Dead reckoning is the process of determining where you are based on a known past position and advancing from where you are to where you want to go. GPS systems have pretty much done away with dead reckoning for day boaters because they give latitude and longitude, distance to and from intended ports, and show the boat among land features on the screen.
But if the GPS kicks the bucket, you don’t have charts and you can’t see land – in fog, for instance – be prepared to dead reckon. Your humble compass and boat speed are all you need in faster boats.
Finding your way home in Faster Planing boats
In a power boat that moves far faster than the speed of off-course drift, it is a simple matter of turning the boat 180 degrees to the return bearing and traveling at a set planing speed. If you are traveling at 26 kph, for example, you will be back near a port 13 km away in 30 minutes (Most speedometers also read in mph, and so a set time can also be calculated for mph, the same being true for speedometers in knots). Do remember to pull up on the throttle before shore leaps out of the fog to grab you.
Finding your way home in Slower Displacement Hull Boats
Estimate your position immediately because your boat is drifting off course based on water current, wind direction and boat speed. The sooner you say, I think I am 13 km from port, the more likely you are close to that spot you plot on your chart.
Determine speed the old-fashioned way: tie a bucket to a line of known distance, throw it over board and stop your watch when the line is taut. That gives you metres per minute, say, and thus speed.
The compass rose on the chart will give you your return bearing. Divide the distance from your position to port by speed and you have the hours to reach it. Charts also give speed and direction of currents and so, accounting for that and wind, you can plot a drift-adjusted course to arrive at a better estimation of where you will end up.
Better Yet, Be Prepared
A boater should always have paper charts and a compass, even on a boat with a GPS. Before each trip, sit down and calculate the distance between ports. Divide this distance by boat speed and you have the hours it will take to do the trip.
And if you have been fishing for hours in fog, and didn’t catch your GPS going down, it may be wiser to sit tight until it clears because you may not have a very good idea of where you are. Before I had a GPS, I once ended up off Lopez Island in the US San Juans for this reason. I didn’t realize how far I had drifted on the flood from the Quarantine Buoy near Constance Bank, where I had started fishing, and thus my dead reckoning was many miles wrong. I got a compass bearing from someone’s fishing charts on the US side and headed back on it, only to have engine problems as I almost hit Discovery Island. I radioed Oak Bay Marina for a compass bearing home, and with my kicker at top rpms, got drifted south into the fog and finally had to radio for a tow in. Most embarrassing.
Alive or Dead from Dead Reckoning
I and my significant other – not a boater – had the scare of a lifetime doing dead reckoning in the fog. Heading for La Conner, I had done a dead reckoning calculation – ahead of time – from Discovery Island to the south tip of Lopez, one hour 13 minutes at a set speed.
On the way across, we passed over the wake of a freighter, something that haunts me to this day. We were very fortunate not to run into it, and have been killed. Also, trying to keep on course with a compass at planning speed is very hard because the needle swings from left to right, back and across the compass bearing you want. The result is making a lot of corrections, either to port or starboard, on gut instinct, to stay on course.
At one hour, I opened the front hatch and strained to see Lopez. Mist settled on my glasses and so my partner, behind the window, actually saw the shoreline before I did. I was standing left hand on the wheel, and right hand on the throttle. Yanking back on the gas, we ended up through the kelp bed and far too close to the rocks.
I backed us out, and followed the shore visually until we hit the south end a half mile later. In other words, dead reckoning was off by half a mile because of the flood tide, as we did indeed come on land after one hour 13 minutes.
But the tale did not end there. I had dead reckoned Lopez to Deception Pass at 12 minutes on a particular heading and speed. And away we went, compass needle swinging back and forth. On the Whidbey Island side, after 12 minutes, I almost ran into someone fishing in the fog. From their charts, I was given another compass bearing angling north, as in I had almost hit the wall a mile south of the channel – in only 12 minutes.
We passed through Deception safely, but that wasn’t the end of the tale. Following my charts, I was going to stay on a contour line until I picked up the channel markers for La Conner, the line of which I had seen on another trip south. Again at planning speed, I had another heart attack, when out of the fog, a ferry rose up like a wall and past back into the fog across my path. It didn’t see me. And again, we were lucky.
The only real luck was that I came right upon the deepest marker where I anticipated it – if you look at the contours on the chart, going closer to shore there is a wide apron of less than 25 feet deep in that area. So keeping just off the ledge line, you should, as we did, luckily, come on the marker, and follow the channel to the docks of our hotel on the inside passage.
As I heaved a sigh of relief, and we were trundling our bags up the dock, we passed a woman so drunk, not only could she not stand up, talking was out of the question, as her head hung low. She was being assisted by two guys, one under each arm pit, and her feet were actually dragging on the wharf behind her.
It turned out that they had left Anacortes and become disoriented in Fidalgo Bay, where the north entrance to Swinomish Channel starts. So, they opted to drift until the fog disappeared and then find the channel. They had drunk so much hard liquor that while the guys were plastered, the woman was FUBARed as the expression goes.
I was able, as the person responsible for our safety, to go through an enormous amount of adrenaline, and as wide awake as possible, get us to port. And there were those lucky things as well. But these people, would simply have been mowed down by a larger boat, so drunk they were. Or they would have presented themselves to a boat like mine, coming out of the fog in a split second, where they would not have been expected and killed us all.
This also turned out to be they day that Princess Diana died. And it is commemorated as a humdinger of a sex poem in, The Hunger, my third book of poems, as The Day Diana Died. May all your fog-shrouded expeditions end in a warm safe place.
Here are the San Juan Islands: https://www.google.ca/search?q=san+juan+islands+wa&tbm=isch&imgil=aN14jCvDotWYRM%253A%253B8Y79rQefvae3GM%253Bhttps%25253A%25252F%25252Fen.wikipedia.org%25252Fwiki%25252FSan_Juan_Islands&source=iu&pf=m&fir=aN14jCvDotWYRM%253A%252C8Y79rQefvae3GM%252C_&usg=__BIM14d-r5qHaPf3Ww8teKVdMz_E%3D&biw=894&bih=450&ved=0ahUKEwjJjKqByNbRAhVI-GMKHUsMDnMQyjcIxgE&ei=JRKFWInmNcjwjwPLmLiYBw#imgrc=aN14jCvDotWYRM%3A.
Here is the chart of Sinomish Channel, La Conner and Anacortes: http://www.topozone.com/washington/skagit-wa/bay/fidalgo-bay/.