16 – Tides And The Rule of Twelfths



Recorded on Wednesday July 8th 2015 with Harry Knight, Ru Hill & Asher King

INTRODUCTION:

Ru is back from New York and Asher is back in the water.

THE NEWS:

Asher brought up the Tracks Magazine article about Taj Burrow sitting out of the European leg of the WLS tour this year, Dane Reynolds being given the Wild Card slot for the J-Bay event and the injury list for that event.

Harry then mentioned Trace signing Jordy Smith, and Ru brought up the new GoPro.

The last thing we talked about was the ISA Adaptive Surfing Championships which are taking place in San Diego in September, which lead Ru to mention Mike Coots who you can watch below.

MAIN FEATURE: Tides and the Rule of 12th’s

Harry gave us a talk about the tides, and the the Wikipedia page can fill in any details on the basics. One thing that came up early on was the existence of Amphidromic Points in the middle of the ocean, and below is a map of where those points are. Dark blue shows no tide, dark red shows big tides.

Tidal Amphidromic Points

We then went into more details about measuring the tide, and we briefly mentioned the French Coefficient system of measurement, before then going on to talk about Chart Datum the Rule of twelfths which you can see visually below.

rule-of-twelfths-chart

To calculate the height of tide at any given time, follow this set of calculations:

Height of High Tide  –  Height of Low Tide  =  Range of Tide
Range of Tide  ÷  12  =  Size of each Twelfth.
Size of each Twelfth  x  Number of Twelfths since the last high or low tide = Change in Tide
Height of last High/Low Tide  -/+  Change in Tide  =  Current Height of Tide

So if I was surfing a beach 2 hours after high tide, when high tide was 9.5ft and low tide was 1.3ft…

9.5ft  –  1.3ft  =  8.2ft
8.2ft  ÷  12  =  0.683ft
After 2 hours we would expect 3 Twelfths to have gone out so…
0.683ft  x  3  =  2.1ft
9.5ft  –  2.1  =  7.4ft

There is also a really neat online calculator for the Rule of Twelfths. For more details on some of the techniques used to navigate the tide in a super yacht, as well as some more details on the Rule of Twelfths, check out this PDF taken from a Yachting Monthly magazine.

SUPERHERO OF SURF: Steve Lis (1951-  )

Asher talked us through the life and times of San Diego kneeboarder Steve Lis, who was the original inventor of the Fish style of surfboard.

Steve Lis Fish

You can read more about Steve on the Encyclopedia of Surfing, and you can see a good breakdown of the Deep Swallow Fish in this Surfline Article. Steve still shapes surfboards under his own label in San Diego and Hawaii.

WHAT TO WATCH:

Harry brought up Reef’s new movie Exit:

Ru mentioned the huge swell that’s been hitting Australia and Indonesia over the last week. Here’s some of the footage from North Point in Australia,

You can follow these links to watch Justin Holland’s Cow Bombie session as well as this great clip from Felicity Palmateer .

There were also some great videos from Indonesia, first up Uluwatu on the island of Bali

And next up is Kandui Left in the Mentawai Islands

Last thing to keep an eye on is the J-Bay Open that will be running all this week.

That’s all for this episode folks, email us at podcast@surfsimply.com

 

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