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A notable character in my story is a notorious pirate known for his ruthlessness. His notoriety makes him a target for other pirates, marine soldiers, and whatever else decided to make a home in the unforgiving depths of the sea. To protect himself, he’s come in the good graces on a metal dragon who, in exchange for a constant supply of precious metals and ores to eat, will use the radiance of its scales to blind, disable, or confuse an enemy. However, making this plausible has become rather difficult for me. Dragons, naturally, weigh much much more than the common human and assorted cargo and boats can only handle so much weight before it sinks. So my question is: what characteristics or features would the ship need in order to handle the weight of the metal dragon? The answer can include what the ship is made of, how big it is, etc.

Details about the dragon for context:

  • The dragon survives on a diet of precious gems and ore, which makes it much heavier than other dragons.
  • It is so heavy that it cannot fly on it’s own
  • It is about 7 meters tall
  • It’s approximate weight is 28,000 pounds, or 14 tons

Guidelines:

  • The boat has to at least be able to carry 10,000 pounds
  • The dragon does not have to be able to stand up or move freely
  • The dragon can be confined to a single part of the ship
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    $\begingroup$ Just a note. A small medieval small ship would have a capacity of about 50 tons cargo. Even in the bronze age Ulysses likely had a ship which would easily carry a puny 14 tons dragon. $\endgroup$ – AlexP Sep 14 '18 at 0:23
  • $\begingroup$ Just for fun, to give a modern number to AlexP's historical ones, a modern cargo ship has no problem clearing 100,000 tons DWT. Ships are big! $\endgroup$ – Cort Ammon Sep 14 '18 at 0:34
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    $\begingroup$ @AlexP ... You can't put a 14 ton dragon at or above the metacenter of a small ship! If the metacentric height goes to zero, the ship's righting moment will be reversed; that is, any roll whatsoever will cause the ship to flip. $\endgroup$ – kingledion Sep 14 '18 at 0:36
  • $\begingroup$ Weight is not the issue. A @kingledion noted, it's size (not only height, which would cause the ship to roll over, but even if it lay down, it would be too long; not enough room to fit). $\endgroup$ – RonJohn Sep 14 '18 at 0:50
  • $\begingroup$ @kingledion: You can if you balance the ship judiciously, by adding ballast for example. I don't imagine that at any time in history mariners didn't know how to balance their ships. (Successful mariners, at least.) $\endgroup$ – AlexP Sep 14 '18 at 1:29
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Size alone

This is a buoyancy problem and is solved through naval architecture. Here are the key concepts:

enter image description here

The center of gravity is the center of mass on the ship. The ship is pulled downwards as if it were a point mass at this point. The center of buoyancy is 'point source' of the buoyancy. These are two forces acting on a ship. When the ship is upright (the picture to the left, above) these two forces cancel each other out.

When the ship rolls to one side or another they are out of balance. If you look at the picture to the right, above, the center of buoyancy is to the left of the center of gravity. That means these two forces will cause the ship to 'twist' in a counter-clockwise direction. This will cause the ship to right itself--this is called righting moment.

enter image description here

In this picture above, the righting arm is positive; that is the righting moment acts in the direction that will right the ship. It is possible, however, to have a negative righting arm. In that case, G will be to the right of Z (and B) in the above picture, and the ship will flip.

The last concept that is important is the metacenter (M in the above diagrams). As the ship rolls from left to right, the center of buoyancy will rotate around the axis of the metacenter. The metacentric height is the (constant) distance between G and M.

If you mount a heavy dragon on the deck, then this move the center of gravity towards the deck of the ship. As G moves upwards, if it ever raises past M then the ship becomes unstable, as any perturbation will move G to the right of B and flip the ship.

What happens to a small ship?

If your your pirate is master of a galley crew (or fleet), then he is likely going to be out of luck as far as landing a dragon. Here is a diagram of a trireme, based on what the Greeks would have used at Salamis. This is a ~50 ton vessel, with a crew of 100-200. Venetian galleys of the 14th century were not much bigger; warships were perhaps 100 tons and merchant galleys up to 300.

enter image description here

For this ship, the buoyant force is 50 tons (same as ship's displacement) and metacentric height is only about one meter. When the ship heels over 20 degrees, the righting moment is the buoyancy times the righting arm; which is

$$ GM \cdot \sin(\theta_r)$$ for shallow angles of $\theta_r$, or roll angle. GM is the metacentric height. Righting arm in this case is about 35 cm. The righting moment is then about 175 kN-m. This is the force that buoyancy puts on the boat to counteract wind and wave and keep you from tipping.

If a dragon is about 14,000 kg, and is two meters above the center of gravity, this changes the center of gravity upwards by roughly 0.4 meters relative to the metacenter. This has a linear reduction of righting arm down to 100 kN-m. This is a pretty solid hit on the stability; almost a 50% drop. Triremes did flip in storms; now they take about half the force to flip with the dragon on board.

How big should the ship be?

The ship should be much larger than the dragon. How much larger depends on the design of the ship. Where the trireme is a long, low ship, with only 2 meters of freeboard (height of the deck above the water level). This is the same freeboard as the reconstructed Nina (of Columbus fame) had at only 40 tons. A larger sailing ship would have a greater height out of the water, therefore a dragon posses a proportionately larger problem.

I would estimate that for a galley-stype ship, you would want at least a 200 ton vessel to safely park a dragon. On a full masted, Pirates of the Carribean stype sailing ship, you probably want more like 500 tons. This isn't that big in the scheme of things, but it is significantly larger than Queen Anne's Revenge (Blackbeard's ship). It is, however, much smaller than USS Constitution at 2200 tons. So, your pirate is going to need a relatively large ship.

On the other hand, the dragon can do a lot to help. First, the dragon has to keep its center of gravity in the center of the boat (just like you have to be careful in a canoe). Second, it would be really nice if the dragon left whenever the wind and waves go high. The danger of flipping is pretty small in calm seas.

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  • $\begingroup$ Triremes (and galleys in general) were not cargo ships, and were never used as such. $\endgroup$ – AlexP Sep 14 '18 at 1:32
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    $\begingroup$ @AlexP But they were pirate ships, which is the point of this question. $\endgroup$ – kingledion Sep 14 '18 at 3:55
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As others already mentioned, it is more a matter of stability and buoyancy than a matter of weight. My idea how to effectively transport that dragon would be a catamaran or trimaran. Both types of ships can carry a huge load compared to their own weight, are really stable in bad sea, and if you put your dragon on a plattform in the center of the ship, nothing bad should happen.

Edit: Since your protagonist happens to be a pirate, he could repurpose the vessels he captures as material for a second or even tertiary hull. Nothing like some rogue MacGuivering!

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  • $\begingroup$ Awesome Idea! It's just that I've never seen or heard of any even somewhat larger catamaran prior to modern times... $\endgroup$ – Hobbamok Sep 14 '18 at 8:57
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    $\begingroup$ @Hobbamok If uncle Wikipedia is to be believed, war canoes (large katamarans) could carry up to 200 polynesian warriors, so maybe not so small at all. It is just that most of these vehicles did not survive to modern day, sadly. $\endgroup$ – DarthDonut Sep 14 '18 at 9:16
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    $\begingroup$ Excellent idea, way to think out of the box. And for the story's sake, maybe this pirate invented the multihulled ship while he was transferring the dragon between his damaged main ship and another one at sea. By running planking between the two ships and affixing them together, he discovered how this was a considerably more stable platform, and over time, discovered that shrinking the hulls some gave superior maneuverability and speed, which assisted him in outrunning his many enemies. $\endgroup$ – ColonelPanic Sep 14 '18 at 9:46
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14 tons dragon + 10.000 pounds of cargo equals roughly 20 tons in total.

Pirate ships like those used in the 18th century on Earth could easily carry such weight and more.

The Queen Anne's Revenge, for example, had a cargo capacity of about 200 tons.

The San Esteban, a Spanish Galleon shipwrecked in the Gulf of Mexico, is estimated to have been able to carry between 167 tons to 291 tons of cargo.

So, the weight itself wouldn't be a problem, it weight distribution that would be. The dragon would have to mostly keep close to the center line of the vessel to avoid creating an imbalance.

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    $\begingroup$ Please expand this answer with quotes from sources. Otherwise it might get deleted. (Already it's been downvoted.) $\endgroup$ – RonJohn Sep 14 '18 at 0:46
  • $\begingroup$ @RonJohn: The Queen Anne's Revenge and the San Esteban are pretty famous ships. I have added hyperlinks to their Wikipedia articles. $\endgroup$ – AlexP Sep 14 '18 at 1:27

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