Tools · Currents

Crossing a current is just vector addition.

A current doesn't really push on you — it carries the whole sea you're swimming through. Your speed over the ground is your speed through the water plus the current, added like arrows: the same triangle a pilot flies in a crosswind, or a rower pulls across a river. The reason it fools divers is that finning feels like progress — but the seabed keeps score on the sum, not on your effort. Point at a mark across a current and you won't arrive there; you'll arrive downstream of it.

So set a current and your own swimming speed below, and the tool solves where to aim to actually reach the mark — and shows you what it costs.

To reach a mark straight across a current, the one thing you must not do is aim at it. Point straight at the mark and the current spends the whole crossing sliding you downstream, so you land somewhere below it. Instead you aim upstream, by just enough that your sideways swimming exactly cancels the current. That angle — the crab angle, or ferry angle — comes straight out of the triangle: sin θ = current ÷ swim speed. You spend part of your effort holding station against the flow, so your speed toward the mark drops to √(swim² − current²) — slower, but along a clean straight line. It's the same trick a ferry captain uses to dock straight across a river, and the same one you'll feel on any drift-prone wall.

There's a hard wall hiding in that formula. sin θ = current ÷ swim only has an answer while your swim speed beats the current — once the current is faster than you can swim, no heading on Earth holds the track. Aim fully upstream, throw everything you have at it, and you still slide backwards at the difference. A fit diver swims maybe half a knot; plenty of real currents beat that easily, which is why "we couldn't make it back to the boat" is almost never a fitness failure — it's arithmetic. The honest response isn't to fight harder; it's to plan a downstream exit, or to dive it as a drift dive and let the water do the work.

And fighting it is expensive in a way that compounds. Drag rises with the square of your speed through the water, so the power you spend — and the gas you burn — rises with the cube. Doubling your speed to claw against a current doesn't double the effort; it roughly octuples it. Worse, to make any headway against a current you must swim faster than it and you only net the difference, so the air cost per metre of ground runs away as your speed approaches the flow — pouring everything in for almost nothing. (You can feel the same cube law in the trim tool's drag, and watch it drain a tank in air consumption.) That runaway is exactly why the old rule is to spend the first, freshest part of a dive swimming into the current, then turn and ride it home when you're lower on gas and energy.

When the current is tidal, you can often just wait it out. The rule of twelfths approximates a tide as a sine wave — over the six hours of a flood or ebb the water moves about 1, 2, 3, 3, 2, 1 twelfths of its range each hour — so the flow is fiercest at mid-tide and falls to almost nothing at the turns. Slack water is the dive window. It's a rule of thumb, not a law of physics — it leans on that sinusoid assumption and real coastlines bend it — but it's a good enough model to plan around.

One honest limit, in the spirit of the rest of these tools: the triangle assumes a single, uniform current. Real water shears with depth — ripping across an exposed surface, slack in the lee of a wall or down in the structure of a reef — so the geometry tells you how to cross a current, not where the fast water is. Dropping low or tucking behind cover is often a better answer than a better heading. This is the same vector addition the Reef Cartographer course leans on to hold a clean survey lane: the tow float lags behind and overshoots on turns for exactly the reason a mis-aimed ferry misses its mark.

A playground for intuition, not a dive plan or training. Real currents shift with depth, tide, and terrain — read the site, ask the local divemaster, and get proper instruction before you trust any of it in the water.