Wednesday, April 18, 2012

Anchor Safely (With A Little Math)

Captain John shares his easy calculations for "peace-of-mind" anchoring...

Guest blog by Captain John
Captain John Jamieson, a regular contributor to the Daily Boater, is author of Seamanship Secrets and publisher of the popular boating education website

If you are anything like me, few things beat a peaceful night at anchor "on the hook". And to stay at peace you will need to put out enough anchor rode (anchor rope and chain or all-chain ) to keep your small sailboat in place. Follow these easy steps before you lower your anchor to keep your sailboat safe all night long!

Depth Transducer Distance Below the Waterline?
Your depth sounder get's its information form an instrument called a depth transducer, mounted somewhere in your bilge. Determine the depth below the waterline of your transducer. If unsure,  make a "best guess". Write this distance down in the ship's log for future reference.

Distance of Your Bow to the Waterline?
The nautical term "freeboard" describes the distance from your deck down to the waterline anywhere along the hull of your boat. On most boats, the bow has the highest amount of freeboard (and the stern has the least). Measure or estimate the freeboard at your bow, because this will be where you will lower your anchor. Write this freeboard measurement in your ship's log for future reference.

How Much Further will the Tide Rise?
Check your Tide Tables or use a free internet site like the National Oceanic and Atmospheric Administration (NOAA)  Tides & Currents  web page, to determine the range of tide at your anchor spot.  You want to know how much further the tide will rise. If you arrive at the anchorage at dead low tide and the tide tables state that the tide will rise 5 feet to high tide, write down 5 feet.

Add all the factors together. For example, let's say you arrive at the anchorage at maximum low tide. Your depth sounder reads 15 feet. You estimate that your depth transducer has been installed 2 feet below the waterline, your bow freeboard measures 4 feet, and determine that the tide will rise 5 feet to high tide.

Add all factors together:  15 feet (depth sounder reading) + 2 feet (transducer depth) +  4 feet (freeboard) + 5 feet (distance to high tide) = 26 feet total .

Calculate how much scope you need based on the conditions. Scope give a ratio of rode to each foot of water. In calm to moderate weather, you want a scope of about 7 feet of rode to each 1 foot of water (7: 1 scope). In extreme weather, it's wise to use 10 feet of rode to each 1 foot of water (10: 1 scope).

Always multiply the scope you desire by the total feet in your calculation. Calculate scope with the factors used earlier:

Moderate weather anchor rode calculation (7:1 scope):  7 X 26 = 182 feet of anchor rode.
Severe weather anchor rode calculation (10:1 scope) : 10 X 26 = 260 feet of anchor rode.

Vessels using all-chain rode can use less rode (5X) in moderate weather, but the skipper must make this determination based on expected wind and swell, wind shift possibility, proximity to shoals or other vessels, sea bottom holding ground, and anchor gear and equipment capabilities .

Use these easy factors for peace-of-mind anchoring to keep your small sailboat safe and sound on the waters of the world. You will sleep better at night and know your crew or partner will too - wherever in the world you choose to cruise!


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Photo of sailboats anchored in Newport, RI, taken in 2010 © Andrew Canepari for Nautical Sites Media


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