Passage from Cocos Keeling to Rodrigues S15:28:967 E083:47:146
Good birthday yesterday – thanks for all those birthday wishes. Still making good progress. The wind abated a bit in the morning (18 – 22 knots) so we shook out a couple of reefs in the genoa and a bit of a reef in the main. We were sailing along quite comfortably until around 2pm then the wind started to get up and the seas built. Back in went the reefs! Winds were more than forecast on the gribs – between 25 – 30 knots gusting up to 33 and the seas were quite ugly! Actually even in their ugliness they are quite beautiful. (I hope I’ll always think that!) As the waves rise higher a mass of rolling water builds then as it reaches it’s peak, the top of the wave becomes a lovely transparent turquoise colour then as it breaks white foam rushes out and around – sometimes the wave will just continue rolling under us, lifting us gently in the air and down again ready for the next one - at other times the wave will crash into us with such force it will send us sideways with a great shoot of spray up in the air. The power and the glory – it really has to be given a lot of respect! I keep trying to get a good photo but it’s quite impossible to capture – the waves just look quite small and insignificant! We talk a lot about waves, so here’s waves 101!
Some typical wave info from the grib files for this trip:
Significant height: 3.88m
Maximum: 4.93m +219 degrees 16s
Primary Swell 2.33m +218 degrees 16s
Secondary swell 1.3 +112 degrees 10s
Wind 2.84m +109 degrees 8s
The swells are those longer period waves generated far away, in our case down in the southern ocean. Because they are long period e.g. 16 seconds between them, they would, on their own, represent a gentle motion because they happen relatively slowly allowing the boat to ride up and down even if they are quite large. Unfortunately the swell does not just come from one direction, nor do they arrive at the same time, so the effect is “not so gentle” or regular. This is what the secondary swell or even a third swell does. These might be the remains of an older or newer weather system close by or far away. After all the swells combine, sometimes like a washing machine if you’re unlucky, (but sometimes not), there are wind waves to be added on top. These are the local waves generated by the local wind conditions. The stronger the wind, the bigger the waves. The period of these is usually much shorter than the swells and they can make a real mess of an already messy situation. So in the grib data which we download, we get the individual elements, then a maximum value which attempts to predict the worst likely combination. Of more use, is the significant wave height. This is defined as the average height of the highest one third of all waves. Because this is statistics, this means that every hour or so you will get one 1.5 times larger, and every 6 hrs or so you will get one 2 times larger. In gales we have had 6m significant waves, with the odd one swamping us at 12 m. Not seen any that big on this trip so far, thankfully. Having said all that, once at sea there is nought you can do to avoid the waves. The only real value of all this is to decide when to stay put in the bar! Thankfully ocean sailing is not always like this. Sometime you get a nice long swell and a simple wind wave on top, making for fast sparkling conditions. Ah, if only!
Back to practical matters - Matt put a preventative patch on a small tear on the mainsail and I actually completed a killer sudoku before it started to get boisterous. We did manage to get our stock up on calories but a quite uncomfortable night being thrown around by those rogue waves every so often. Wind down a bit now to around 22 – 25 knots and seas a bit less. According to the gribs we will have another couple of days of this then maybe a bit less wind for a couple of days before it builds again as the next high marches east to the south of us.
Date and time: 19th September 10.15 local (GMT + 6.5 hrs)
Position: S15:28:967 E083:47:146
24 hours distance: 199 nm