Watch Those Currents !

Before setting sail offshore, I used to think Sea Currents occurred in broad, long streams along the surface. And whereas the Gulfstream may be such case, most often offshore currents happen in the the form of relatively small, swirling eddies. And they can play a relevant role on a skipper’s routing, as they did during our recent passage between Fiji and New Zealand. Read on:

As you can see on the image below, Sea Currents occur in eddies across the surface:

Surface Currents in the Pacific Ocean. Note the swirling eddies. Image from earth.nullschool.net

 

The most relevant eddies tend to accumulate along the equator, with a secondary accumulation along the Tropic of Capricorn, at the western side of the ocean.

Because of that, we didn’t have to deal with relevant currents during most of our cruising this year, from Tahiti to Fiji. Moreover, the little we had to deal with happened during downwind sailing, when minor route adjustments are trivial.

This story changed when we left from Fiji to New Zealand – note the prevalence of current eddies , particularly during the first half of the trip:

Sea Currents along our passages of 2017. Nothing relevant between Tahiti and Fiji, but quite a few eddies to deal with on the way to New Zealand. Image from earth.nullschool.net

 

As we were to learn on this passage, dealing with currents on upwind passages requires a bit more consideration than on downwind ones.

Let’s take a look.

 

1.) A LITTLE BIT OF CURRENT BY MY SIDE (trigonometry of Cross-Currents, Apparent Wind and Heading for the upwind passage where to buy codeine promethazine ):

Upon departure from Fiji, our first 12 hours-or so of sailing were in an area devoid of relevant currents:

Leaving Fiji, sailing in an area of mild currents.

 

Winds were blowing at 14 knots from the ESE, around 112 degrees magnetic, and Pesto was doing a good 7.5 knots speed over water, the bow pointing to 185 degrees toward New Zealand:

Note how the boat’s movement causes the perceived wind speed to accelerate from 14 to 18 knots on deck, and the wind angle to close from 73 degrees to 49 degrees off the bow. That’s business-as-usual for upwind sailing.

 

Those were nearly ideal sailing conditions.

We then entered a patch of transversal currents, flowing nearly at a 90-degree angle to our route at about 1 knot:

Pesto entering an eddy of 1-kt current flowing Eastwards, which pushed us to windward

 

The current pushed us to Windward, causing Pesto to drift at about 7 degrees off her route (in other words, the boat’s bow was still pointing to 185 degrees, but the boat was moving at a heading of 178 degrees):

A drift of 7 degrees caused by a 1-kt cross current.

 

Since the drift was to Windward, we just had to point the bow a few degrees to Leeward to get back to the desired heading, sailing on a slightly more comfortable angle, and gaining a bit of extra speed in the process:

Course-correcting for a windward drift isn’t much of a problem. We gained a few degrees on apparent wind, and one fourth of a knot in the process.

 

The Eastward cross current therefore had a beneficial impact to us, with a slight gain on speed and bonus of 7 degrees on the apparent wind angle.

Some 12 hours later, however, we entered another patch of current, this one from the opposite direction, with a Westward motion (from East to West), nearly at a 90-degrees angle to our route, pushing us to Leeward, and flowing at about 1 knot of speed.

Westward current pushing us to Leeward

 

The mechanics (or, rather, trigonometry) of the impact to our navigation were similar, but the effects were different:

Westward current pushing Pesto to Leeward, causing a 8 degree drift versus the intended heading

 

Notice that the angle of Drift (that is, the deviation from course due to the transversal current, in degrees) was slightly higher when the current pushed us to Leeward. Why?

Correcting the course from a transversal current, requiring us to point Pesto’s bow further to Windward

 

This time, in order to compensate for the Drift to Leeward, we had to turn Pesto’s bow INTO the wind. As we did so, we took a small penalty on boat speed, in the order of 0.5 knots. And less speed over water requires a bit more of an angle of attack to compensate the Drift:

As Pesto’s speed over water reduced from 7.5 to 7.0 knots, we had to turn her bow further to Windward to compensate for the 1-knot drift.

 

Moreover, a portion of the boat’s speed over water was “spent” in compensating for the Drift. Thus, the net advance towards New Zealand was reduced to 6.9 knots.

Moving along with a Leeward-oriented current. Pesto is heading to 185 degrees, as intended, but we had to keep her bow pointed to 177 degrees to do so. Apparent wind angle tightened to 42 degrees, and boat speed reduced to 7.0 knots over water, and 6.9 knots towards New Zealand.

 

As this happened, we were into our third day of passage. Slightly seasick, but conditions weren’t bad. We were frustrated with the effect of the Leeward-oriented current on our progress, but the experience of going through two currents of opposing direction had been quite didactical:

Pesto’s progress under currents from opposite directions. Note the significant differences in speed and apparent wind angles

 

The learning so far had been that, when going Upwind:

  • a Windward oriented cross-current can have a positive impact to navigation: a slight relief on Apparent wind angles, coupled with a small gain on boat speed (or, an alternative benefit is the possibility of sailing further into the wind without having to tack)
  • a Leeward oriented cross current will have a negative impact to navigation: tighter apparent wind angles to compensate for the drift, coupled with a penalty on speed

Note the distinction between the “can” and “will” above, for the Windward and Leeward current situations respectively, and that was something we were about to learn a few hours down the line.

 

2.) CAUGHT IN THE CRUNCH ZONE (how Opposing Currents and Winds affected our motion significantly):

South of the two cross currents that we just mentioned lay an area with a relatively long patch of South-going currents, and I had been eying it since the planning stage. It was about 300 to 400 miles long, with currents ranging from 0.7 to 1.5 knots. It was narrow but long, and appeared in different forecasting models. If we were able to find and ride it, we could shave as much as 7-10 hours off our overall passage time:

Note that patch of South-going fast currents present at the time of the passage. We drove into it and rode it on the way to New Zealand to gain a boost of speed. And we got a lot more than we had signed up for …. images from www.earth.nullschool.net

 

So, here’s what happened.

We were successful to find the patch, but by the time we got into it the winds also increased from 14 knots to 20 knots, gusting to 25. Winds of that magnitude are not too bad, neither are the waves they create. However, winds and currents were of a nearly opposing direction. When this happens, the wind-generated waves get compressed and much steeper than normal. They sometimes also stack, creating higher than normal waves. Finally, note on the image above how the current changes direction slightly along the  patch. I believe that this change of direction caused the seas to be more confused than they would otherwise have been.

This is naturally something that didn’t occur to me during the passage. But as I reviewed the current data for this post, it became clear that currents in opposition to the stronger winds were a sure cause for the unusually steep seas that we had and made the motion so difficult and wet. For us and for all other boats we know that made the same passage at the same time. So, our new learning from this episode was that:

  • in winds above 15 knots, a relevant opposing current will create very steep seas, and cause unusual, very discomfortable, and potentially gear-breaking motion to the upwind-oriented vessel

 

3.) IT TAKES PLANNING. A LOT OF IT:

Finally, one last item I wanted to highlight in this story is related to the planning stage, before casting the lines off. To me it is clear now that currents MUST be accounted for during the planning stage of a passage, much specially if it entails upwind sailing. And it is even more important to do this analysis BEFORE leaving. And here’s why:

  • There are multiple sources of current data and forecasting, and they don’t agree as well as their atmospheric brethren. The pre-trip analysis then must look for patterns, and major discrepancies between the different models
  • All analysis MUST be done BEFORE departure, because current forecasting data are much heavier than atmospheric ones, making it much more difficult, almost impossible, to get relevant current data while underway
  • The corollary of the two items above is that the planning stage must include a comprehensive analysis of all relevant current scenarios, and lay out a “what-if” based plan for once you are out there.

 

LEARNINGS:

So, this is what we took from this experience:

  • Currents swirl in eddies across the ocean surface
  • They are strong around the equator, and also around the Tropics of Cancer/ Capricorn at the western side of the Pacific Ocean
  • Currents have a significant effect on navigation for upwind sailing passages
  • With Leeward-oriented currents, it’s better to let the boat drift, whenever possible. Compensating for the drift will lead to tighter upwind sailing and slower speed
  • Windward-oriented currents can have a beneficial effect (better upwind angles, more speed), but only IF the winds are not strong enough for the waves to pile up
  • Accounting for currents on passage planning requires all work to be done before departure, when internet is available, because surface currents data files are heavy and time-consuming to download while underway

 

APPENDIX:

A generic trigonometry schematics for some of the analyses on this post:

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