60 D St.
Welcome to Lesson 2 in Newbie Navigation. 🙂 If you missed Lesson 1, you may want to check out the CADET post.
Disclaimer: I think it would be wise to avoid studying something as important as ‘how to not get lost in the middle of an ocean’ from a couple of self-proclaimed newbies who have owned a boat for less that a year. You have been warned!
Back in June I mentioned watching a Navigation DVD that talked about, among other things, Dead Reckoning. The last three week’s Seamanship classes have covered a lot, including the process of creating a DR (Dead Reckoning) plot on a chart. DR allows us to calculate an approximate position based upon the course travelled from a previous known position (a fix), taking into account the time underway and your boat’s speed. Because it does not factor in the effect of currents and leeway, it may not be perfectly accurate, but it still forms the basis of navigation.
Assuming that we are starting from a known position (a fix), we would hopefully be able to tell the course we are traveling (have traveled) via the compass heading. We can also use our ship’s instruments to give our boat speed (perhaps an average for a boat under sail) and the time we have been underway (via our watch. duh!). The one bit of info that we don’t know in this equation is the distance we have traveled. This is where the we need to use some math (or cheat and use a handy little tool, more to come on that).
The image of the street sign above is a reminder for the formula that we use: 60 x Distance = Speed x Time
If for example we are traveling at 5 knots for 30 minute our distance would be calculated as follows:
D = (5 x 30)/60
D = 2.5 nm (nautical miles)
There is a lot of excellent info on this stuff already on the web so it doesn’t make sense for me to try to regurgitate it here into newbie terms. I suggest that if you’re interested in learning more about it you could do as I have done and exercise your Google skills.
Oh, and if you want to cheat a bit on the whole 60 D St. math thing, you could always pick up a slide rule tool like this one. We have one and it seems to work quite well.
The thing about GPS and dead reconing, in my expereience, is this; when you need to begin a DR plot because the GPS quit, you need to have a starting point. That means, you need to keep a possition/speed/course log ~ every 30 minutes when you are somewhere where losing you place would matter. Then, it is easy to back-calculate where you are.
This is something I only do at night in unfamiliar loactions where I cannot manage with dependable ATN and shore bearings. Fortunatly, the 2 times my GPS quit, though I was headed out on long passages, I was still in site of a good landmark.
After my last coastal run in the wake of a huricane, with sandbars moved and multiple ATNs off-station, I believe only what I see.
Makes sense Drew. I think one obviously needs to be disciplined about keeping that log and not get complacent, relying upon the ease of the GPS. I wonder what percentage of people who cruise do this.