Wakesurfing: Some Wakes are More Equal than Others

Australasian Coasts & Ports Conference 2015

Ruprecht, J. et al.

15 - 18 September 2015, Auckland, New Zealand

Wakesurfing: Some Wakes are More Equal than Others

Wakesurfing: Some Wakes are More Equal than Others Jamie E. Ruprecht1, William C. Glamore1, Ian R. Coghlan1 and Francois Flocard1 1 Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Australia, Manly Vale, Australia; [email protected] Abstract Recreational boat usage and ownership in Australia is at an all-time high. Every vessel that moves through the water generates wake waves. Of particular interest has been the proliferation of recreational vessels designed and manufactured for the sport of wakeboarding and more recently, wakesurfing (a popular alternative activity to wakeboarding). Wakeboarding/wakesurfing vessels are designed, through the use and control of ballast and customised trim, to maintain a breaking wave at the optimal operational speed (typically 10 knots for wakesurfing and 19 knots for wakeboarding). The Decision Support System (DSS) developed by [6] provides a standard methodology for assessing the vulnerability of a shoreline to erosion, providing management recommendations on the likely impact of recreational boat wake waves along a waterway using an evidence-based approach. The DSS is underpinned by a database incorporating extensive field measurements of boat wake waves for water skiing, wakeboarding and wakesurfing activities. To test the hypothesis that wakesurfing waves are equivalent to wakeboarding waves, a series of field measurements were undertaken on three late model wakeboarding vessels at a range of operating speeds and ballast configurations. The tests were undertaken in a controlled environment (deep water, no currents, controlled boat speeds, repeat runs etc.) using state-of-the-art measuring equipment. The results of the field tests indicate that the wave energy associated with the single maximum wave height (Energy Hmax) for the wakesurf “operating conditions”, is approximately four times that of the wakeboard “operating conditions”, and twice that of the wakeboard “maximum wave” conditions. Keywords: wakeboard, wakesurf, waterski, riverbank erosion, waterway management. 1. Introduction The sport/recreational activity of wakeboarding involves being towed behind a customised towing vessel at elevated speeds (typically about 35 km/hr or 19 knots) in a manner similar to water skiing. As in water skiing, wakeboarding is usually undertaken in low energy environments such as inland waterways, ports and harbours. However, in contrast to water skiing, wakeboarding involves utilising the wake waves to conduct manoeuvres, using the wash as a launching ramp. It is important to note that the largest possible wake wave is not necessarily the best wave for wakeboarding. Typically, the optimal wake wave is one that is large enough to provide a sufficient ramp for aerial manoeuvres but that simultaneously contains the optimal slope (i.e. wave shape) for approach. A wave that is too big will break and thus not provide the adequate angle for lift.

operating at slower speeds then water ski vessels; and (v) installing elevated towing platforms. More recently, with sophistication in the design of specialised wakeboarding vessels incorporating WEDs, the sport has seen a rise in a popular alternative activity to wakeboarding, being that of wakesurfing. Wakesurfing involves creating a large wake on one side of a boat that can be surfed without a tow rope. The creation of a large wake at the optimal operational speed (typically about 18 km/hr or 10 knots) is assisted by placing the majority of the ballast near the aft (i.e. stern) corner on the side the vessel to be surfed (biased ballasting). Consequently, the wake generated off the opposite side of the boat is considerably smaller. The predominant factor that has limited the popularity and growth of wakesurfing as a sport has been the lack of boats capable of making optimal, surfable waves at a safe distance behind the boat.

To maximise the amount of lift without creating a breaking wave, wakeboarding vessels began to use wake enhancement devices (WED). The primary purpose of the WEDs is to increase the speed at which the vessel can maintain its critical Froude condition, thus ensuring that a large displacement wave is generated at speeds of between 12 to 19 knots. WEDs are now designed to optimise wake waves by (i) increasing the ballast in the vessel (either through inflatable water bags or internal ballasting); (ii) modifying the hull design; (iii) installing wedge platforms on the stern of the vessel which impacts vessel trim; (iv)

The impact of boat waves generated from recreational vessels on the shoreline has become an increasingly important field of research [2][4][5][6][8]. A significant step in understanding the difference between boat wake waves generated from wakeboarding and water skiing vessels is presented in [5]. [5] presents the work of [7] which measured wakeboarding and water skiing vessel generated boat waves from controlled experiments conducted on Manly Dam, Sydney, NSW, between 2004 and 2005. The 1


Ruprecht, J. et al.



results presented in [5] show that the wave energy produced by wakeboarding and water skiing vessels is not significantly different. In fact, the results contradict previous anecdotal evidence regarding wakeboarding wave height which have suggested wave heights >0.5 m [1][3].

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However, how do these results compare with boat waves generated from specialised wakeboarding vessels equipped with WEDs designed to increase the size and enhance the shape and location of the wake for wakesurfing activities? Ultimately, are wakesurfing waves equivalent to wakeboarding waves? This paper provides the methodology used to answer these questions and addresses the implications of wakesurfing activities on current best practice for shoreline management.

To measure the propagation of a wave train from a test vessel, an array of equipment was deployed across the site. A 250 m long sailing line was setup using four floating buoys. At distances of 22, 35 and 75 m from the sailing line, three submersible wave probes were deployed. Each probe was a battery powered RBRduo TD pressure transducer which logged data internally at 6 Hz. Each wave probe was secured to a portable mounting rig composed of modular pipe lengths and a weighted base. Following deployment, GPS waymarks were taken at each sailing line float and each wave probe.

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2. Methodology The objective of the field testing program was to accurately measure wake waves from three different late model wakeboarding vessels travelling at a range of speeds, to update and expand the existing DSS database (for wakeboarding and wakesurfing) and improve statistical robustness. For this experiment, the best practice methods outlined in [5] were again applied for measuring wake waves. These methods emphasise full scale testing of vessels using multiple wave probes deployed at distinct distances from the sailing line at a location without strong currents or wind energy, with water sufficiently deep so that the waves are not depth affected. Importantly, this method is repeatable and allows for subsequent comparison of wake waves without external impeding factors and is enhanced by extensive quality assurance checks.

The site was located in a straight stretch of the river which provided an optimal sailing line for the approaching vessels; and At the time of the field testing program, erosion was observed at riverbank sections in the vicinity of the site location.

During the testing program, two ‘Control’ vessels were anchored approximately 50 m downstream and upstream of the sailing line. The downstream ‘Control’ vessel used a calibrated radar gun to check the speed of an approaching/departing vessel prior to it passing the line of wave probes. Note that vessel testing was undertaken alternatively in both directions; upstream and downstream (except for wakesurf testing which was only undertaken in the downstream direction). A laser rangefinder was used to accurately calculate distance between the wave probes over water. The weather conditions throughout the testing program were considered ideal. Wind speeds at the site varied between calm and up to 5 knots. There was no rain during the vessel tests. The testing program was undertaken on a falling tide; currents were slack at commencement (on approximately high tide) and directed weakly downstream at its conclusion.

The 2014 field experiments were carried on the Clarence River, near the Junction Hill Boat Ramp, in Grafton, NSW. The site location was selected from a range of possible testing locations because: • The river was moderately deep throughout (i.e. waves would not be depth affected but it was shallow enough for wave probe stations to be deployed); • The river bed at the site was very even (i.e. the water depth was spatially constant) • The river width was sufficient for deploying three wave probe stations; • The site was partially sheltered from wind energy and, as such, the measurements were not greatly influenced by background noise; • During the test program, the site was not influenced by strong currents that would affect the measurements; • The sloping shoreline (composed of complex sediments and reeds) absorbed the majority of the wave energy, thereby eliminating wave reflections (which would be measured by the wave probes);

For this investigation, three wakeboarding vessels were tested on 9 May 2014 (Table 1). During testing, each boat was operated by an independent boat captain familiar with the vessel. Each boat was tested under a range of trim and ballasting arrangements. To obtain a statistically robust data set, a comprehensive testing program was developed. Each vessel was tested at a complete range of speeds including 4, 8, 10, 14, 19, 24 and 30 knots. All vessels were tested with full ballasts (except 10 and 30 knots), without towing a rider and with 1 to 4 people onboard. Biased ballasting was used at 10 knots to undertake an examination of waves generated in association with wakesurfing. Empty ballasting was used at 30 knots for comparison with waves generated by waterski vessels at their 2


Ruprecht, J. et al.



operational speed. Six replicate runs were completed for each vessel at each speed (except at 10 and 30 knots with only 3 runs each). This resulted in a total of 36 test runs per vessel.

derived from the 18 vessel runs at 4 knots due to the very small wave wakes produced (i.e. it was not possible to differentiate between boat wake waves and small wind waves for these tests).

Table 1: Vessels Tested

ID

1

2

3

MakeModel

Malibu Wakesetter VLX (2014) Tigé RZ2 Platinum Edition (2011) Super Air Nautique G23 (2014)

Engine (hp)

Lengthbeam (m)

Boat Type Wake/Ski

Speed (knots) at Critical Froude Fnl=0.5

Indmar 409

6.55/2.53

Wake

7.8

PCM 450

6.71/2.59

Wake

7.9

PCM 409

7.01/2.59

Wake

8.1

To remove the influence of small wind waves present in the boat wake data, the “significant” wave height was set to 0.04 m (consistent with [7]). That is, the minimum value considered in boat wake wave analysis was 0.04 m and wave heights smaller than this were excluded from calculations. 4. Results Time-history plots of wake waves were generated for each vessel run. The individual plots were stacked for each wave probe to provide a graphical representation of the wave train evolution over time and distance. To help illustrate the findings from the study, an individual wave trace from Boat 2 at 19 knots is provided in Figure 1. The top (red) line indicates the probe closest to the sailing line (22 m), while the middle (green) line indicates the wave by the time it reaches the middle probe (35 m) and the bottom (blue) line shows the wave at 75 m from the sailing line. In general, Figure 1 shows that while wave height attenuates with distance, wave period remains fairly unchanged. The wave traces also indicate that the total wave train energy remains largely constant between measurement probes.

3. Data Analysis Following the field testing program, the pressure sensor data from each wave probe was downloaded. Initially, a high-pass filter (>0.25 Hz) was applied to the raw pressure data to remove the tidal signal. Then the raw pressure data was converted to water surface elevation time series using the technique of Nielsen (1989), reproduced in Equation 1. (1) = water surface elevation corresponding where to the nth central gauge pressure reading (m); = nth central gauge pressure reading (Pa); ρ = water 3 density (998 kg/m ); g = acceleration due to gravity 2 (9.81 m/s ); δ = sampling period of the data (1/6 s ≈ 0.17 s); yp = height of the pressure transducer above the river bed (m); D = water depth (m); (-); (-). Note that the water depth at each wave probe varied between 4.2 and 3.7 m on a falling tide during the field testing program. Similarly, the wave probes were located between 1.0 and 0.5 m below the water surface during this time. The converted water surface elevation data was normalised to a distance off centreline and low-pass filtered (