Navigation at Speed

Prerequisites

You need the navigation from the Intermediate Powerboat course: chart symbols, true and magnetic bearings and compass variation, plotting a course to steer allowing for tidal stream, tidal heights, buoyage (IALA Region A) and light characteristics, and setting up and following a route on a chart plotter. You should also have read the earlier lessons in this course, especially High-Speed Handling and Wave Theory and Rough-Water Driving, because the sea state and the boat's motion are what make fast-boat navigation different.

Learning Objectives

By the end of this lesson you should be able to:

Explain why navigation at 20 to 30 knots must be prepared before departure, not done on the move.
Work out time, speed and distance quickly at planing speeds, using the six-minute rule and minutes-per-mile.
Plan a route with safe waypoints and prepare a leg card for each leg.
Construct and use clearing bearings, clearing lines and transits.
Use a chart plotter safely at speed, knowing its limitations and how to back it up.
Keep a pilotage notebook for harbour entries by day and night.
Run a fast passage with a navigator and helmsman working as a team.

Why Fast Navigation Is Different

On a yacht at 6 knots, the navigator can go below, plot a fix, work out a course to steer and come back up, and the boat has moved about half a mile. On a powerboat at 25 knots, in the same ten minutes, it has covered more than 4 miles. The motion of a planing boat in any sea makes it almost impossible to use a chart, plotter controls or a pencil while moving.

The consequences for fast-boat navigation are:

All planning is done ashore, or stopped, before departure: the route, waypoints, courses, distances, times, dangers, clearing lines and what you expect to see.
On the move, you check, not calculate. The navigator compares what is seen with what the plan predicted: the right mark, at the right time, on the right bearing.
Time is your most important measurement. At a steady speed, a stopwatch tells you how far you have run more accurately than you could ever plot it in a bouncing boat.
If in doubt, stop. Coming off the plane to check the chart takes a minute and costs nothing. Charging on at 25 knots when unsure where you are is how fast boats run aground.

Time, Speed and Distance

Quick methods

Two mental methods cover almost everything:

The six-minute rule. In 6 minutes (one tenth of an hour), a boat covers one tenth of its speed in nautical miles. At 25 knots, that is 2.5 nm in 6 minutes.
Minutes per mile. Divide 60 by the speed. At 20 knots, 1 nm takes 3 minutes; at 30 knots, 2 minutes. To find a leg time: minutes = distance × 60 ÷ speed.

The diagram below compares distance covered in six minutes at a yacht's speed and at two planing speeds, with a quick-reference table.

Distance covered in six minutes at 6, 25 and 30 knots, with a table of distance per minute and time per mile

Choosing a planning speed

Plan at a round speed you can actually hold in the expected sea, not the boat's top speed. Speeds of 20, 24 or 30 knots make mental arithmetic easy: at 24 knots, 1 nm takes exactly 2.5 minutes, and 6 minutes is 2.4 nm. If you have to slow down for a sea state, re-time the remaining legs at the new speed.

Tidal stream at speed

Tidal stream has much less effect on a fast boat than on a yacht, but it is not zero. The angle you must steer into a cross-stream is roughly:

Offset angle (degrees) ≈ 60 × stream rate ÷ boat speed

A 2-knot cross-stream at 24 knots needs about 60 × 2 ÷ 24 = 5 degrees of correction. Ignore it on a 15-minute leg, and you will be about 0.5 nm off track at the end. That is enough to miss a harbour entrance in poor visibility or to stray inside a clearing line. On short legs, aim off by the estimated angle; on longer legs, work it out properly before you leave.

Planning the Route

Waypoints

A waypoint is a position stored in the plotter or GPS that the route uses to turn. Good waypoints are:

In safe water, never on a buoy, rock or headland. Place them a safe distance off any mark, so that if two boats use the same waypoint or the GPS is slightly in error, nobody hits the mark or each other.
At points you can check visually, such as a cardinal mark close by, a transit, or a clear bearing on a headland.
Not too close together. At 25 knots, legs of less than a mile give the helmsman too little time to settle on the new heading.
Checked on the chart, at large scale, along the whole leg between them. A straight line between two safe waypoints can cross a rock or sandbank.

Name waypoints meaningfully (for example "SCard Ledge" or "Fairway") rather than "WP017", so the navigator can call them out.

The leg card

A leg card gives the helmsman and navigator everything needed for each leg on one waterproof sheet, in large print readable at speed. For each leg, it should show:

Leg number and waypoints (from, to).
Course in degrees magnetic: the track, plus the course to steer where a tidal offset is needed.
Distance in nautical miles.
Time at the planned speed.
What to look for: marks, headlands, lights and their characteristics, and when you should see them.
Dangers and the clearing bearing or line that keeps you off them.
Depth checks: what the echo sounder should read.

The diagram below shows a three-leg route from a harbour to a fairway buoy, and its leg card.

A three-leg route sketch with a leg card showing course, distance, time at 24 knots, what to look for and dangers on each leg

Clearing Bearings, Clearing Lines and Transits

On a fast boat, you will rarely take a three-point fix. Instead, you use simple checks that answer one question: am I on the safe side of this danger?

Clearing bearings

A clearing bearing is a bearing of a charted object, drawn on the chart so that it passes safely clear of a danger. As long as the object's bearing stays on the safe side of that line, you are clear.

To construct one:

Pick a conspicuous object (a lighthouse, church tower, headland or beacon) that you will be able to see when you need it.
Draw a line from that object passing clear of the danger by a safe margin.
Measure the bearing of the line from the boat towards the object.
Hatch the danger side of the line on the chart.
Decide whether the safe bearings are NMT (not more than) or NLT (not less than) the clearing bearing, by checking a point on the safe side.

The diagram below shows a clearing bearing past a ledge that dries at low water.

A clearing bearing of 015 degrees True on a lighthouse keeps a northbound boat east of a drying ledge

In this example, heading north towards the headland, the clearing bearing is lighthouse NMT 015°T. A boat on the safe side sees the lighthouse on a bearing of 002°T: less than 015°, so it is east of the ledge. A boat that has drifted west, towards the ledge, sees it on 029°T: more than 015°, so it is in danger and must alter to starboard.

On the move, you check a clearing bearing with a hand-bearing compass, or by putting the steering compass heading on the object for a moment. Remember to convert to magnetic for a compass: with a variation of 2°W, a clearing bearing of 015°T becomes 017°M.

Clearing lines from depth

A depth contour can also act as a clearing line. If the chart shows that you are safe outside the 10 m contour, an echo sounder reading of less than 10 m (corrected for height of tide) tells you that you are standing into danger. This is a useful check along a steep-to coast, at night, and in poor visibility.

Transits

A transit is two charted objects in line. When you see them in line, you are somewhere on the line joining them. A transit needs no compass and is very accurate, which makes it ideal for fast boats.

Leading lines are pairs of beacons or lights set up to lead along a safe channel. Keep the front and rear marks in line, and you are on the channel.
If the rear mark appears to move left of the front mark, you are to the left of the line; steer right to bring them back in line. If it moves right, steer left. In short, steer towards the front mark (the nearer one).
Clearing transits are lines that pass clear of a danger, for example "keep the church open west of the pierhead". As long as the two are not in line, you are clear.
Natural transits can be planned from the chart, such as a headland edge in line with a summit.

Transits are best checked looking ahead. Back transits (looking astern) work, but they need the lookout to watch them rather than the helmsman.

Using a Chart Plotter at Speed

A chart plotter is the main navigation tool on most fast boats, and it is very good. It is also the reason for many groundings, because it is so easy to trust.

Setting up before departure

Build the route ashore and check every leg at the plotter's largest scale, zoomed in along its entire length. Electronic charts hide detail at small scales: a drying rock or a shoal can disappear when zoomed out. In 2014, the round-the-world racing yacht Vestas Wind ran onto a reef in the Indian Ocean after the route was checked at a scale that did not show it.
Set alarms: cross-track error (XTE), for example 0.1 nm in pilotage and 0.25 nm offshore; arrival at waypoint; shallow water; anchor watch when stopped.
Set the display so the helmsman can read it at a glance: course-up or head-up for pilotage, large numerals for speed, heading, bearing and distance to the next waypoint, and time to go.
Dim the screen to night mode for night passages.

On the move

Use the plotter to confirm what you see, not to replace looking.
Keep the next mark and time in mind, and check them against the plotter and the leg card.
Watch the XTE. A steady drift off track means a tidal stream or leeway you have not allowed for.
Have the MOB button within reach of the helmsman.

When it fails

Plotters fail: water gets in, batteries flatten, the GPS signal is jammed or lost, or the screen is unreadable in bright sun. Back it up with:

A paper chart or chart book, in a waterproof case, with the route drawn on it.
The leg card and pilotage notes.
A handheld GPS with the same waypoints, and spare batteries.
The steering compass, deviation-checked. Electronics, speakers and steel near the console can cause large compass errors on small boats.

If the plotter fails, slow down, note the time, and continue on the leg card: course, time and visual marks. That is exactly what the leg card is for.

The Pilotage Notebook

Pilotage is navigation close to the shore and into harbour, by eye, using marks, transits and bearings. On a fast boat, you prepare a pilotage plan for each harbour or tricky passage in a notebook.

A good pilotage page includes:

A simple sketch of the approach, north-up or in the direction of travel, showing marks, dangers and the planned track.
Bearings and distances for each leg, with times at the planned approach speed (usually much slower than cruising speed inside a harbour).
Transits and clearing bearings.
Buoy and light descriptions: colour, shape, topmark and light characteristic. At night, write the characteristic in full, such as "Fl(2)R 5s", and count it with a stopwatch.
Depths expected, and the height of tide at arrival.
Speed limits, VHF channels for the harbour and marina, and any traffic signals.

Keep the notebook in a waterproof cover, write in large clear print, and talk it through with the crew before you arrive. At night, use a red light that does not spoil night vision, and be ready for background shore lights to hide navigation lights.

Working as a Team

At speed, the helmsman should keep their eyes outside. The navigator does the checking. A good pattern is:

At each waypoint, the navigator calls the new leg: "Leg 2, steer 092, 6 miles, 15 minutes, looking for Gull Rock to port at 7 minutes." The helmsman repeats the course.
The navigator starts the stopwatch.
At the expected time, the navigator calls out the mark to look for, and both confirm it.
If the mark does not appear by the expected time plus about 10 percent, the navigator calls for the boat to slow down or stop and check.

Worked Example: A Fast Passage at 24 Knots

You plan a passage from your harbour (WP1) to the fairway buoy (WP4) off a neighbouring harbour, at a planned speed of 24 knots. The route and leg card are those in the leg card diagram above.

Distances and times.

Leg	Course	Distance	Time at 24 kn
1: WP1 to WP2	128°M	4.0 nm	4.0 × 60 ÷ 24 = 10 min
2: WP2 to WP3	092°M	6.0 nm	6.0 × 60 ÷ 24 = 15 min
3: WP3 to WP4	041°M	3.2 nm	3.2 × 60 ÷ 24 = 8 min
Total		13.2 nm	33 min

Tidal stream. On leg 2, the tidal atlas shows the stream setting north at 2 knots. The offset is 60 × 2 ÷ 24 = 5 degrees. The stream pushes you north (to port), so to stay on a track of 092°M you steer 5 degrees to the south of it: 097°M. Write this course to steer on the card next to the 092°M track. Without this, you would be set about 0.5 nm north in the 15 minutes of the leg, towards Gull Rock.

Dangers. Black Ledge lies to port of leg 1. The plan keeps WP2 south of the south cardinal mark (VQ(6)+LFl 10s) that marks the ledge. Gull Rock lies about 1 nm to port of leg 2. On leg 3, the harbour leading line (two beacons in line on 041°M) brings you to the fairway buoy (red and white stripes, LFl 10s).

Running it. You leave WP1 at 1400. The navigator calls leg 1 and starts the stopwatch. At 1408, the south cardinal is fine on the port bow, right on time, and at 1410 you pass south of it at WP2. On leg 2, the sea builds to a short chop, and you slow to 20 knots: the navigator re-times the remaining 4 nm of the leg at 3 minutes per mile, giving 12 more minutes. Gull Rock comes abeam at the re-timed point, and WP3 is reached at 1427. On leg 3, you pick up the leading beacons, keep them in line, and reach the fairway buoy at 1435. You slow to 6 knots, and the navigator opens the pilotage notebook for the harbour entry.

If the plotter had failed. You would have slowed, noted the time and position on the paper chart, and continued with course, time and the visual marks on the leg card, which is everything you need.

Common Mistakes

Trying to plan on the move. Every course, distance, time and clearing line should be on the leg card before you leave.
Placing waypoints on marks or hazards. Always place them in safe water, offset from the mark.
Checking a route only at a small scale on the plotter. Zoom right in along every leg.
Ignoring the tidal stream because "we're fast". A 2-knot stream sets you 0.5 nm off track in 15 minutes at 24 knots.
Not using a stopwatch. Without time, you cannot predict when to see a mark or know when you have run too far.
Getting the NMT or NLT the wrong way round on a clearing bearing. Always check by testing a point on the safe side.
No back-up for the plotter. Carry a paper chart, leg card, handheld GPS and a deviation-checked compass.
Not slowing down when unsure. Stopping costs a minute; a grounding at 25 knots can cost lives.

Summary

At 20 to 30 knots, you cannot do chartwork on the move: plan everything before leaving, and check rather than calculate while moving.
Use the six-minute rule (distance in 6 min = speed ÷ 10) and minutes per mile (60 ÷ speed). At 24 knots, 1 nm takes 2.5 minutes.
Allow for tidal stream: offset angle ≈ 60 × stream ÷ speed.
Put waypoints in safe water, check every leg at large scale, and name them clearly.
A leg card gives course, distance, time, what to look for, dangers and clearing lines for each leg.
Clearing bearings, depth contours and transits answer the question "am I on the safe side?" quickly and reliably.
Set plotter alarms, use it to confirm what you see, and back it up with paper, a handheld GPS and the compass.
Keep a pilotage notebook for harbour entries, including light characteristics for night.
Work as a team: navigator calls the leg, times it and confirms marks; the helmsman keeps their eyes outside.

Check Your Understanding

How far does a boat travel in 6 minutes at 30 knots, and how long does 1 nm take at 24 knots?

Answer: At 30 knots, 3.0 nm in 6 minutes (one tenth of the speed). At 24 knots, 1 nm takes 60 ÷ 24 = 2.5 minutes.

A leg is 5.4 nm. How long will it take at 18 knots?

Answer: 5.4 × 60 ÷ 18 = 18 minutes.

You are crossing a 1.5-knot tidal stream at 20 knots. Roughly how many degrees should you steer into the stream?

Answer: About 60 × 1.5 ÷ 20 = 4.5 degrees, so about 4 to 5 degrees into the stream.

Why should you never put a waypoint on a buoy?

Answer: If you navigate precisely to the waypoint, you will hit the buoy, and other boats using the same position may converge on the same point. GPS or chart errors make it worse. Always place waypoints a safe distance off marks, in safe water.

What should a leg card show for each leg?

Answer: Leg number and waypoints, course to steer in degrees magnetic, distance, time at the planned speed, what to look for and when, the dangers with their clearing bearings or lines, and expected depths.

A clearing bearing on a lighthouse is NMT 015°T. You take a bearing of the lighthouse and get 022°T. Are you safe?

Answer: No. The bearing is more than 015°T, so you are on the danger side of the line. Alter course away from the danger (in the diagram example, to starboard) until the bearing is 015°T or less.

You are following a leading line into harbour, and the rear mark appears to the right of the front mark. Which side of the line are you on, and which way should you steer?

Answer: You are to the right of the line, so steer left until the marks are back in line. The nearer (front) mark always appears displaced to the side you need to steer towards.

Why should you check a plotter route at the largest scale along every leg?

Answer: Electronic charts hide detail at small scales, so a rock or shoal can disappear from the display when zoomed out. A route that looks clear at small scale can cross a hazard. The Vestas Wind grounding in 2014 is an example of what can happen.

Your plotter fails halfway along a leg. What do you do?

Answer: Slow down, note the time and estimate your position from the time run since the last waypoint. Mark it on the paper chart, then continue using the leg card: course, time to the next mark and visual checks. Switch to the handheld GPS if carried, and stop if you are unsure where you are.