CURRENT & TACTICS V: COURSE OVER GROUND and VMG

It may seem quite an obvious statement, but the influence of the current on our Course Over the Ground and VMG is one of the biggest subject to discuss when we speak about racing in current.
We’ve seen in the article “The wind triangle” how current affects the wind we sail into, now we are going to see how it affects the movement of the boat itself.
image1First of all, there is a quote which is quite useful to remember an important fact. The quote says: “The less you heel, the more current you feel”. This is great to remember that the slower our boat goes, the more the current will affect, in percentage, our course. Nevertheless, even the super-fast foiling catamarans of the last America’s Cup where always concerned about the current in the bay of San Francisco. So, remember the quote but don’t think that, if you are fast, you can forget about the current!

Having said this, it’s hard to write general rules valid for different types of boat. What we can try to do is to understand what we are looking for.
We know that a current pushing us from leeward brings a lift (a good wind shift!). But this doesn’t mean that it will actually bring us a real advantage compared to a boat sailing in the same Ground Wind but in no current! In fact, our VMG becomes better only if the current is coming from a direction that is more then 90° different from the direction of the wind! The figure shows the 3 cases: no current, current on the leeward side bust still pushing us back and current on the leeward side actually pushing us forward. It’s clear to see who gains and who loses. Of course this is true if the change in TWS caused by the Tide Wind doesn’t allow the boat to plane. Remember, there is an exception to any rule.
Overall, we can say that we must always think of our VMG and not just look at the windshifts.
Keeping the current to leeward gives us a good windshift, but it may also slow us down a lot if the current is coming from the “wrong” direction.
As a general rule it is important to highlight that, if the current is the same along the course, there is absolutely no gain in going to one side or the other; it’s just like sailing on a skewed course. The geometry changes (laylines etc) but there is no gain or loss in going to one side or the other. But it doesn’t happen often to sail in constant current!
So, keep an eye on your VMG and try always to get as much distance “in the bank” as soon as possible!
Figure 2 shows an example where the strategy “put distance in the bank” paid off with the green boat taking advantage of the lift (and the push) of the current that is coming from 90° from the wind. When the current stops to run, the green boat is ahead.

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Author: Gilberto Pastorella

Missed the previous chapter of our CURRENTS & TACTICS?

Read it here: CURRENTS & TACTICS IV: THE IMPORTANCE OF THE COASTLINE SHAPE AND UNDERWATER BATHYMETRY.