If you are familiar with boating, you may know that some ports are best entered at high (or low) water.
It’s simple enough to check your tide tables to determine the exact time of high water, but you are then left wondering how long high water actually lasts.
Although it is impossible to give a precise answer, many years of experience in handling ships has given me a good rule of thumb.
High water lasts for 1 hour, from approximately 30 minutes before until 30 minutes after the high water time at your location.
Although it is a good rule of thumb, it’s best to remember that the precise duration of the high water tidal window actually depends on the tidal range at your location.
In my career, I regularly experienced tidal ranges in excess of 10m (33ft).
With a range that large, it was important to enter ports at high water because that was when slack water occurred.
If you are interested, you should check out this article next: When Is Slack Tide?
I would always time my arrival to enter the port 30 minutes before high water because I knew that it gave me approximately one hour to complete my manoeuvre before the tide started to ebb.
While 1 hour was the approximate duration of high water in my area, in most other areas around the world, the high water length can be considerably longer.
If there is a very small tidal range, you could almost say that there is a 6-hour high water window as the tide is either “high” or “low”.
More normally, however, the duration will be somewhere between the two.
Why does the length of high water vary?
Around the world, tides follow an approximately sinusoidal pattern.
The graph starts at low water and increases slowly until it reaches maximum flow approximately halfway between high and low water. Then, as it approaches high water, the tide slows down again.
After high water, it turns and follows a similar pattern while it ebbs.
In the graph above, you can see the approximate tidal heights throughout the tide. The precise numbers are not too important because we are interested in the flat part of the graph, indicating high water.
As you can see, with a relatively large tidal range, the graph quickly starts to drop away after high water occurs.
With a small tidal range, it is very different.
In the graph above, you can see that the relatively flat area centred on the time of high water is much larger.
This shows that high water itself apparently lasts longer in areas with a small tidal range.
Isn’t high water instantaneous?
In the graphs above, you could argue that the precise time of high water is instantaneous.
In reality, however, there is always a period of time where the tide appears to stay at high water.
The period is shortest in areas with a large tidal range, but even in those places you do get a window of approximately 1 hour that you could label as “high water”.
During my time manoeuvring ships, I experienced large tidal ranges, but as it approached the precise time of high water it still appeared like the tide was not going in or out.
Although it was still either rising or falling, when you are reading the tide board minute-to-minute it was imperceivable whether the tide was flooding or ebbing.
Even when handling the ships on the rivers, I would assume the effect of the tide was minimal within the 1-hour high water window.
From a physics perspective, high water did occur at a single point in time. From a boating perspective though, high water has a duration that depends on your tidal range.
What about the “rule of 12ths”?
The rule of 12ths is a good approximation of the sinusoidal curve that the tide follows.
In the 6 hours before high water, the tide will rise by a different amount each hour.
|Time (hour ending)||Rise|
You can see that in the hour ending at HW, the tide rises by approximately 1/12 of its total tidal range.
With a small tidal range, the movement will be barely perceivable for the entire last hour.
With a larger tidal range, the movement will be greater, but within the final 30 minutes, you can still say that you are within the high water window.
Why do you need to know the duration of high water?
Lots of different people ask about the duration of high water, for all manner of different reasons.
- Photographers like to know the duration of high water because it tells them how long they will have to capture their images.
- Swimmers like to know the duration of high water because it is safer to swim at slack tide, which occurs at high or low water.
- Boaters like to know the duration of high water as it forms a vital part of their passage planning process.
- Fishing enthusiasts like to know the duration of high water because different fish appear at different stages of the tide.
Why do boaters need to know the duration of high water?
The tide affects boaters in lots of different ways, so I have actually written an entire article on that which you can read here: How Do Tides Affect Boats?
In terms of the duration of high water, however, the main reason boaters need to know it is because that is the time when the tidal flow is at a minimum. The water is slack.
It makes it easier to enter ports and berths that are located perpendicular to the usual flow of current.
High water is also, fairly obviously, the time when there is most water under your boat. This is important for entering places where you have little under-keel-clearance.
If you know that the duration of high water is around 1 hour, you know roughly how long you will have while the tidal height is relatively stable for manoeuvring in shallow water.