Subsection 1.1.1 How do we measure speed
We don’t typically discuss speed in terms of feet per second. Is 5 feet per second fast or slow? To make this more relatable, let’s convert feet to miles and seconds to hours. Remember, there are 5,280 feet in one mile, 60 seconds in one minute, and 60 minutes in one hour. Here’s how it works:
Step 1: Convert seconds to minutes:
Step 2: Convert minutes to hours:
Step 3: Convert feet to miles:
Now, we’re using a unit of measurement that’s more familiar to us - miles per hour.
Subsection 1.1.2 Measuring the speed of a waʻa
If we’re driving in a car, we can estimate our speed by dividing the distance by time. For instance, if we know the distance between utility poles is 125 feet and it takes 2 seconds to travel between them, our speed would be:
We can convert this to a more familiar measurement of speed, miles per hour, using the steps above. In this case, we combine all three steps at once:
Out on the water, we don’t have familiar landmarks to measure speed. Instead, we use the canoe itself as a reference. We know how long it is.
To find out how fast the canoe is moving, we time how long it takes to go from the front to the back. It’s not always easy to tell how far we’ve gone, so we use things like bubbles or even things like orange peels to help. When these things pass the front, we start counting seconds until they pass the back, and we write down that time.
Now that we know how long the canoe is and how long it took to cover that distance, we can figure out the speed. Speed is just how far we traveled divided by the time it took. This helps us measure the canoe’s speed accurately, even without usual markers in the vast ocean.
Example 1.1.1. Estimating Speed on Hōkūleʻa.
As Hōkūleʻa sails, the crew spots bubbles in the water and prepares to time the passage. The distance from the front ʻiako (crossbeam) to the back ʻiako is 42 feet, and it took 5 seconds to cover. What is Hōkūleʻa’s speed? In the ocean, speed is commonly measured in knots. One knot is defined as one nautical mile per hour. One nautical mile is approximately 6,076 feet.
Solution.
We calculate the speed of Hōkūleʻa:
To express our answer in knots, we’ll first convert seconds to minutes:
Next, we convert minutes to hours:
Finally, we convert feet to nautical miles using the fact that one nautical mile is 6,076 feet:
Thus, if it takes Hōkūleʻa 5 seconds to sail past an object, then Hōkūleʻa is sailing at a speed of 5 knots.
Subsection 1.1.3 Magic Speed Number
Now that you know how to find Hōkūleʻa’s speed, you might wonder if it’s a lot of work every time. But here’s the trick: the canoe’s length and the conversions we use for distance and time stay the same. So, if we figure out everything except the time (which can vary), we’ll get a special number we’ll call the
magic speed number. When we divide this number by the time in seconds, we’ll get Hōkūleʻa’s speed in knots for any time.
Combining the steps from the example into one line and omitting the specific time (5 seconds), we get the following expression for our
magic speed number:
which has units of knots times seconds. Dividing the magic speed number by our time in seconds, we obtain units of knots, which is our desired speed:
We can confirm this formula with the example, where we calculated that if it took Hōkūleʻa 5 seconds to sail past an object:
only works if the distance between ʻiako is 42 ft. If you are on a canoe with a different length, you will need to calculate a new number. You can obtain that number by replacing 42 feet in the previous example with the length of your canoe.
Example 1.1.2. Estimating Speed on the Makaliʻi.
On the Makaliʻi, the distance between the first and last ʻiako is 25 ft.
Determine the magic speed number for Makaliʻi, rounded to the nearest integer.
If it takes Makaliʻi 3 seconds to pass an object in the ocean, use the magic speed number to calculate its speed.
Solution.
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We’ll use the same method as we did for Hōkūleʻa, but since Makaliʻi is shorter, we’ll replace 42 ft with 25 ft.
So, the magic speed number for Makaliʻi is 15. For any given time, we can calculate her speed with this equation:
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If it takes Makaliʻi 3 seconds to pass an object, her speed can be calculated as: