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View Full Version : Can a Marine Sextant Be Damaged, Lead One Astray?



mfarraday
05-21-2014, 07:35 AM
In my novel, a character has an interest in marine sextants. He is using one that he has been told is inaccurate and possibly damaged, and not of good quality. While sailing, he is using the calculations from his damaged sextant when he discovers a small island off the coast of the USA somewhere. The island previously belonged to the US government, but is no longer used, and has some buildings that have been abandoned.

He is insane, and he believes the end of the world is imminent, and he is very wealthy. So he brings his daughter there after he loses his job and his wife in the same month. He basically imprisons his daughter with him. She doesn't know how to sail away on her own.

Only one other person knows about his habit of navigating using an inaccurate sextant - the daughter's friend. He has to deliberately miscalculate the coordinates produced by a sextant to find her and rescue her.

Is this a plausible scenario? Can a person obtain faulty coordinates from a sextant in this way and end up in places they didn't expect?

Thanks for any input! :)

jclarkdawe
05-21-2014, 04:19 PM
Sextants are very fragile and easily go out of whack. Even temperature changes can cause problems. So it's easy enough to believe someone is using a sextant that's inaccurate.

Whether you can damage a sextant intentionally to mimic another sextant's mistake is possible depending upon what is out of whack. For example, a misadjustment on the mirrors is definitely possible to replicate.

But if you're going to argue that there's an unchartered island off of the US, no sailor is going to buy it. The US coast is well chartered and frequently sailed. Many day sailors during the summer head to specific islands as a destination. Fisherman are constantly out there.

I'd be wondering why he's worrying about wrong coordinates to mimic when I know the island is on a chart. Here's a link to a standard navigation chart - http://www.charts.noaa.gov/OnLineViewer/11509.shtml off of Georgia.

Best of luck,

Jim Clark-Dawe

King Neptune
05-21-2014, 04:24 PM
Yes, and it could be really funny. One of the essential parts of a sextant is a mirror on which the Sun is reflected. If the mirror is not properly aligned, then the readings would be off. The misalignment would have to be small, or it would be apparent, but the sextant could be two or three degrees off without it being immediately obvious. And being off by three degrees over a few hundred miles would be enough for your story; I think.

whiporee
05-21-2014, 04:45 PM
You might think about the barrier islands of South Carolina. There might the some uninhabited ones there. Or in the Keys -- if you go north from the main chain, there are some uninhabited ones there that might even be out of sight range from other islands.

A better bet, though, might be the Bahamas. Lots of islands, and a lot of them uninhabited. Would be plausible to find old plantations there.

Can't help with the sextants. If it were set 30 years ago, he could misinterpret Loran lines. They were very easy to screw up.

ULTRAGOTHA
05-21-2014, 05:43 PM
jclarkedawe, the OP doesn’t say it’s an uncharted island. In fact the OP says it used to be owned by the government and has abandoned buildings on it which implies thoroughly charted to me.

It could be on every chart in the world and the boyfriend still won’t know which abandoned island the daughter is on.

But I think unless the boyfriend knows exactly how the sextant is buggered, which implies crazy rich man knows as well, it might just be easier for boyfriend to haul out charts of where the island is likely to be, records of old US abandoned installations and, if this takes place in modern times, a bunch of Google satellite views and find her that way. It’s got to be far enough from other islands, shipping lanes, fishing areas, air traffic corridors and other frequented parts of the ocean that it isn’t passed easily by chance. One presumes even if the daughter can’t sail, she can light a smoky fire or write SOS on the roof of the house.

BTW, you don’t need a buggered sextant to accidently re-discover an abandoned island. Inattention, some current drift, a squall or storm, or just random sailing around for fun could re-discover it, too.

ETA: Or checking out remote islands listed on charts. ;-)

Kregger
05-21-2014, 05:49 PM
On top of what has been previously said, a sextant only gives you the degrees away from the equator. That's it. Using only whole degrees of latitude (as an example) there 720 points 30 degrees away from the Equator. That's a lot of places around the world. To navigate you need a N-S coordinate and an E-W coordinate. The E-W coordinate is done using time. Either way, coordinates won't get you far without a chart to relate where you are on the planet. Unless your madman has memorized Earth's cartesion coordinate system, he's as good as lost.
Suggestion? Have your madman know exactly where he's going with a GPS and have him misdirect the hero with false GPS readings.
I know few people that can navigate with a sextant. I can do it on land, with a clear sky and sitting on a bench. A boat is another story. The sextant technology has been replaced with GPS technology. Being able to use a sextant is akin to being able to hand grind and reseat the valves on a Model T engine. Which is unfortunate because it is neat to use the sun as a reference point to where you are physically standing (give or take a few miles). It's almost like being without one of those new-fangled smart phoney thingies.
Kregger

jclarkdawe
05-21-2014, 06:21 PM
But I think unless the boyfriend knows exactly how the sextant is buggered, which implies crazy rich man knows as well, it might just be easier for boyfriend to haul out charts of where the island is likely to be, records of old US abandoned installations and, if this takes place in modern times, a bunch of Google satellite views and find her that way. Itís got to be far enough from other islands, shipping lanes, fishing areas, air traffic corridors and other frequented parts of the ocean that it isnít passed easily by chance. One presumes even if the daughter canít sail, she can light a smoky fire or write SOS on the roof of the house.


This is what I meant to say and was too lazy to write out. If the island is on the charts, then you can start ruling out islands pretty quickly. You're not going to be talking unlimited coastline, as you know the port the crazy guy is using. From there, you can calculate a radius based upon boat speed and time.

Then you start looking at all the islands within that radius. If it has buildings on it, there was a reason, and some way of providing water. And water supply is probably a big issue still. This way would be quicker then trying to make your sextant match and then navigate to it using that.

And anyone good enough to navigate that way can probably figure out the error factor and come up with a pretty good guess from shore. Any good search at sea spends a lot more time on shore figuring things out. Searching at sea is slow and very time consuming, and frequently expensive.

Best of luck,

Jim Clark-Dawe

PorterStarrByrd
05-21-2014, 06:45 PM
Good info from others, but why would he need a faulty sextant? Better to have one he can rely on, in case he needs it. It's not like anyone is looking over his shoulder. Boats and ships don't file 'flight plans' and are not required to report destinations to any official agency. He can use any route he wants to get where he wants to go, though, once he is out to sea, it would be most economical to set a direct course. Perhaps if he encountered another vessel on radar (or visually) he could change course for a while. If he manually sets a course back toward the island, that's when he might want a healthy sextant.
Also very few modern boat owners could use a sextant if they had one. With GPS there is very little need for one anymore.

The sextant wrinkle seems a unnecessary knot in the plot.

Big peeps can track him by satellite if they have an interest. Most other people wouldn't care.

frimble3
05-22-2014, 06:50 AM
In my novel, a character has an interest in marine sextants. He is using one that he has been told is inaccurate and possibly damaged, and not of good quality. While sailing, he is using the calculations from his damaged sextant when he discovers a small island off the coast of the USA somewhere. The island previously belonged to the US government, but is no longer used, and has some buildings that have been abandoned.

He is insane, and he believes the end of the world is imminent, and he is very wealthy. So he brings his daughter there after he loses his job and his wife in the same month. He basically imprisons his daughter with him. She doesn't know how to sail away on her own.

Only one other person knows about his habit of navigating using an inaccurate sextant - the daughter's friend. He has to deliberately miscalculate the coordinates produced by a sextant to find her and rescue her.

Is this a plausible scenario? Can a person obtain faulty coordinates from a sextant in this way and end up in places they didn't expect?

Thanks for any input! :)
I can understand a wealthy collector having an 'inaccurate and possibly damaged, and not of good quality' sextant. Maybe it has sentimental value, or is a souvenir of some famous ship or journey, etc. But I cannot understand why he would try to use it. Not if he's a collector and has more than one to work with.
Would it work if he found/bought/obtained a chart of the area, that looked interesting and led him to island-hunt? Maybe the chart was jammed into the box the sextant came in?

Once!
05-22-2014, 11:50 AM
I must admit I'm struggling here. Let me see if I've got this right. The insane guy uses a faulty sextant to navigate to an uncharted island. He leaves his daughter there. Then he goes back to the mainland with his faulty sextant. A friend of the daughter gets hold of the sextant and has to figure out how to use it in order (allowing for the fault) to find the missing island and rescue the daughter?

But a sextant only tells you where you are at the moment. It doesn't even do that fully. What it tells you is your latitude (not longitude). In effect, it tells you half of the coordinates you need to know where you are right now.

A sextant would not tell you how to get to somewhere unless you had a map or chart with the coordinates of the place you wanted to get to.

I suppose that we could just about make this work. The bad guy leaves daughter on island, writing in his diary that the island is at coordinates (X,Y). Whether done deliberately or not these coordinates are wrong and the only way to compensate for them is to use the faulty sextant in conjunction with the faulty coordinates in the diary.

I suppose it's just about possible but I'm struggling to say plausible. Because a sextant only gives latitude, the longitudes in the diary would be correct. So to find the island all you would need to do would be to sail or fly along that longitude until you came across an island not on a chart.

WeaselFire
05-23-2014, 04:38 AM
As someone who has been on a boat that sailed to the wrong island, you don't need much to be off in many areas. It would depend a lot on where the island is, how big and how far the sailing distance is, but hitting the wrong island can be common for even experienced sailors with GPS and accurate maps.

Now, how far off, how close to the coast and how believable would be part of your writing. We sailed right past the correct island thinking it was a different one on the chart, landed on what we believed was the correct one and spent a few hours trying to figure out why it didn't match the description we had. Broad daylight with decent navigation equipment. In the Florida 10,000 islands everything looks the same from water level.

I'm not entirely sure this works for the premise of your story, but it could be written believably. Even without a sextant.

Jeff

ULTRAGOTHA
05-23-2014, 05:44 AM
Once, it doesn't appear the crazy rich dad in the OP finds an unCHARTED island. He appears to find a rather remote and unINHABITED island that used to be used by the US government.

As jclarkdawe pointed out, an uncharted island in US territorial waters, especially one with an abandoned US government installation, would stretch one's suspension of disbelief to the limit.

OTOH, if the boyfriend does manage to figure out how the sextant is buggered, then he can draw a line straight east (or west if this is the Pacific or gulf) along the buggered sextant latitude and somewhere near that line is the island.

Once!
05-23-2014, 11:43 AM
Whether the island is uncharted or uninhabited doesn't really make much difference if the key point is that the rescuer doesn't know where it is.

jclarkdawe
05-24-2014, 02:32 AM
It makes a lot of difference as to whether it is uncharted compared to uninhabited. It's mainly a question of narrowing down your search area. If I can't use the chart, then I have no idea where in an arc from his home port to start looking.

If the island is on a chart, I start with his home port and draw an arc of his cruising range. Then I look at every island in that arc to see whether it is a likely source. At that point, I will be able to reduce the number of islands by at least 50% because they're too small to support humans at all, or they are people living on the island, or the island is on a major water route and has a bunch of boats passing it daily, or the island is a favorite site for sailors.

For example, if you look for islands off the New Hampshire coast, the most likely to meet the OPs requirements are the Isles of Shoal. First problem is that people live on them, second problem is that they are a favorite site for day sailors. So those islands won't work. I can rule them out of a search.

Best of luck,

Jim Clark-Dawe

mfarraday
05-24-2014, 02:47 AM
Thank you all for the information. This is very useful to me. I hope what I write can pan out as being believable/plausible. I am mostly just listening to what you all - people more experienced than me - are saying.



I must admit I'm struggling here. Let me see if I've got this right. The insane guy uses a faulty sextant to navigate to an uncharted island. He leaves his daughter there. Then he goes back to the mainland with his faulty sextant. A friend of the daughter gets hold of the sextant and has to figure out how to use it in order (allowing for the fault) to find the missing island and rescue the daughter?


I am not planning to have him leave his daughter, abandoned and alone, on an island by herself. But I might have him, or his imprisoned daughter, use his cell phone. Then the boyfriend might be able to locate them via gps. Does this make sense?

jclarkdawe
05-24-2014, 05:48 AM
Cell phones only work a certain distance off of the coast. Ten miles is about the limit.

Second question is how is he accessing her GPS information from her phone?

Best of luck,

Jim Clark-Dawe

Bing Z
05-24-2014, 07:50 AM
Instead of a cell phone, the insane dad can use a satellite phone (some info here (http://www.satellitephonefaq.com/)). At least one of the services providers, Thuraya, has even made satellite adapters for iphones (http://www.thuraya.com/SatSleeve) & Android phones. But I have no idea how can the boyfriend access the GPS information. Maybe there is an iPhone app that can display the current GPS location and the daughter tells boyfriend verbally? Sounds too easy?

mfarraday
05-24-2014, 07:55 AM
Interesting...maybe? I'll look into it. Thanks again...to both of you!

Megann
05-25-2014, 12:07 PM
Ha, this is my industry, but before I can help you with your question I need to know in what era your story plays out? Also what type of boat has the crazy, wealthy dude got and has he got GPS and ECDIS (electronic chart display and information system) on board? How old is his boat and how large is it?

Also you might want to check out Maidentrip, which is a documentary of how Laura Dekker sails around the world. They also show a bit of how she navigates.

cbenoi1
05-26-2014, 12:40 AM
If the guy is a collector of antiques and the sextant / chronometer are of French origin, then maybe the calculation tables that come with the kit are based on the Paris meridian, thus the error. Not a big deal up North, but a larger snafu down in the lower Caribbeans.

-cb

Matchu
05-28-2014, 05:54 PM
There's a deserted US island off the tip of Cuba, an old guano mine. It's on wikip, and if it's not Cuba, it's Barbados or Haiti or some other island in the Caribbedian. Otherwise, Pacific dots are good for spook. Nothing tops Bouvet tho...that Bouvet Island mystery tops my pops...

Bouvet: the most remote spot on planet Earth, a desolate lump of ice rock that lies 6000 miles from Cape Town, and 6000 miles from Antarctica. It was here in 1964 that the Research vessel Kruschevstalina discovered the remains of an abandoned and mysterious wrecked lifeboat. Who were the occupants of this lifeboat, and what became of these shipwrecks...were they mariners stranded on the loneliest spot on planet Earth? We shall never know. Next week: Yeti Island, home of the big steppers etc umm...