Trip Plan

muckypup

muckypup

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I'm planning my first trip on Harlequin to stay overnight somewhere. Not being too adventurous for my first trip, I thought of a going from BYH to Bradwell Waterside overnight and back again.

It's about 18nm which I would leave about 2.5 - 3 hours for in Harlequin for a steady ride.

The question is that just off Ray Sands there is a narrow channel with a drying height of about 1.1m. Even at neaps, high tide is about 4m which should leave 2.5-3m of depth. Is there any reason why this wouldn't be safe at high tide?

Steve
 
I came through there for the first time in June about an hour and a half before HW springs (5th June) and had 0.2 under my 1.7 keel so nearer HW would have been even better. The shallow bit is fairly small.

I don't know what Harlequin draws but I can manage 3 hours either side of HW into Bradwell where my berth is. The shallowest bit is in the creek past the second and third buoys. If you can get up the creek then there is no problem into the marina.

Oh - just realised you are a speedy mobo thingy. No problems then with water anywhere. Enjoy your first trip!
 
So, say high water springs for Burnham is 4.8m and low water is 0.5m thats a 4.3m tidal range or 0.36m per 1/12. So 1 hour before high tide it's 3.94m, and 1.5 hours before high tide it's 3.58m.

From the charts, 1.5m drying height is about the highest point, which leaves 2.08m clearance. Your keel is 1.7m, which gives 0.38m (very close to the 0.2m you mentioned). Harlequin's draft is 1m which would give her 0.7m clearance. As you suggest, a little nearer HW to be on the safe side.

I like it when the math works out as it means I know how to work it out correct in the future.

mmm... mobo thing yes, speedy no. Harlequin is a sedate old lady, flat out at 13knts but happier at about 8.

S.
 
Use the 12ths rule

1/12 first hour after LW
2/12 second hour
3/12 third hour
3/12/ hour
2/12
1/12

works a bit better as the tides are slower in and out at low and high water.
 
I did (I thought)...

4.3m range divided by 12 is 0.36m per 1/12th. So in the first hour 1 1/12th @ 0.36m and in the second hour 2 1/12ths @ 0.72m. MoodySabre mentioned an hour and half before HW, so 0.36 + (0.72 / 2) = 0.72m below HW.

or doesn't it work like that?

oops just spotted what I did wrong. I took the tidal range of 4.3m as the height of tide, rather than more correctly the 4.8m of depth above LAT. So an hour and a half before high tide the depth of water should have been 4.08 above LAT which with a 1.5m drying height should leave 2.58m of depth.

I really need more practice with this stuff. Was thinking about an online Yachtmaster theory course over winter (I did 95% of Day Skipper theory and then had to abandon it due to work commitments).

S.
 
[ QUOTE ]
oops just spotted what I did wrong. I took the tidal range of 4.3m as the height of tide, rather than more correctly the 4.8m of depth above LAT. So an hour and a half before high tide the depth of water should have been 4.08 above LAT which with a 1.5m drying height should leave 2.58m of depth.

[/ QUOTE ]

Sorry for my ignorance, am a sailor in training /forums/images/graemlins/smile.gif , but can I ask what you mean by your LAT. I thought this was to do with Latitude. Its probably something that I have overlooked through all my reading material hence my ignorance.

Reading so much lately about sailing and currently navigation that pea has become overloaded and is now mushy. /forums/images/graemlins/smile.gif
 
lowest astronomical tide

Which I think means the lowest the tide is ever expected to be. Presumably the 'astronomical' bit refers to the pull of the moon and therefore doesn't allow for other factors that might affect tidal height such as strong wind and air pressure. But what would I know?
 
LAT = Lowest Astronomical Tide. I officially should have referred to my calculations as against CD (Chart Datum) not LAT as CD is the plane against which all depths are measured against.

From the admiralty website

"Chart Datum is the plane below which all depths are published on a navigational chart. It is also the plane to which all tidal heights are referred, so by adding the tidal height to the charted depth, the true depth of water is determined. By international agreement Chart Datum is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide.

The highest and lowest predicted tides that can occur are deemed Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT) respectively. These levels are the highest and lowest levels which can be predicted to occur under average meteorological conditions, and under any combination of astronomical conditions; these levels will not be reached every year. HAT and LAT are not the extreme levels which can be reached, as Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur) and pressure effects can significantly alter the times and / or heights of the observed tide.

In order to determine the manner in which tidal levels vary along any given stretch of coastline, it is necessary to refer all levels to a common horizontal plane. Chart Datum, being dependent on the range of the tide, which varies from place to place, is not a suitable reference plane."

S.
 
Hunson,

LAT = Lowest Astronomical Tide.

A google search produced lots of info such as:

"HAT - Highest astronomical tide
LAT - Lowest astronomical tide
Highest astronomical tide (HAT) is the highest level, and Lowest astronomical tide (LAT) the lowest level that can be expected to occur under average meteorological conditions and under any combination of astronomical conditions. HAT and LAT are not extreme levels, as certain meteorological conditions can cause a higher or lower level, respectively. The level under these circumstances is known as a 'storm surge' ('negative surge' in the case of level lower than LAT). HAT and LAT are determined by inspecting predicted sea levels over a number of years."

Hope this explains it OK. A google seach will gives lots more info.

Catseven
 
"Les informations de diagramme sont l'avion au-dessous dont toutes les profondeurs sont éditées sur un diagramme de navigation. C'est également l'avion auquel toutes les tailles de marée sont référées, ainsi en ajoutant la taille de marée à la profondeur dressée une carte, la profondeur vraie de l'eau est déterminée. Par le diagramme international d'accord des informations sont définies comme niveau tellement bas que la marée ne tombera pas fréquemment au-dessous d'elle. Au Royaume-Uni, ce niveau est normalement approximativement le niveau de la plus basse marée astronomique."

/forums/images/graemlins/smile.gif
 
Thanks guys, thought I was going mad there for a moment. I am sure I would have got it once I had had some more sugar intake and my pea started to perform.

I will have to put that down in my list of things to promptly get confused about with abbreviations. No doubt there are more of them that have double meanings.
 
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