Dinghy access times to a drying pontoon ?

[sarabande]

I keep my dinghy on a pontoon which dries out towards low tide (springs and neaps).

Let's say that I can't use the dinghy for an hour before LW, and an hour after LW neaps.

I asked a highly qualified and experienced friend to check out my maths for predicting when the pontoon would become useable at LW springs, and intermediate ranges.

He indicated with lots of complicated diagrams and tables that the time available for access (i.e. when I could row to the pontoon rather than find mud) would be different for springs and neaps, but empirical observation seems to indicate that the drying time is still about 1hr before and 1 hr after LW.

Disregarding low or high air pressure, wind direction, rain surge, etc, I think that it matters not whether the tidal range be large or small, the tide will reach the level needed to float the pontoon at the same time for neaps and springs.


Or have I discovered an anomaly in my maths or the tide tables ? It would be handy to look at the tide times and say we can't back to land until LW plus 1 hour - always.

 

[prv]

Most places that express a tidal limitation in hours either side of high/low water, don't specify springs or neaps. Which implies that it doesn't much matter.

I prefer to know what rise of tide I need, and read that off the graph in my phone's tide app to find the corresponding time. So I don't pay too much attention to fixed times either side of high or low water, but to the extent that I've noticed, my experience is the same.

I'd expect it to possibly vary more in places with "interesting" asymmetrical and non-siney tidal curves.

Pete

 

[TangaraToo]

It does change but not by much.
Using 'Absolute Tides' app for Harwich and needing a depth of 2mtr it works out as 3hrs 7 mins from low water for neaps and 3hrs 11 mins from low water springs. I also looked at 1mtr depth but that is very close to low water Harwich on neaps so there is a greater variance due to the slacker tide either side of low water.
So it would seem the close the depths is to low water neaps depth the greater the variance will be, rule of 12th coming in to play. As the pontoon dries at both springs and neaps the variance is likely to be small as not close to low water tide height so probably not in the 1/12th tidal rate band.

 

[RobbieW]

Wonder if this is explained by the flow rates at springs being greater than that at neaps ? Although the water drops more at springs it comes back faster, thus time-to-float is roughly equivalent across the board. Cant offer any proof of that other than gut feeling

 

[TangaraToo]

It seems to be more affected by how close to low water neaps depth the required depths is. The closer you get to low water depth the more tidal rate affects the variance between springs and neeps. If the required depth is on the steep part of the tidal curve then little difference springs to neaps, Closer to either end the the variation becomes more pronounced.

 

[lw395]

First thing is that the tide is quite often significantly different from the predictions. You don't need headline grabbing weather to move the time or height of HW noticeably.
Secondly, most people build in a big, but variable, safety margin.
Thirdly, the springs and neaps published curves only apply to those exact days, at those exact places. The curve is assumed 'near enough' for a place half a mile from QHM's tide gauge two days after springs.

But spring tide lows are often much lower than neaps. Places that dry at springs don't dry at neaps, so the access times must vary in reality.
But if the figure is close to HW+/-3, then that's close to mean sea level. And mean sea level neaps might be quite similar to mean sea level springs.

 

[tatali0n]

I guess everything's relative.

If you've got a tidal range of only a few meters, then there probably isn't much difference for access times between springs and neaps. For comparison, here on the Bristol Channel there's about an hour's difference: Portishead lock is operational three hours either side of HW on springs, but 4 hours either side on neaps.

But you are looking at a tidal range of around 7m for neaps and about 12m for a springs.

I guess the other way of looking at it is that it takes that much of a tidal range to make even an hour's difference

 

[VicS]

I keep my dinghy on a pontoon which dries out towards low tide (springs and neaps).

Let's say that I can't use the dinghy for an hour before LW, and an hour after LW neaps.

I asked a highly qualified and experienced friend to check out my maths for predicting when the pontoon would become useable at LW springs, and intermediate ranges.

He indicated with lots of complicated diagrams and tables that the time available for access (i.e. when I could row to the pontoon rather than find mud) would be different for springs and neaps, but empirical observation seems to indicate that the drying time is still about 1hr before and 1 hr after LW.

Disregarding low or high air pressure, wind direction, rain surge, etc, I think that it matters not whether the tidal range be large or small, the tide will reach the level needed to float the pontoon at the same time for neaps and springs.


Or have I discovered an anomaly in my maths or the tide tables ? It would be handy to look at the tide times and say we can't back to land until LW plus 1 hour - always.

It depends on the tidal range. You need to consult the tidal curves for springs and neaps for the port concerned.

In this diagram for a neap tide ( at a West Country port) I have highlighted the two hour period , -1hr LW +1hr, you mention. You can see that this period covers the time during which the tidal height, above chart datum, is less than 2.0m

If you now examine the curve for a spring tide at the same port you will see that the period during which the tide is below 2.0m has increased to approx 4 hours, -2hr LW +2hr

 

[ShinyShoe]

Rule of twelfths?

So 1 hour after LWS will be 1/12th of the tidal range. Lets say the range is 4m. So you should have 33cm of water if there was 0cm at LWS (rather than a negative value).

If the tidal range is only 2m, you will have 17cm of water. Might sound different but in the grand scheme of passage making I doubt that makes that much difference. What makes the difference is waiting a further hour when you'd have 99cm and 45cm both of which sound quite "sailable" in a small rubber duck...

When the range is bigger the depth of the trough is usually lower so even if 33cm sounded a big improvement from 17cm, chances are the 17cm wasn't as low a tide in the first place...

 

[Seajet]

At my half tide mooring, to the NW end of Chichester Harbour, there's definitely lots more water around generally inc low water at neaps rather than springs; but it's at the meeting point of two harbours and their tides so a bit unusual.

In general around Chichester and other harbours I think it fair to say neaps give you a bit longer, but not so much as to test it to the last inch with a fin keeler in a place with a rocky bottom !

 

[VicS]

At my half tide mooring, to the NW end of Chichester Harbour, there's definitely lots more water around generally inc low water at neaps rather than springs; but it's at the meeting point of two harbours and their tides so a bit unusual.

In general around Chichester and other harbours I think it fair to say neaps give you a bit longer, but not so much as to test it to the last inch with a fin keeler in a place with a rocky bottom !

Depths are always greater at LW neaps than at LW springs, unless you are somewhere where the tidal curves are very peculiar.

My boat dries completely on its C3 mooring at LW springs but just about stays afloat at neaps .

However if you look at slipways etc that are well above MSL you have a much shorter period of access at neap tides that you do at spring tides. At Paynes you have to be on your toes not to delay coming in in the dinghy at neap tides.

 

[jac]

Assuming that the mean height of the tide is the same for springs and neaps ( i.e. that Spring High is the same height above Neap High as Spring Low is below Neap Low) then what you will see is an exaggerated curve, with steeper sides. So at the same relative point in time, the amount of the tide left to fall will be similar. During springs of course it will fall far more in absolute terms but the relative fall should be the same.

As for your drying pontoon, I think you probably have a lucky fluke.

Take for example a pontoon that that just retains 1cm more water at LWN than is needed. AT LWS, ( unless the range is miniscule) the depth will fall below that of LWN at some stage - depending on the range of the tide.

Another example is that the tidal curve can change between Neaps and Springs. Taking the Solent as an atypical example, the classic double high tide can turn into a long stand. The almanacs show slight differences as well in the theoretical shape but I can't believe that these are significant enough in reality to make a difference!

So in another location if the tidal curve changes or the pontoon is close to the limit of drying at LWN, I don't think this observation would hold true.

 

[VicS]

Assuming that the mean height of the tide is the same for springs and neaps ( i.e. that Spring High is the same height above Neap High as Spring Low is below Neap Low) then what you will see is an exaggerated curve, with steeper sides. So at the same relative point in time, the amount of the tide left to fall will be similar. During springs of course it will fall far more in absolute terms but the relative fall should be the same.

As for your drying pontoon, I think you probably have a lucky fluke.

Take for example a pontoon that that just retains 1cm more water at LWN than is needed. AT LWS, ( unless the range is miniscule) the depth will fall below that of LWN at some stage - depending on the range of the tide.

Another example is that the tidal curve can change between Neaps and Springs. Taking the Solent as an atypical example, the classic double high tide can turn into a long stand. The almanacs show slight differences as well in the theoretical shape but I can't believe that these are significant enough in reality to make a difference!

So in another location if the tidal curve changes or the pontoon is close to the limit of drying at LWN, I don't think this observation would hold true.

Mean sea level ???