Calculate the height available on its slipway

François-Xavier RicardouPosted onFollow

From my garden overlooking the harbor of Brest, I admire the beautiful slipway that I have just built in my last nocturnal dream. Unfortunately, the superb semi-rigid that I have just acquired - also in a dream! - remains ashore until further government orders. In order to keep my spirits up, my mind wanders towards new maritime adventures… But at what time will I be able to launch this beautiful skiff?

1 - Chart zero level2 - Lowest point of my hold (30cm above sea level)3 - Current tide level

No one being perfect, the amateur builder of virtual hold that I am, was not able to build an "all tide - all hour" hold. Indeed, Tuesday March 24, 2020 at 10:10 a.m., the lowest point of my hold is 30 cm above sea level.

Consequently, as for any slipway built on the foreshore, SHOM would note this sounding by a figure printed in italics and underlined indicating the height of a rock discovered during the lowest (coefficient 120) of the low tides.

But what number would be noted on the card if SHOM was interested in my dreams?

Here is a way to calculate it:

1 - find tide information:

Please note that the times are given in legal time

2 - Use of the rule of twelfths

As a first approach, I use the rule of 12ths to know the theoretical water height at 10:10 am, the time at which I took my measurement. This is only a theoretical calculation, because the weather also influences the height of water by the pressure exerted on this liquid mass. For this 1st calculation, I knowingly forget this data, but it can be crucial when the anticyclone is well established and the tide “rises” less than expected!

number of “tide hours”:

tidal range (difference between the water heights at high tide and at low tide):

value of "1/12", i.e. the difference of water at each twelfth:

As a reminder, the rule of twelfths says that:

Conclusion: At 10:10 a.m., the theoretical height of the water will be 1.39 + 0.45 + 27% x 2 x 0.45 = 2.08m

3- Calculation of the height of my probe

I calculate the height of my wedge in relation to the "zero" of the cards

4 - Representation on the map

The Shom would mark this probe as follows: 2 ₄ , or rather (2 ₄ ), because this probe would not be placed at its exact point (drawing of the hold) but nearby.

To avoid calculations

This is all well and good for the sweet dreamer that I am, but for the homo-internicus that we have become, isn't there an easier way?

The SHOM thinks of us, and of the hours when we allowed ourselves to be entertained, during middle school mathematics, by the clouds which brightened up in the opening of the classroom window, or more prosaically, by the beautiful hair of our neighbor or classmate (to your taste!). See Landévennec hold sheet.

By moving the cursor over the sinusoid representing the water level during the day of March 24, you will find 2.02 m at 10:10 a.m.

You will tell me that my calculation announced 2.08 m and that the SHOM announces 2.02 m. Believe me, trust SHOM to have developed a method of calculation closer to reality than the approximation that we all make using the good old method called "twelfths".

So, if I believe the SHOM website, the lowest point of my virtual hold is 2.32m above the “zero” level of the sea charts.

What time can I get in the water?

To conclude, knowing that:

I have to wait for the theoretical water height to be 3.30 m to be able to launch or take out my beautiful skiff.

Discover all the features with the Bateaux.com bilge guide

The importance of the pilot's foot

Note that the pilot's foot that I give myself for launching or putting on a trailer is not the same as the pilot's foot that I give myself during my navigations. Yes, when sailing, you have to take into account the swell, my speed, the load (the fish in the bucket is heavy!) and therefore the sinking of my transom...

Back to our sheep, if we consider that the containment will be lifted for May 1st (let's be crazy!), several questions arise for me:

SHOM is still there to help me with these metaphysical questions:

And I discover that my day is going to be magnificent, because I will be able to use my boat from 7:35 a.m. to 4:40 p.m.! There you go, life is beautiful, and the lockdown lighter...