Jump to content

How are your navigation skills?


Recommended Posts

38 minutes ago, Kevin McCready said:

Great purple prose. But I smell a rat. How the hell do you estimate latitude 56°55’?

Heh, I spotted that too.... looking at a chart, that's where the continental shelf rises from a depth of 4000m to 1000m and then goes abrupty back to 4000m and the pacific and atlantic mix.  I suppose it is like crossing a reef or a tide line where you can see the colour changes in the water?

Thanks for linking - a good read.

Link to post
Share on other sites

He completed that voyage anyway, steering by the stars. And if it was possible to navigate without instruments by night, he reasoned, perhaps it was possible to do so by day.

“By the time I got back to shore two and a half weeks later, I had figured out that daytime steering was no problem at all,” he said. “We would use the wind as a reference; we would use the waves as a reference.”

doesn't sound very efficient during the day, but wasn't this pretty much how they believe oceania was navigated before 1500?

but with the advantage of good charts

Link to post
Share on other sites
16 hours ago, Kevin McCready said:

How the hell do you estimate latitude 56°55’?

While I think being as accurate down to minutes like that is a hell of a big ask for all but small handful of probably old Pacfic Island gents, estimating degrees of latitude is not that tricky if you know your stars and have a good clock. Even with no clock you can get pretty damn close if your good enough.

It's all about angles.

Link to post
Share on other sites

This made me remember an anecdote told to me by a fellow at usenet news rec.boats when that forum was still active. My source had worked either for the U.S. Navy or the Coast Guard in southern California back in the day when hippies were a thing. One day they came across this sail boat heading straight out into the Pacific Ocean and since they were already some distance from the coast they decided to check things out.

The boat was crewed by a bunch of young, bearded men claiming they were heading for Hawaii. Sure enough they had food and water to last the trip but very little navigational aids and no radio. "How are going to find your way to Hawaii?"

"Easy", said one of the crew and pointed at the con trail of a jet liner passing over head.

/Martin

Link to post
Share on other sites

Some facts:

He rounded Cape Horn without hitting it.

Cape Horn lies at 55 degrees 59 minutes S.

He had charts.

He had a sandglass.

He didn't see land during the rounding.

He estimated his latitude at rounding to be 56.9S, in round numbers, one degree of latitude (60NM) south of the cape.

Reasonable assumption:  He knew his boat and was able to estimate speed through the water - he may even have been using a traditional log line, which would have been consistent with his navigation approach.

He wasn't racing, so his course would have been considerably north of the cape until it was necessary to get south and around it.  You will not be aiming to make landfall at the cape but you DO want to make landfall before you approach Cape Horn.  But, with prevailing westerly winds that can be dangerously strong, you do not want to get closer to the coast than you have to. 

There is a peak of about 1,700 feet (518m) 16 NM north of the cape.  7 NM north of the cape is a 1,000 foot peak. The cape itself is not high, and the land immediately to the north does not exceed 500 feet.

So starting with Cape Horn and in ideal conditions, and assuming an eye 10 feet above the waterline, these points of land might be seen 36 miles south, 35 miles south and 30 miles south respectively.  In other words from the south you would see land behind the cape before you see the cape itself.  From 60 miles south of the cape you would need land of around 2,500 feet in elevation to see it in perfect conditions - and there is none.  However there is plenty of land of that altitude and higher around Isla Thomas (and not far inland), 89 NM north west of Cape Horn.  What does this mean?  You cannot rely on seeing land from a safe distance off when rounding Cape Horn, even in the event you get ideal weather.  Without accurate navigation, you need a good fix off the land north of Cape Horn before you put sea room between your boat and the cape itself.  Because you cannot rely on good weather, you need a period of time - perhaps a week - in advance of rounding Cape Horn - to establish your position well north of the cape by reference to high points on the land.

Now speculation:

You need to round Cape Horn without GPS, sextant, compass, weather forecasts etc and you are in a small boat.  You are going to want two things - plenty of sea room and a good departure fix.  (You young folk may have to look that one up).  You will make landfall something like 600 NM north of the cape, and the coast at that point is also considerably west of the cape.  Assuming perhaps 120 miles a day or less, you will keep the coast at a safe distance and approach - weather permitting - only close enough to make observations of high land until you believe you have correctly identified that land.  This is done primarily by reference to height above sea level on the chart and sufficient observations of points of high land over a couple of days to achieve a match on the chart as you sail south. 

You now have a fix as to latitude and, by observation of loss of a high point below the horizon (in good weather) you also have distance off (approximate - because you have to estimate the bearing).  This is a fix - with errors but a fix nonetheless.

You then continue to proceed down the coast - with the land in sight intermittently depending on the weather.  There be mountains here - so in good visibility the high points can be seen well out to sea, where they appear as distant "islands" above the horizon.  From the chart you have an expectation of which high points will be seen, and when they are seen, you use these to update your position.  Again - you approach closer to the land when the weather permits but probably never closer than an estimated 30 NM.  There are probably extended periods with no horizon and clouds down to almost sea level, so patience is needed and frustration is certain.  You may heave-to at night if safely offshore, or sail a reciprocal track so that you can confirm a high piece of land with a second or third look.

Over three days you have comfort that you have a reasonable position and track.  Now you need sea room and you need to get south.  You abandon any sight of land and note your final departure fix.  Perhaps this uses the snow covered mountain peaks around Isla Thomas if the weather is good.  Your plan is simple, by DR and steering by the waves, sun and stars you run due south if the wind allows.  You log your estimated speed through the water every hour or so, and you adjust for currents (noted on the charts) and estimated leeway.   Using your departure fix as your starting point and your DR track you note when you believe you are at about 56S (approximate latitude of Cape Horn) and judge that you want one degree of latitude between you and the Horn.  You continue to sail, time and distance, until you believe you have run a further 60 miles south.

You turn to port and attempt to maintain the same latitude until you believe you have passed the Horn.

So your plan is to pass the Horn at 57S, using DR from a fix about 24 hours old.  You continue to update your DR track and for whatever reason estimate some northing and note 59.9S in your log.

Years later, this log entry is quoted in an obituary.

I don't see purple prose or trickery here.  I see logic.  This is much the same as coming back from the Islands and planning a landfall at Cape Brett even if the destination is Auckland.  In the days before Transit, and then GPS, we always aimed at high land in order to have the best chance of establishing a position line from the coast before we got close.

 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...