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Old 11-07-2011, 11:30   #1
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Calculating Safe Depth - Bar Crossing

With our draft of 7'6" we have to calculate rise of tide and safe depth with great care. In April, I discovered that very high barometric pressure can reduce the rise of tide by quite a bit.

There are a number of interesting ports in our cruising area where we can barely get in around high tide because of sand bars at the entrance. To name just a few: Bridport in Lyme Bay, Beaulieu River in the Solent, Littlehampton near Selsey Bill, Salcombe.

The Beaulieu is gorgeous and worth the effort, but at high tide and smooth water there is barely a foot of water under the keel! It is terrifying.

Last weekend we were in Littlehampton, the entrance of which actually dries. The weather was slightly rough, and I was stumped by the question: What allowance do you have to make for the troughs of the waves? Smaller seas don't really move the boat up and down very much. But the waves tend to stand up at bars. What if they are, say, 3 meters high?

Does that mean you have to deduct 1.5 meters from the amount of water you think you have? So that you don't bang your keel on the bottom in the troughs?

How do you guys do it?
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Old 11-07-2011, 11:41   #2
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Re: Calculating Safe Depth -- Bar Crossing

There's too many variables. The bar height can vary day to day depending on wave action...etc...etc. You go in slowly and try not to bump. Can depend on how easy it is to plow a furrow, too. Best to experiment on a rising tide. You can lead with the dinghy and a handheld sounder. I sometimes back in as there is more power in forward to unstick.

You haven't really sailed until you've run aground a few times.
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Old 11-07-2011, 12:04   #3
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Re: Calculating Safe Depth -- Bar Crossing

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There's too many variables. The bar height can vary day to day depending on wave action...etc...etc. You go in slowly and try not to bump. Can depend on how easy it is to plow a furrow, too. Best to experiment on a rising tide. You can lead with the dinghy and a handheld sounder. I sometimes back in as there is more power in forward to unstick.

You haven't really sailed until you've run aground a few times.
Sailing in SW Florida with a one foot tidal range and skinny water everywhere, I used to run aground at least once a year, sometimes more. With a Towboat US contract it was no big deal. With a one foot tidal range, you will generally not smash up and downflood the boat, so you just wait for help if you can't kedge off.

Sailing in the English Channel is an entirely different ball of wax. With a tidal range of at least 3 meters and up to 13 (!) meters, you will lose the boat, more likely than not, if you run aground on a falling tide. The boat will fall over and downflood as the tide comes back up. So I do pilotage here with an entirely different level of care and conservatism. As a result, in two years, I have not yet (thank you, Lord!) touched the bottom, and fervently hope to keep it that way.

Yes, I do do bar crossings strictly on a rising tide. The problem is that I don't usually have enough room to spare to do it with a lot of tide rise left. It needs to be pretty near high tide. So there won't be much rise left to lift me off if I do get stuck. So those calculations need to be as precise as possible.

So still hoping for an answer about considering wave troughs -- half the wave height?
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Old 11-07-2011, 12:12   #4
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Re: Calculating Safe Depth -- Bar Crossing

Going aground on a channel entrance bar is not where I'd want to be. Move to a boat with significantly shallower draft or drop certain ports from the itinerary?

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Old 11-07-2011, 12:33   #5
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Re: Calculating Safe Depth -- Bar Crossing

I think it would be unwise to rely on detailed computations if running aground is not an option.

As for waves: Bowditch would have the answer. The trough is not nearly as deep as the wave crest is high. It's not a half-and-half thing. That would be for mature waves, not newly formed wakes and such.
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Old 11-07-2011, 12:45   #6
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Re: Calculating Safe Depth -- Bar Crossing

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Going aground on a channel entrance bar is not where I'd want to be. Move to a boat with significantly shallower draft or drop certain ports from the itinerary?

Yes, well, the rougher the weather, the greater margin of error I would want, and pretty soon you just don't go in at all.

But done carefully and in the right weather and with an appropriate margin of error, and on a rising tide, I don't see why not.

Beaulieu is the tightest one I have done, and I risked it only in dead calm weather and exact timing (one hour before high tide). The charts and my calculations turned out to be very accurate, and I never saw less than 30cm under the keel.

By the way, Tennyson wrote one of his more famous poems about crossing the bar at Salcombe, which I have crossed:

Sunset and evening star,
And one clear call for me!
And may there be no moaning of the bar,
When I put out to sea,

But such a tide as moving seems asleep,
Too full for sound and foam,
When that which drew from out the boundless deep
Turns again home.
Twilight and evening bell,
And after that the dark!
And may there be no sadness of farewell,
When I embark; For tho' from out our bourne of Time and Place
The flood may bear me far,
I hope to see my Pilot face to face
When I have crossed the bar.

Crossing the Bar by Alfred Lord Tennyson
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Old 11-07-2011, 13:20   #7
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Re: Calculating Safe Depth - Bar Crossing

When computing a margin for a shallow channel, I keep in mind that the wave height mentioned in weather bulletins is the "significant wave height": the mean value of the highest third of waves. Then, many waves are higher (and deeper) than the "significant wave height". About one in a hundred is twice as high (and as deep).

I also take into account that waves are higher and deeper in shallow water. And I remember that at speed, a displacement hull will squat deeper in shallow water. Then, I consider a total margin about 1.5 times the wave height.

In fact, I got caught once: the sandbar was so steep that I was aground before I had time to turn. Fortunately, it was on a rising tide and when I threw my (then) brand-new NZ-made Rocna anchor at the stern, it held with 9m scope (1 boat length) in 1.6m water (1 boat draft) . It prevented the waves to push me higher on the bar. Then, I managed to turn the boat and get away.

Now, I often visit Etel (south Brittany) and I request radio guidance from the signal station. The lady there keeps track of the shifting sand banks and she guides boats going in and out.

Alain
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Old 11-07-2011, 14:22   #8
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Re: Calculating Safe Depth - Bar Crossing

It does sound like a more complex calculation.
Its why I try to have an alternate plan. So if one arrives out side the channel and sees the waves are going to cause a problem there's already another port in mind.
But from what I understand about you lot is you transported half your island as well. Its now just mud shallows and tide races.

Very tricky.

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Old 11-07-2011, 14:47   #9
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Re: Calculating Safe Depth - Bar Crossing

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Originally Posted by Hydra View Post
When computing a margin for a shallow channel, I keep in mind that the wave height mentioned in weather bulletins is the "significant wave height": the mean value of the highest third of waves. Then, many waves are higher (and deeper) than the "significant wave height". About one in a hundred is twice as high (and as deep).

I also take into account that waves are higher and deeper in shallow water. And I remember that at speed, a displacement hull will squat deeper in shallow water. Then, I consider a total margin about 1.5 times the wave height.

In fact, I got caught once: the sandbar was so steep that I was aground before I had time to turn. Fortunately, it was on a rising tide and when I threw my (then) brand-new NZ-made Rocna anchor at the stern, it held with 9m scope (1 boat length) in 1.6m water (1 boat draft) . It prevented the waves to push me higher on the bar. Then, I managed to turn the boat and get away.

Now, I often visit Etel (south Brittany) and I request radio guidance from the signal station. The lady there keeps track of the shifting sand banks and she guides boats going in and out.

Alain
Good God! That's a hair-raising story!

I'll be cruising your beautiful waters (Britanny) for much of the month of August; any other spots like that I should watch out for?
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Old 12-07-2011, 14:24   #10
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Re: Calculating Safe Depth - Bar Crossing

Dockhead,
I didn't run aground in Etel but in the mouth of the Aven (the river going to Pont-Aven). There are some sand banks, moving with the winter gales. But it is gererally safe to enter or exit 2 hours before and after HW. The anchorage in Port Manec'h isn't well sheltered: in case of any swell, you shouldn't wait there for the tide. Immediately to the east, the mouth of the Belon open to the SW: here too, a bar may appear during the ebb.

Besides Etel (where you should go only in good conditions in daylight and request guidance on VHF ch 13, 2 hours before and after HW, call sign "Sémaphore d'Etel"), the other really dangerous river mouth in south Brittany is the Laïta (between Le Pouldu and Guidel Plages): the sand banks move very quickly (no usable buoyage), the channel is shallow and the current of the river can be strong (depending on the rain in the previous days). No VHF guidance here but following a local boat might help.

In Etel as in the Laïta, it is possible to be trapped for a few days if the swell is strong outside. But it is really worth it to go to Etel: the waterfront is amazing and people at the marina are very nice.

The other rivers in south Brittany are deeper: you just have to consider the tidal streams.

In north Brittany, I ran aground once in Aber Ildut; the moving bank at the entrance isn't sand but shingle . And we were on a bilge keeler: both keels were aground at the same moment. Fortunately, the sea was calm. A passing motor boat towed us. When I go there now, I keep to the middle of the channel.

Alain
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Old 12-07-2011, 16:41   #11
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Re: Calculating Safe Depth - Bar Crossing

Getting to my regular mooring involves a sometimes rough and always shallow bar crossing which always makes me a bit nervous. My general rule is 2 hours after low or 2.5 hours before. When it is rough or there is a lot of wind or a pressure system, I use the rule of 12's to calculate the expected depth and also check a graphical tide table. For a twelve foot tide, the distribution throughout the hours approximates 1,2,3,3,2,1 although there are places that this is not true at all.

As the bar is never totally calm (even on a totally calm day there will be plenty of powerboats throwing large wakes), I try to allow a minimum of 2' clearance. When the waves kick up, I estimate their height divide by 2 and then add it to the minimum desired clearance. As the waves get bigger, I increase the clearance number. When there are 8' breakers, I use a clearance of around 5' in my calculations. I have found this to be pretty conservative but it works for me.

Hopefully this helps. I tend to be pretty conservative coming from working on much larger vessels but it makes me more comfortable when surfing in on a huge breaker.
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Old 12-07-2011, 16:53   #12
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Re: Calculating Safe Depth - Bar Crossing

I hate to say it, but I don't think I would want to cross any bar entrance if the waves were 3 meters high and I had any question at all about the depths. With those waves you want a deep-draft harbor of 10 or more meters depth, at least. When waves are that high there will be breakers with anything close to your boat's draft, and you will not want to cross that bar. I used to keep my boat in a harbor with a bar that was about 6 feet at MLW. When there were ten-foot seas running outside, no matter what the state of the tide, there were solid breakers across that bar even though there would be enough depth for my boat. I witnessed a Morgan OI 41 nearly flipped over and a Grand Banks 40 something stood up right on its transom in those conditions. In other words, there was sufficient depth for these boats to cross the bar, but still the conditions were extremely dangerous.
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Old 12-07-2011, 18:27   #13
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Re: Calculating Safe Depth - Bar Crossing

I'm with Kettlewell on this one. I looked at the bars you mentioned via google earth.
I must say that you have some very interesting sailing grounds. Great coastline!
Those bars must be a real challenge in anything but decent weather.
The entrance at littlehampton looks to be only 100 feet wide. ( only 2x your length)
With only inches under your keel there's no way that I'd cross that bar in 10 foot seas...Unless..Jennifer Anniston was waiting for me on shore..

I have to imagine that 10 feet seas would pile up in some of those inlets with relatively short periods...You'd have no margin for error.....
I don't have an mathematical answer...on the troughs..
I'd gather as much local knowledge as I could in areas like that.
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Old 14-07-2011, 13:00   #14
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Re: Calculating Safe Depth - Bar Crossing

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... I use the rule of 12's to calculate the expected depth and also check a graphical tide table. For a twelve foot tide, the distribution throughout the hours approximates 1,2,3,3,2,1 although there are places that this is not true at all ...
The Tidal Rule of 12 states that, in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence - 1:2:3:3:2:1.

If we assume the tidal curve to be a perfect sinusoid with a period of 12 hours, the height changes over the full range in the six hours between HW and LW.
* During first hour after HW the water drops 1/12th of the full range.
* During the second hour an additional 2/12th.
* During the third hour an additional 3/12th.
* During the fourth hour an additional 3/12th.
* During the fifth hour an additional 2/12th.
* During the sixth hour an additional 1/12th.
Hence, two hours after the HW the water has fallen 3/12 (1/4) of the full range.

Corrolorary - Rule of Thirds: It’s readily apparent that the (2) 6 hour half-cycles
(during which the tide either rises or falls) can each be broken down into thirds.
- During the first third (2 Hrs) the water level changes 1/4 of full Range.
- During the middle third (2 Hrs) the water level changes ½ of full Range.
- During the last third (2 Hrs) the water level changes 1/4 of full Range.

Rule of Seven To interpolate between spring and neap tides. Since the change from spring range to neap range can be assumed linear (instead of sinusoid), each day the range changes with 1/7th of difference between the spring and neap ranges.
Hence, the daily change in range is (spring range - neap range) ÷ 7.
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Old 14-07-2011, 13:15   #15
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Re: Calculating Safe Depth - Bar Crossing

There are many areas, e.g. the south coast of England, where the tide is very far from a sinusoid. If using the rule of 12 there, you need to add a BIG margin, say 2 m/6 feet, for safety.

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