the Portland Speed-Time-Distance Calculator Slide Rule effectively for navigation.

**NOTE: Our guides are intended as just that, a guide. Please seek professional instruction before attempting to use these tools yourself.**

### What You’ll Need:

**Portland Speed-Time-Distance Calculator Slide Rule****Known Speed**(in knots or nautical miles per hour)**Known Time**(hours and minutes)**Known Distance**(in nautical miles)

### 1. **Understanding the Slide Rule**

The **Portland Speed-Time-Distance Calculator Slide Rule** by **Blundell Harling** is a physical tool that functions by aligning two scales. The key components are:

**Sliding centre scale**: Used to align your known values (speed or distance).**Fixed outer scales**: These contain the reference values for time and distance.

The basic formula still applies: **Distance = Speed x Time **

With this formula, if two values are known, the third can be calculated.

### 2. **Setting Up the Slide Rule for Use**

Using the Portland Slide Rule requires you to know two of the three variables: speed, time, or distance. The slide rule will then allow you to find the missing third value.

#### A. **Step 1: Familiarise Yourself with the Scales**

- The sliding centre section typically shows speed, and this is the scale you will adjust.
- The fixed outer scales represent time and distance.

#### B. **Step 2: Align the Scales**

- By moving the sliding scale to align the known values, you can find the unknown variable.
- The scales are logarithmic, allowing for rapid and precise calculation.

### 3. **Calculating Distance**

If you know your speed and travel time, the Portland Slide Rule will help you find the distance travelled:

#### A. **Step 1: Set the Known Speed**

- Find the speed value in knots on the sliding scale.
- Move the sliding section so that this speed aligns with the time value on the fixed scale.

#### B. **Step 2: Align with the Time**

- Use the outer time scale to set the number of hours and minutes travelled.

#### C. **Step 3: Read the Distance**

- The corresponding value on the distance scale will give the distance travelled.

**Example:**

**Speed**: 12 knots**Time**: 3 hours**Distance**: The slide rule will show 36 nautical miles.

### 4. **Calculating Time**

If you know your speed and distance, you can calculate how long it will take to complete the journey.

#### A. **Step 1: Set the Known Speed**

- Start by moving the sliding scale to match your speed in knots.

#### B. **Step 2: Set the Distance**

- Find the known distance travelled on the outer scale and align it with the speed value.

#### C. **Step 3: Read the Time**

- The corresponding value on the time scale will indicate how long the journey will take.

**Example:**

**Speed**: 15 knots**Distance**: 45 nautical miles**Time**: The slide rule will show 3 hours.

### 5. **Calculating Speed**

If you know the distance travelled and the time taken, the slide rule can be used to calculate the speed.

#### A. **Step 1: Set the Known Distance**

- Find the distance travelled on the fixed outer scale.

#### B. **Step 2: Set the Time**

- Move the sliding scale until the time travelled (in hours and minutes) aligns with the distance on the outer scale.

#### C. **Step 3: Read the Speed**

- The corresponding value on the speed scale will show your speed in knots.

**Example:**

**Distance**: 60 nautical miles**Time**: 4 hours**Speed**: The slide rule will show 15 knots.

### 6. **Practical Examples Using the Portland Slide Rule**

#### Example 1: Calculating Distance

**Speed**: 10 knots**Time**: 5 hours- The slide rule will show that the distance covered is 50 nautical miles.

#### Example 2: Calculating Time

**Speed**: 8 knots**Distance**: 32 nautical miles- The slide rule will show that the journey will take 4 hours.

#### Example 3: Calculating Speed

**Distance**: 90 nautical miles**Time**: 6 hours- The slide rule will show that your speed is 15 knots.

### 7. **Tips for Using the Slide Rule**

**Always check your units**: Ensure that your speed is measured in knots and distance in nautical miles for accurate results.**Avoid rounding too early**: Keep your values precise, especially when dealing with long distances, to avoid errors.**Use it with other navigation tools**: While highly reliable, it is good practice to verify your results with other navigational aids like GPS or logbooks.

The **Portland Speed-Time-Distance Calculator Slide Rule** is a straightforward yet highly effective tool for mariners and navigators. By knowing two variables—whether speed, time, or distance—you can easily calculate the third, allowing you to make quick, accurate decisions while on the move. Mastering this tool is essential for traditional navigation, especially in environments where electronic tools might fail.