Estimating Time of Arrival
Assume that you will leave your marina on a trip to a nearby restaurant for lunch.
- You have made reservations at the restaurant for 1200.
- The restaurant is 29 nautical miles from your marina.
- You plan on a leisurely cruise at a speed of 12 kts.
What time must you leave your marina to arrive at the restaurant at your reservation time?
This is a simple Distance, Time, Speed question. The formulas to use for the calculations are made simple if you remember, and actually write in a corner of your chart, the following diagram.
The "D" represents distance, "S" represents speed and "T" represents time. In order to solve a distance, speed, time problem you need two of the three values and then must solve for the third.
Using the diagram, cover the unknown value with your finger and what you have left is the formula to solve the problem.
In our problem we know the distance and the speed so we cover up the "T" and the resulting formula is "D" over "S" or "D" divided by "S".
Using this formula, the first step is to calculate how long it will take you to get to the restaurant by cruising at 12 knots for 29 nautical miles.
29 / 12 = 2.42 hours
Note: This is not two hours and 42 minutes, it is 2 hours and 42 hundredths of an hour. We now have to convert this decimal to minutes by multiplying .42 X 60.
.42 X 60 = 25 minutes - So...our trip will take 2 hours and 25 minutes.
Now that we know how long the trip will take, we simply need to subtract that time from the time of our 1200 reservation.
1200 - 225 = 0935
1160 - 225 = 0935
Note: that in order to subtract we had to borrow an hour from the hours column. 1160 is the same as 1200.We must leave the marina at 0935 (9:35 am) in order to arrive at the restaurant by noon.
If you would like to learn more about Time, Distance, Speed calculations or other aspects of navigation, check out our Nautical Know How Coastal Navigation Course.