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Anke and I are building our next boat, and writing about it at ABargeInTheMaking.blogspot.com. Access to the net comes and goes, so I'll be writing in fits and spurts.

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Fair winds!

Dave and Anke
triloboats swirl gmail daughter com

Saturday, July 6, 2013

Home, Home on the Ranges: Line of Sight Navigation

Rounding Point Brandt by Peter Quigley
 
Sailing is easy, so long as you remember that you're never going where you're pointed!
-- A. Cangiamilla


Home, Home on the Ranges: Line of Sight Navigation

Ranges are some of the most useful tools in a sailor's toolkit. They can tell us where we are, how we are moving over the water, and how we are moving in relation to other points of interest, whether fixed or moving. Crossed ranges provide fixes for ourselves or to locate hidden or hard to spot features.

We learn to navigate using ranges from the moment we begin to crawl. We learn without thinking to navigate our way through a door, rather than crashing into the wall on either side. Without a thought, we cross a stream of fellow pedestrians, performing complex calculations of velocity and trajectory. All below the level of awareness and all Line of Sight.



Sailing is far less complex, but we want to lift these skills into conscious awareness to navigate in an environment that seems less familiar.

Let's start with the useful distinction between our vessel's heading (where it's pointed) and its course (where it's actually traveling). As sailors, we must constantly remind ourselves that these two things are NOT the same. Currents, leeway and vector physics pull our course away from our heading... something that doesn't at all draw attention to itself. Ranges help us get a handle on how much they diverge and what to do about it.

The rowboat is HEADED to the right, while its COURSE angles down and across the current...
The Vector Velocities are extra credit.


In its broadest sense, a range is any line formed by two, observable points. If our eye falls along that line as a third point, the range is both a Line of Sight and Line of Position.

Points are usually fixed by some connection to terra firma. Coastal features such as mountains, points, capes, islands, rocks and even trees are fixed. Up close and personal, water disturbances from rock or reef are fixed. Navigational aids such as buoys, lights, daymarkers and such are fixed. Anchored moorings, pot markers and vessels are fixed.

Some points – vessels underway, gillnets, flotsam, whirlpools, whales – are moving. Range techniques are most useful when their course and speed are steady, but help provide info, even when not.



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Here's some of the fun we can have with ranges:


Fixes -- Where two or more ranges cross they establish a fix – a specific point at their intersection. The wider the angle between ranges, the more positive the fix (two at 90deg or three at 60deg are ideal, but not always available).

Fixes can be used to locate ourselves or any point 'out there', such as a shrimp pot or that one rock in the middle of a wide estuary.

We won't go into them deeply, here... just consider that, while fixes sound very precise, they are not. We use them as guides, but maintain skepticism in inverse proportion to our confidence in them, especially when they designate dangers.


Range Markers -- Common for navigating a narrow channel are range markers. These are two markers in line with a channel (safe passage portion). The further marker is higher than the nearer.

When your eye is on the Line of Sight which they establish, the one appears directly the other.

If  you stray to the left, they appear to go out-of-line, with the further (taller) one to the left of the nearer.

If you stray to the right, they appear to go out-of-line, with the further one to the right of the nearer.

Correcting your course until they come back in line, then running down that line has you following the safe passage portion of the channel. The line itself is invisible but the ranges guide you along it.


Danger and Safety Ranges
– A danger range is a range that closely marks one edge of danger.

Let's say, on entering a cove, that a submerged reef lies 'somewhere' in the middle. If we can note two fixed features that identify one edge of the reach, we have a good idea where not to sail. We can approach that line, safely, but not touch it.

Better yet, a safety range will add a margin of safety to that edge. We  may safely sail to that line, but not across it. It tells us when to tack, or acts as a 'handrail' for sailing close by the danger.

Secondary ranges at near right angles to the channel fix the danger... they help us similarly to know when we are approaching or past the danger.

Both types of information, once identified, can be charted and logged. Consider describing and sketching the range features, as well as the danger they demark. The line itself can be penciled in on the chart for future reference, with features noted and the date (for log look-up).


Opening, Closing, Collision
– Let's say we've got to round a point, ahead of us, and can see a fixed feature beyond it.

If we can see more and more of the background, we are opening the point, and will clear it safely (assuming conditions stay steady). If all we can see is a single point, beyond, the apparent gap between it and the point will increase, in this case. Fingers held upright at arms length are handy ways to measure the gap.

If we see less and less of the background (or the gap is narrowing), then we are closing the point. If we continue on this course, we will strike the land somewhere this side of the point. We must change our heading (and course) if we wish to clear.

If there is no apparent change between point and background (the gap is steady), then we are on a collision course with the tip of the point (that is, we're making a course straight for it). Consider changing heading to clear.

Magically (well, insofar as math and geometry are magic), this works even if the foreground feature is moving! If it is a boat, the same rules apply, though the terminology might change. A vessel opening against the background will pass aft of us (regardless of our respective speeds), one closing will pass ahead of us. If the background is steady, we are on a collision course and appropriate action should be taken.

Note that one needs review one's observations frequently. If wind, current, drift, course or speed change, so does the situation. This is especially important in crossing situations with another vessel.


Finding the Course Line – Let's say we're crossing a Strait, heading for that cozy cove on the far side. How we doing? We look for our course line to see whither we're actually faring. Scan the far shore for foreground/background features which stay in line (neither apparently moving in relation to the other). A good choice might be remarkable trees or bolders at beach level vs. trees or notches in the ridge above (and behind) them. Got it? That's our course, the line we're travelling along. Features to one side of that line will be opening, and closing to the other. We're on a collision course with the shore along the course line. If happy, carry on. If not, take corrective action.

This technique is especially important in strong currents. They can make course and heading diverge wildly, and it can be of utmost importance to determine the actual course.

In very strong currents, consider the advice to swimmers caught in one... swim (or sail) at right angles to the current. This puts your full force into positioning, rather than either fighting the current or correcting slowly with it.

In zero wind, strong current situations, we find that drifting broadsides with the current with an oar over the side, we can give a lazy sweep forward or back for positioning, and let the water do the work.


Night Ranges – At night, many of the main features of the coast disappear, leaving only broad silhouettes. Foreground and background disappear, for the most part, and must be inferred (the exception being from lit navigational aids). Still, much can be inferred...

When coursing along a strait, a topographical atlas (for example, the Gazeteers available for every state) helps identify the valleys (and sometimes ridges) that empty into the strait. As one runs along, the valley is, as we approach, occluded (can't see up it, but only across its opening, which remains lost in darkness). As we draw abeam, however, the valley opens visibly, providing a range along its approximate midline. As we pass, it closes behind us. This lets us monitor our progress along the strait.

Often, we sail an offset course for the inside of an unseen point we wish to round. Eventually, the background closes behind it, and the point  stands out against the lighter sky. From there we can proceed visually, relative to its silhouette.

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This is just the bare bones of what can be done with ranges. As you use them, they will suggest other fun and useful applications. They definitely take a lot of the guesswork out of gliding over liquid surfaces!

The inside passages of Cascadia – roughly stretching from Puget Sound, in northwest Washington, to Skagway, in Southeast Alaska – are rich in salient features. Plenty of fodder for Line of Sight navigation.

Anke and I find that this accounts for probably 99% of all the navigation we do. Speed, Time, Distance calculations rarely work out for the engineless in these flukey, current riddled waters.  Compass navigation is rarely necessary, and is complicated by large, deviant areas. Depth contours are only useful where we're not sailing over inland abyss (I suppose we could break down and get an electronic depth sounder).

You could say Line of Sight navigation is lax and lackadaisical... yet, in our range, it doth suffice!

2 comments:

  1. "The Practical Pilot" by Leonard Eyges quantifies and elaborates on these techniques. He is very good on enabling one to transition between qualitative and quantitative position finding with a minimum of equipment or chart work.

    Glenn
    Marrowstone Island

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    Replies
    1. Thanks Glenn, I'll check that one out.

      I'll get back, soon, with another piloting reference that I really like. One of the nuggets gleaned from it was the principle:

      In piloting, it's often more important to determine where you are NOT than precisely where you ARE.

      Dave

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