Land ho

May 6, 2005 1:29 AM

So I watched Master and Commander again last night...

And as a result I have two burning questions:

• how are hourglasses made so that the sand precisely measures out an hour before the glass is sealed?
• after the first Acheron attack, Aubrey has three or four small boats "row" the Surprise into fog; how can these small boats generate enough momentum to pull a tall ship of that mass?

posted by AlexReynolds to science & nature (26 comments total)

1) If the hourglasses were made in a factory, or at least by someone who makes a lot of them, I'd say there would be a standardized cup or bowl, maybe with a little fill line that would mark how much sand to put in the hourglass. And that size/amount could be worked out simply by trial and error.

2) If you were standing on land you'd be surprised how big a ship you can pull along. The men in the boats of course don't have the leverage you do standing on land, but it seems plausible to me....I don't remember that scene, myself.

Wow, a historical engineering question and a physics question. I have a question for you, how did you like the movie??
posted by zardoz at 2:11 AM on May 6, 2005

It's probably the ninth time I've seen it over the last year. Well written, well acted. The SFX don't feel plastic and fake like they did in LotR — I was very disappointed it did not win more Oscars. I haven't yet begun to read the books.
posted by AlexReynolds at 2:41 AM on May 6, 2005

Tried the books--the first one, anyway--and it was quite dry. I'm imagining the story and action pick up later on in the series. Loved the movie, seen it twice. Peter Weir is a genius.

/just in case you were wondering
posted by zardoz at 2:54 AM on May 6, 2005

Generally when making anything that will be sealed in glass, I think they just leave a small protruding tubular hole at the bottom (or top), put the sand in (or whatever), get the timing right, and only then seal the hole using a flame to melt the glass around it.

You can buy glass ampoules for a few cents each like that - you put stuff in them, melt the tubular top with a flame to seal it, and they're then permanently sealed. If you look at anything sealed in glass, such as vacuum tubes / valves, you'll typically find a little stem of glass poking out somewhere out-of-the-way with a pointed melted tip. That's where the glass is sealed once the stuff has been put inside.

As to pulling a big ship, it's just normal physics. It's the same way the ion drives they use on satellites (or solar sails they're soon to use) can push a satellight in space, even though the force these drives generate is so tiny we couldn't even feel the effects and the satellite is so big and heavy we couldn't push it with all our might if it was sitting on the ground. We could push it if it was on wheels though. The point is that the boat is not sitting on the ground, it's not even sitting on wheels, it's floating in water, which is like uber-wheels.

It doesn't matter that the force generated by the boats is small - the resistance to motion of water is much smaller, so the only limiting factor is the inertia of the big ship, and that's not really limiting at all - it just means it will take you longer to pick up speed (and likewise, it will take longer to lose that speed once you cease rowing - so the higher weight isn't really costing you anything, the ship could be twice as big and you could still move it, providing you are exerting more force on it than wind or other opposing forces.
posted by -harlequin- at 3:28 AM on May 6, 2005

I should probably add, the low resistance to movement you get in water is highest at low velocity. The faster you try to move through water, the more resistance you face from the inertia of the water itself. But if you're talking about moving something very slowly, like rowing a huge ship, the resistance from water is much lower than you'd get using wheels on land. At higher speeds, the opposite becomes true.
posted by -harlequin- at 3:32 AM on May 6, 2005

how are hourglasses made so that the sand precisely measures out an hour before the glass is sealed

It's not precise. But when you're on the sea, and without your Timex, it's "close enough" not to matter.

how can these small boats generate enough momentum to pull a tall ship of that mass

The resistance of a boat on water, as harlequin mentions, is quite minimal. The real problem is the sheer mass, but once you get it going, it tends to keep going (See: Newton). The issue wasn't "let's outrun the Acheron with our trusty rowboats," after all. Remember, when the alternative is no wind/movement, even a couple guys rowing your sorry ass into fog is better than nothing.
posted by Civil_Disobedient at 4:59 AM on May 6, 2005

Civil-Disobedient is probably right about the glasses not being perfect measures of one hour, close though, and relatively consistent. The time on board a ship at sea (navy) as I understand was always measured by local noon. This is when the sun has reached its highest point in the sky and begins to move lower. These noon shots were done everyday and the time keeping was started anew. A good hand with a sextant would actually be able to calibrate their sand glasses. Local noon was the standard for time on land too before the telegraph became widely used.
posted by flummox at 5:15 AM on May 6, 2005

Loved the movie, love the books even more. ("Dry"? To each his own, but I found it gripping from the first page.) Since the poster's questions have been taken care of, let me put on record a minor complaint: I'm pretty sure that in the movie a Frenchman pronounces the ship's name as "ah-shay-RO(N)." Somebody didn't do their homework: in French, as in English, ch in Greek words is pronounced k, so it's ah-kay-RO(N).
posted by languagehat at 6:06 AM on May 6, 2005

Blimey! Good skills, Languagehat.
posted by nthdegx at 6:47 AM on May 6, 2005

languagehat- That's not necessarily the case. Consider the French verb acheter (to buy), which has a "sh" sound. Or vache (cow). Actually, I'm having trouble thinking of a French word pronounced with a "k" sound. Got an example?
posted by mkultra at 7:00 AM on May 6, 2005

(btw, this is a cool post!)
posted by mkultra at 7:00 AM on May 6, 2005

mkultra: l-hat specifically said "ch in Greek words".
posted by kenko at 8:46 AM on May 6, 2005

Ah, ah, ah, ok, I see...
posted by mkultra at 8:50 AM on May 6, 2005

Local noon was important, but many days cloud cover could make it difficult to take a measurement.

I found longitude to be an interesting book about nautical time-keeping and navigation.
posted by fleacircus at 10:25 AM on May 6, 2005

The question of nautical time-keeping is also a prominent subplot (or three) in Mason & Dixon
posted by mr_roboto at 10:36 AM on May 6, 2005

I just noticed- Alex, dude, what's up with the tags?
posted by mkultra at 10:42 AM on May 6, 2005

I've never sailed any distance, but I've read a lot of nautical novels. To move a becalmed ship, one description had a small boat row out ahead maybe 100 yards with the ship's anchor, which was then dropped. The crew on the ship then used the anchor winch to move forward.

It sounds good, but I have no idea if it'd work or not.
posted by Daddio at 11:02 AM on May 6, 2005

mkultra:


"The only traditions of the Royal Navy are rum, sodomy and the lash." - -- Attrib to Winston Churchill, i think it was actually his assistant. The very exact definition of pith. Great Pogues album too. Great movie, great books. Half water grog all around.
posted by Divine_Wino at 11:23 AM on May 6, 2005

I think the only reason they used the row boats is because the Surprise wasn't rigged for oars. If you imagine the force being generated by the boats as being generated on the ship directly it seems a lot more plausible.
posted by Mitheral at 11:40 AM on May 6, 2005

I've never sailed any distance, but I've read a lot of nautical novels. To move a becalmed ship, one description had a small boat row out ahead maybe 100 yards with the ship's anchor, which was then dropped. The crew on the ship then used the anchor winch to move forward.

Actually, this does work!

My dad and I sail around Essington, PA, just south of Philadelphia International Airport.

There's a small island across the shipping channel that we grounded upon once, to have some lunch. Unfortunately, we finished lunch as the tide receded, so the boat ground deeper into the silt and mud below.

We spent a good hour trying to push the boat out into water but nothing helped until he threw the anchor into the water and pulled the line, moving the boat along as if tied to a pulley, as I pushed from behind. The anchor was ground deep enough in the water to unfix our boat.

The only problem with this answer is that I can only imagine the anchor working along depths where the anchor can hold onto the seabed. In deep seas I can't see the physics of this working, where the row boats are like bars of soap on a countertop.
posted by AlexReynolds at 12:11 PM on May 6, 2005

The pull along the anchor technique is really only used when boats are aground or in shallows or to pull out of a tricky harbor/cove when the winds are against them. Anchors are designed to hold better when the pull against them comes at an angle, this is why it is often only possible to pull an anchor out of the bottom when the boat is vertical to the anchor.

Alex, begin to read the books, they are brilliant. You'll walk around for hours saying "He is not quite the thing" and "oh we have reached our lee shore" and "I say why do you thread... why do you transpierce that crumpet? Whom is that crumpet for? For whom is that crumpet?"
posted by Divine_Wino at 12:30 PM on May 6, 2005

i just finished the 20th--and last--book. i'm thinking of starting from the beginning again!

i wonder if many women like the series, or have read all of it.
posted by subatomiczoo at 12:34 PM on May 6, 2005

My friends moms put me on to them. FWIW.
posted by Divine_Wino at 12:58 PM on May 6, 2005

i wonder if many women like the series, or have read all of it.

subatomiczoo- I'm currently on Book 20. And I am thinking of starting again from the beginning. In the meantime, however,
this is next on my reading list.
posted by ambrosia at 1:48 PM on May 6, 2005

The "ch" in "anarchie" is pronounced "sh".
posted by Wolof at 11:24 PM on May 6, 2005

Right, I spoke too loosely -- I didn't mean "words derived from Greek," I meant "Greek proper nouns": Charybde, Echo, and the like. But of course it's only a tendency, not a general rule (Archimède is pronounced -sh-); it probably would have been less confusing if I'd just said "Achéron is pronounced with -k-" and left it at that.
posted by languagehat at 6:52 AM on May 7, 2005