O Canoe Wave Height Calculator

For paddlers, wind and big waves on the open water can be unpleasant at best and deadly at worst. Fortunately, we have a variety of good free tools at our fingertips these days to help us anticipate when and where the waves might be bad. What are the safe limits for wave height? That depends on you, your paddling party, your watercraft, and all the other factors specific to your activity.

Inexperienced paddlers enjoy the best initial learning experiences in calm winds and sheltered waters. Later, while building experience in windier open-water conditions, it's a good idea to take note of the increasing wave heights and actually measure them for future reference. You might find it takes surprisingly few inches of wave height to make your paddle craft feel uncomfortably unstable. Never ignore that feeling.

To predict the height of the waves we might encounter on the open water, three key things we look at are (a) the forecast wind speed; (b) the forecast wind direction; and (c) the open-water distance (called wind fetch, or fetch length) over which that wind will be able to generate waves in that direction.

For wind & weather forecasts, there are several amazing phone apps including Windy.com, Windy.app, and PredictWind. These apps allow the user to compare hour-by-hour forecasts generated by the world's leading weather-forecast centres. Some of them can paint a picture more than a week into the future (but don't expect much reliability in such a far-reaching forecast!). The free versions of these apps have their limitations but are still useful. We also look at Environment Canada's hourly forecasts using their excellent, free WeatherCAN app.

Windy.com wind speed

The dedicated wind apps cleverly present wind direction graphically with easy-to-interpret moving arrows on a map. WeatherCAN on the other hand gives the wind direction in text only, e.g. "NW" (for northwest). Keep in mind, this is the direction the wind is coming from.

Windy.com wind fetch length

Wind fetch length can be measured right on the maps built into some of the wind forecasting apps. We also use Google Maps for this purpose (and for getting to our paddling places, and measuring our planned paddling routes, and keeping track of where we are, and so on). We picture exactly where we'll be on the lake or river we're considering, and then visualize the direction the wind is forecast to come from, and then we use the Google Maps "Measure distance" tool to measure the over-water wind fetch length along that line between us and the distant shore. Measuring distances in Google Maps is quick and easy; I've explained exactly how to do it here. (I hope you'll find that useful for more than just measuring wind fetch lengths!)

O Canoe wave height calculator example

The final tool is the wave height calculator, which I built for this purpose at the top of this page. This paper by Etemad-Shahidi et al. in the Journal of Coastal Research compares three of the mostly widely accepted wave height forecasting methods, and tests them against statistical wind and wave data from Lake Ontario. The data suggests the long-established Sverdrup-Munk-Bretschneider (SMB) model is usually the most accurate, so that's the method I've chosen here.

The SMB model boils down to the two equations below, which I've used to program the calculator in JavaScript. H_s is the significant wave height in metres. X is the wind fetch length in metres; g is the local acceleration due to gravity in m/s²; U is the wind speed in m/s; and t_min is the time required for the wind to build the waves up, given that fetch length. (My JavaScript for this page takes a few additional steps to convert to the more familiar units we see in paddling.)