Thursday, August 21, 2014

Tides, Current, and Anchoring.

Mackay is within easy striking range of the Whitsundays.  There are islands as far as you can see.  We have even been referring to this area as the AVI's (Australian Virgin Islands) because they are so similar to the British Virgin Islands, only so many more islands covering so much more area.  Whale sightings are now almost as common as dolphins and we are still 30 miles inside the Great Barrier Reef.

Even though the Barrier Reef is blocking much of the Pacific Ocean swell, in many of the anchorages the boats get rolling and uncomfortable at times.  The real issue however are the tides.  They are huge at 23ft in Mackay and 18 ft in the Whitsundays.  The hydrodynamics of this area are similar to the Chesapeake Bay with the Barrier Reef being similar to the Eastern Shore with similar depths.  However, the Chesapeake has 2 ft tides and still has some pretty good current.  Imagine what a 23 ft tide would do with the volume of water in the Chesapeake.

So even when anchoring at an isolated island, there is current as if you are in a coastal river and the boat will swing 4 times daily according to the tide and not according to the wind.  Since you are not pointing in to the wind but covering 180 degrees of swing, there are times when the ocean swell, wind chop, or current counter wind will be hitting the side of the boat and there you go, rocking and rolling.

Normally we put out an anchor chain length of about 4-5 times the depth of the water.  This is called 'scope' and make a low angle at the shank of the anchor which keeps the anchor from pulling out.  With these big tides, you have to check your depth and tide when picking a spot to anchor to make sure that you won't be sitting on the bottom when the tide goes out.  You also need to add the tide to your depth so you have scope to keep from pulling your anchor out when the tide goes high. 

Anyone for a Vegemite Sandwich

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