Where does it happen?
In areas where recreational boating activities occur, it is important that boat operators are aware of the potential impact of their wake. In some popular boating and cruising areas, the effects of wake damage have been severe.
True or false? Some facts many people get wrong.
- Boat wake can’t cause any more damage than wind-driven waves. Yes, it can. Wind driven waves tend to travel along the length of the waterway and directly approach the shore only at bends in the channel. But boat wake may travel almost directly towards the bank and can cause erosion along the entire length of the waterway.
- Most watercrafts are small and light enough not to cause any wake problems. No, they’re not. Speed is just as important as size, and both factors must be considered together. For their size, most outboard powered boats can travel much faster than almost anything else on the water.
- On the plane, the smaller wake causes less damage. No, it doesn’t. Even though a wake reduces in height as the boat planes, the waves are moving faster, further and travelling outwards from the vessel track. When a planeing vessel travels parallel to a sheltered shore, the wave energy is directed towards that shore.
- You can see what your wake’s doing from the boat. No, you can’t. Even if you’re fairly close to the bank (say 164 feet away) the peak wake impact of a boat travelling at 23 mph only occurs as the first 5–10 waves hit. By that time, you’re half a minute and 1,000 feet away. The only way to really observe the impact of wake is to stand on a soft shore and watch the impact of wake in all three phases of boat speed. Every boat driver should do this sometime – you may be surprised at what you see.
There’s a pattern to what you leave behind
Every craft moving over the water leaves a wake. A boat wake has two distinct sets of waves – one following the vessel and the other spreading outwards from its track. The mixture of these two sets of waves forms the wake pattern, which varies with vessel length, speed and water depth. Vessel size and hull form have little influence on the wave pattern but do influence wave height. Only waves that are larger than those a shoreline is normally exposed to will cause erosion. But large does not necessarily mean high, at least not in deep water.
Because it travels faster, a long low wave can carry much more energy than a short high one. There are four wave patterns that a typical powerboat can generate, some of them are dominated by long period waves that can cause a lot of damage where shores are well sheltered from any swell. Understanding when your boat produces these different wave patterns are the key to controlling its impact.
- The Shallow Water Wave Pattern – As a vessel moves into shallow water the wake waves start to feel bottom and slow down. This changes the wave pattern again. Even if the boat is running at displacement speed (but this needs very shallow water) the following waves can’t keep up because of drag against the bed, so all the waves spread outwards from the vessel track. The leading wave has a characteristically straight crest, and all waves are much straighter than for the planing wake pattern. All waves are feeling bottom and potentially stirring up the bed.
- The Displacement Wake Pattern – Displacement hulls generate bigger wake as their speed increases. They are efficient up to a point (sometimes known as hull speed) beyond which an increasing proportion of additional propulsive power is spent making waves for little increase in speed. Generally speaking, the longer the displacement hull the faster the cruising speed. Below hull speed most of the energy lost to wave making is spent on short waves that follow the track of the vessel.
- The Transitional Wake Pattern – The high power-to-size ratio of high speed and planing vessels means they can power through the hull speed barrier into transition mode. Here the stern “digs in” as it falls into the trough of the bow wave. The largest possible waves are generated, as fuel consumption skyrockets. As speed increases the wave pattern changes because the waves following the boat start having trouble keeping pace and begin to die away, with waves spreading out from the vessel track becoming dominant.
- The Planing Wake Pattern – When a boat rises onto the plane there is less hull in the water to make waves. The total energy of the wake is less than in transition mode but still greater than in displacement mode. By now the following waves have all but disappeared, so in a confined waterway all the wave energy is heading towards the banks. If you have a close look at the planing wake pattern, you’ll see that the waves curve outwards and become longer away from the vessel track. Long waves don’t just carry more energy – their motion also reaches deeper into the water column and can stir up a muddy bed 16 or even 30 feet below the surface. This can mean problems in surprisingly large water bodies where even the strongest winds only create a relatively short chop.