Secondary ports tidal heights Bembridge

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FairweatherDave

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Thought I would revisit my rusty dayskipper stuff and see if it correlated with last weekend. So as an academic exercise I started on the tidal heights and curves. In my 2009 Reeds it gives Portsmouth MLWS as 0.8 and Bembridge harbour as -1.0. Does that mean that when I look at the Special Tidal curve section (which has a specific curve for Bembridge and Rye) that I subtract minus 1.0 from 0.8 and get 1.8 as my LW height above CD?
I would appreciate confirmation of this as my head is starting to get a bit fuzzy! The alternative of 0.2 below CD seems unlikely as it would be off the tidal curve table. Apologies for the numpty question but it is for fun....

In reality we crept into Bembridge on a rising tide but pretty much at low water springs on Saturday (the joys of a 1m draft bilge keel) but there was not much water according to the tide guage at the entrance.
Thanks for any help.....
Dave
 
You know that the big bit at the front of Reeds has full explanations for all of this stuff?
 
There's something a bit odd about Bembridge. Chart datum in the harbour is 1.34m below Ordnance Datum but in the approaches it is 2.73m below Ordnance Datum (same as Portsmouth). So the same water level will be shown as 1m higher above datum in the approaches than in the harbour because of the two different datums. Easy tide for low water today shows +1.1m in the approaches and -0.1m in the harbour.
 
Lustyd
That's what I thought but mine is the Channel Almanac and as far as I can see it did not help on how to actually utilise the tidal curves. I have looked at Tom Cunliffes Day skipper book which has refreshed my brain fairly well but (to use a phrase) I was looking for confirmation from another source, particularly as my brain started to fuddle.
 
Easy tide seems to confirm that -0.2m chart datum at LW springs is to be expected so it looks like you should be subtracting 1m from the Portsmouth LW level. But it looks like a lot of this correction is to account for the change in datum.
 
FairweatherDave said:
Lustyd
That's what I thought but mine is the Channel Almanac and as far as I can see it did not help on how to actually utilise the tidal curves. I have looked at Tom Cunliffes Day skipper book which has refreshed my brain fairly well but (to use a phrase) I was looking for confirmation from another source, particularly as my brain started to fuddle.
Click to expand...
Ah sorry in that case I recommend buying the "proper" almanac next time. It leaves very few questions unanswered including some medical ones and procedures for entry and exit of countries!
 
Thanks DJE. I feel more confident now, typical to choose an weird secondary "port" to get my brain back into gear. The different datums issue might do my head in again though :).

No worries Lustyd. It is a Reeds Channel Almanac so it is reasonably proper, if slimmed down. The Solent, not the World is my oyster, the full almanac at the moment would be overkill .........
 
0.8 + (-1.0) will give you the MLWS for Bembridge harbour, but just because a particular day is springs doesn't mean that low is at the mean spring low level - you need to interpolate/extrapolate based on the actual low for the day and the standard port spring / neap low heights (P'mouth 0.8 and 1.9 : BH -0.2 and 0.6, so 1.3 at P will give 0.16 at BH).

The datum changes are not relevant. Ordinance Datum is a land based leveling system, and we don't use it. We use Chart Datum, which is normally approximately LAT (in the UK), and ro which the tide heights and chart depths are referred.
There are frequent changes between the OD and CD levels, particularly in narrow entrance harbours and up long rivers. If CD was locked to OD we'd need loads more standard and secondary ports, and much more complex tidal calculations.
 
Thanks John. Understood. From my understanding for the day in question (last Saturday) the springs were close enough to the means for me not to take that any further. It's been a good academic exercise for me.
The best bit of the real world exercise was getting into Bembridge and realising I had left my "back of an envelope" passage plan with tide times etc at home. My mobile with dying battery then gave me tide times but I failed to correct for BST. Handy that since everything we did was later than I intended each day so it worked well enough.
 
I have just had a walk out to the Bembridge entrance and at 20.15 today the red can No8 was well aground. It seems that No8 is now on the inside of a sweeping bend and should not be approached in a straight line from either No10 outbound or No6 inbound as there is solid sand if you are a bit tight on depth.

Keep well over towards No9.
 
Surely you should get out more? :D

These days it is simpler to look up the tide of Bembridge directly on an app and use the time you would have otherwise wasted doing an inaccurate calculation to go to the pub.
 
IanL said:
I have just had a walk out to the Bembridge entrance and at 20.15 today the red can No8 was well aground. It seems that No8 is now on the inside of a sweeping bend and should not be approached in a straight line from either No10 outbound or No6 inbound as there is solid sand if you are a bit tight on depth.

Keep well over towards No9.
Click to expand...

Much appreciated - last time I left early on the ebb, there also seemed to be a shallow(well, shallower than last year) patch around No3 (I think).
 
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