A Few More Things About Tides (6)

The sand was still visible under the boat but with so much yet to talk about, Ahab knew he would have to be quick because even though high tide was some hours away, what he needed to talk through with Ernest, his sailing student and nephew, was quite a lot…

“Remind me about tides, Ernest and what we have done so far.”

“OK. Tides are caused by the moon and the sun a little bit too. When they line up, the two bulges of water meet and we get a huge tide but when they are not so much in line, the tide is really low and this is called a neap tide. How’s that?!”

“Great, Ernest just great. So how can we use the tides in our favour?”

“Well, we need to make sure we have enough water to float. That’s our draught!” Ernest was triumphant, maybe some of this stuff was going in after all.

“Yes, it’s a bit simple but that is why you need to know your draft. But there are other ways of using tides and some other things you need to be aware of. And guess what?” Ahab asked, “It links back to charts!”

“On the charts, you will see a series of numbers, dotted all over the place. These are drying heights or chart datums. This figure tells you how low the water drops when it is at the lowest astronomical tide. So, in other words, on a neap tide, how much does the water drop in certain places. Why do we need to know this?”

Ernest sat and thought. He knew Ahab well and realised he was asking him because he already had the information in his brain. “Think, “he said to himself, “think!”

“Draft” the word fell out of his mouth before he knew but picking up courage, Ernest realised he knew the answer. “Will there be enough water under the boat to float at a very low tide!”

There was a certain look of pride of Ahab’s face as he realised that slowly, but surely, Ernest was starting to link things.

“Yes, you are right. If you are sailing in shallow water, you need to be super-confident that not only are you clear of rocks and other submerged objects but that you have enough water beneath your hull too. Getting beached on a sandbank is never a good look…”

“Have you done that?”

“In my younger years, Ernest. But not anymore because I take note of the tide times for the day that I am sailing, as well as taking note of what the charts for that particular part of the sea are telling me, “continued Ahab “For example, I can leave harbour three hours before high tide but I have to be back on my mooring in the three hours after high tide. If I don’t, there may not be enough water under the boat to bring her back in.”

“So, we are going to sail on the next tide?” Ernest was now, indeed, very earnest to get sailing.

“Maybe,” replied Ahab, “maybe”.

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