Boating & Sailing Calculation Guide

📚 Overview

Time on the water rewards a little arithmetic: how fast a hull can go, how much anchor line keeps you in place, how high the tide will be under a bridge, and how far the fuel will take you. The calculators here turn well-known boating rules of thumb into quick answers for passage planning and learning. They are planning aids only. Charts, tide tables, the boat manufacturer data, and the skipper judgement always come first.

📐 Hull Speed from Waterline Length

A displacement hull pushes a wave system along its sides whose length grows with speed. When that wave is as long as the waterline, the boat sits between a crest at the bow and a crest at the stern, and extra power gives little extra speed. That point is a speed-length ratio of about 1.34, and the formula gives the matching speed in knots.

🔢 Worked Example

Scenario: A cruising sailboat with a 30 ft waterline that motors along at 6 knots.

• Waterline length: 30 ft
• Cruising speed: 6 kt

1. Square root of 30 = about 5.48
2. Hull speed = 1.34 × 5.48 = about 7.3 kt
3. Speed-length ratio = 6 ÷ 5.48 = about 1.1
4. Share of hull speed = 6 ÷ 7.3 = about 82%

💡 Pro Tips

• Let out anchor rode for the high-tide depth, not the depth at the time you drop the hook, and add more scope when the wind rises.
• Use the rule of twelfths for a quick tide height, and check the tide table when depth is tight.
• Keep a fuel reserve of about a third, and count wind and waves against your range.
• Add the antennas and instruments to your air draft before you plan to pass under a bridge.

🧮 Boating & Sailing Calculators (11)

Put the formulas above into practice with these free tools:

❓ Frequently Asked Questions