School Boats and Tidal Heights?

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Simes

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Sat on board Talisman at our mooring in the River Medina last night / early evening. Just before low water a few keel boats ground to a halt. One of these was a school boat a benateau "Tresco". After struggling for a short time the Instructor freed them from the soft mud, backed down stream then turned around and tried again but much faster this time. Failing again he backs down and now tries to force his way detween the moored vessels in the forlorne hope that there may be deeper water there.
My suggestion that he run a quick Sec' port tidal height and stop messing around was greeted with silence.

We have taken to betting on how many vessels will go aground here at Springs. The best one was just after the RTI, a boat full went aground on a falling tide, but about 2 hours before LW. they were there for 5 hours.

Do you do tidal heights and sec port calc's or are they a waste of time?

Have fun,

Simes
 
Back in '88 did a course on Dartagnan, a brand new school boat from Dartmouth. Was asked to do a tidal calc to get us up the Yealm in time for the pub. Did it and said it was a non starter. Instructor looked at my calc and then the sounder and said I was way off so we went up river....and promptly ran aground well short. That was folowed by someone shouting that they had just found a slip of paper in the log stating that the sounder needed to be calibrated............Dooh
 
Tidal Hights

I normally check just to see if SWMBO should wind the keel up .. Otherwise its a mad dash with the winch handle .. Only been aground twice .. Both mine ..
 
I wouldn't bother for the Medina - I wouldn't rely on the accuracy on any of the charted depths. The channel and depths have changed significantly south of Island Harbour in the last few years, I would be very surprised if they haven't changed further down the river too.
 
Only needing 2'6" to float I don't normally bother. I do have though tidal graphs for various areas that I anchor and just put in the LW or HW time, tidal heights and have a educated guess as to how accurate my pencil lines are !!
 
Ahem. A couple of years ago, I was at the helm of a Sailing School Dufour 34, heading to Cowes from the Folly one morning, a little after LW. That's when I discovered that, not only is the channel quite shallow (you don't notice these things in a mobo) but also that there's a shallow patch that comes out from the east bank of the river, into the channel, a few hundreds north of the Folly. Luckily, a swift turn to port and a burst of throttle let us continue on our way.

No, I hadn't done any tidal calculations :)
 
Just before low water a few keel boats ground to a halt. .... Do you do tidal heights and sec port calc's or are they a waste of time?

________________________________________________________

Low water approx 1845 bst. Range approx 3.1 metres. Sheltered river. Soft mud.

Don't think I would waste time calculating anything.

I'm more interested in why you felt it necessary to publish the name of the "offending" boat. What good does that do anybody?
 
Depends on the sailing school instructor I guess, I had a good one - among the first things he did was check the depth gage with a lead line, then check the tides.

For what we were about to do it was also a good job he also knew the area like the back of his hand.

Andrew
 
Doing the calculation only gives a rough idea, local factors, wind direction , air pressure may all conspire to make reality different .. so in soft mud nothing wrong with an attempted plough through IMHO!
 
Soft mud will confuse some sounders more than others, but there is no substitute for a sounder that you know is calibrated.
Doing tidal calcs to the nearest cm is pretty futile, a look at http://www.pol.ac.uk/ntslf/networks.html
will often show variations of a foot from prediction.
Harbours silt up through the season.
And remember that a yacht ploughing soft mud can sometimes find some less-soft mud and stop or change direction rather suddenly! Don't follow too close!
No excuse for not keeping a grip on the basics though!
 
A few peeps are likely to get quite a surprise around 2 March next year, when just about all the water in the Bristol Channel drains away down towards Lundy Isle and beyond, and the Avonmouth/Newport end dries out to sticky black mud, at -0.59m. below chart datum - according to Proudman Labs.

Then it all comes back in again, as a Range of over 14.5 metres. The Bristol Channel Cruising Assocn. guys are going to need long mooring lines..... ;)
 
Simes said:
Sat on board Talisman at our mooring in the River Medina last night / early evening. Just before low water a few keel boats ground to a halt. One of these was a school boat a benateau "Tresco". After struggling for a short time the Instructor freed them from the soft mud, backed down stream then turned around and tried again but much faster this time. Failing again he backs down and now tries to force his way detween the moored vessels in the forlorne hope that there may be deeper water there.
My suggestion that he run a quick Sec' port tidal height and stop messing around was greeted with silence.

We have taken to betting on how many vessels will go aground here at Springs. The best one was just after the RTI, a boat full went aground on a falling tide, but about 2 hours before LW. they were there for 5 hours.

Do you do tidal heights and sec port calc's or are they a waste of time?

Have fun,

Simes
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One of the benefits of running aground as an instructor, is to maintain that it was a deliberate action, enabling a further lesson on how to get off the putty.

PS, before taking any school boat off a mooring (D/S or YM courses), I suggest that the pupils use a leadline, to compare instrument reading against actual. Tends to concentrate their minds.
 
I got caught out when the charter company advised me that the draft of their Beneteau was 1.8m (the original invoice said it was 1.8m) when in fact it was 2.1m. And 2.1m doesn't go over a 2m concrete sill!! The repair yard measured the keel and confirmed the draft at 2.1m and since then the charter company has advised its clients the draft is 2.4m. So to be really sure measure it yourself.
 
Lots of reasons regarding accuracy, or lack of it, weather etc to justify not doing tidal calcs. I take it you all ignore the Standard ports and other printed tide tables for the same reasons ???

There are times when they are useful and other times when not needed. Glass high or low, gale blowing, mobile sea bed, whatever - make adjustments accordingly and use an even greater under keel clearance just in case.

Personally, I do a secondary calc before entering an anchorage - gives me a target minimum depth to anchor in that will keep a safe LW under keel clearance during my expected stay. I do not like the feel of keel on bottom except when deliberately drying out. I also know at what echosounder reading my keel will touch and that is put in to the equations. Also, I suppose that should the worse happen I can show my insurers the calculations as supporting evidence that I had done all I could to remain safely afloat.
 
srm said:
Lots of reasons regarding accuracy, or lack of it, weather etc to justify not doing tidal calcs. I take it you all ignore the Standard ports and other printed tide tables for the same reasons ???

There are times when they are useful and other times when not needed. Glass high or low, gale blowing, mobile sea bed, whatever - make adjustments accordingly and use an even greater under keel clearance just in case.

Personally, I do a secondary calc before entering an anchorage - gives me a target minimum depth to anchor in that will keep a safe LW under keel clearance during my expected stay. I do not like the feel of keel on bottom except when deliberately drying out. I also know at what echosounder reading my keel will touch and that is put in to the equations. Also, I suppose that should the worse happen I can show my insurers the calculations as supporting evidence that I had done all I could to remain safely afloat.
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Some truth in that, but some people seem to think calculating there should be enough water overrides observing that there isn't.
And the calculations are often on a dubious basis. You don't always have a valid curve for the secondary port and the interpolation between neaps and springs may not be linear.
The calculated corrections are aften small depths compared to what I would accept as clearance under the keel. You're right we should always check, and I do unless I have a good justification for knowing I don't need to.
The range of corrections is perhaps an indicator of the uncertainty that should be assumed in the answer?
 
I was along side the pier wall at Loch Aline and we had a competition to calculate at what time the boat would float off. She had a foot less than draught. The floating time was when you could place one hand on the wall and push. If she was floating she would not rock.

It was a secondary port with interpolation. The 4 crew failed, I counted down to my time, 30 seconds before the boat rocked as I pushed. Then my time arrived, pushed and off she floated.

Luck - of course not. Its all black and white if you do the sums properly - I bull*&it - of course it was luck.

Seriously though the height of tide calculation is quite accurate. As has already been pointed out there are many factors that can introduce an offset to your calculation.

Later on the cruise I cut a corner and bounced over a rock at a very high spring tide. I thought I had given it a good offing based on echo sounder depth - completely fooled by the extra high tide that day.
 
lw395 said:
And the calculations are often on a dubious basis.
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In practice (and used with common sense and a basic understanding of the factors likely to affect the tide) it is possible to calculate with a very adequate level of accuracy. I think most east coast sailors (such as I) are used to doing these calculations all the time - they become second nature. Not only are they needed when anchoring, but also when sailing in many places where the track takes us through very shallow waters, some of which are coloured green on the chart.......

I generally use the rule of twelfths and have no problem with the results. I can't imagine anchoring without knowing the range. Without that, I do people know the depth to anchor in or how much scope will be needed?
 
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