Are you familiar with the Coriolis Effect and the Indonesian Throughflow (ITF)? When sailing in this part of the world, they’re pretty important concepts to be familiar with.
The currents around Lombok regularly run at 3 knots, even making it to 10 knots on occasion. Sailing here is impossible if you’re trying to go through the wrong way at the wrong time.
Navigating the Notorious Currents of the Lesser Sunda Islands
We found ourselves questioning the powerful forces of the ocean. Why are the currents in this area so strong? Is it the Coriolis effect, or are these tidal currents? In this part of the world, the currents are notorious, and the channel between the Indonesian Sunda islands of Lombok and Sumbawa, where we are now, is no exception.
In the northern part of this channel, the currents are running at a gentle three to four knots. However, we know of boats that have had to turn around because the currents were running faster than they could sail or motor. We’re going to talk more about currents, both local and global, in a bit. But first, a gentle sail eastwards towards the sleeping giant that is Sumbawa.
Sailing Towards the Sleeping Giant
As we approached the west side of Sumbawa, we were struck by its sheer size compared to Lombok. It’s no wonder it’s been called the “sleeping giant” of this chain of islands along the bottom of the Java Sea. This nickname also alludes to the fact that Sumbawa was home to the biggest volcanic explosion known to man, which occurred on its east side. The port here is incredibly busy; we’ve never seen so many ferries going backwards and forwards.
Despite the traffic, it was a beautiful sail. We did have a bit of a current against us, which meant our heading was about 30 degrees off our course. As we navigated, we kept a close eye on the water depth as it started to get shallow. We carefully passed a single rock with a long, thin, shallow patch coming out from the south of it. The scenery was just beautiful, and we looked forward to tucking ourselves behind one of these islands. The water depth went from 800 meters down to 22 meters quite quickly, signaling it was time to get back to the helm.
A Beautiful Anchorage at Kanawa Island
That was a really nice little sail across, especially considering we had all those counter-currents when we left. Weighing anchor was fun and games, but we got out and slowly motored down the side of the island. Once we were aimed in the right direction, we got the sails out, and it was a really quite gentle journey.
We arrived at the island of Kanawa, which has a big peak on it. This spot was recommended to us by a local lad who works at a dive resort on the east side of Gili Gede. We had bumped into him in his village and got chatting. He loves coming here, bringing his scooter across from Lombok on one of the hundreds of ferries. He leaves his bike at the port, gets a little boat over to the jetty, and then walks up to the top of the hill. We decided we would do that ourselves and send him a picture to say thank you for the recommendation.
Understanding the Currents: Global and Local
Ocean currents are driven by three main factors: gravity, water density, and wind. They are continuous and predictable, moving either horizontally or vertically, and they influence climate and weather patterns around the world. As global winds drag on the water’s surface, the water builds up and moves in the direction of the wind.
This movement is affected by the Coriolis effect. In the Northern Hemisphere, the wind is deflected to the right, and in the Southern Hemisphere, it’s deflected to the left. This pattern of deflection, along with the Earth’s rotation, creates five major spirals of ocean-circling currents, also called gyres. Perhaps the most famous current is the Gulf Stream, which begins in the Gulf of Mexico and extends towards Europe, responsible for the warm climates in those areas.
Sailors have been taking advantage of steady ocean currents and winds for millennia. Since these change throughout the year, there are well-defined times in which to make ocean passages. For example, in February of this year, a passage northwest through the Java Sea would have been difficult. With the winds blowing from the west across the Flores Sea, a journey eastwards would be more desirable, while a journey northwest would have been almost impossible, fighting both wind and counter-currents.
For much of the time, the currents are running from west to east in this area. But during the summer months, when the winds are blowing from the southeast, the currents switch, making a passage through the Java Sea quick and lively. If you miss a weather window to travel in a certain direction, you have two options: head into a challenging sea for an uncomfortable or even dangerous passage, or sit and wait until the next one comes around, which could be up to nine months.
While global patterns are predictable, local currents across the Sunda Islands are less so. You can’t talk about currents in this area without mentioning the Indonesian Throughflow (ITF). This is the monstrous amount of water continually pushing from the Pacific Ocean through Indonesia’s islands into the Indian Ocean. This, combined with tides, monsoons, and local anomalies, creates huge and sometimes confusing currents. The bathymetry—the underwater landscape of deep trenches and basins—weaves the ITF around, creating upwells, eddies, and downdrafts. When you add in the tides, currents, and trade winds, you have a recipe for unpredictable and challenging conditions.
The ITF is strongest during the southeast monsoon (June, July, and August), when currents have been known to reach 10 knots. Keeping a continual check on the sea surface and using local knowledge when possible can save you from an unrewarding sail against impossible currents.
Navigating with an Eye on the Tides
As we left our beautiful anchorage at Kanawa, we were once again met with a strong current against us. We checked our charts and saw that we had about an hour before the tide turned. Rather than fight it, we decided to drop anchor and wait. It was a good decision. After about an hour, the current abated, and we were back up to a respectable speed, ready to continue our journey.
Accompanying Video
In Episode 352 we explain in detail what happens to the currents in our seas around the planet. And we look at how the Indonesian Throughflow from the Pacific Ocean effects the waters of this vast region.
While explaining and filming the turbulence going on in the water, we captured some stunning footage of remote Indonesia…
For a fuller explanation, including Jamie’s animation and overlays, find out more in our video…
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