The Science of Wake Waves

The-Science-of -Wake-Waves-min

In the years since we have been fortunate enough to write this column for the magazine, requests to our office for methods to regulate motorboat usage on inland lakes have increased dramatically.

In our personal experience, these regulatory requests rarely stem from the noise that motorboats make (fourstroke low emissions outboard motors and PWCs have reduced traditional lake noise sources), and almost always stem from the wake wave size that newer motorboats are making while towing riders or surfers.

Lakefront property owners have recognized, firsthand, that the size of wake waves on Michigan inland lakes have increased in recent times, but quantifying that increase in wake wave size is something we have had a very hard time doing…until now. In February, the University of Minnesota St. Anthony Falls Field Laboratory conducted a field study of four recreational boats on a freshwater lake. In sum, the researchers quantified, using vibration sensing equipment and a set and repeatable test pattern, how motorboat size, dry weight, ballast weight and wake enhancing
devices effect boat wake wave size and power and various distance intervals.

I will not delve into the science behind the paper, and highly recommend reading the painstaking research yourself (a link is in this article.) But what was most interesting to me, from a lake regulation and policy point of view, is that the paper tested: a common 21ft “bow rider” inboard/outboard (in the style of a Sea-Ray or 4-Winns), a common performance course oriented waterski boat, and two ballast and wake-wedge equipped wakeboard boats. Also, crucially, the researchers tested each boat at the speed they are typically used in towing a rider (slower speeds for wakeboard boats with a wakesurfer, faster for ski boats with a skier.) The researchers determined that, “under this comparison, the two wakesurf boats produce substantially higher (~2-3 times) maximum wave heights than non wakesurf boats at 100ft, and approximately ~2 times higher after 600ft) (see p.80.)” In sum, the researchers supported what lakefront owners are experiencing…that the wakes impacting their shorelines are demonstrably larger at times as boat composition changes to reflect newer watersports.

From a legal point of view, this information merits special consideration. For example, in a previous article, we discussed seawall replacement permitting. Since 2021, EGLE has amended its “Replacement of Seawalls” permit application to require the applicant to select one of four different initiatives to “mitigate the loss of habitat”, including reducing seawall length by 25%, and the incorporation of “coarse woody structure” on the shoreline. On a lake with frequent slowspeed tow sport traffic, a riparian owner’s choice of mandatory mitigation measure could result in increased maintenance and more frequent repairs. There are numerous other implications that merit forethought on shoreline management in the future, as we begin to quantify how we are changing our inland lakes over time. If you have regulatory concerns, or need legal assistance planning shoreline projects, please do not hesitate to contact us. Article available at: