Most arguments on a boat start with one number: when do we get there? Someone guesses out loud, everyone else plans lunch around the guess, and the cool box sits open an hour longer than it should. Working out an arrival time does not need complicated gear. It needs two numbers you already have, speed and distance, plus one habit most boaters skip: a realistic buffer.
Arrival time equals distance divided by speed. That is the whole calculation.
Worked example: 18 nautical miles at 12 knots. 18 ÷ 12 = 1.5, which is one hour and thirty minutes of pure running time. Add a 20% buffer: 1.5 × 1.2 = 1.8 hours, or one hour and forty-eight minutes. Round it to 1 hour 50 and give your family that figure, not 1:30.
If minutes are easier: 18 miles at 12 knots is 90 minutes, and 20% of 90 is 18, so 108 minutes. Same answer by a different road.
The formula assumes a constant speed. Local water rarely cooperates.
Say you launch at Shuwaikh and run to a mark 26 nautical miles north, cruising at 14 knots. 26 ÷ 14 is about 1.86 hours, roughly 1 hour 52 minutes. Add 20% and it becomes about 2 hours 14 minutes. Add fifteen minutes for the ramp and fuel, and your honest arrival is around two and a half hours after you reach the marina, not after you untie the lines.
Calculate the return leg separately. The boat is usually heavier coming back, the afternoon wind builds, and nobody wants to do arithmetic in the dark while the ice box slides across the deck.
Write down your actual arrival time for a few trips and compare it with the estimate. After three or four runs you will know your own buffer, which beats any general rule. The B7R app gives you sea state, tide times for Shuwaikh and Shuaiba, and navigational tools to measure the route properly instead of guessing, so you leave the dock with a real number.
Want your trip planned on real numbers? Download B7R free on Google Play: https://play.google.com/store/apps/details?id=com.q8dm.b7r — tides for Shuwaikh and Shuaiba, sea state, fish guide, catch log and an SOS button.