That's it, my boat is ready for the season. This one well fixed on the trailer, I am about to leave home, but a doubt takes me... At what time will I be able to launch this beautiful skiff? Is the water height sufficient?
1 - Zero level maps
2 - Lowest point of my hold (30 cm above sea level)
3 - Current tide level
This Tuesday, March 24, 2020 at 10:10 am, the lowest point of my hold is 30 cm above sea level.
Therefore, as for any hold built on the foreshore, the SHOM would note this probe by a number printed in italics and underlined indicating the height of a rock discovered during the lowest (coefficient 120) of low tides.
But what number would be noted on the chart if the SHOM was interested in my dreams?
Here is a way to calculate it:
1 - find the tide information :
- the site of Bateaux.com proposes the wedge guide with the tide table. For the example, we have chosen the hold of Landévennec :
Please note that all times are given in legal time
2 - Use of the rule of twelfths
As a first approach, I use the 12th rule to know the theoretical height of water at 10:10 am, time at which I made my measurement. This is only a theoretical calculation, because the weather also influences the water height by the pressure exerted on this liquid mass. For this first calculation, I deliberately forget this data, but it can be crucial when the high pressure is well installed and the tide will "rise" less than expected!
- calculation of the duration of the tidal hour noted HM
- 11h30 âeuros 5h09 = 6h21
- 6h21 / 6 = 1h 03m 30s
- or one MH = 63 minutes
- number of "tidal hours":
- 11:30 âeuros 10:10 = 1h20
- so 63min + 17min, i.e. the last MH + 27% (17/63) of the penultimate hour
- tidal range (difference between the water level at high and low tide):
- 6,82 âeuros 1,39 = 5,43 m
- 1/12" value this is the difference in water at each twelfth:
- 5.43 /12 = 0.4525, or about 45cm per twelfth. On the downstream side, the water height will drop by 45cm per twelfth.
As a reminder, the rule of twelfths states that:
- at the first "tide hour", the sea rises or falls by 1 twelfth
- to the 2nd HM, of 2 twelfths,
- to the 3rd HM, of 3 twelfths,
- to the 4th HM, of 3 twelfths,
- to the 5th HM, of 2 twelfths,
- to the 6th HM, by 1 twelfth.
Conclusion: At 10:10 am, the theoretical height of the water will be 1.39 + 0.45 + 27% x 2 x 0.45 = 2.08m
3- Calculating the height of my probe
I calculate the height of my wedge in relation to the "zero" of the cards
- 2.08m + 0.30m = 2.38m
4 - Representation on the map
The Shom would rate 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 close to it.
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 is thinking about us, and about the hours when we let ourselves be entertained, during the college math class, by the clouds that brightened up in the opening of the classroom window, or more prosaically, by the beautiful hair of our classmate (to your taste!). See the card of the hold of Landévennec .
If you move the cursor over the sinusoid representing the water height during the day of March 24, you will find 2.02 m at 10:10 am.
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 calculation method closer to reality than the approximation we all make using the good old method called "twelfths".
So, according to the SHOM website, the lowest point of my virtual hold is 2.32m above the "zero" level of the charts.
What time can I launch?
To conclude, knowing that :
- The RIB of my dreams has a draft of 35 cm,
- It is necessary to add 30 cm for the engine,
- I am not a propeller manufacturer, so I give myself a 30 cm margin if the sea is flat (non-kamikaze pilot's foot),
I have to wait until the theoretical water height is 3.30 m before I can launch or take out my beautiful skiff.
Discover all the features with the bateaux.com slipway guide
The importance of the pilot's foot
Please note that the pilot's foot I give myself for launching or putting on a trailer is not the same as the pilot's foot I give myself during my sailing. Yes, when sailing, I have to take into account the swell, my speed, the load (the fish in the bucket weighs a lot!) and therefore the sinking of my transom...
Back to the point, considering that the lockdown will be lifted by May 1 (let's be crazy!), several questions arise for me:
- What time will I be able to launch on Labor Day?
- At what time should I come back to avoid sleeping on my semi-rigid, not really designed for that?
The SHOM is still there to help me in these metaphysical questions:
- I go to the page https://www.bateaux.com/plaisance/cale/
- I am looking for my hold, here the hold of Camaret-sur-Mer
- I click, at the bottom of the page, on the threshold option, and I indicate the threshold of 3,30 m
And I discover that my day is going to be beautiful, because I will be able to use my boat from 7:35 am to 4:40 pm! So, life is beautiful...
