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-   -   Dead Reckoning (http://www.sailnet.com/forums/general-discussion-sailing-related/86985-dead-reckoning.html)

Cruisingdad 05-03-2012 03:15 PM

Dead Reckoning
 
It came up in another thread, and I thought I would ask how many of you have done it? Were you at sea and for how long?

I have done it at sea. Just once. Puckered my butt. The small cable came out of the back of my autopilot on a ST6001 (I think that is the right model???). It is part of a system with our E80's. Just a word of warning, when that happens, the whole system gets really screwy... not just the single item you lost. At least it does to my system. Anyways...

We plot in our log (which we make at sea):

Time, SOG, COG (M), Coord, Wind (t), and sea state. I think that is all... I am probably forgetting something. We also lay a vector between our targets and plot on paper maps usually around every 30 mins (in bad weather) to 1-2 hours.

So, when this happened, I was amazed at how difficult it is to keep on a heading. Remember, you are hand steering now. Just hand steering to a compass heading is very hard especially if there is much wind. My short story is that I figured outhe problem an got us back up but within an hour or so, we were off course from our way line.

A good bit of practice and seamanship to do it when you have your electronics, incidentally. Especially if you crew with a family and you typically do the driving because when it goes down, you (the dad or capt) will be the one trying to fix it and mom or kids are having to hold the rhum line.

So, how many of you have been forced to Dead Reckon? At sea or coastal? How long? What were your experiences?

Brian

Cruisingdad 05-03-2012 03:18 PM

Re: Dead Reckoning
 
Oh, just another cautionary tale I learned the hard way, don't write you log book in red ink. You cannot see it at night with red lights. Not sure what reminded me to say that, but there goes.

DRFerron 05-03-2012 03:25 PM

Re: Dead Reckoning
 
Sigh. I'll open myself up to all sorts of ribbing but in the winter when the boat is asleep, I love to fire up the wood stove, put on the blues playlist, spread a chart out on the drafting table, gather my tools and plot DR routes. I don't do it for the open sea as I don't know celestial nav, but I do like to try to follow my routes come the sailing season and see how accurate I can be. Plotting in advance also gets me familiar with the area we'll be sailing before we get there. Takes some of the adventure out of it but it also takes some of the unpleasant surprises out of it.

Our nav computer went down once in a dicey channel and I did use paper to get us out.

jackdale 05-03-2012 03:55 PM

Re: Dead Reckoning
 
In the early 1990's before I had a GPS, I hasd to dead reckon across Haro Strait in the fog. We had a good visual; just before we got socked in. We set the throttle to maintain a speed of 4 knots and laid out a course that would take us to Mandarte Island which has few trees and would be recognizable. Just as we expected to sight Mandarte, the fog lifted and voila.

I still lay out a DR track when going up Georgia Strait and going from Winter Harbour to Hot Springs Cove via the South Brooks ODAS. Also transiting Juan de Fuca.

rockDAWG 05-03-2012 04:06 PM

Re: Dead Reckoning
 
DR is an essential part of seamanship. I don't use it at all for the routine day sail or weekend sail in the familiar water. However I do DR off shore and always reconcile the coordinates with GPS. Mentally gets a pic the differences.

All my ASA courses I took, we used no Chartplotter or GPS, so I learned DR well. I found C Bay or coastal DR is more difficult comparing to off-shore. The errors in DR off shore is very easy to make up when compared to DR in the bay. Without a proper way to fix the coordinates, you can be off a lot. To make the matter worse is there are so many indistinguishable water towers and radio towers on land. The mistake can be dangerous. The only way to positively ID the coordinates is by buoy or channel marker. So a good German made binocular with compass is essential and saves lot of time. WM binoculars suck. :)

When making entry on the chart, I use writable and removable Scott tape and pencil or inked pen. So I can keep my chart clean without many confusion with the multiple entries.

Between the on board chartplotter, handheld garmin GPS, handheld VHF with GPS, and iPad chartplooter, DR becomes less important. But is is a fun game to teach the little one (niece) to use the chart. But the knowledge and tools are there if I need it.

DRFerron 05-03-2012 04:09 PM

Re: Dead Reckoning
 
When I've taught the navigation portion of our aux sailing class (includes DR and using paper charts) I've had to separate couples who started yelling at each other as they worked through the problems. Fortunately, in the class this past Tuesday all was calm.

travlineasy 05-03-2012 04:32 PM

Re: Dead Reckoning
 
I've had to DR my way home from a dozen offshore fishing locations, some as far as 70 miles from the coast. The biggest problem is not knowing all the parameters you're faced with, tidal currents, wind drift, etc... There's a spot known as the Parking Lot, a good size lump rising up from the Atlantic's floor that is located about 45 miles southeast of Chincoteague, VA. It was a great location to catch big bluefin tuna, some weighing in excess of 200 pounds.

My boat at the time was an 18-foot Starcraft Mariner, an aluminum center console boat with a 75-HP Johnson outboard. The boat was really fast, skipped along at speeds to 45 MPH on flat water, but when you're in the ocean those days are really rare. The navigation instruments consisted of a chartbook, compass, and a Loran-C. Loran-C provided an accuracy of 1/100th of a nautical mile, which was just fine for most purposes. However, when you're 45 miles offshore and a thundershower comes over the western horizon, that Loran-C becomes useless. A message came across the screen that said "No Signal."

I fired up the outboard, pulled in the fishing gear, and DR'd my way toward Chincoteague Inlet. The rain came down in torrents, lightning flashed in sheets, which is really scary when you're dodging bolts in an aluminum boat with a half-dozen graphite fishing rods sticking up in the air. When I finally was in sight of land, all I saw was miles of endless sand in both directions. At this point, I figured that the earlier winds were from the southwest, therefore I probably drifted north a few miles. It turned out to be a good guess--I was about three miles north of the inlet.

For the most part, DR, at least in MHO, is a wild-assed guesstimate, particularly when you really don't have accurate information of all the variables involved in setting a course to your destination. In a powerboat, I think it was not as difficult as it would be in a sailboat. In either boat, however, I manually steer all the time and never had a problem holding a solid compass course.

Good thread,

Gary :cool:

jackdale 05-03-2012 04:39 PM

Re: Dead Reckoning
 
BTW - A DR position is necessary in when doing celestial navigation. You need a sense of where you are before the sextant shots.

Minnewaska 05-03-2012 05:08 PM

Re: Dead Reckoning
 
DR was the second thing I learned on the water, after tying a knot.

My Father used to have me measure the distance on a chart to the next buoy, tell me our speed and give me a Wiz Wheel to determine how long it would take to get there. Then I would wait and see if I was right. I loved playing with the wheel to determine the missing T-D-S variable. I'm sure it was mostly to keep me busy as a young kid, but it worked.

That exact same Wiz Wheel was necessary in my earliest pilot training, where you must visually navigate over a chart. That's a bit more intense as you cover miles while doing the calcs.

klem 05-03-2012 05:20 PM

Re: Dead Reckoning
 
I have sailed all over New England with just doing DR, I feel that it is an essential skill. The first 2 summers I sailed the Maine coast, we didn't have a GPS and we sailed fog or no fog. On the sailboat, I use my GPS now when the visibility is poor but I plot out all of the courses beforehand on the chart including distances so I can get times. In the sea kayak, I basically never turn on the little handheld GPS that I have (it is so old it only gives coordinates). The trouble with sea kayaking in reduced visibility is that your speed is so low that current can throw you way off so you need to do a good job of estimating it (boy do I love lobster pots for this).


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