Tuesday, September 17, 2013

Paddling On A Compass Bearing

From the National Park campsite at Yanks Jetty on Orpheus Island to Taylors Beach on the mainland is a straight line distance of about 15 km. As is usual on the north Queensland coast, the flood tide runs south. Frustratingly, our nautical chart gives no indication of current strength. On the day we paddled back to the mainland from Orpheus Island, we left at our usual early hour of 7.20 am when the ebb tide had another hour or two to run and the winds were all but calm. We should have been roughly near slack tide, and the what current there was should have been running north.

Morning fog and cloud lay along the mainland and that, combined with the low non-descript nature of the landscape on the mainland meant that when we started paddling, and for the first couple of hours, we had no landmarks to aim for. Accordingly, we set off on a compass bearing. I took a bearing off the map aiming slightly south of the buoy that marks the channel into Taylors Beach and came up with a bearing of 282 degrees. I aimed slightly off (to the south) so that when we reached the mainland we would know whether to paddle to the north or south. Using the GPS in our mapping package on our mobile telephone to take a bearing right off the buoy we came up with a bearing of 288 degrees.

We started out paddling at 7.20 am and decided we would check our location with the GPS after one hour. I mounted my compass on my deck and did my best to keep us on course. When the hour was up, Orpheus Island looked along way behind us, but the GPS indicated we were only about 4 km off the coastline as our course had sagged to the south. We took a new bearing (roughly 293 degrees) and set off again, this time deciding to stop after half an hour to check our position.

In half an hour, we had progressed another 2 or 3 km from the shore, but, had sagged yet further south. Another bearing, another course adjustment, and off we went again. We repeated this procedure at half hourly intervals and found our bearing, by the time we could see the red buoy marking the channel at Taylors Beach had changed a whopping 35 degrees from 288 degrees to 323 degrees.

Paddling on a compass bearing, like navigating a glacier in a white-out by compass is a somewhat disorientating procedure. You are never exactly sure where you are, and relying on an instrument to travel is always somewhat daunting. Without a GPS to check our position we would have drifted very far off course and would have faced a long and tiring (as the north wind came up) paddle up to Taylors Beach.

In the end, that straight line crossing of 15 km took us about 3.5 hours. That's just over 4 km an hour, which seems a little slow given such calm weather. But, I'm sure we didn't travel a straight line, more a big looping curve, so we likely paddled more like 17 km.

There is a way to calculate a vector angle that takes into account currents and paddling speed so that travel on a bearing is more accurate. However, you have to know current speed and your own paddling speed. Neither of which I was sure of in this instance. It would be interesting to paddle the route under the same conditions using a vector corrected bearing to see if it proved more accurate than our ad-hoc correct on the go option.

Another day of learning in the kayaks.

Some where out there is the mainland

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