Can a low-tech sailing ship tell how deep the water is?

Question

I'm trying to figure out the challenges faced by a low-tech civilization mapping its world. They have access to roughly Renaissance level technology, plus some (not very abundant) magic.

One question that occurs to me is mapping the edges of continental shelves. In a sailing ship, can you tell whether you are over the continental shelf or deep ocean?

You can drop a weight on a rope, I suppose, though this is a relatively slow and awkward procedure. What sort of depth is it good for?

I remember when I was in an airliner flying over the Atlantic, there was a visible difference in wave patterns, which I took to be very long waves building up on the ocean and then bunching up and becoming higher when they hit relatively shallow water. Would this be noticeable in a sailing ship?

Are there any likely ways of doing it with the sort of magic you get from access to low-level D&D spells? They aren't going to have actual sonar.

Are there other techniques I'm not thinking of?

Answer 1

You're right, dropping a weighted rope is indeed a possibility - as you might already know from historical records - but it can actually operate in deeper water than you might imagine.

Shallow water lead lines were used to depths of about 20 fathoms (~36.6m, or about 120 feet, as 1 fathom = 6 feet). This might not seem very helpful, as the continental shelf ends, on average, at depths of about 200 feet, though there's rather large variation - it can stretch up to around 300 feet in places. Fortunately, longer lead lines have also been used, reaching as long as 50 fathoms (~91.4m), or 300 feet. That should indeed be enough to map out most of the deeper regions of the continental shelf.

Waves do behave differently at different depths; for instance, for a given wavelength, there's a particular depth beyond which a wave will not break. Sure, you could attempt to measure the average wavelength of nearby waves, and try to characterize them (breaking, swells, etc.), and from there estimate the depth of the water. However:

• Waves are turbulent and travel in many different directions, leading to interference and complicated motion.
• The ship itself is traveling through the water, and therefore affects the waves - your plane does not, presumably, unless it's essentially skimming the water.
• Measuring wavelengths accurately is . . . not easy under even the best circumstances, let alone when you're on a rocking ship in the middle of the ocean.

I think lead lines are honestly your best bet. Besides, they've been in use for centuries and centuries - I think the sailors might have gotten it right. . .

Answer 2

The easiest one would be to start talking to the dolphins and other marine mammals who already have an in-depth (pun intended) knowledge of sea bed topography.

Or would that not suit your porpoise?

Answer 3

How about sonar?

Obviously people in the age of sail did not have the technology in its modern form, and prior to the 17th century they didn't even have the basic physics to understand the principle.

However, with hindsight, they could have done it, though it is pure speculation how well it'd work:

In the keel of the ship you mount a large bell poking through the hull, shaped to focus a sonar ping downwards. Next to this is a conical trumpet, also pointing downwards into the water, with essentially an Edison phonograph pickup at the end. Instead of a needle, this is connected to an inked quill. The bell is struck and then immediately muffled, and the same action simultaneously releases a sandbag that causes a long strip of paper to be drawn past the quill at constant speed. So you have a chart recorder that will record the initial ping as a large mark, and its echo as a smaller mark, with the distance between them measuring the time taken and therefore the distance to the sea floor.

The mechanics aren't beyond medieval engineering. The materials are no problem – brass works OK in seawater, and the membrane for the pickup could be mica or isinglass. The acoustical engineering of the bell seems to require some math that late-medieval people didn't have – but then, you could say that about bells in general, and they managed to figure it out by experimentation, without understanding any of the math. It's not a stretch to imagine that someone noticed how the length of an echo relates to distance.

Would I bet money that this would work? No. But would I believe you if you told me Elizabethan ships had such devices? Probably!

As Justin Thyme the Second points out, what sail-age people did not have was the ability to measure their position on the open ocean. On long voyages, they were lucky to hit the right continent. However, within sight of land, they could measure their position with very good accuracy – nearly as good as we have now – using trigonometric techniques known since antiquity. If you have a couple of ships just in sight of land, and a third ship further out taking readings from those ships, you could probably do this up to about 30km from the coast, or quite a bit further with the help of well-placed mountains or rockets.

Generally, the continental shelf is more like hundreds of kilometers out, so I don't think you could accurately map where it is with this technology. But Wikipedia says it's only 140m deep, so you might at least be able to tell when you were above it.

Answer 4

Magic solutions like low-level D&D spells. Hmmm.

Soggy Rope trick

"Rope Trick" is a 2nd level spell that makes a rope stand up to 60ft in the air and opens an extradimensional portal for an hour.

It should be trivial to invent a similar spell that used a rope to determine precise water depth. Such a spell would be simpler (no interdimensional shenanigans) or at least last longer.

Conjure something wet and intelligent

A tiny water elemental might intrinsically "know" how deep the water is, though I couldn't find the stats for anything but "large" water elementals. IIRC Giant sea otters have all-but-human intelligence (6 int according to one online source).

Send a person down

Water breathing is a level 3 arcane spell. It'll keep them from drowning, but not necessarily from being eaten!

Make up a spell

If all you really want to know is "how deep is the water", that's probably a cantrip, though I imagine it wouldn't be all that precise (say within 10% of actual depth).

I can also imagine a cantrip that lets you see twice as far through obscurment (smoke, fog, water) for up to a minute, and one that lets you know the distance to anything you can see.

Magic navigation

Given two magic beacons far enough apart, and a way to accurately point to both (and tell which is which), you could triangulate your position to a fair degree of accuracy. These beacons need not be man-made, they could be some "natural" phenomenon, like an intersection of lay lines for example. Combine that with a compass, and you've got an excellent idea of your position and heading. Accurate depth maps would result soon after.

Answer 5

Water color is actually a pretty good indicator of some hazardous conditions at sea. To a certain extent, seeing a change in water color is a warning of a depth change. In very clear water, it is more of an optical problem where the amount of light being reflected from the bottom changes, but there are often several reasons what can can be more than that.

With shallow water you can have more sea-life and more sea growth, and also more particulates. That can provide more visual contrast.

Due to more sea life, birds may be attracted to the area since schools of fish may be more likely to be in that area.

There can be more particles and organic matter in the water from wave action or from biological activity.

Near river mouths, water will be more brown due to silt, and perhaps more yellowish due to dissolved organic matter, and could be more green due to wash off of nutrients resulting in more plankton.

If you have been to the deep ocean coming back to shore it there are noticeable changes in color from very clear blue to clear greenish color.

Shoaling is the technical term for the wave height changing due to a change of depth. Usually it refers to the increase in wave height as you approach the shore. However if there is a change in depth, and the wave height is being disturbed that can change the surface "texture" or the pattern of the waves going over it.

I think early sailors were very observant, and the navigators and captains to a certain extent kept records or describe these kinds of things to each other.

Sounding to obtain the depth, is certainly a valid way to both know where you are and if you are approaching a hazard. This kind of information I believe was logged regularly as well as some other things like weather conditions and water temperature by whalers (might be a later time period than you are interested in)

Using the sun and stars, works for location to a certain extent. Finding latitude is not that hard, but knowing your longitude typically requires knowing the time precisely, but in your era I think sectants were availible. Certainly, there was substantial knowledge of astronomy. Many cultures had knowledge of ocean currents and some form of celestial navigation in the pacific.

The height of eye is important and determines the distance to the horizon. 1.17 times the square root of your height of eye = Distance to the horizon in nautical miles.

If in sight of land - getting a fix off of landmarks is actually very accurate. But you can see from the height of eye equation you need to be relatively close to shore.

Most likely, Dead Reckoning, is how a lot of ships away from shore would estimate their position. This is basically estimating your speed and course heading and keeping track of how long you are going in a direction. With a well trained crew, this actually worked fairly well until there was a storm. Traveling in groups, ships would keep separate dead reckoning logs and then compare their positions. So in addition to depth, knowing the speed is important,

The term knot dates from the 17th century, when sailors measured the speed of their ship by using a device called a "common log." This device was a coil of rope with uniformly spaced knots, attached to a piece of wood shaped like a slice of pie.

The simplest piece of magic would be a very accurate time piece and "shooting the stars" to get a celestial fix to know your location. Other magic, could be a kind of sonar, where you detect how long it takes the magical energy to bounce off the bottom, if you know the speed of the magical pulse in water, you could "bottom sound". A strong ray of light could also be used to look into the water, but unless magic not that far in most water conditions, Blue-Green light, travels farther in sea water compared to yellow-red light. Of course light has been used to attract sea creatures, in our would fish being of interest, but who knows what a magical ray, or magical sonar might attract.

By the way, the distance of the continental shelf is something that can very a lot. On the East coast of the US it extends much further than the west coast. Take a look at some undersea maps, and you can probably find some kind of undersea topography to fit your story. The shore line can sometimes give hint of what the shelf is like. In addition to the major oceans, the Mediterranean might be a good place to look at undersea topography.

Answer 6

It depends on where your civilization is located.
If it is someplace similar to the Bahama Islands, the depth of the water is very visible.

In this picture, the lightest green is shallow sand. The slightly darker green is a shallow reef. Past the reef the water gets progressively deeper until at the line depicted by the dark blue water, it drops off to several hundred feet.

Answer 7

In the early nineties, I did an analysis for an aerial surveillance company that wanted to proved to the Canadian Navy that you could accurately measure the depth of the Beaufort Sea from a helicopter with a device of some kind. The control measurements, deemed "ground truth" by the Navy, were soundings obtained by two men in a boat with a plumb line. In the 1990's, this was still the "correct way". The new method did indeed have a higher variance than the plumb line, but you got a lot more measurements more cheaply, hence the advantage. You don't need to be constantly sending out men in boats, since navigation charts contain this information and are the result of centuries of observations.

Answer 8

As noted in comments, depth measurement is necessary but not sufficient for making charts. You also need to know where you are horizontally to know where on the chart to record that depth mark! That's the hard part; unfortunately most of this answer covers depth-finding ideas.

However, flying high in the air could give sight of land. That's possible with D&D 5e spells or for an 8th-level druid (Wild Shape into a flying animal).

Are there any likely ways of doing it with the sort of magic you get from access to low-level D&D spells?

D&D doesn't have a lot of spells for mapping; the main navigation spell is 6th lvl Find the Path which shows you how to get somewhere but not your own absolute location. There's also teleport to a known location. There are a few class features / skills for navigation on land (e.g. Ranger class stuff), but there are several spells for going underwater / talking to animals.

Perhaps your best option would be a druid that can Wild Shape into an animal with echolocation, like a dolphin. If this allows accurate depth mapping like sonar tech, you're all set. With other magic, people on the ship can communicate with them in dolphin form to relay and record measurements. (e.g. the Message cantrip cast by someone on the ship, which allows a telepathic reply. Or maybe a Speak With Animals spell.) Or give the dolphin a board of numbers to point at.

If we look at actual 5th edition D&D spells:

First level spellcaster character get first-level spells, 3rd level character for 2nd level spells, 2*n - 1 char level gets at least 1 spell slot of level n. (See table for druids for example). And Cantrips are easy spells that can be cast an unlimited number of times per day, unlike higher level spells. But "most people" in a D&D setting aren't lvl1 of any class, and people of higher than 1st or 2nd level are rarer still.

The official sourcebooks don't have a depth-measuring cantrip, but it would reasonable for a wizard to research a new cantrip that just measures the depth of water with some limit like 300 feet. Some damage cantrips like Firebolt or Eldritch Blast have ranges of 120 ft, also Message. (You could maybe use such cantrips as range finders, to see if they reach the bottom...) A 1 mile limit would probably warrant being at least a 1st level spell, but could be something you could maintain concentration on for 10 minutes or an hour instead of just taking one depth sample in a single place.

The lowest level D&D spells with more than 120 foot range are Earthbind (2nd; 300 ft), Skywrite (2nd, "Sight", i.e. any point in the sky you can see), and Clairvoyance (3rd level, 1 mile). So it's rare for magic to affect or detect anything outside a pretty local area near you, until you get to higher level spells. But Clairvoyance actually lets you see or hear from that point for 10 minutes; a more specialized spell could trade more limitations for that range at a lower spell level. Or you could get cheesy and decide that the range is how far you can be from the point on the surface of the water, and the effect is telling you the water depth at that point. So the range doesn't have to reach to the bottom; that would be the area of effect of the spell.

There are some "feats" like Magic Initiate that let someone learn a cantrip (and a 1st level spell) without being an official Wizard or other spellcasting class, so if you're modelling a world after D&D you could have sailors / oceanographers who know a depth cantrip but not other spells with a weaker version of this feat.

Official pre-existing D&D spells let you communicate with animals (including aquatic ones), or go underwater yourself. Any of these can give you at least a rough idea of depth even without measuring equipment, or could help you use measuring equipment. If magic is highly available, it would be plausible that it gets used for this instead of just weighted sounding ropes, although ropes of known length do work well.

• Cantrip: Light (Bard, Cleric, Sorcerer, Wizard) - make the weight at the end of your rope light up like a lantern ("bright light" in a 20ft radius), making it easy to visually see for some depth below the water.

• 1st level Find Familiar (Wizard) - You gain the service of a familiar, a spirit that takes an animal form you choose: ... including fish (quipper) and sea horse. While your familiar is within 100 feet of you, you can communicate with it telepathically. So that gives you a precise 100 foot reference point to tell if water is shallower than 100 ft, or just a bit, or a lot, deeper. Or if you're not at water level, +- whatever. Additionally, as an action, you can see through your familiar’s eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has.

• 1st level Speak With Animals (Bard, Druid, Ranger) will let you ask fish or aquatic mammals about the depths, as suggested by @David Hambling . If necessary, Animal Friendship is also 1st level, and will "charm" an animal for 24h. (Also a Bard, Druid, or Ranger spell)

• 2nd level Alter Self (Sorcerer, Wizard) lasts 1 hour, and has an Aquatic Adaptation option: gills / webbing: underwater breathing and a swim speed = walking speed. Presumably also tolerance for cold water, although that's not mentioned because D&D 5e doesn't usually bother about that for the intrepid heros who will be casting these spells on themselves / each other.

• 3rd level Water Breathing (Druid, Ranger, Sorcerer, Wizard) lasts 24h on up to 10 creatures, and can be cast as a ritual (not using up a spell slot but takes an extra 10 minutes to cast, plus the normal 1 Action ~= 1 round = 6 seconds). Repeated ritual castings of that could get whole crews of people able to descend a rope and swim / walk around on the bottom surveying. It doesn't help you swim better; if you don't have mundane fins + wetsuit you'd want 4th level Freedom Of Movement (Bard, Cleric, Druid, Ranger), lasts 1 hour.

  D&D also doesn't make a big deal out of cold water, but in many oceans you'd need some protection from the cold. (Protection from Energy (Cold) is a 3rd level spell, single target and requires concentration.)

• 4th level Control Water (Cleric, Druid, Wizard) would let you "part" the water and make a trench up to 100 ft deep (100 ft cube of no water for up to 10 minutes), giving you easy view of the bottom if it's shallower than that, or not much deeper. The precise depth limit of the spell could be a useful depth reference. But it takes a spell slot so you can only do it a few times per long rest (day), depending on character level. A warlock that gets this spell somehow could do it 2 times per short rest (1 hour break during a day), or more at higher levels.

• 5th level Commune With Nature (Druid, Ranger) can give you knowledge of "terrain and bodies of water"; arguably you could use it to map the ocean floor from a ship in a 3 mile radius. It costs a 5th level (or higher) spell slot so you can only do it a few times per day. (Once for a 9th level caster). This is not a low-level spell.

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Starting at 4nd level, a Druid's Wild Shape allows turning into an animal with a swim speed (for "a number of hours equal to half your druid level"). So you could become a dolphin, sea otter, shark, or other animal that's comfortable in the local water temperature. There's a tradeoff between having hands vs. needing to breathe air (aquatic mammal) or not having a very high sustained swim speed (octopus).

You retain your mind so you could totally use this to check out the bottom, like maybe swimming along the bottom while holding one end of a rope and keeping it directly below a ship. People on the ship can count knots / marks on the rope to record depth without having to stop and reel the rope all the way in between depth soundings. IDK if this is much better than just normal depth soundings with a weighted rope.

(Having a Wizard polymorph someone would be somewhat similar, although that replaces the person's physical and mental stats with the creature's stats for the duration. But if you only had a wizard, not a druid or ranger, it might be an option.)

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Other non-spell class abilities

D&D also has lots of class abilities and features other than spells. One of the most relevant might be the Ranger's Favored Terrain / Natural Explorer feature, which they get at 1st level. If Coast applies to open water near the coast, this could be very valuable. Open ocean with its lack of landmarks is harder to justify as working with this feature, though; immunity to becoming lost is way too good.

You are particularly familiar with one type of natural Environment and are adept at traveling and surviving in such regions. Choose one type of Favored terrain: Arctic, coast, Desert, Forest, Grassland, Mountain, swamp, or The Underdark. When you make an Intelligence or Wisdom check related to your Favored terrain, your Proficiency Bonus is doubled if you are using a skill that you're proficient in.

While traveling for an hour or more in your Favored terrain, you gain the following benefits:

• Difficult Terrain doesn't slow your group's Travel.

• Your group can't become lost except by magical means.

• ...

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I'm sure other classes have some other relevant features.

Answer 9

A nice simple and dignified magic way would be to drop some kind of object that a magic user could see through the water.

You can drop it, travel a known distance, and then look back at it, using a sextant to measure the viewing angle between the horizon and the underwater object. From this angle you can calculate how deep it is.

Answer 10

Prior to the advent of sonar, mariners used lead lines to take systematic 'soundings' of the seafloor, which enabled them to produce early depth charts and bathymetric maps. A "Sounding Line" was a sunken lead weight. When the line began to float, it was pulled taught and let go again, this measurement of the length of rope determined the rough depth of a given position. By the 19th century they developed a new line measuring device , a mechanical one. A clock mechanism with a trigger lock was water proofed and incased, when it was placed in the water it sank, when it hit the bottom the trigger would Lock the mechanics so it would give a determination, the device was basically a vertical odometer.

Answer 11

Most vessels with a deep keel are larger vessels and would likely be commercial or militaristic in nature. Commercial vessels would likely keep to trade routes and military vessels would likely spend most of their time patrolling those routes. These well known routes would likely be well known and charted; the tricky bit is when your vessel gets further inland.

Well traveled ports will have the bottoms dredged out; if they have the economic and technological means to do so. Otherwise ports would have been strategically placed in protected channels or inlets with good depth. Some ports would be shallower and trickier to navigate just like Boston's streets are narrower and busier than rural Montana's but this would be common knowledge to a professional sailor.

Different hull shapes have also evolved to combat different environment. Also, breakers are a distinct (and scary) indication that you're likely a bit too close to land.

Essentially, unless your crew are cartographers trying to find new passages or the first inhabitants of a new land and sea, most of the places you'd want to go would be mapped or known as a safe route; much the same as anywhere I want to go being able to be googled for directions.

It's also worth noting that modern sailboats have completely different rigging (not square rigging) and self righting keels that pierce much deeper for stability reasons. A tender (rowboat or skiff) preforms the in between functions in both cases.