Scuba divers should learn the basics of currents and tides to make their progress in the water much easier. Currents usually alter their force and direction from hour to hour without any warning. Swimming against a current forces scuba divers to exert more energy and burn oxygen much faster. Scuba divers should orient themselves frequently to stay on course and make progress without exerting too much effort.
After a relaxed exploration along the reef, your pressure gauge says you’ve used half of your available air, minus 700 psi for reserve. So you turn around to swim back to the boat. Immediately, you find yourself heading into a strong current. Now you have to fin hard to make headway, doubling your air consumption. Will you have enough to make it back to the boat?
Like rude in-laws, currents seem to show up unannounced at the most inopportune moments.
This is often a matter of perception, You hardly notice a current when it is at your back and helping you – until you turn around. If you swim at 1/2 mph with a 1-mph current at your back, you achieve 1 1/2 mph over the bottom. But when you turn around, you’d have to swim 2 1/2 mph – five times as fast – to make the same progress.
To make matters worse, currents may alter their force and direction significantly from hour to hour and, what’s more important, from one coral head to the next.
Table of Contents
- Is This Really a Big Deal?
- But Currents Can Be Predicted, Right?
- Is Local Knowledge Helpful?
- Can You Read the Current by Looking at the Water?
- Once in the Water, Can You Sense the Current?
- Making Your Go/No-Go Decision
- Eight Ways To Fight Back
- Special Current Problem: Beach Diving
- Special Current Problem: Slack-Water Diving
- SPECIAL CURRENT PROBLEM: Drift Diving
- SPECIAL CURRENT PROBLEM: Rips
Is This Really a Big Deal?
It can be. Currents are powerful forces, and your fins are not. Most divers cruise between 1/4 and 1/4 mph; only a few can maintain more than 1 mph over any distance. But a l-mph ocean current is only moderate, and 2 mph is common. The current in places like Cozumel frequently exceeds 3 mph, and tidal flows through narrow passes can top 8 mph. By contrast, the fastest sprint over 300 feet ever recorded in ScubaLab fin tests was just under 4 mph, and at the end of that 300 feet, the exceptionally fit test diver was exhausted.
A little more perspective: a 2-mph current carries you 176 feet per minute. How many minutes will take you out of sight of the dive boat? Obviously, dealing with currents requires using your brain as well as your fins.
But Currents Can Be Predicted, Right?
Not easily or consistently. You may have been told that large-scale ocean currents circulate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. So the prevailing current along the West Coast of the U.S. is north to south, and along the East Coast is south to north. But that’s only the background to the picture.
* Wind Currents. Currents generated by winds far out to sea follow their own course.
* Tidal Currents. Currents are generated by the ride flowing up onto the continental shelf and into bays, then reversing direction every five or six hours (in most places).
* Freshwater Currents. Streams and rivers can produce currents felt miles out to sea or down the coast.
All these components combine, conflict and react to the shoreline and the bottom contour, skipping across bays, bouncing off headlands or eddying behind them, accelerating, slowing and even circling back on themselves. Sometimes, differences in temperature or salinity keep currents from blending. As a result, currents may lie side by side or on top of one another for many miles, with an easily visible line between them. That’s why it’s not uncommon to have, say, a northbound current at the surface and a southbound current 50 feet below it.
Is Local Knowledge Helpful?
Yes. Any particular dive site has a fixed topography, therefore only a limited bag of tricks. So an experienced local diver, who knows the weather and the state of the tide, can predict pretty accurately what currents you’ll find there. When boat diving in an unfamiliar area, one of the most important reasons you should listen to and understand the pre-dive briefing is to get information about currents.
Can You Read the Current by Looking at the Water?
To do so with any degree of accuracy requires much experience. Normal clues can be deceiving.
- Wind-driven waves
They may look like moving water, but in fact only the wave form moves. The water itself remains stationary or, more likely, moves in a different direction.
- Direction the dive boat points
After it has settled back on its anchor, the boat’s bow would seem to point into the current. But this is only helpful if there is little or no wind. Most dive boats have high superstructures, so they’re influenced by wind more than by current. You might expect the boat to point toward some compromise between wind and current, but a moderately strong wind may cause a boat to do strange things, veering off to one side of the wind or oscillating back and forth – even when there is no current at all.
- Current line
A better clue may be the floating current line laid out by the boat crew, unless there is a large, high-floating buoy on the end of it to catch the wind.
Some divers watch the direction and speed of debris floating in the water. They look for bits of weed and algae, the cigarette butt dropped by that fool at the bow – there’s nearly always something tiny in the water floating by. Again, this can be misleading. Very light debris, like cigarette filters, may be driven by wind as much as by current, and the direction of any debris close to the boat will always be along the side of the boat regardless of the direction of the true current.
More reliable current indicators:
- Divers in the water
Divers who have entered the water before you and are still on the surface behind the boat are a better indication of the current. Which way are they drifting?
- Current wakes
A buoy or a stake will show a wake from the current, and a buoy may lean with it, as will kelp or seaweed below the surface.
Once in the Water, Can You Sense the Current?
Only if you have a stationary reference. One of the best references is the anchor line or descent line. Before you leave it, adjust for neutral buoyancy so you can hang without tinning and see which way you drift.
Since it is rare for the current to remain consistent in force and direction for your entire dive (unless you stay in a small area), as you move through your dive, occasionally check on the current by stopping and watching your drift in reference to the bottom, a wall or kelp.
In general, currents will follow the bottom contour, but expect surprises where the topography changes radically, such as at reefs, outcrops, walls, pinnacles and the underwater extensions of points of land. lt’s similar to the behavior of the wind in a breezeway or near the corner of a building.
Be especially alert for vertical currents: upwellings and downwellings caused when a current meets a vertical wall. When the water meets the wall face, it has to go somewhere, and it is likely to go up or down as well as to one side or the other. Or when it flows from a shallow area over the top of a wall: it may flow down the wall face causing a downward current, or it may flow out and eddy back, causing an upward current at the wall face. Either situation can quickly take you deeper or shallower, although you may have no sensation of moving through the water. If you are swimming along the face of the wall, it may not be apparent that you are also going down it. Remember, 2 mph is 176 feet per minute.
Making Your Go/No-Go Decision
As always, it’s easier to avoid problems than to solve them. The apparent strength of the current should be one of the factors in the go/no-go decision you make before you even get wet. Remember that as a mere human, you will be among the least powerful swimmers in the ocean.
Again, when you’re hanging on the anchor line gauging the actual current at depth, you should make a conscious go/no-go decision.
Your next decision should be pretty obvious too: start your dive swimming against the current so it helps you on your return to your entry point. This is a good rule even when the current is very weak. No matter how weak, it will seem strong when you are tired and bucking into it. Also, a weak current may become much stronger when it reaches a point of land, creating a barrier that will be difficult to cross on your return.
Eight Ways To Fight Back
Despite the best-laid plans, it may become necessary to fight a strong current. How can you minimize the current and maximize your effort?
- Sidestep it. Instead of fighting the current head-on, move to where the current should be weaker. If the current is accelerating around a point, swim wide of the point. If it is flowing down the face of a wall, swim away from the wall. If a current “rip” (incorrectly called a “riptide”) is carrying you away from the beach, swim parallel to the beach. If the current appears suddenly as you descend below 60 feet, get shallow. But most of the time you’ll probably find less current close to the bottom or the shoreline, where friction slows the flow of water.
- Fin efficiently. A short, rapid stroke with a full, nearly Straight leg is most efficient. When you make large strokes or bend your knees, your legs and fins cause more drag. Resist the temptation to sprint, unless you only need to penetrate a short zone of high current. Instead, find a pace you can maintain for a distance. If you are on the surface, go down a few feet where your fins are far more efficient or swim on your back. Concentrate on steady breathing and a steady fin stroke.
- Get neutral. Adjust your buoyancy carefully so all your energy is spent on propulsion, not maintaining your depth, and so that your legs are horizontal, not hanging down and increasing drag.
- Streamline. Clip or hold accessories close to your body in the most streamlined position possible.
- Orient yourself. Check your compass frequently so you don’t veer off course and increase the distance you have to swim.
- Grab something. If you’re not making adequate progress, you may have to go to the bottom and hold onto something, despite the no-touch rule we are all taught. Be as friendly to the environment as you can – grab dead coral and rocks, for example – but in the last resort, your safety comes first. You can pull yourself from rock to rock along the bottom with far less energy than you would use tinning.
- Use surge. Sometimes you can use the rhythmic surge of wave action to help you through a zone of strong current, such as where it speeds around a point. When the surge coincides with the current, hang on to a rock. When the back-flow negates most of the current, sprint to the next rock.
- Know when to say when. When all else fails and you just can’t make progress against the current, surface and signal for help. Don’t delay, because with every second the current is taking you farther from the dive boat.
Special Current Problem: Beach Diving
There is almost always a current along the shore. Despite the direction the mass of water comes from, when it reaches the shoreline it has to turn and follow it. But be careful about deep bays and headlands, where you can expect eddies and countercurrents. Even along a straight coastline, sometimes the in-shore current runs in the opposite direction of the main current farther offshore.
Any beach dive has to start with determining the direction of the current through the dive site, then locating an alternate exit down-current. When in doubt of the actual current, find an alternate exit in each direction.
Special Current Problem: Slack-Water Diving
What is slack water?
Probably the strongest and most changeable currents are generated by tides. In areas of extreme tides, like Puget Sound, it is important to plan dives around the time of slack water at high or low tide. That’s when the tidal current slows to a stop (in most places) before reversing direction. Usually there’s a half-hour or so of slow, diveable current on either side of slack water.
How do I find it?
Not necessarily in the tide table printed in the newspaper. Tide tables predict only the vertical movement of water, not the horizontal movement of tidal currents. Slack water usually does not occur at the same time as high or low tide. Instead, it may be as much as two hours later than high or low tide, depending on the site and the day.
You may need to consult a current table, which is harder to find but is available from some marine supply stores and the U.S. government printing office. An alternative is $130 of software called “Tides & Currents For Windows” which predicts both through the year 2100. One source for it is www.tides.com, where you can also click on the “Daily Predictions” link and search by destination for a daily tide or current table.
SPECIAL CURRENT PROBLEM: Drift Diving
The idea is to use the current for propulsion, drifting through the dive site without expending much energy. In its usual form, “live-boat diving,” the dive boat follows your bubbles and picks you up far down-current of your entry point. Cozumel and Florida Gulf Stream diving are two popular examples.
The sensation of hanging motionless while Cozumel’s Palancar Reef sweeps by makes this kind of diving seem as effortless and safe as riding a tour bus, but there are obvious dangers. One of the biggest is separation from the group and, therefore, from the dive boat. You will notice that the group tends to spread out without meaning to and the divemaster, if she’s good, will spend much of her time herding charges back together. She may, for example, use the still water behind a rock outcrop as a “bus stop” to re-form the group. Don’t miss the stop!
If, for some reason, you have to leave the group and surface, inform the divemaster if at all possible. Look out for boat traffic and when you reach the surface, inflate your sausage unless you are spoiled and picked up immediately. Alone or with just one buddy, you are not as visible as the group.
Another danger is strong upwellings and downwellings at walls and buttresses. It’s important to monitor your depth gauge even more than in areas of mild currents.
SPECIAL CURRENT PROBLEM: Rips
Another beach dive problem is the “rip” current caused by a gap in a sandbar just off the beach. Between the sandbar and the beach is, in effect, a lake. As this “lake” is filled and emptied by the surf, water flowing through the narrow gap can accelerate to high speeds. If you’re caught in a rip carrying you to sea, swim across it, not into it, until you are out of the rip area. Swim parallel to the shore and down-current, so you’re not carried back into the rip by the current.