# Hull Speed Calculator

URL: https://speedycalculator.com/calculators/sports/hull-speed/
Last updated: 2026-10-30

> The hull speed calculator finds the theoretical maximum speed of a displacement boat from its waterline length – or the waterline length needed for a target speed – with an adjustable speed-length ratio.

## Inputs

- Find
- Waterline length (LWL) (ft, m)
- Target speed (kn, km/h, mph)
- Speed-length ratio

## Results

- Hull speed
- Waterline length needed

## Formula

```
Hull speed (knots) = 1.34 × √(waterline length in feet)  ·  LWL = (speed ÷ 1.34)²
```

## Explanation

A displacement hull makes a bow wave whose length grows with speed. When the wave becomes as long as the boat, the stern sinks into the trough and the boat effectively hits a wall – the hull speed.

### How to use the Hull Speed Calculator

- **Find**
- **Waterline length (LWL)**
- **Target speed**
- **Speed-length ratio**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Choose what to find and enter the waterline length or the speed. Adjust the ratio for fine or heavy hulls.

### Tips

- Check local conditions and safety rules before going out.
- Start with conservative equipment choices and adjust with experience.
- Every field has its own unit menu.
- Everything is calculated in your browser.

### Common mistakes to avoid

- Using estimates instead of a proper check in the water.
- Mixing feet and metres, or knots and km/h.
- Overestimating your skill in strong wind.

### Inputs explained

FieldWhat to enterFindChoose one: Hull speed, Waterline length neededWaterline length (LWL)Units: ft, mTarget speedUnits: kn, km/h, mphSpeed-length ratioA number

### What each result means

- **Hull speed** = [Speed-length ratio]×sqrt([Waterline length (LWL)])
- **Waterline length needed** = ([Target speed]÷[Speed-length ratio])^2

### Example values

How the main result changes when **Waterline length** changes and everything else stays at the example values above:

Waterline length (LWL) (ft)Hull speed (kn)155.189822.56.3562307.339537.58.2058458.9896010.3796

## Worked example

Inputs — Find: Hull speed; Waterline length (LWL): 30 ft; Speed-length ratio: 1.34.

Results — **Hull speed = 7.3395 kn**.

## Key terms

- **LWL**: Length at the waterline.

## FAQ

**Can a boat go faster than hull speed?**
Planing and multihull boats can; displacement boats need huge power to exceed it.

**Why 1.34?**
It is the speed-length ratio where wave length equals waterline length.

**Is LWL the same as boat length?**
No – it is the length at the waterline, usually shorter.

**Does heeling change it?**
Heeling can lengthen the waterline slightly.


## Sources

- [US Sailing – Basic Keelboat](https://www.ussailing.org/)
- [Marchaj – Sailing Theory and Practice](https://www.worldcat.org/)