Please select your home edition
Edition
Marine Resource 2016

Use magic depth curves to keep clear of danger!

by John Jamieson on 2 Jun 2012
Note this coastal approach chart to the Krenitzin Islands in Alaska. Soundings are in fathoms. Depth curves are staggered at 10 fathom intervals. Note the 50 fathom curve about 5 miles offshore. Closer inshore, you see the 40, 30, and 20 fathom curves. The thick blue line represents the base sailing track of course 135° Magnetic. The light dashed blue lines show what would happen if your course was in error by a few degrees on either side. You would still receive warning via your depth sounder John Jamieson
Imagine that you and your sailing partner are on your small sailboat, surfing down the waves on a broad reach. Ahead in the distance lies your destination, a tiny cove on the northwestern side of an island. What simple sailing navigation technique could you use right now to back up your 'black box' navigation systems to enhance your sailing safety?

Assume that your sailing navigation will be in error. Small vessels have unstable platforms. Heeling, pitching, rolling, yawing. All of these motions cause a small vessel to wander back and forth across her intended course. Consider that navigation errors can include leeway, current, steering, compass deviation, and the 'fatigue factor'.

Leeway Error: Expect your vessel to experience leeway--or 'side-slip'--in a downwind direction. The amount of leeway depends on hull and keel design, vessel displacement, and point of sail. Sailboats experience more leeway when beating, close- or beam-reaching and minimum leeway when broad-reaching or running. Make your best estimate of leeway, and apply it to the course line in an upwind direction.

Current Error: Currents can push a vessel to the right or left of track (current from the side), or increase speed (current from astern) or reduce speed (current from ahead). Even with careful planning, currents along the coast or offshore can change direction and strength as wind and sea conditions change. Check current tables and marine forecasts along with local knowledge for best estimates of tidal, coastal, or ocean currents.

Steering Error: Steering a small boat in a seaway can be quite difficult. Legendary sailor John Mellor once remarked that a person steering a small sailboat would do well to keep within 5° to 10° of the course. Even with careful tracks drawn onto the nautical chart or routes plotted onto a chart plotter, your sailing vessel will often make a snake-like track above and below an intended trackline. Rotate the helm at more frequent intervals in rougher weather.

Compass Deviation Error: Even the best adjusted compass can accumulate error from unexpected magnetic or electrical influences (called 'deviation'). That's a good reason to spend the money to hire a pro compass adjuster before you leave on a cruise or voyage, or learn to do this yourself. Caution the crew to leave cell phones and computers below or well away from the compass. Twist wires nearby to help cancel out electrical influences.

'Fatigue Factor' Error: Add in the factor of crew fatigue. On a small boat, you grab, squat, kneel, go up and down companionway ladders, hang on when heeling, pull yourself up from a sitting position. Multi-tasking becomes a natural element in sailing. And it's tiring. Fatigue leads to errors; in more than one case it has lead to disaster. Plan on being fatigued. Rest when you can, but plan on a bit of 'fuzzy headedness' from time to time. Just another good reason to lessen the length of watches in thick weather.

How to use magic depth curves
Back up your black box navigation with the magic of depth curves (also called depth contours). These provide the short-handed navigator with a good safety factor for the errors described earlier. Look on your nautical chart for lines or non-concentric circles near the coast. Depth curves show equal depths all along the line or circle. Scan along the line and look for a number that indicates the depth of the curve.

First, check the margins or title block of your chart to determine whether the soundings shown are in feet, fathoms, or meters. Also note the datum for those soundings. On US charts, the usual datum will be Mean Lower Low Water (MLLW); British Admiralty charts often use a slightly more conservative datum called Lowest Astronomical Tide (LAT). In all cases, never assume when it comes to soundings. Check your nautical chart before you begin to navigate!

Depth curves are often staggered in increments of 6 or 10. In inland or coastal waters, you will see depth curves numbered 6, 12, 18, 24, and 30. Further offshore, depth curves might be labeled 10, 20, 30, and so forth. Set your depth sounder alarm to warn you when you arrive at the outermost depth curve near a hazardous coastline.

Realize that this will be approximate; high waves could alter this reading, so take an average over a period of time. Calculate the tide (in particular for close coastal or inland waters) for your time of arrival and apply this correction to the reading displayed on your depth sounder.

Note in the illustration how the offshore 50 fathom depth curve warns you of your approach to this Alaskan Island. Further inshore, depth curves continue at 10-fathom intervals of 40, 30, and 20 fathoms.

In this case, it would be prudent to set your depth sounder to alarm when you arrive at the 50 fathom curve. That curve lies about five nautical miles offshore. You could also set an alarm for the remaining depth curves closer to the coast. But do not rely just on an alarm for warning (they can fail to sound). Alert the crew on watch to log the depth readings at frequent intervals each hour. Once the boat crosses the 50 fathom depth curve, the person on watch can alert the other crew in order to prepare for the final approach.

Use these sailing navigation tips to find vital depth curves to keep your small sailboat in safe water. Backup your black box navigation with valuable sailing tips like these to keep your crew safe and sound--wherever in the world you choose to cruise.

John Jamieson (Captain John) shows you the no-nonsense cruising skills you need beyond sailing school here. Sign up for a free sailing tips newsletter or join for hundreds of sailing tips articles, newsletters, eBooks, and live discussion forums.
Barz Optics - Melanin LensesWildwind 2016 660x82Newport Boat Show 2016 660x82

Related Articles

Free $US3,000 Carbon Vang with SouthernFurl boom orders in July
Southern Spars is giving a free carbon vang - valued at US$3,000 - with SouthernFurl in-boom furlers ordered in July Southern Spars is giving away a free carbon vang - valued at US$3,000 - with all of their SouthernFurl in-boom furlers ordered in July. Carbon gas vangs make a great addition to the furling boom package, though if you’d prefer to keep your existing one, Southern Spars will offer you a 5% discount on the price of your boom instead.
Posted on 29 Jun
Newport Bermuda Race - High Noon takes honours
As the Newport Bermuda Race fleet rushed to the finish line on Monday in the wake of the first-to-finish boat, As the Newport Bermuda Race fleet rushed to the finish line on Monday in the wake of the first-to-finish boat, the powerful 100-foot grand prix Comanche, to the surprise of many they were led by an unusual boat and crew. High Noon, at 41 feet, is fully 59 feet shorter than Comanche and tens of feet shorter than many other entries.
Posted on 22 Jun
Platino recovery - Family confirms that tug has made rendezvous
Reports in social media say a salvage tug has made a rendezvous with the Platino earlier than expected. Reports in social media by family and friends of Nick Saull, the crew member killed during a catastrophic incident abroad the 66ft yacht Platino say the salvage tug which left on Tuesday night has made the rendezvous earlier than expected. The Facebook report says the tug, Sea Pelican, arrived on Friday morning, the weather in the area has eased and with a more favorable outlook.
Posted on 16 Jun
Royal Queensland Yacht Squadron to ban bottled water
Approval has been given to create a ban on bottled water that comes in plastic containers. The RQYS Management Committee has confirmed that approval has been given to create a ban on bottled water that comes in plastic containers. This will place the club as a leader in environmental impact management in Australia and around the world. The Royal Hong Kong Yacht Club earlier this year did likewise. Who’s next?
Posted on 16 Jun
SouthernFurl In-Boom furling systems withstand Sydney Hobart test
Southern Spars design team have developed the SouthernFurl Boom smaller budgets and yachts from 35-70 feet. Have you ever scared your family when you got stuck with too much sail up when you should be reefed? Southern Spars’ range of SouthernFurl booms are the answer – letting you reduce sail quickly and easily without leaving the safety of the cockpit.
Posted on 23 May
2016 Garda Trentino Olympic Week - Day 4
Garda Trentino Olympic Week 2016 draws near to the conclusion: the weather once again proves difficult. Garda Trentino Olympic Week 2016 draws near to the conclusion: the weather once again proves difficult. Tomorrow the medal race is scheduled for the Lasers and another two races for the three paralympic classes.
Posted on 13 May
CCA presents RCC Award to Scott and Mary Flanders
CCA has announced that Scott and Mary Flanders are the recipients of the organization’s 2015 Royal Cruising Club Trophy. The Cruising Club of America (CCA) has announced that Scott and Mary Flanders are the recipients of the organization’s 2015 Royal Cruising Club Trophy.
Posted on 22 Apr
Cyclone Winston Relief Fund – Help the people of Fiji
Sea Mercy is sending volunteer fleet of small and large vessels, loaded with shelter, food and medical supplies to Fiji. Sea Mercy is once again sending our volunteer fleet of small and large vessels, loaded with shelter, food, water and medical supplies and teams to Fiji.
Posted on 27 Feb
Golden Globe Race - 30 sailors from 12 countries named for challenge
Sailors from America, Britain and France head the preliminary entry list for the 2018 Golden Globe solo round the world Sailors from America, Britain and France head the preliminary entry list for the 2018 Golden Globe solo round the world race. The 29 men and one woman – have each paid an initial A$3,000 entry fee. Other entrants hail from Austria, Australia, Brazil, Estonia, India, Ireland, Italy, New Zealand, Norway, Palestine, Russia and Switzerland.
Posted on 18 Feb
Passage making and the responsibility to others—World Cruising news
Editorial Editorial
Posted on 29 Jan