Shipwreck Bay: A Case Study
Shippies is a pretty magical place. However, accurately forecasting the surf at this spot also requires a bit of magic.
October 4, 2024
Sam's been flat out all week, but it's finally Saturday morning. He lives in a small town in Northland, just 20 minutes away from one of the best surf spots New Zealand has to offer: Shipwreck Bay.
First thing in the morning, he gets an email from Swellify and it’s calling for solid fun-sized waves—1.5 to 1.8 meters at Shipwreck Bay, perfect for a long, fun session. But when he looks at another forecast, it’s claiming the swell is going to hit nearly 3.5 meters. He hesitates for a moment, imagining massive, unmanageable walls of water.
Gosh, 3.5 meters is HUGE! Surely, both forecasts can't be true?
Despite the conflicting reports, he chucks his board in the car and heads to Shipwreck Bay.
When Sam arrives, the waves are just as Swellify predicted—glassy and right in the sweet spot, around 1.6 meters or about shoulder-high, and some waves look about head-high. He paddles out and waits his turn. First ride is a long, peeling perfect left. Amazingly, there are just a few other locals in the lineup! Meanwhile, other surfers, spooked by the massive 3.5 meter forecast stayed home and gave it a miss. Sam has the perfect day, scoring wave after wave pleased that he made the right call.
Blissed out and reminiscing about the day, he thinks to himself:
How were the forecasts SO different?
The answer is actually pretty simple. Let's get into it, shall we?
First thing to understand is that the forecasting sites that were calling for 3.5 meters weren't actually wrong, they were just telling you the swell height (size of waves in the open ocean), not surf height (size of waves at the beach).
And as we know from the Swellify Learn section, there's a massive difference between "swell height" and "surf height".
Here's a visual explaining the scenario that Sam experienced at Shipwreck Bay that day:
When the swell wraps around the headland, it changes direction. When it does, it loses height according to the formulas that dictate refraction and the conservation of wave energy.
So while it started off as a massive 3.5 meter swell out in the ocean, it only ended up being 1.5-1.8 meter surf at Shippies. The amount of height lost when it wraps around the headland depends on a few factors like swell period, but the main factor is how much it has to change direction. In this case, the swell has to bend nearly 135 degrees!!
This means because of the headland, there will be waves in the inner bay that break in nearly the COMPLETE OPPOSITE direction of the original swell. Absolute. Bonkers.
So bonkers in fact that when I was developing Swellify, it completely broke my algorithms (umm, rude!) but that was great. I went back to the drawing board to account for swells that can bend greater than 90 degrees and Swellify is better for it.
Now, you might be wondering
Tobias, what happens if there's a massive swell from the West, instead of the Southwest?
Well, the swell doesn't have to wrap and change direction as much, so it won't lose as much height. A swell from the West of the same height and period will arrive at Shippies as larger surf than a Southwest swell of the same height and period.
So next time you're out there looking at a surf forecast, remember that swell height doesn't mean surf height.
Fortunately, you don't need a calculator or a math degree, just Swellify. In case it wasn't clear, the app takes care of all this complex math for you, estimating surf heights at over 420 New Zealand spots, for every hour, of every day.
Thanks for reading! Now get out there, touch water and get some surf.
