Anyone who spends their sailing life in the Baltic or the Mediterranean could be forgiven for thinking that the sea is a constant. Either it’s there, or it’s not. Sure, there can be significant current where water funnels through narrow spaces such as the Messina Strait or the Strait of Gibraltar, but no tide to speak of. Once you get to UK or North American waters, though, it’s a completely different story.
King Canute and the tide
In the 12th century, a chronicler named Henry of Huntingdon wrote a tale of King Canute. The king, who ruled Denmark, England and Norway, was getting a little irked by the brown-nosery of his courtiers, who had decided he was basically a little bit of a god. He set out to prove them wrong. According to Huntingdon, Canute had his throne placed on the beach, and, in order to demonstrate that he was merely human after all, commanded the rising sea to cease, and under no circumstances to wet his feet. Once his sandals had been given a thorough soaking, he stepped back from the water’s edge and said: “Let all men know how empty and worthless is the power of kings, for there is none worthy of the name, but he whom heaven, earth, and sea obey by eternal laws”. Huntingdon writes that Canute hung his crown on a crucifix, and swore never to don it again as a mark of honour to god.
Apropos of nothing except that we’re an English-Polish crew, we will also note here that Canute was the son of the Polish princess Świętosława, daughter of Mieszko I, who was the first king of a unified Poland and the man who introduced Christianity to the country. Świętosława had originally been married off to Eric the Victorious of Sweden, but then for reasons involving a battle became the spouse of Sweyn Forkbeard of Denmark, and it was from this second marriage that Canute sprang, thoroughly human.
Anyway, tides. What causes them?
Before reading on, we recommend that you watch our short video about tides. Click on the version you want to watch, and it will open a new window direct to the film on our YouTube channel. If you would like to watch in English, click the image on the left, and for Polish click the image on the right.
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We know that the movement of the sea is caused by the gravitational pull of the sun and the moon. Again, if you’ve lived your sailing life in the Med, it might come as a bit of a shock to learn that these forces are powerful enough to do what Canute could not, and actually move oceans. Seas in most parts of the world are hauled in one direction for six hours, then flood back for the next six (more or less).
Note that we round up or down, usually on the basis that the moon orbits the earth once every 28 days. In fact, it takes 27.32 days. Likewise, we think of each tidal cycle as 24 hours, when in fact it is 24 hours and 50 minutes. This is okay, because when thinking about tides we are working with predictions which can be influenced by outside factors. For example, high pressure and an offshore wind can decrease the depth of water expected, and low pressure with onshore wind might increase it. Put simply, it doesn’t matter to us as sailors if the depth of water is 2.5 metres or 2.51 metres, and we don’t care if the tidal stream is flowing at 3 knots or 3.1 knots.
Diurnal, semi-diurnal and mixed semi-diurnal tides
Some places have two high tides and two low tides approximately every 24 hours. When these highs and lows are roughly the same, it’s called a semi-diurnal tide. When they are different, it’s a mixed semi-diurnal tide. And, if there is only one high and one low tide each day (for example, in the Gulf of Mexico), it’s called a diurnal tide.
Springs and neaps
Tides cycle approximately every 28 days in response to the position of the moon in relation to the sun and the Earth. In each 28-day period there will be two spring tides, and two neap tides. Spring tides have very high high water, and very low low water. The difference between high tide and low tide – known as the range – will be quite large. Neap tides generally have moderately high high water, and moderately low low water. This means the range of tide will be moderate too.
High tide, low tide, ebb and flow
When we talk of high tides and low tides, we mean the maximum and minimum height of water above chart datum. Chart datum can be found on all navigation charts, and represents the lowest level of water that can be expected in the area (hence the commonly used phrase “lowest astronomical tide”, or LAT). On the flow, a rising tide will increase the depth. So, if the chart datum is two metres, and the tide is expected to rise four metres, you will end up with six metres of water before the tide starts to ebb (fall) again. Note that you might see, in your tide table for the day, a negative figure (we’ve seen it for Falmouth on exceptional spring tides). That means that the water is in an ornery mood and is going to drop below the level of LAT.
Drying heights
Remember also that the areas marked (usually) green on your chart will show chart datum underlined. These are drying heights, which means that these areas stick up out of the water at low tide. So, if you see a drying height marked as two metres and you have a four metre tide, the first two metres of the tide is going to cover the land, and at high water the depth will be two metres.
Mean high water, mean low water
Mean high water springs (MHWS) is the average high water of spring tides, and mean low water springs (MLWS) is the average low water of spring tides. The difference is the mean spring range.
Mean high water neaps (MHWN) is the average high water of neap tides, and, you’ve guessed it, mean low water neaps (MLWN) is the average low water of neap tides. The difference is the mean neap range.
Measurements of height
If a land-based object appears on a navigation chart, you will often see an indication of how high it is. Or, that’s what it might look like at first. In fact, the height of a lighthouse, for example, is given as its height above MHWS. This is worth knowing, because when you then look at the information on the chart about the lighthouse range of visibility, you can adjust it according to how high (or low) the tide is.
To confuse matters, the clearance for bridges, overhead cables and so on is generally given in reference to the highest astronomical tide (HAT). That’s the highest that, theoretically, the tide can ever be.
Tidal streams
To close this post, let’s look at tidal streams. They will decide if you get to the pub before it closes, or whether you might be bobbing about a mile from land for the next six hours.
Imagine you are sailing up the Bristol Channel, which has one of the most impressive tides in the world (with a range of around 15 metres, it’s beaten only by the 16-metre tidal range in Canada’s Bay of Fundy). Now, with enough water rushing in or out to raise the overall depth by 15 metres, there’s got to be some movement backward or forward too. Taking our example of Bristol, all that water tears into the Severn Estuary reaching a speed of up to eight knots. That’s all very well if you are going with the tide; you can drop the sails, turn off the engine, and you’ll still be about eight nautical miles closer to your destination after an hour. If you think your yacht is going anywhere against this kind of tide, think again.
Let’s consider an example from dry land. What’s the average walking speed? We’ll say it’s three miles an hour. If you could step on an escalator three miles high and going up at a rate of three miles per hour, you could stand still and reach the top in 60 minutes. If you step on it and walk at your average pace, you will reach the top in 30 minutes (your 3mph, plus the escalator’s 3mph). On the other hand, if you wanted to go up on an escalator moving down at three miles per hour, you could walk at your average speed and you would never, ever, get away from the bottom.
That’s why tidal streams are so important for sailors. If you go with them, you get a lot of “free” speed. If you go against the tide, you will lose a lot of your speed through the water, and may even end up going backwards; even though your instruments tell you that you are moving at five knots through the water, that’s not a lot of use if the tide is pushing you six knots the other way. In other words, if you’re sailing up the Bristol Channel against a massive spring tide, you’re going to end up somewhere near the Isles of Scilly, if you go anywhere at all.