My post here is 2 fold…
Provide some competitive banter amongst mariners about whether the USCG databank provide ANY realistic education for upcoming mariners - I would argue yes and no, but can make a strong case for no. *Of the 86 Tides and Current problems on lapware, I have successfully completed about 75.
Educate mariners that want to learn more - This will consist of an excerpt from your boy Nathaniel Bowditch as well as small piece I put together on a pretty cool book I’m reading called Tides: The Science and Spirit of the Ocean by Jonathon White. I am about halfway through this one, but so far, I can recommend it for anyone looking to gain further knowledge on the matter. The author is passionate about Tides. He ran aground on a sailing vessel in his younger days due to tides to currents and vowed to never do it again by understanding this matter further (he discusses this in the intro). So he we go…
The tide tables are from 1983 - that’s a long time ago, lets update that. All of these “calculations” are inaccurate due to sea level rise. Also, your ECS or ECDIS does this automatically.
From Bowditch “One should exercise extreme caution in using general rules. The belief that slack occurs at local high and low tides and that the maximum flood and ebb occur when the tide is rising or falling most rapidly may be approximately true at the seaward entrance to, and the upper reaches of an inland tidal waterway. But generally this is not true in other parts of inland waterways. When an inland waterway is extensive or its entrance constricted, the slacks in some parts of the waterways often occur midway between the times of high and low tide. Usually in such waterways, the relationship changes from place to place as one progresses upstream, slack water getting progressively closer in time to the local tide maximum until at the head of the tidewater (the inland limit of water affected by a tide) the slacks occur at about the times of high and low tide.
The daily rise and fall of the tide is important to every mariner."
More from Bowditch “One should remember that heights given in the Tide Tables are predictions and that when conditions vary considerably from those used in making the predictions, the heights shown can be considerably in error. Heights lower than predicted are to be anticipated when the atmospheric pressure is higher than normal or when there is a persistent strong offshore wind. Along coasts where there is a large inequality between the two high or two low tides during a tidal day the height predictions are less reliable than elsewhere.
The current encountered in inland waters is due primarily to tidal action, but other causes are sometimes present. The tidal current tables give the best prediction of total current, regardless of the cause. The prediction of a river may be considerably in error following heavy rains or a drought.”
Some tidbits I found interesting the book mentioned above.
There are 370,000 miles of shoreline on the planet and all of them are affected by tides in some manner. Examples of coastline that have small tide ranges are the Gulf of Mexico, Mediterranean Sea and South Pacific Islands. Oppositely, the tide change is very large and noticeable in Northeastern Canada, northwestern Australia, Patagonia and the United Kingdom.
Sea Level Rising
The famous Piazza San Marco in Venice Italy occasionally can receive up to 18 inches of water in the square at high tide these days. While having the water occasionally jump the wall in the past hasn’t been unheard of, this has happened more frequently in recent years. In 2015, this famous square was recorded as receiving water over the seawall more than 100 times. The Italians call high water acqua alta.
Bay of Fundy, Nova Scotia
Bay of Fundy has the largest tidal range in the world, recorded officially at 54.5 feet at its greatest. At certain times of the year, the tide range will be 52 to 53 feet on average at its maximum. The mud shore at Bay of Fundy combined these unusual and drastic tide changes results in the unusual flight migration of the Sandpiper en route to its destination in Suriname near the equator.
Mont Saint Michel, Normandys Atlantic coastline in France
The 45 foot tidal range here brings tourism to its famous monastery that is a common ending spot to pilgrimage hikes across Europe. “Brochures today caution tourists not to walk on Mont Saint Michels flats without a guide. Besides the risk of quicksand and the tide, lightning is also a danger: anyone walking in the tidal zone is surely the tallest thing on the horizon. “Once the tide hits your feet, two minutes later it will be at your waist and take you away.”
Chinas Qiatang River Tidal Bore
A tidal bore is a tidal phenomenon in which the incoming tide forms a wave (or waves) of water that travels up a river against the direction of the river current. This strange phenomenon is present in many parts of the world, but here is it the most drastic and most violent, responsible for many deaths each year. This “tidal bore” can get up to 26 feet high and travel upriver at speeds of up to 20 mph in certain stretches of the river. The 2nd largest tidal bore known occurs in the Amazons Pororca and reaches heights of 12 feet and can travel upriver for almost 500 miles. “All tidal bores have two things in common, the first is a funnel-shaped river mouth with a shallow, gently sloping bottom. The second is a large tide at the rivers entrance.” The Qiatang River, is about 90 miles south-south west of Shanghai and the well known Yangtze River.