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Cormorant · 28-11-2011, 08:57

Quote:

Originally Posted by capn_billl

The Vikings used a calcite crystal to find the sun...

My old college prof said cordierite was also used as a Viking nav instrument.

There's a roadcut for one of the exits on Route 9 north of Old Saybrook in CT where you can hack a cordierite out the bedrock.

Blue Stocking · 28-11-2011, 09:03

Reading the opening chapters of James Michener's Hawaii would give you a pretty good idea.

rebel heart · 28-11-2011, 09:12

No way in hell are you going to cross an ocean and be lined up for a channel with no tools to do it with. You could keep the boat in a +/- 30 degree heading but that's a lot of error.

Blue Stocking · 28-11-2011, 09:19

Quote:

Originally Posted by rebel heart

No way in hell are you going to cross an ocean and be lined up for a channel with no tools to do it with. You could keep the boat in a +/- 30 degree heading but that's a lot of error.

What would you do now, if you were forced to make a landfall for which no charts existed?
I think that was part of DOJ's context.

**David M** · 28-11-2011, 09:27

It would be pretty easy to sail by latitude using Polaris. All it would take is a simple protractor to measure its elevation above the horizon. Previously known islands could be found by sailing at their known latitude by sailing east or west towards a particular island. No chart is needed to already know a latitude.

The sun could be a rough compass during the day. Everyone knows it rises in the east and sets in the west. When it is at its highest (Local Apparent Noon) it is to the South. (above the Tropic of Cancer for all of the time) At civil twilight you pick a star rising in the east and use that as a rough indicator of direction at night.

ADMPRTR · 28-11-2011, 09:49

Quote:

Originally Posted by callmecrazy

For myself, I'm pretty sure I can find North south east and west without a compass. + DR guestimating speed... Hitting an Island might be a bit difficult, but landing in Europe or Africa would be no problem

As long as you don't care where you land.

ADMPRTR · 28-11-2011, 09:50

Quote:

Originally Posted by David M

The sun could be a rough compass during the day.

And prevailing winds too, I'm sure.

rebel heart · 28-11-2011, 10:37

Quote:

Originally Posted by Blue Stocking

What would you do now, if you were forced to make a landfall for which no charts existed?
I think that was part of DOJ's context.

Honestly? I'd aim for a huge land mass (like head east across the Atlantic) and then sail up or down the coast until I could drop anchor and/or ask someone where the hell I was. When I was completely dumb and new to the water I got myself lost on a coast line and had to buzz past a pier and yell up to the fisherman and ask them where I was so it won't be the second time I've done such a thing.

You couldn't even keep a DR plot running without a compass.

rgscpat · 28-11-2011, 11:03

If you can maintain heading to some sort of reasonable accuracy, there's always the standard offset technique of aiming to one side of the known destination and then running down the coast to it. Someone who had memorized some trig ratios and knew the origin and destination latitudes could probably make a sort of protractor out of cardboard to use to make an angle from Polaris.

**estarzinger** · 28-11-2011, 11:07

There is a terrific on almost exactly this called "Emergency Navigation" by David Burch. I have always carried it on board, both in case we get completely fired by lightening (loosing compass and watches in addition to the gps), but also simply because its a fascinating way to approach navigation, from the very elementary ground up.

Quote:

Originally Posted by rebel heart

You couldn't even keep a DR plot running without a compass.

Yes you can and I am sure you would. It just would have a much lower accuracy level than a 'normal' DR. Since your speed estimate would probably be better than your course estimate, it would really be more of an arc of deduced position than a point. But the aim would be to guess when you were say starting to be within about 200nm of land and then start keeping a really good watch all around (and not just in front) for the many small signs of land.

The above book suggests very roughly that you might/could expect about a 25% error rate (eg after an estimated 1000 miles sailed a circle of position with 250 mile diam) in your deduced position without using the stars. Using the stars (eg particularly Polaris and steering stars) it suggest you could do rather better.

rebel heart · 28-11-2011, 12:05

A normal DR course will give you a reasonably accurate position, something that in my experience is nearly a pencil lead's width away from a GPS. A 3K mile trip (Atlantic crossing) using your math would give me a 750 mile margin of error? That's probably no better than "keep aiming towards the sunrise" and maybe even worse.

A handheld compass or even a needle in some water would make short work of this whole problem.

callmecrazy · 28-11-2011, 12:09

But we're talking about doing without a compass...

mikefp60 · 28-11-2011, 12:27

The Bare Foot Navigator by Jack Lagen is a fantastic reference. A very interesting study of the teqniques used by the ancient mariners. Using the teqniques discussed in the book I try to fix my position as we make passages and then compare to actuals on the chart plotter. Hey, four hours is a long watch in the middle of the night when you haven’t seen a boat for a week. It will give you something to do. Using crude noon fixes for longitude, (Ok I cheat and look at GMT) and North Star fixes at night (no cheating) I can usually put us within 50-100 miles of lat and about double that on long. That’s using a stick for a shadow and knuckles for elevation of the North Star. Of course only good in the northern hemisphere. Not good enough to find Bermuda but I know I could hit the USVI or BVI 8 days after leaving the east coast.

Get the book. It quite good.

Mike
S/V Vivo

**estarzinger** · 28-11-2011, 13:04

Quote:

Originally Posted by rebel heart

A 3K mile trip (Atlantic crossing) using your math would give me a 750 mile margin of error? That's probably no better than "keep aiming towards the sunrise" and maybe even worse.

either you are a rather better natural navigator than I, or perhaps you need a bit more realistic perspective.

a simple mathematical fact: if you are going 3000 miles and your aim is due east along the equator - if you have a constant 10 degree steering error (say in fact steering 80 rather than the intended 90) then your distance error at the end will be about 450 miles (you end up at about 7.6 lat rather than the intended 0 lat). That of course doubles if you don't know which way you are 10 degrees off. If you have the distance exactly right, you are only off by 25 miles in your longitude, but if we add a 10% error in your speed/distance/time estimate they you have another 300 mile error (again double if you might be 10% low or high). Put those together and you have a circle of error with radius 375nm (diameter 750)

I don't know about you, but a 10 degree steering error and 10% distance error sounds like pretty decent estimating without a compass or accurate mechanical log. Could you do better than that 'aiming at the sunset'??

I believe what all this suggests, is if you are going to cross oceans without compass, you need either (1) shorter legs, like the northern route across the Atlantic, or (2) to be simply aiming at a very big target like the whole continent and not too concerned about where exactly you end up, or (3) to be able to somewhat correct your position estimate along the way (like using Polaris and run down latitudes).