Published on February 25th, 2017 | by Mark A Smith0
Yesterday Rich Osbourne did this 90k flight, starting with a sea thermal at Woolacombe beach.
He’s done coast to coasts from Woollacombe before.
It’s certainly possible to connect with inland thermals from sea thermals and do big cross-country flights. Epics.
Paragliding sea thermal tutorial
Thermal tracking & horizontal speed
Here are some tracklogs of get-aways from inland thermal sites (Greg Hamerton and Hugh Miller). Note the angle at which they climb (which of course will be in part a function of wind strength as well as thermal strength).
Compare with these sea thermal take-offs (Graham Phipps and Richard Osbourne). The climb angle is typically much shallower and the circles are elongated in comparison – skimming back with the fast drifting thermal. The inland thermals have ‘loops’ in the GPS tracks, while the sea thermals do not.
I calculated the angle from horizontal and the speed of some typical inland thermal vs sea thermal flights, and the figures I got were:
Inland thermals (e.g. from Forest Farms): 15 deg angle, and 8.5 – 9 mph (14 kph)
Sea thermals (from West Cornwall coast): 7 deg angle and 15.5 mph (25 kph)
So sea thermals are double the speed and twice as shallow!
Because the wind is typically stronger at coastal sites than inland sites when you fly them, and because sea thermals are weaker than inland thermals, with sea thermals there tends to be a much faster ground speed – skimming along lower, with a weaker climb.
Rich Osbourne has noted the shape of the circles over land (not with reference to the airmass
I went back to basics and studied the local maestros, seagulls. I found a place where I could watch the gulls from side-on as they left the hill. Bingo! They revealed their secret. What I saw was that they flew long, flat, ovalised 360s as they tracked back with the thermal. When they faced into wind, they turned quickly, but critically, only when they had found the upwind edge of the thermal and not a second later.
Next they drifted back to the downwind edge, travelling what looked too far, before facing up again and repeating the process. That’s when the penny dropped as to the shape of the thermal as it tracked back. Because of the fresh breeze, the weak thermal was laid over at a shallow angle and that’s why I was flying out of the upwind edge so frustratingly. I realised too, that once you lose this thermal, you’re likely to descend at the same sort of angle as the thermal, thus explaining why I rarely found them again.
I couldn’t wait to try again, but though I got immediate results, I still lost the lift far too easily. I worked out that our wings are slow to respond to input, and this fact was causing me to whizz past the edges of the thermal. The solution was to learn to anticipate where the edges were and turn within them. I was getting it now. I could reliably stay in the lift, but still something else was needed. The lift is so delicate that often just the physical action of turning was enough to critically unsettle the wing. So I practised turning the wing even flatter, even slower and with massive amounts of weightshift to increase efficiency.
The results were stunning.”
Rules about sea thermalling
1. Blank out the ground. Sea thermals are disorientating. They are faster and more shallow than you would expect if you use the ground for reference. You may try to ‘compensate’ by pushing more into wind – losing it as a consequence. So it’s best to blank out what’s below you completely (while skimming over the terrain super quickly and lower down!)
2. Fly constant circles – not adjusted circles. In land thermals the wind is likely to blow you out of the back of the thermal but not with sea thermals. Inland the thermals travel slower than the wind (the wind rushes around them) but with sea thermals they travel at the same speed as the wind. So in this case there’s no need to make centering corrections by pushing up-wind. Better to fly constant bank/speed circles. On the GPS track over the ground the circles LOOK elongated (without a loop as you turn into wind), but this is only because of the speed of the airmass you are circling in.
3. Bubbles – not columns. Sea thermals cycle like land thermals, but land thermal sources typically create leaning columns so you can join the same thermal even 1000 ft below someone else. Sea thermals work like bubbles or fat cigars, and if you don’t stay in it as it breaks from the source you won’t be able to connect lower down a leaning column. You have to keep up!
4. Use heavy weightshift. Sea thermals are often weaker (and very shallow) and so easier to fall out of the bottom of if your own sink rate is high e.g. with heavy break circles. Also the very shallow angle (6-7 degrees) is a 1:9 glide ratio which means (unlike land thermals) you will not connect with it again pushing upwind. So use weightshift much more while circling and don’t fall out the bottom!
5. Use ‘zig-zagging’. The strongest sea thermals often break away from the sea surface further out to sea due to their higher heat energy. The weaker thermals are more likely to come off the sea cliffs in compression. The sea cliffs here act as triggers for thermic air that has skimmed off the sea surface. These drift back fast and are weaker. But they can be like stepping stones to get you higher so you can THEN push out on glide and connect with the bubbles further out to sea – which you often wouldn’t be able to find if you pushed out to sea directly from ridge lift.
This is similar in concept to the ‘zig zag’ method for paragliding small hills inland.
Here is an example from one of my own sea-thermal flights at Morvah, West Cornwall – first gaining height with a weaker, fast-drifting thermal, and then pushing back and connecting with a stronger one at 400m that takes me inland.
Extra thermic input & triggers
Rich Osbourne talks about how important additional ground sources and triggers are for the early part of sea-thermal flights. What are possible sources and triggers to add thermic energy to or help trigger sea thermals?
- Dark rocks/sand (e.g. at low tide)
- Cliffs warmed by the sun
- Towns / tree-lines / buildings, etc
From experience in West Cornwall, having a hill within 1 km or so behind the sea-thermal site helps you get away. This could be because the thermic air allways flows ‘uphill’ and a prominent hill collects thermic air travelling over the landmass resulting in hill-based thermic cycles that can add energy to the thermal you are drifting back in.
Down here in Penwith there are a number of such sea thermic sites – shown here (the numbers indicate the distance in km to the trigger).
Sea breeze convection
When there is sun inland, not only is there an opportunity for inland thermals to connect to but also the chance for sea-breeze convection setting up, adding inland drift to the thermals as they come in from the sea.
When to try sea thermal flights?
Conditions should be unstable OVER THE SEA as well as over land on RASP! This is best if the air temperature is several degrees colder than the sea temperature, while inland is also booming. This will typically be in cold late-Winter or Spring conditions.
For conditions here in Cornwall, a useful rule of thumb is that if you’ve gained 250 m (>800 ft) above take-off and are going up in a thermal in anything consistently over a 0.5 up – then try going over the back towards your hill or other thermic trigger. With more skill you could do this at 200m ATO no doubt.