The computer models that generate the data used by our weather routing program are run on some of the largest super computers in the world, but no matter how super they might be, they are not the real world. When you rely on computer simulations, you ALWAYS have to leave room for questions and doubt.
For our trip south from Fort Lauderdale to Key Biscayne the model was forecasting about a 6 hour run in front of a brisk North to Northeast wind. The wind was pretty much exactly as forecast, yet we arrived in a bit over 4 hours. Why the large discrepancy?
The issue is the forecasts for the local ocean currents are not very precise. From Miami to Fort Lauderdale the Gulf Stream dominates the ocean, roaring northward at speeds of 3.5 to 4 knots. If you try to sail against it it can slow you down dramatically. Here is were the models fall apart.
All the models predict a strong North-bound current right in to the beach. That, however, is not reality. In the shallow water (less than 100 feet) close to the shoreline, the current is not 2 knots toward the North, but instead it is 1 knot toward the SOUTH. The net difference of 3 knots in total boat speed over the ground was the difference between an average predicted speed of 4.5 knots, compared to what we actually saw of close to 7.5 knots.
Figuring this out as you sail is a challenge without a full suite of instruments. We have both a mechanically measured speed through the water and a GPS measured speed over the ground. Using these data, our instruments present to us the actual speed and direction of the current. This lets us know in real time if we have sailed a bit too far away from the shore, and are getting into a contrary current.
A few hundred yards in the position of the boat make a HUGE difference in total trip time while making your way down the coast. It’s not too often that the difference can be this dramatic, but the speed and direction of the current is always an important input to the smart navigator.
While some of the computer models can have issues, overall they are hugely important to us. Right now we are waiting here in Key Biscayne for the wind to switch around to the south. All the models agree that Wednesday will give us the perfect chance to leapfrog out to the central Bahamas in one long, easy, 36 hour sail. Here is to hoping they are right!
In the meantime, we are exploring the wealthy enclave of Key Biscayne. Packed full of exclusive condos, gated communities, and even more exclusive waterfront estates. There are a couple of delightful state parks, but otherwise there isn’t a lot to attract us to this spot except for the grocery store and the anchorage which is well positioned to wait out north and east winds before jumping off to the islands.