Lubber's Loops

Boats, unlike cars, rarely go where you point them. This is, of course, old news to anyone who voyages under sail; likewise for those who navigate in coastal areas with strong currents. Those of us with small, fast powerboats tend not to care about this quirk once we leave the dock- how much of an effect can a light breeze have on a 20-plus-knot boat?

Quite a bit, in fact. The "lubber's loop", the course that's sailed when cross-track error is ignored, gets all of us in a bit of trouble now and then.

The problem, of course, is that aiming the bow at a mark doesn't mean the boat's course is toward that landmark. Current and wind will push you sideways, ever so gradually. Our landlubber skipper, noticing that the boat has now drifted off course, compensates slightly- aiming the bow at the mark once again. The current and wind continue to do their thing, the skipper adjusts his course again, and where the boat ought to take the green line, it ends up following the red one:

Courses over ground from Big Sandy Bay to Simcoe Island

In this case (from a run I did last weekend), the four-mile-long loop sticks out into open water. But let's put some rocks a bit off the intended course, and one might end up in an unpleasant situation.

Correcting for current, of course, is a simple matter of trigonometry: take a guess at the speed of the current, take a guess at the speed of your boat, draw arrows to scale representing your course and the current, and adjust your course to compensate. A 10-knot boat, running perpendicular to a 2-knot current, would aim (arctan 2/10) = 11 degrees up-current of its intended target.

Wind is a bit trickier, especially in a light, fast boat. In the course shown above (it's a rough guess, not a GPS track), the cross-track error is caused entirely by wind, as there's almost no current here. This is what Sunset Chaser does if she's aimed directly at the destination- lighter, slower boats would blow farther off course, while faster ones would barely deviate from the green line even without any cross-track compensation. Since I don't take Sunset Chaser into open water very often, I thought I'd try to measure how big an effect this is:

Courses over ground from Big Sandy Bay to Simcoe Island

In force 3 conditons, aiming directly for the landmark took us over half a mile off course on a four-mile leg. The cruise speed here is about 14-15 knots. So, now we know how far upwind we have to steer when crossing an open stretch like this: about ten degrees in force 3, and twenty degrees in force 4, keeps Sunset Chaser nicely on the rhumb-line course.

Why is this important, in this era of GPS/plotter/radar linked autopilots that correct for such errors automatically? Why would you bother trying cross-track error experiments in the wind with your own boat? Simply put, it's about knowing your boat, knowing how she reacts, and knowing how to get her home in safety and comfort, no matter what.

And, of course, it's a fully legitimate excuse to get out on the water and enjoy your boat. And isn't that good enough?



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