Wake Up- The Science Behind Creating a Wake and Why Size Matters
By: Bill Jennings
Do people throw tomatoes at you when you boat past their cottage? Perhaps your breath is bad, but more likely, it’s your “wake.” Here is what you need to know about your boat’s wake, for you to stay popular.
Speed limits for boats are much more critical than those for highway vehicles. Most people driving cars will push the speed limit a little because they want to save time and they believe they can get away with it. And on land there is no dangerous trail of water moving out both sides of your car. But in boats, there is such a trail and it is 800 times more dense than air. It is called a “wake.” The speed limits on water are not just about safe speeds, they are also about the damaging wake trail. Watching videos we have made of wakes hitting sensitive shorelines and you can see dirt being pulled away. It is therefore important for a good boater to fully understand their cause and effect.
The first step is to understand the difference between a displacement hull and a planing hull. A displacement hull moves through the water, while a planing hull moves on the top of the water. Generally, trawlers and sailboats are examples of displacement hulls, while runabouts and catamarans are examples of planing hulls. A displacement hull, such as a 25’ sailboat will not go onto plane, but a planing hull, such as a 25’ runabout will. Here is where it gets interesting: you can install a 400 horsepower outboard on that sailboat and you will only manage to achieve a limited top speed, called “hull speed.” If you found that interesting, how about this? The approximate maximum hull speed in knots of any displacement hull can be calculated by using the formula, 1.34 times the square root of the water line length of the boat, in feet. Therefore, for the 25’ displacement sailboat we have 1.34 x 5, which is 6.7 knots, or a maximum speed of 7.71 miles per hour. This also answers the question why most 25’ sailboats use a 9.9 hp outboard. Any higher hp would be a waste of money. Conversely, planing hulls do not have a specific ‘hull speed’ because while planning across the top of the water they encounter significantly less drag.
But every planing hull begins its journey as a displacement hull. It converts to a planing hull when horsepower is applied and the strakes and hull design lift it over its bow wave to skim across the water. We know this transition as “coming onto plane.”
To leave the dock, you move the throttle lever forward into a notch, just ahead of neutral (in full inboards, there is no notch). At this throttle setting your boat will move forward and make a very small wake. Once you advance the throttle slightly, you begin the process of going onto plane and in that process the bow rises and the stern drops. As the stern digs deeper, it displaces water in the form of stern waves, or wake. It is the largest and most damaging wake that your boat can produce. And remember- adding throttle and lifting the bow, you also shorten your waterline length. Now applying the displacement hull speed formula, you can see that your speed may actually be slower with the throttle advanced than at idle. All you do is consume more fuel and produce an offensive wake.
To research some wake size numbers, we tested a popular 18’ runabout and a 21’ bowrider on flat water conditions. With a measuring stick planted with the water surface set at “zero”, we drove these boats past the stick from an idle speed to full throttle in fractional increments. As the transformation from idle RPMs to higher RPMs began, the boats made a wake up to 600% larger than when running at idle. And the wake size at just at 15 kilometers per hour was even larger than the wake the boat produced at full speed.
Unfortunately, the laughable regulations in Canada don’t help the wake problem. Boats in Canada are delivered with a speedometer that reads in ‘miles per hour.’ Mariners measure speed in ‘knots.’ Our speed limit signs, of course, read in ‘kilometers per hour.’ And few boat speedometers register any speed at all, until the boat is moving over 10 km/h.
Here's a simple test for the next time you are boating through a speed zone. Pull your throttle completely back to neutral for one second. If your bow drops, you are a wake maker. If your bow does not drop you are travelling at a considerate speed with minimal wake. Why let’s clean up those tomatoes, be aware of wake science and be glad to know that you don’t have bad breath.