The Tactics of Pursuit

[HenryC]

An abbreviated version of this appeared in Florida Wildlife, Jan/Feb - 2010.  This is the full Monty, written for sailors.

The Tactics of Pursuit

There is a scene in the film Master and Commander where the captain attempts to explain the concept of  "weather gage" to  his nautically challenged ship's physician.  The good doctor does not get a very clear explanation, and neither does the audience, unless they are sailors.   Holding the weather gage meant that a warship had a favorable wind for battle.  Specifically, it meant that a ship was to windward of its opponent.  During the age of fighting sail, single ship actions were often decided before the first shot was fired as the two opposing skippers engaged in elaborate maneuver ballets to try to gain the weather gage.  The ship to windward had the advantage of being able to control the battle, it could engage the enemy at any distance it chose, delay close contact or quickly swoop downwind on her opponent in the event the latter was damaged or mishandled.   Furthermore, heeling towards her enemy, any hits the windward vessel took would be above the waterline. The vessel to leeward, in addition to exposing her bilges to cannon fire, would have to beat upwind to exploit any opportunity that might present itself, exposing herself to potentially devastating and unanswered fire into her bow or stern as she came about or wore ship.  One final advantage of the weather gage, and not an inconsequential one, was that smoke from your guns would be carried away from your ship and not blown back into your eyes. With black powder, smoke was considerable.
There were certain benefits to being to leeward.   If disabled, it was easier to escape, provided there was plenty of sea room.  The ship downwind also had the advantage of heeling away from her opponent, exposing the heaviest ordnance in the lowest gun deck.  Depending on the state of the seas, the windward ship might have her lower tier rolled too near the water level and not be able to open those gun ports at all.  These considerations also affected gunnery tactics since the  ship with the weather gage often had to elevate her cannon just to fire parallel with the sea surface.  Her opponent  would have her guns aimed higher and often took advantage of this to disable her enemy's rigging using munitions specialized for the purpose.  But as a rule, captains preferred the position that offered them the initiative.

Today's recreational sailor need not concern himself with these grisly details, but even with the advantages of a weatherly fore-and-aft rig and an engine in reserve, there are still good reasons to be aware of the geometry of wind in the same way as our tall ship forebears.  Racing skippers are very cognizant of this, both when jockeying for position at the starting line or maneuvering for a right-of-way privilege when rounding a point.   Something very analogous to the weather gage occurs when the windward vessel in a race uses her wind shadow to degrade the sailing performance of a downwind adversary.   There are situations when thinking like a frigate captain might be useful or maybe just plain fun for the weekend sailor. 

Consider the challenge of maintaining a straight course over the ground in a strong cross-current, or when forced to sail shoal waters with the centerboard up.  In either case, extreme drift, whether induced by wind or current, can be a serious problem.  The unfortunate modern tendency to rely on GPS waypoints and headings can lead to embarassing conclusions when a boat refuses to go where it is pointed.  But under the right conditions, you can get this effect to work for you.  I've seen boats able to maintain a course almost directly into a 30 knot wind, with a little help from a six-knot tidal current abeam.   San Francisco Bay sailors do this routinely!   It isn't always possible to navigate precisely, especially when one is alone or has a crew of neophytes aboard.  This is where a little understanding of how the wind and current can affect a boat's motion, and how to predict this effect accurately by eyeball, can be very useful.

Every sailor knows that when an object is on collision course with his boat  its compass bearing does not change.  What is not always obvious is that this object need not be another boat.  It can be a stationary point on shore or some hazard out on the water.  It can even be a place where you may want to go.  This rule of thumb also holds true regardless of whatever leeway or drift you (or it) are experiencing.   An object whose compass bearing remains constant will eventually collide with you, unless, of course, it is travelling  parallel  to you at the same speed.   Another unexpected corollary of this triangular geometry is that the relative bearing of the object will also remain constant, as long as your heading does not change.
   
It is perhaps easiest to visualize this condition  with an example.  Suppose you are proceeding down a long narrow channel with hazards to either  side.   You are navigating by range markers, knowing that as long as the two ranges line up one above the other you are on the straight line defined by them, presumably right down the center of the fairway.   Let us further assume that there is no current and leeway is negligible.   This is the easy case, you just point your bow down the range and make a note of the steering compass heading.   You will be centered up on the channel, your compass bearing (in this case, both your course made good and your heading) will remain constant, and the relative bearing will be "dead ahead".   

So far, so good.   But let's add another wrinkle.    Let us assume that, unbeknownst to you, you suddenly get hit by a strong current at right angles to your direction of travel.   Most people can't tell when they're in a current, so your first indication will be that the range markers start moving relative to your compass course.  You continue to steer towards the markers but you can see that you are being diverted away.   The boat is still pointed at them, but you are being set to one side of the channel.   It should be clear that the situation will appear this way even if there is no range;  you will make the same observation if only a solitary light marks the channel, but it is easier to visualize if you think of pairs of range markers.   By now, as you drift further off course, the nearer of the two markers appears to move away from the further, in the opposite direction of the set of of the current.  If you haven't run aground by now, it should still be possible to steer in such a way that the two markers are again lined up, and you are in the channel once more.

Your compass bearing  to the markers will now be the same as your course made good, but your heading has changed because you are steering into the current, compensating for drift.  The range markers are no longer dead ahead, i.e., the relative bearing has changed, but as long as the current does not change again,  it will remain constant.  In fact, you could steer accurately without a compass just by keeping the range markers over some convenient spot on the deck, such as a stanchion or cleat.   Sailors in coastal areas all know this instinctively, those more familiar with inland waters may find current drift disconcerting.  The effect of leeway is usually much less, and is often compensated for unconsciously by the helmsman, but it too can become substantial when sailing with the centerboard up.  In our example, the helmsman wants to keep his true course (and true bearing) to the range constant, regardless of what heading he has to steer to do it or how the markers will wander around the relative bearings.   In other words, he wants to collide with the range markers.   

This is all pretty straightforward on a clear day in familiar waters, but far from home, at night, particularly in a crowded harbor or in bad weather, it can hopelessly befuddle even the best skipper.   GPS has the potential to cut through the confusion, but it often has the opposite effect.   If you steer by waypoints, the satellites will tell you whether or not you are heading towards your destination but they won't tell you if an island or reef is in the way.  Unfortunately, the skipper may not be able to leave the helm and go to his nav station and consult his chart, or he may just be out of practice.    The technical term for the result is a GPS-assisted grounding.

The same principles which apply to stationary objects also can be used in maneuvers with respect to moving ones.   Let's play frigate captain!  Your orders: to intercept another vessel, perhaps one which is faster than yours but traveling at a constant course and speed.   For example, you spy a boat in the distance you'd like to get a closer look at, perhaps you suspect some friends of yours are aboard or you just want to test your rendezvous skills.   Of course, it may be impossible to actually catch the other boat, it may be too fast or too far ahead, or both.  But you might just surprise yourself, give it a try.   Again, what you want to do is provoke a collision.  Steer in such a way that your compass bearing to the other vessel remains constant and it won't matter what the current is doing, as long as it remains constant.    Simply make your boat go as fast as it can go, and steer ahead of your quarry.    After your boat has reached equilibrium with the new conditions and you are steering a steady course, start taking bearings, or simply note the position of the other vessel in relation to some object on deck, say a shroud or winch.   You are now on converging courses, and if the target appears to be falling behind you , steer a little more towards it.  If it appears to be pulling away, steer a bit further ahead.   Unless it has already gotten away from you, you will quickly find a point where the bearing to the other boat remains unchanged.  You are now on a collision course and you will eventually come alongside.  Assuming you are far enough ahead of your quarry, you can even  rendezvous with a faster boat this way, much to the consternation of its crew if they have been paying attention.

This simple principle is often applied while carrying out one of a boater's most common tasks:
placing his vessel into a narrow berth in a crowded dock.  For safety, you approach very slow, so your boat responds sluggishly to rudder commands.  The dock is busy, a crowd is watching, the pressure is on you, skipper!  Invariably, there is a strong crosswind or current, maybe both, and your boat doesn't want to go where you point it.
 
Start your approach from far off so you'll have plenty of time to line up and room to  abort the maneuver and try again in case you don't.  As you are set off-course, steer into the drift until you see your boat is "crabbing" sideways straight where you want to put it, even though the vessel is pointing up-drift.  You will note the spot on the dock you are aiming at is approaching in a straight line, lined up with some spot on your boat, even though your bow is not pointed at it.  Your course and your heading are not the same. As you approach, adjust your speed so you don't strike the dock too fast, which may require you to alter your heading again to compensate for the new drift at the slower speed.  It's like parallel parking in three dimensions!

The remarks above do not specifically take the wind into account and thus also apply to powered craft.  They are also restricted to the situation where the pursued vessel is making no effort to escape.   Let's now consider the case of two sailing ships where the quarry is determined not to get caught.   You are a privateer chasing down a prize, or a man-o'-war trying to close with an enemy!   Again, steer ahead of your quarry and trim your sails for maximum speed.   Keep an eye on the bearing  until it becomes stationary, and hang on for a long chase.   If your prize is downwind of you, it becomes a race.   You can expect your opponent to employ the fastest possible point of sail for his vessel and the one which leads her away from you as quickly as possible.   If she is faster she will escape no matter what you do, if not, she will attempt to prolong the race until nightfall when she can lose you in the dark. 

But what if the pursued vessel is upwind of her pursuer?   That is, she has the weather gage.  Now you must be faster and more weatherly to overtake her.   If you are both roughly neutral in relation to the wind, your opponent may attempt to get upwind of you, especially if she is a better sailer to windward.   This is the only case where either captain has an option, that is, the pursued vessel's commander must decide whether to run for it, or try for the weather gage.   The latter choice is the correct one if the pursued vessel is slower but more weatherly than the pursuer.   

So the race is on, your prize has the weather gage, but you are faster and sail closer to the wind.  You must work your way upwind while gaining ground simultaneously in the process.   The chased vessel can only get away if daylight runs out or in case of accident or blunder on your part.  Of course, she will do everything she can to prolong the contest as long as possible.   The pursuer's optimum strategy will be to steer an intercept course at maximum speed until the quarry is directly to windward.     From this point on the hunter can head up just enough to work her way a bit closer to the quarry, even if it means slowing down a bit.   In other words, trade excess speed for upwind distance.  The pursuit ship's optimum strategy is to keep her quarry in the eye of the wind, using every bit of additional speed to claw up closer to her target.  The pursued's optimum strategy will be to come about the moment the hunter reaches the dead downwind position and deny her that opportunity.  She will certainly do so if you get past that spot, leaving you storming away from her in the opposite direction, further away and further downwind every second.  You have no choice but to tack when she does, keeping her in the eye of the wind.   Since the hunted vessel can decide when to come about and it will take you time to respond, she will pull ahead a little each time she does so, but you will eventually gain on her.  Your quarry may try trickery, false tacks, jibing or even wearing ship,  in the hope you get caught in irons trying to match her, or that your vessel is slow to maneuver or clumsily handled in stays.  What your enemy wants is to get you into a position where you are on opposite tacks, heading away from each other; every moment you are doing so you are losing ground that will have to be made up later.   

It's difficult to conceive of a situation where a weekend sailor will need to have mastered the tactics of pursuit, but sailing is, after all, not the most practical form of water transportation available: we sail for other reasons.  An understanding of these maneuvers will make us better sailors and practicing them will give us another way to enjoy our boats, especially if the exercise is enhanced by constricted waters or changing conditions.    Most important, it will put us in touch with the historical community of mariners, reaching back to a time when tacking duels were conducted for bigger stakes.