Time and Tide Part 1
This article in the ‘View from the Harbour Entrance’ blog looks at some of the problems caused by tides. Given how incredibly accurate tide predictions can be, and how easy it is to get hold of those predicted tide times and heights, you might think that watercraft users would never suffer tide-related problems. Unfortunately, in practice, you would sometimes be wrong. That’s not to say that everyone hits problems caused by tides – far from it. But there is one particular event that you can almost guarantee we will observe at almost every spring tide.
Before we launch into that, let’s review where you can get tide information. Well, there are plenty of almanacs and harbour guides available containing tide information (note that times are invariably given in GMT, so you have to adjust by one hour in the summer). You can also get a tide app for your smartphone.
Next, the QHM (Queens Harbour Masters) website gives tide information for that day; a good place to go is here as you can also get information on shipping movements…..
https://www.royalnavy.mod.uk/qhm/portsmouth/shipping-movements
For the current depth of tide, take a look at channel marker BC4 as you go past it; this has tide gauge rings spaced every half metre; the top represents a depth of six metres.
In the NCI Gosport lookout tower, we keep careful track of the tides (as does every other NCI station in the country); we use an application on our PC which shows the predicted tide height minute by minute. And we have a clear view of BC4. So if you’re stuck for information, give Gosport NCI a call on Channel 65.
So why do some skippers have a problem ? Well, it concerns ebbing spring tides. When the tide ebbs, the millions of gallons of water in the harbour all try to get out through the small pinch point formed by the Round Tower on the Portsmouth side and Fort Blockhouse on the Gosport side. This causes what is called a Tidal Stream of fast flowing water; and its speed is dependent on the levels of water inside and outside the harbour. At its peak, about three hours after high water, the Tidal Stream can be running at four knots or more, maybe as high as six knots. Since this effect is most marked during Spring tides, it tends to occur mid- to late afternoon.
If you are leaving the harbour at that time, especially if you are near the centre of the channel, you will be leaving mighty fast; we have sometimes seen very small vessels get out of control for a short while.
The bigger problem though is if you are trying to enter the harbour, against this stream, particularly if you are near the centre of the channel. Your vessel has to be making at least five knots in order to even move forward against the Tidal Stream. We so very often see low-powered vessels fighting against the Tidal Stream to come into harbour, engines at max and dirty smoke pouring out of the exhaust, but moving at snails’ pace. Some make no real progress for tens of minutes at a time, and waste a lot of fuel in the process.
So, how do you avoid getting caught like this ? One answer is to understand the tides, know when the Tidal Stream will peak, and simply avoid this period of time. All the information you need for this is on your charts in the Tide Diamond tables, you just need to understand how to use it. You can always call us on Channel 65 and ask about the state of the tide before you attempt to come in.
Another answer is to simply give up; if you find you are making no real progress then turn away, enjoy sailing for a bit longer, and come back an hour or so later.
The last solution is to avoid entering near the centre of the channel, and positioning more accurately within the Small Boat Channel (SBC). You should be in the SBC anyway, but the advantage in this situation of being in the SBC is that you are closer to the shore, and the Tidal Stream here is much lower… possibly to half what it is in the centre, maybe even less. It is sometimes marked by large patches of calm water. So, if you are in danger of blowing up your engine trying to get in against the tide, and you see other boats going past you on your port side getting in with far less effort, do the obvious thing… copy them. Move to port (but still keeping the red channel markers on your port side), throttle back, and (if this suggestion has helped you) give us a wave.
It would be wrong to finish without mentioning one other situation. If the tide is ebbing and a strong Tidal Stream develops, and is there is a strong southerly wind, you get a condition called “Wind Over Tide”. Without going into the hydrodynamics of it all, the bottom line is that you end up with a strong and very rough Tidal Stream. Large standing waves can be formed. Notwithstanding all the suggestions above, small vessels should avoid this altogether; even large vessels will find it uncomfortable.
Note - if only a few posts are visible in this blog, click on the ‘Older Posts’ link towards the bottom of the page to see more.
