Cowes Small Craft Channel.

[XDC]

There are two tide gauges, one outside, one inside the harbour.

http://www.cowesharbourcommission.co.uk/small_craft_channel

This morning the inner one showed a tidal height of 1.6m, the outer one perhaps 10 minutes later was 2.0m.

I’m pretty sure we passed the western end of the breakwater at 08.25 and were initially motoring at 5.5 sog passing the first one then sailing at 6.5+ sog with no time lost raising sails past the second one.

There is a tidal curve here:

http://www.cowesharbourcommission.co.uk/cowes_tide_tables

but I can’t see a rise of 0.4 in just 10 minutes.

What am I missing? Apart from depth

 

[Topcat47]

Do they measure the depth over the putty at that particular point, rather than the depth over chart datum, perhaps?

 

[XDC]

I did wonder about that but think that telling you that you have x depth at the entrance is a recipe for numerous groundings if depth y inside the harbour (which only the people in the harbour can see) is less, as in this case.

 

[prv]

Do they measure the depth over the putty at that particular point, rather than the depth over chart datum, perhaps?

The Harbour Commission website linked in the OP says they measure rise of tide, but that the shallowest depth in the channel is only 20cm below datum so in practice you should simply compare your draft directly against the figure on the gauge.

So provided the sea isn't on a slope, they should be reading the same depth at the same time.

Pete

 

[langstonelayabout]

The Harbour Commission website linked in the OP says they measure rise of tide, but that the shallowest depth in the channel is only 20cm below datum so in practice you should simply compare your draft directly against the figure on the gauge.

So provided the sea isn't on a slope, they should be reading the same depth at the same time.

Pete

And if the sea is on a slope... Get your water skis out immediately

 

[Elessar]

And if the sea is on a slope... Get your water skis out immediately

The sea is on a slope or there would be no difference in tide heights.
But not that much of a slope they should be the same.

 

[Angele]

The sea is on a slope or there would be no difference in tide heights.

It is probably like the difference between Bembridge Harbour and Bembridge Approaches.

This morning:

Bembridge Harbour:
LW= 0.0m
HW= 3.0m

Bembridge Approaches
LW= 1.1m
HW= 4.6m

I always imagine a metre high standing wave waiting to crash over the foredeck as soon as you pass St Helen's Fort - a bit like the Severn Bore, only far more spectacular.