Wake up

Mark Robinson | VOLUME 22, ISSUE 5

We look at the complex science of waves and the simple steps you can take for the protection of moored boats, especially your own.

We’ve all seen those languid waves slowly rolling across a channel in the wake of a vessel. As waves go, they’re not very high; they almost seem innocuous. But these seemingly mild waves pack a considerable punch. And judging by our claims histories, it’s a punch that frequently causes considerable damage to boats.

It can be tempting when traveling through ‘No Wake’ zones to speed up just a little. When the wide open sea beckons, it’s hard to putter along patiently and when the day is over, who hasn’t felt the urge to gun it down the home stretch to the dock? You might think a little extra won’t really hurt, but the truth is, it most certainly can. According to marine scientists concerned with shoreline erosion, a boat speeding just two knots above a posted five-knot limit creates a wake with a far greater force and a far greater potential for damage.

To understand why this is the case, we need to look at the dynamics of a wave. Waves are actually energy. Energy – not water – is actually what moves across the water’s surface. Water particles inside a wave travel in a circular motion, moving forward on the crest and backwards on the trough (depression in the wave), with vertical movement occurring between the two. The net movement of water is in the direction of the wave.

To picture this, think about how often we see objects such as birds, boats and even people floating on the water surface being passed by wave crest after wave crest, yet remaining in the same position.

WAVE AWARENESS

Another way to understand waves as energy is to think of a long rope laid on the ground. If you pick up one end and give it a good snap, there’s a ripple effect all the way to the other end, just like the waves on the ocean. When energy is applied at one end of the rope, it moves to the other end. At the other end, the energy is released, just as the energy of waves is released on shores or on immovable objects adjacent to the shore.

Ocean waves are generated by wind, and that wind can be many thousands of nautical miles away. But the forces generated are so powerful that the energy travels huge distances before rolling ashore in the form of surf or breakers. Wake waves have a different genesis to those of the open ocean. They’re formed by pressure differences along a vessel’s hull. But just like ocean waves, wake waves are characterised by height and period. The period of a wave is the time the full wave takes to pass a given point. To measure a wave period, pick a stationary point and count the seconds it takes for two consecutive crests or troughs to pass it. Taken together, wave height and period give a wave its power, with height affecting power twice as much as period.

Every moving vessel leaves a wake, which is made up of two distinct sets of waves. One follows the craft, the other spreads outwards from its track. The combination of these two waves forms the wake pattern.

The degree of wake formation is affected by the two most common distinct hull modes: pure displacement and pure planing. However, hulls also operate in the transition region between these two modes as they accelerate and decelerate.

Displacement speed – when the craft is travelling slowly and the bow is down – creates the least wake. Transition speed creates the largest wake. The transition region is entered when power is increased while attempting to get onto plane. The rising of the bow causes the stern to plow through the water. At planing speed, the bow drops back down and a lesser amount of the hull contacts the water and thus creates less wake than the transition period – but still much more than at displacement speeds.

At this point it is tempting to draw the conclusion that at planing speed, the wake effect might not be too bad. Sure, it’s more of a wake than at displacement speed, but when looking astern, the wake doesn’t look too bad. Wrong. Despite the fact that a wake reduces in height as a boat planes, the waves created move faster and further and travel outward from the vessel track. As a planing vessel is traveling parallel to a shore, the wave energy is directed to that shore.

PROTECTION

So much for making waves, but what can we do to protect our moored or docked vessel from their effects? The first thing to do is to understand and appreciate the amount of force wake waves generate. To understand this, we need to consider the impact of these waves when they meet a solid structure. When waves meet seawalls, jetties and other steep or solid structures, the wave energy is reflected back out in the direction of the wave source. This reflected wave energy is then added to the next incoming wave. And there, in the middle of all this incoming and outgoing energy, sits your pride and joy.

Protecting your craft requires the judicious use of fenders of sufficient size and quantity and knowing how to best secure your craft with the appropriate lines. The rule with fenders is ‘the bigger the better,’ so err toward oversize rather than undersize. Sure, they can be a pain to stow, but with a little creative thinking, solutions can be found. And don’t think that just because a certain number and size of fender came with your megadollar vessel that no further thought is required. Manufacturers all too often sacrifice craft protection for aesthetics and place a minimum number of the smallest-sized fenders on a craft.

Dock lines in sufficient numbers and appropriately-placed fenders are the only things preventing your boat from smashing against the pier, seawall or mooring pontoon so proper use becomes paramount.

You should have at least six dock lines, consisting of two bow lines, two stern lines and two spring lines. The dock lines should be at least two-thirds of the length of your boat and the spring lines should be at least the full length of your boat. Although spring lines have many handy uses when docking or undocking, they also help to prevent damage by preventing fore and aft movement of your craft.

The size (diameter) of your lines is dependant on the size and weight of your boat. The following is an approximate guide. If your boat is heavy for its size, consider going up one size in diameter. Although a boat can’t be damaged by having its dock lines a bit oversized, lots of damage can occur because lines are too small.

Line Diameter Guide:

• Boats under 6m: 10mm

• Boats 6-9m: 13mm

• Boats 9-12m: 16mm

• Boats 12-18m: 20mm

• Boats over 18m: 24mm

A knot will reduce a line's breaking strength by as much as 50 per cent so you should avoid them as much as possible. The only knots a dock line should have are those that are tied around cleats or pilings. The art of tying nautical knots and the proper use of bow, stern and spring lines are essential skills that all boaters should learn and use.

In summary, you can protect your vessel and avoid damaging others by:

• Keeping your speed down in No Wake zones or anywhere else you observe any likelihood of your wake creating damage

• Waiting until you are well clear of areas where there are craft moored and then moving promptly from displacement mode to planing speed without lingering in the transition mode

• Having a sufficient number of well-sized fenders and knowing where to place them

• Using strong dock lines tied in a seaman ship like fashion

Ultimately, nature tends to take its own course when it comes to wave generation. The vast majority of the time, waves are moving back and forth across any given body of water. But equally, in the vast majority of cases, marina-based and berthed craft enjoy relative protection from the elements. In these cases, it’s the movement of all the other craft in the vicinity that poses the greatest threat.

If you’re the one doing the moving, spare a thought for those boat owners whose craft are berthed around you and take the attitude that you will proceed as though your own boat was in the path of your wake. Similarly, if you have your boat berthed, ensure that it is secured to allow for the worst possible scenario.

Berthed craft will always be vulnerable to damage from wave movement. But being aware of the possibilities will at least give owners the opportunity to keep damage to a minimum.


Tags
How to-Safety
Share
Subscribe
Previous
Next