Low Water & Safety Margins.

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Ian_Rob

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I am wanting to leave the River Beaulieu on the morning of the 1st June but will need to wait for some guests, who can't be there much before before 09.30hrs. Allowing 60mins to get them aboard, the earliest we could be ready to go would be say, 10.30.hrs. LW is predicted as 11.10 hrs/1.2m. Reeds advises that access is possible +- 2hrs MLWS but what should be possible at neaps (Neaps is June 2nd). Obviously any safety margin will depend upon the particular conditions but what margins are appropriate. We draw 1.9m.
 
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It might be helpful to compare the depth you observe on the way in with the tidal height indicated at Bramblemet?
Assuming the issue is at the entrance, the tidal should be fairly closely tied to Bramble Post?
Is there a tide gauge much nearer you can read from deep water before leaving?
A look at sea level networks will show you that the tide can easily be 0.3m off prediction in the Solent, without the weather being extreme.
Personally I'm nervous unless I have 0.5m to spare, unless I know the bottom is soft, there are no big waves and the keel ploughs well...
 
Within 1 hour of low water assume it is low water, so 1.2m over CD. You draw 1.9m so require at minimum 0.7 m below CD but I would double this, especially if there us any swell at the bar. So you require a minimum depth of 1.5m below CD.

My Navionics chart shows min depth over the bar as 0.4m, so you need to cross latest 2 hours before or 2 hours after LW (for your boat Reeds re spot on) If flat you might be ok 1 hour either side, but watch your depth carefully and hope no one as left a shopping trolley in the way !

May not impress your guests if you do touch, so either get them there earlier or have some beers at the Master Builder and head out after lunch. If you are going to be close always better to do so when the tide is rising. On my boat the rule is simple - if yo do not want a very large bill for new stern gear (props shafts P brackets and rudders) DO NOT TOUCH BOTTOM !
 
If you have to ask this question on a public forum should you be the master of the vessel- especially as you have not quoted any time zones.

There are plenty of good hints in earlier replies.
 
Ian_Rob said:
I am wanting to leave the River Beaulieu on the morning of the 1st June but will need to wait for some guests, who can't be there much before before 09.30hrs. Allowing 60mins to get them aboard, the earliest we could be ready to go would be say, 10.30.hrs. LW is predicted as 11.10 hrs/1.2m. Reeds advises that access is possible +- 2hrs MLWS but what should be possible at neaps (Neaps is June 2nd). Obviously any safety margin will depend upon the particular conditions but what margins are appropriate. We draw 1.9m.
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I think this would be a good opportunity to revise all your tidal height and depth calculations.

I dont have relevant tide tables handy to be able to work it out for you

But you say the height of LW on that day is 1.2 m . The charted depth off the entrance is only 0.8m ( on the chart I am looking at) so at LW there will only be 2.0m. 0.1 m under the keel is not enough of a safety margin. ( If the charted depth is only 0.4m as suggested by superheat then you dont have enough water anyway)

Even looking at the tidal curves the half an hour or so you will be leaving the river before LW will only give you a less than a tenth of the tidal range extra . Guesstimating now that that will be about another 0.2 m at best. 0.3 m under the keel still is chancing things except in calm conditions and when you can confirm that meteorological effects are not depressing the tide below predicted levels.

But look up all the data and work it out properly.
 
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I have already done the tidal height and depth calculations (though based on a min depth of 0.8m that's on my charts rather than 0.4m). I have always tried to leave 0.8m min spare even in calm conditions but was just trying to get a handle on the safety margins that others use and whether I was being overly cautious.
 
Why don't you tell your guests to go to Lymington and pick them up from there? All you have to do is get up early and go with the ebb?
 
If I can't get them to arrive at the Beaulieu earlier that may be the better option. Leaving the Beaulieu at 14.45hrs (0.8m margin) is really too late - hence the question.
 
Ian_Rob said:
I am wanting to leave the River Beaulieu on the morning of the 1st June but will need to wait for some guests, who can't be there much before before 09.30hrs. Allowing 60mins to get them aboard, the earliest we could be ready to go would be say, 10.30.hrs. LW is predicted as 11.10 hrs/1.2m. Reeds advises that access is possible +- 2hrs MLWS but what should be possible at neaps (Neaps is June 2nd). Obviously any safety margin will depend upon the particular conditions but what margins are appropriate. We draw 1.9m.
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Have everyone standing on the lee side, to reduce draught if worried.
 
'tide and time waits for no man' Your departure time must be governed by the safe tide times. The crew must be ready to sail. If they can't make it in time, get them to come the night before.
 
Under keel Clearance minimum safety margin 20% of draft is a common recommendation. For my boat at over 6ft draft or approximately 2m. 20% would be 40cm or 16 inches.
I'v a feeling I'd want at least twice that. Probably a meter would make me happier
 
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Could it be that you know in your heart that this is not a seaman like thing to do, but that the social pressure from non sailors to conform to your guests timetable is distorting your judgement? Hence your appeal to the forum to try and justify a course of action you know is wrong? Your instinct is right. Modify departure time to suit the tides and don't hit shallows on a falling tide.
Didn't Drake himself delay the deployment of the response to the armada until the tide turned?
I only speak as one who has, against my better judgement, altered departure times to suit non sailors and regretted it.
'tide and time waits for no man' is as true as ever was. Maybe time for your guests to learn that while their timetables are "really important", natures timetables are timeless and immutable!
 
John the kiwi said:
Could it be that you know in your heart that this is not a seaman like thing to do, but that the social pressure from non sailors to conform to your guests timetable is distorting your judgement? Hence your appeal to the forum to try and justify a course of action you know is wrong? Your instinct is right. Modify departure time to suit the tides and don't hit shallows on a falling tide.
Didn't Drake himself delay the deployment of the response to the armada until the tide turned?
I only speak as one who has, against my better judgement, altered departure times to suit non sailors and regretted it.
'tide and time waits for no man' is as true as ever was. Maybe time for your guests to learn that while their timetables are "really important", natures timetables are timeless and immutable!
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I think you have hit the nail on the head to some extent re social pressure and decision making. I was never intending to leave at Low Water but I was trying to establish the earliest safe times that I might achieve for various sea conditions and from that decide what the best plan of action is.

The whole exercise got me into thinking about 1] what does allowing +- 2Hr MLWS actually mean and more particularly what draught it is based upon & ii] the safety margins that others work to.

I seem to recall having read somewhere that these +- times for MLWS were based upon 1.5m draughts?

With regard to margins, one poster has suggested 0.5m, another 1.0m min. I have always allowed [0.75m - 0.8m min]. The difference between these allowances on that particular day, equates to departure times of 13.45hrs or 15.15hrs respectively; 13.45 is OK for me, 15.15 too late.
 
To take a single clearance figure on it's own is flawed IMHO.

You have to relate it to likely sea conditions and the accuracy of the data you're working to.

For example. Entering beaulieu in a flat calm with glassy water leads less of a clearance than entering say Chichester with a 2m swell running. The former you'd probably be happy with 0.5m under the keel, the latter 3m might be interesting.

Treat tide tables with a pinch of salt. They are projections, and even the projections differ slightly. Round them to the nearest 1/4 hour and you'll be about right timewise but remember that extremes of pressure, prolonged heavy winds, heavy rainfall in a river basin will all impact height.

Best bet is to use actual height, e.g. A local tide gauge which tells you as is. With accurate data you can have a finer clearance than if using a projection.
 
It's interesting when one starts examining the 'rule of thumb' margins in this or any other game, to understand the assumptions and to learn when one might be able to refine things when really needed. For example, how accurate will the charted least depth be now? It will almost certainly have changed since the last survey information was reported to the Hydrographic Office, the relevant chart amended, and you/me accessing that info on board. Why not ask the local harbourmaster for the most up-to-date info about 'least depth'?

Then there are the constraints of the method used to calculate predictions for LW time and height of tide. One method uses about 240 harmonic factors in such a calculation; another uses 8. One might assume that the calcs for the local Standard Port are reliable ( NOT infallible ) but Beaulieu is a Secondary Port and published info will be generated by the aggregate of several 'fudge factors'. What are the sources of the primary and secondary info in the publication you intend to use, and how much 'fudge factor' is built in?

The place of interest is, of course, in an area subject to a distorted tidal regime. While Low Water times/heights are better predictable than HW, the best of the graphs published are, at best, only fair approximations.

Meteorological conditions of barometric pressure, current and recent, have a known effect on levels. The approximation or 'fudge factor' of plus/minus 1 cm per 1 hectoPascale difference from the Standard Atmosphere 1013.25 hPa is itself only a reasonable approximation, and there is time-lag. The effects of wind direction/period in causing 'heaping' or its opposite is known, understood, but hard to quantify. All this stuff and more is what's expected of a wizened RN 'Dagger' Navigator when trying to squeeze an aircraft carrier or laden RFA fueller in and out of little ports not intended for big ships. With your draft of 1.9m. you have a similar problem....

You might care to drop down the river on the end of the previous morning ebb, armed with the harboumaster's latest info, and see how you get on. Should you manage all right, you'll likely have a slightly better margin on the morning your guests turn up. But don't forget your draft is influenced on their combined
weight..... And what's wrong with leaving just after the turn of the tide? Then any 'embarrassment' is likely to be temporary and short-lived.
 
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We draw 1.85m and often go in/out Bealieu at low water.
Spring low you wouldn't get all the way up but at Neaps I've not had an issue.
It's often used as a training exercise to 'test' the depth guage.
On the plus side you'll be on the turn of the tide so if you're not comfortable just stick the kettle on again. You'll not be waiting that long.
Most groundings I see are at the Bealieu River entrance, It can be quite tricky for people to work out which side the deeper water is.
 
Ian_Rob said:
I think you have hit the nail on the head to some extent re social pressure and decision making. I was never intending to leave at Low Water but I was trying to establish the earliest safe times that I might achieve for various sea conditions and from that decide what the best plan of action is.

The whole exercise got me into thinking about 1] what does allowing +- 2Hr MLWS actually mean and more particularly what draught it is based upon & ii] the safety margins that others work to.

I seem to recall having read somewhere that these +- times for MLWS were based upon 1.5m draughts?

With regard to margins, one poster has suggested 0.5m, another 1.0m min. I have always allowed [0.75m - 0.8m min]. The difference between these allowances on that particular day, equates to departure times of 13.45hrs or 15.15hrs respectively; 13.45 is OK for me, 15.15 too late.
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Like Talulah, we go in and out of the river at lw and we draw 1.8m. It goes without saying that with low margins, we SLOOOW right down to a couple of knots until the depth starts going up again.

On a calm day I'd be happy with 6", 0.15m clearance with a chop I'd want 1m. FWIW, there is a tide guage on the RSYC pontoon at Gins Farm which is accurate.
 
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