Wisconsin Watersports Coalition

Wave Height and Shoreline Erosion

We cannot make Wisconsin the most RESTRICTIVE state in the nation.

A comprehensive review of studies leads to enacting a 200' from shore rule.

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Summary

"Biased" comparison

Anti-wake boat groups, have in effect weaponized the data and studies to create fear by exaggerating surf wave heights. A comprehensive review of the studies reflect that the average height of the wave at 200 ft. is 10" and that both wave height and energy will attenuate by 60% at 100 ft. and over 70% at 200 ft. from the boat's path of travel.

Anti-wake boat groups often cite findings from studies like St. Anthony Falls among others to validate their recommendations of 500 to 700 feet setbacks from the shore. These studies, many of which are crowd funded or paid studies looking to obtain a particular outcome, establish unrealistic criteria to make their comparisons. A great example of this is the St. Anthony Falls Study. They created their assumptions based on the ski boat's performance at 20 mph using 200 ft. as the baseline instead of the current 100 ft. setback. These anti-wake groups will often claim in presentations that they have "nothing" against the boats, but just the performance during the activity of surfing and while using ballast.

Recreating the data points from one of the St. Anthony Falls' wake boats in Figure 1 below, it highlights the performance of the VLX with and without ballast, and even includes the results of operating without ballast at 20 mph. St. Anthony's own data shows, that regardless of ballast and speed, any boat with sufficient displacement and water line length will produce a wave that cannot meet this study's "criteria" it established for the surf wave within 500 ft. Furthermore, it highlights several key points:

An accurate comparison, to assess the impact of wave heights, is to compare similar boats and set the distance that a boat can legally pass a shoreline beyond a no-wake zone (100 ft.) as the criteria. (At 100 feet, the ski boat generates an 8" wave)
A wake boat at surf speed, with or without ballast, will have comparable wave heights starting at 100 ft. from the boat's path of travel.
Any boat with sufficient displacement, will have a comparable wave at 100 ft. to wake boat surf waves at 200 ft.

Additionally, these groups continue in their sensationalist narratives, often myopically citing a single data point from Goudey as the "industry's data," reporting a 16-inch wave height 200 feet from a boat, while ignoring other findings from the same study, as well as the six additional studies citing wave height measurements, which measured wave heights between 6-11 inches. These groups also attempt to explain this wave height is due to the increased testing depth, adding that this depth represents reality. However, they fail to communicate that the Australian Maritime College (AMC) Study was performed at greater depths with the wave height measurement being recorded at a depth of 15 ft., eliminating any interference from any shallow water. The AMC data showed an average of 10.5" at 200 ft. for all the boats tested in surf mode at a max wave setting. In addition, the study incorporated the "other" wave from Goudey's study for analysis, but did not use this data point. This 16" wave is considerably greater than even the AMC's Study results, establishing this data point as an outlier. Overall, this measurement is inconsistent with six other studies that show an average wave height of 10 inches at the same distance. Figure 2 incorporates all the leading studies to date and highlights the point that comparable boats, without ballast, will create similar wave heights to wake surf waves at 200 ft.

Unfounded Claims

Furthermore, shoreline erosion has been studied extensively for over 40 years. Research consistently shows that the primary drivers are natural and environmental factors, such as ice movement, runoff, fluctuating lake levels, natural wave action, and shoreline development—not boat wakes. Water inherently seeks equilibrium. When a boat generates a wake, gravity and the physics of boat wakes quickly dissipate the wave’s energy. As the wake moves away, it disperses into smaller, less energetic waves. Research shows that after just 100 feet, the maximum wave height is the same regardless of whether a boat is using ballast (MacFarlane 2018, Marr et al. 2022).

Effective shore erosion assessment focuses on wave energy derived from wave height and period- the time between waves. Wave period is more important than wave height. (Cox and McFarlane 2019) This energy dissipates rapidly: more than 70% of a wake’s energy is dissipated 200 feet from the boat falling below the impact of waves produced by a 20 mph wind over a mile of water (Goudey and Girod 2015). Runabouts and fishing boats tubing at slow speeds 100 feet from shore are widely accepted. McFarlane demonstrated that a 4000 lb fishing boat at 100 feet produces as large a wake as a wake boat at 200 feet.

Maintaining a distance of 200 feet from shore ensures that wake energy decays sufficiently so the waves do not differ significantly from natural conditions and do not contribute notably to shore erosion. There have been six independent studies on wave heights from wake boats. The average wave height at 200 feet from all six studies was 10”, the size of a small iPad, with a 2-second period. Cox and McFarlane (2019) conclude, “Wave wake studies are inevitably a study of orders of magnitude and not small percentages.” While smaller fetches will result in lower energy waves, wake boat wakes are not orders of magnitude different and are still similar to those generated by other boats 100 feet from shore and significant wind driven waves, which is the current state regulation.

Maintaining a minimum distance of 200 feet from shore ensures that wake height and energy decays sufficiently so the waves do not differ significantly from natural conditions and other boats operating closer to the shoreline.

Figure 1. St. Anthony Falls Max Wave Height: Ballast/No Ballast/20 mph Comparison

(recreated from the St. Anthony Falls Study, 2022)

Figure 2. Leading Studies Max Wave Height Decay: No Ballast(100 ft.) vs. Ballast(200ft./300ft.)

AMC (Australian Maritime College) Study (MacFarlane)

The 2018 report by the Australian Maritime College (AMC) offers one of the most comprehensive and credible data sets on wave heights, led by expert Professor Gregor McFarlane, a leading authority in boat wake research. McFarlaneand his students have spent over 30 years studying and modeling boat wakes, building a database of over 6,000 watercraft. This study measured wave heights from various boats, including wake boats (Malibu V Ride, Nautique G21, Axis T23, Centurion Ri217), a fishing boat (Thunder Jet Alexis), a ski boat (Ski Nautique), and a runabout (Reinell). The data was also compared to a 2015 study by Goudey and Girod, which used a Nautique G23.

Many previous wakeboat studies are plagued by inconsistencies, non-standard procedures, poor recording, and oversimplified results. In contrast, AMC’s methodology ensures broader applicability beyond the test site.

The trials took place on the Willamette River near Coalca Landing, Oregon, chosen for its straight course and consistent 40-foot water depth, minimizing wave reflection. Buoys marked lateral distances of 100, 200, 300, and 400 feet from the wave probe to guide boat operators.

Unlike typical studies using less precise pressure transducers, this research employed a highly accurate MK-VI capacitance wave probe, with rigorous lab and on-site calibration ensuring reliable data. Instead of measuring total wave energy within a wave packet, Gregor Macfarlane, a key contributor, focused on calculating the maximum wave energy from wave height and period, providing more precise results.

The study’s systematic approach across various watercraft, speeds, and distances yielded key findings:

• Beyond 100 feet, wave heights from wakeboats are similar with or without ballast.

• A slow-speed fishing boat without ballast or wave-shaping devices can produce wave heights and energies comparable to wakeboats.

• At 200 feet, waves from wakeboats are similar or smaller than those from a fishing boat at 100 feet, with most wave heights ranging from 8 to 11 inches, except for two unusual 10 mph data points.

• The findings align with Goudey and Girod’s shallow water results, despite AMC’s deeper bathymetry.

• Displacement vs. time traces confirm that 20-23 foot boats generate waves with ~2-second periods, comparable to a 20 mph wind.

• Wakeboats operating at 20-24 mph produced 6-11 inch waves at 200 feet, comparable to those at surfing speeds.

WSIA Study (Goudey)

Goudey's study was one of the first wake surfing studies performed, in 2015. The study looked at two distinct testing parameters, described as a "shallow" test and a "deep" test. These parameters referenced the slope of the lake to get to the testing depth, with the deep parameter having a reduced distance to get to a testing depth. Goudey's work also brought in the comparison of wind energy to understand the affect on a shoreline versus a wake wave.

Goudey used a large wake boat, a Nautique G23 with a full ballast while adding 1400 lbs. of additional ballast. The study resulted in measuring the wake surf wave at several distances. At 200 feet, the wave height for the shallow parameter was measured at 10", while the deep was 16". The shallow parameter data of 10" falls in line with the rest of the studies while the deep parameter result appears to be an outlier, being 60% higher than most of the other data points. Furthermore, this study also provided data showing that between the two parameters, the energy of a surf wave decreased between 62-85% in the first 200 feet of travel from the boat.

This study also incorporated the use of wind energy and it's effect on wave energy in relation to a wake surf wave. In summary, what Goudey found was that the wind generated wave energy from a 20 mph wind over a 1 mile fetch was approximately the same energy as the energy calculated from the G23 wave at 200 feet. Considering that wind and storms can last hours, if not complete days, it's calculated that the wake boat would need to pass the shoreline roughley every 3.5-4 minutes to equal the wave energy resulting from a 20 mph wind.

As a result, Goudey goes on to state, "Boat wakes coming ashore are discrete events and while they do convey energy and can have impacts on some shorelines, they typically are a minor perturbation compared to the persistent arrival of wind waves. In all but the most protected of shorelines, it would be difficult for boating to match the role of wind waves and natural currents on shaping shorelines."

The St. Anthony Falls Laboratory Study (Marr)

Critical Issues with the Marr et al. MN St. Anthony Falls Wake Study

Boat wakes have been a subject of scientific inquiry since the 1960s, initially focusing on wave energy to estimate vessel resistance. Numerous studies have examined the wake characteristics and impact of wake boats. The 2022 study by Marrs et al. at the MN St. Anthony Falls Lab is frequently cited by groups opposing wake boats despite the fact his study was not novel, and was never published in a scientific journal, nor did it follow peer review protocols. This study suffers from severe flaws in data collection and analysis. It fails to adhere to standard scientific conventions. This work does not rise to the level required for policy decisions. Below are the key issues that undermine its credibility:

1. Data Collection: Massive Variability Lacking Precision

The studies used two different types of sensors, creating discrepancies in the data. Their acoustical Doppler current sensors consistently measured higher wave heights than the pressure sensors, yet they used both data sets in their analysis of wake boats, but not the remaining boats, introducing systematic error. There was no cross-calibration of the different sensors nor explanation for the discrepancy. Both of these sensors are inferior to capacitance sensors other researchers have used. State-of-the-art researchers use above-surface radar sensors, which are simpler and more robust.

The wave height data collected in this study is riddled with inconsistencies, rendering it impossible to analyze confidently. For instance, wave heights measured 200 feet from the MXZ boat varied wildly, with reported values of 9 inches and 18 inches. A 100 % discrepancy is unacceptable. The standard for reliable data is no more than a 20% variance. Similar prior studies exhibited 10%. The significant scatter in the data effectively renders it useless for any meaningful analysis. Even though their mast sensors produced up to a 50% variation, the mast data can provide useful information.

The authors attempted to obscure this lack of precision by applying regression to fit a curve. Averaging data with such a high degree of scatter yields poor R-squared correlation coefficients, indicating they are not explaining the data variability with their equation. The R-square for the wake boat studied was 0.7-0.5. Anything less than 0.8 is considered unacceptable. Averaging data to reduce random noise is acceptable, but averaging to reduce systemic errors is not. The exponential coefficients of their fit are not consistent with prior work.

2. Failure to Compare Data with Prior Studies

In any rigorous scientific study, it is essential to compare new data and analysis with existing research, identifying and explaining any discrepancies. The authors of the St. Anthony Falls study failed to do so, disregarding significant relevant prior work. For example, they include a reference to Ruprecht J, et. al., which uses similar watercraft, but they never compare their energy calculations to that work. Goudey and Girod conducted an extensive measurement analysis of wakeboard waves in 2015, followed by a similar study by McFarlane in 2018. Both studies offer superior data quality and analysis. However, the St. Anthony Falls study ignores these efforts, claiming they were not journal articles which their report isn't either. This is particularly disturbing since their analysis is inconsistent with the prior work. In addition, they attempt to calculate total wave power while other experts such as Greg Cox and McFarlane have shown that max wave energy and period are the most useful parameters to consider in erosion. (Cox and McFarlane 2019)

3. Flawed Analysis & Energy Calculations

Poor-quality data undermine the study’s attempts to characterize waves. Further degrading the usefulness of this paper is the analysis approach. The best practice for this type of work is to accurately measure wave height and period and then calculate the max wave energy (Cox and McFarlane 2019). Due to the dispersive nature of boat wakes in this regime, the initial large wave observed at the boat breaks up into multiple lower-amplitude waves. The further from the sailing line, the more waves and the smaller the height. Using the max wave height is the standard practice for this type of work. The St. Anthony Fall’s authors did not adopt this approach but rather attempted to calculate the total cumulative energy of all waves in a boat wake wave packet. This approach is fraught with challenges even when using good data. The authors need to determine the duration of the packet. They assume the endpoint of the packet is when the next wave contributes less than 1% to the total wave energy. Given this was an experimental study, it inherently had noise from wind drive waves, reflected waves, and other boat waves. This can be seen in Figure 12. Their data for the unique boat wake waves are confounded by the superposition of other waves from wind, reflection, and other boats. In the example they present, they summed no less than 16 individual waves with no definitive way to know if these are actual boat wakes or waves from interference.

The authors also adopt a nonstandard approach, converting their wave periods to wavelengths. Even when wave characteristics appear similar to previous studies, the energy calculations presented in this study are 3 to 8 times higher than previously reported results. For instance, the study calculates a wave energy of 16,000 J/m at the boat for the Malibu MXZ; Goudey and Girod, who used a similar approach, calculated a total wave energy of 1800 J/m. To demonstrate how absurd this value is, The Australian Maritime College measured the wake of a 118-foot, 116-ton catamaran at 14,000, 3 boat lengths away from the craft.

The St. Anthony Falls study reports wildly inconsistent energy values. At 100 feet from the boat, they report both 9,000 J/m and 4,800 J/m—raising the question, which should be believed? Goudey and Girod’s calculations for similar conditions yielded 1,200 J/m in deep water.

These analysis errors are particularly troubling since wave energy calculations are critical for comparing different watercraft and forming the basis for recommended setbacks from shorelines.

Conclusion

The St. Anthony Falls wake study was severely criticized by its reviewers, and at least one of them stated they were not given an opportunity to view a revised manuscript to ensure their comments were addressed. It is fundamentally flawed, both in its data collection and analysis. The combination of unacceptable scatter in the experimental data and erroneous calculations makes it impossible to draw reliable conclusions from this work. Given the availability of superior research from more experienced scientists, the conclusions drawn from this study should not be considered credible or used in policymaking.

References

Cox, G; MacFarlane, Gregor (2019). The effects of boat waves on sheltered waterways – thirty years of continuous study. University of Tasmania. Conference contribution. https://hdl.handle.net/102.100.100/522918

Cox, Gregory, (2000) Sex, Lies and Wave Wakes, Proceeding of the International Conference on Hydrodynamics of High Speed Craft- Wake, Wash and Motion Control (HHSC 2000), London UK

McFarlane, Gregor (2018), Technical Report Wave Wake Study-HB4099 Motorboat Working Group

https://static1.squarespace.com/static/5a0ba0f9e5dd5bce46ef4ed2/t/5c01dec34d7a9cb0b6f25937/1543626456377/AMC+Wave+Wake+Study_HB4099+Motorboat+Working+Group+REPORT.pdf

Goudey, Clifford, and Girod, Lewis, (2105)Characterization of Wake Sport Wakes, and their Potential Impact on Shorelines. WSIA, Wave energy – 2015 https://www.wsia.net/wp-content/uploads/2020/03/WSIA_draft_report_Rev_II.pdf

Ruprecht, J.E.; Glamore, W.C.; Coghlan, I.R.; Flocard, F. (2015). Wakesurfing: Some Wakes are More Equal than Others, Proceedings of the Australasian Coasts & Ports Conference 2015, Auckland, NZ.

Terra Vigilis (Phase 1/Phase 2)

Terra Vigilis is best known for the sediment resuspension work done on North Lake in Waukesha, WI. However, both phases of the study did wave height studies on wake boats and even a 20 ft. pontoon boat.

In Phase 1 of the study(2020), the testing resulted in a wake surf wave height of 10" at 200 feet. However, in Phase 2 (2021), the test came back with a wave height of 8" at 200 feet. The study went on to deem this result a concern and ultimately referred to the St. Anthony Falls Study, stating their results were similar. In addition, the pontoon also generated a wave height of 8" at a cruising speed a similar distance from shore. This isn't surprising because energy and height are affected by both displacement and the length of the waterline for the boat tested.

The study also provided the wave period data, so that allows us to calculate the wave energy for these boats. Both of the wake boat data points fell comfortably within the normal ranges that have been seen for both the wave heights and wave energies across other studies.

The calculated energy for the pontoon was somewhat of a surprise. Due to the waterline, the period of this undersized pontoon (compared to what's on the water today) was 2.2 seconds. When calculating the energy, the pontoon exhibited a wave energy that was within the wave energy grouping from the wake boats in surf mode at 200 feet.

Big Payette Lake (Ray)

Similar to the Terra Vigilis Study, the Big Payette Lake Study by Ray is probably most cited for his modeling work and sediment resuspension. However, Ray also conducted a wave height study. The findings resulted in data points measured at approximately 135 ft. and 175 ft. from the boat. At 135 ft., the wave surf wave height was 9" and at 175 ft. the wave height was 8.5".

Both of these data points are also within the range of the more comprehensive wave height studies that have been performed.

Tera Vigilis Lake Waramaugh

The Terra Vigilis Lake Waramaug report is self-published with no editorial review, lacks elements of scientific rigor, and fails to provide credible evidence to support its conclusions about wake boat wakes and prop wash. The study employs non-standard methods, such as drone video analysis, to measure wave height but provides no explanation of how or if this was validated against established measurement techniques like capacitance or pressure sensors. The report provides no information on the variation in the experimental measurements. This methodological shortfall undermines the reliability of the wave height data presented, which is inconsistent not only with more comprehensive studies but also with the author’s own prior work.

ISSUES:

● THERE WAS NO THIRD-PARTY REVIEW OR VERIFICATION; THIS WAS A SELF-PUBLISHED STUDY.

● USED NON-TYPICAL SURFING BOAT SPEED TO PRODUCE MAX WAVE HEIGHT

● NON-STANDARD TECHNIQUES- LACK CALIBRATION

● INCONSISTENT WITH MORE COMPREHENSIVE STUDIES INCLUDING THEIR OWN PRIOR WORK.

● USE INCOMPLETE COMPARISON RELYING ON SKI BOATS AT PLANE

In this study, the wake boat was operated at 9 mph, compared to a typical wake surfing speed of 11 mph. Operating at 9 mph with a 23-foot boat results in a Froude number of 0.51, which yields the maximum wave height. Conversely, at 11 mph, the Froude number increases to 0.63, leading to waves that are expected to be about 33% smaller. This study inappropriately compares a wakeboat operating at 9 mph to a ski boat at 22-24 mph. A more appropriate study would be to examine the wake of a deckboat or fishing boat operating at tubing speed (10-15 mph).

This report employs drones and video to measure wave dynamics. In contrast, other studies have relied on direct measurements using capacitance or pressure sensors or stationery above-water radar. Using drones and video analysis to assess wave heights presents several challenges compared to the more established method of capacitance probes. Video analysis entails complex algorithms that can introduce errors if they are not calibrated or applied correctly. It is susceptible to inaccuracies and can be influenced by distortions such as camera angle and lens curvature. Since the authors did not provide information on how they calibrated the wave heights or discuss how their system compared to previous studies, it remains unclear how quantitatively their results can be interpreted. Conversely, capacitance probes utilized by most studies offer a more direct and reliable method of measuring wave heights by physically interacting with the water surface, resulting in highly dependable data. Additionally, capacitance probes are less susceptible to interpretive errors inherent in video-based techniques, establishing them as the gold standard for accurate and consistent wave measurements.

The wave heights discussed in this study do not demonstrate the same reduction with distance as seen in all previous research, including their own referenced works. For example, at 300 feet, a wave height of 12 inches is reported. The average wave height from six other studies utilizing more precise techniques is 8.4 inches, suggesting that their results are 50% higher than those from earlier research. Even the authors prior work only reported a wave height of only 8" at 200 feet. This study reports a wave height of 8”at 500 feet, whereas, in their own prior work, they reported 8 inches (2022)- waves were observed at 200 feet with no explanation given for the 2X difference. The authors cited references include Ray, who reported 8 inches at 300 feet, and Marr et al., whose findings indicate wave heights ranging from 7 to 11 inches 300 feet from the boat. The authors never acknowledge or address these inconsistencies, even with their own research. Why should the reader accept this data? There are also inconsistencies in the report itself. In Figure 9, the authors say the wave heights are from 100, 300, and 500 feet, but the wave traces report 150, 300, and 500 feet. There is a note saying they did the shallow test at 150 feet, but the deep test also says 150 feet.

Finally, within this flawed data set, the rate of wave decay is much less than in all other reports, bringing further doubt to the validity of these results. Six other studies of wave decay vs. distance have an average of 13.7” inches 100 feet from a wake boat and 8.4” 300 feet away, a 36% decrease, with some studies showing a >50% reduction. Much of the data in previous studies was done in deep water where there is no interaction with the lakebed. This study claims there is only a 2”, or 17% change (14’ to 12”) from 100 to 300 feet. This study fails to calculate wave energy and uses a simplistic approach to assign energy as a percentage to the results. This is critical since their wave heights and energy estimate are the basis of their suggestion of a 500-foot setback. Since their data is inconsistent with all other data, their setback proposal lacks sufficient proof to be taken seriously.

The authors rely on an extreme example of a ski boat on a plane and its questionable higher wave heights to justify a significant setback distance of 500 feet. This approach is misleading because, like some previous studies, the study compares wake boats and ski boats operating at speeds that minimize their wave profiles rather than comparing them to other heavy boats of similar length. Ski boats are specifically designed to produce minimal wakes.

A more accurate comparison would involve a pontoon/tritoon or a comparable weighted runabout at speeds of 10 to 15 mph representing other activities on the water such as tubing. These boats generate wakes that are equal to or larger than those produced by wake boats operating at 11 mph at a distance of 200 feet. Data from prior studies also indicates that other boats with substantial water lines or displacement, traveling at speeds of 22 to 24 mph, would require distances of up to 500 feet to match the wave height of a ski boat at 200 feet. For example, a 2024 Crownline 22SS has a dry weight of 4,370 pounds. When fully loaded with a full tank of fuel and four adults, its weight exceeds 5,000 pounds, highlighting the significant impact of heavier boats on wave generation.

References:

Cox, G; MacFarlane, Gregor (2019). The effects of boat waves on sheltered waterways –thirty years of continuous study. University of Tasmania. Conference contribution.https://hdl.handle.net/102.100.100/522918

Clifford Goudy and Lewis Girod, (2015)Characterization of Wake Sport Wakes and theirPotential Impact on Shorelines, https://www.wsia.net/wp-content/uploads/2020/03/WSIA_draft_report_Rev_II.pdf

McFarlane, Gregor (2018), Technical Report Wave Wake Study-HB4099 Motorboat WorkingGroup https://static1.squarespace.com/static/5a0ba0f9e5dd5bce46ef4ed2/t/5c01dec34d7a9cb0b6f25937/1543626456377/AMC+Wave+Wake+Study_HB4099+Motorboat+Working+Group+REPORT.pdf

Marr, J., Riesgarf, A., Herb, W., Lueker, M., Kozarek, J., & Hill, K. (2022). A field study of maximum wave height, total wave energy, and maximum wave power produced by four recreational boats on a freshwater lake (St. Anthony Falls Project Report No. 600). University of Minnesota.

Ray, (2020) Analyzing Threats to Water Quality from Motorized Recreation on Payette Lake, Idaho; Western Colorado University

Tyre, et, (2021-2022) "A Phased Study of the Water Quality and Wave Propagation Dynamics Currently Impacting a Small Southeast Wisconsin Freshwater Lake: Waukesha"; (Phase 1 - 2021; Phase 2 - 2022) Terra Vigilis Group