Calculating the weight of your fantasy creature

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Robinne Weiss

Full Member
May 19, 2015
New Zealand
This is for you, @Nmlee :), but I thought others might at least be amused by it ...

For my current WIP, my characters have to move a 40-metre long dragon off a mountaintop. The dragon is a marine species, adapted to life in the Southern Ocean (long story how it ended up on top of a mountain, unable to get down on its own).

So, of course my main characters think they might helicopter the thing off, but to know if they can do this, they need to know how much the dragon weighs.

Which means I, the author, needed to know how much the dragon would weigh.

So I geeked out.

First, I considered what animals would be comparable in size or weight. It was tempting to look at weight estimates for dinosaurs, but I saw two problems with this: dinosaur weights are estimates, themselves (though probably pretty accurate), and dinosaurs didn’t live in freezing water and knowing that fat and muscle have quite different densities, I expected they wouldn’t accurately reflect my dragon’s physiology (If I were calculating the weight of a tropical, terrestrial dragon, yeah, I’d have looked at dinosaur weights).

So, big marine animals ... I checked out some whales and the largest shark.

Blue whale: 29.9 m long, 173 tonnes

Gray whale: 15 m long, 36 tonnes

Whale shark: 10 m long, 19 tonnes

If we break their weights down to tonnes per metre, they look like this:

Blue whale: 173 tonnes ÷ 29.9 m = 5.8 tonnes/m

Gray whale: 36 tonnes ÷ 15 m = 2.4 tonnes/m

Whale shark: 19 tonnes ÷ 10 m = 1.9 tonnes/m

Obviously, the larger the animal’s overall body size, the greater its weight per linear metre, because it’s bigger around.

If a dragon’s body was shaped like a whale’s, we could just use the blue whale’s weight per metre as a decent approximation, and multiply by the 40 metre length of our dragon:

40 m x 5.8 tonnes/m = 232 tonnes

But a dragon’s body isn’t shaped like a whale’s—it has a long neck and tail that are significantly thinner than a whale’s stocky body, so using the blue whale’s weight per metre gives us a number much too high.

If we draw a dragon stretched out in a straight line, its neck, body and tail each occupy about one-third of its total length (at least in my sketch). For our 40-metre-long dragon, that’s 13.3 metres each.

So the gray whale’s weight per metre figure is probably closer to our dragon body’s weight. Using the gray whale’s 2.4 tonnes/m, the dragon’s body weighs:

13.3 m x 2.4 tonnes/m = 31.9 tonnes

Now the maths gets harder. Looking at my dragon sketch, I estimate that the tail and neck average about one-third the diameter of the body.

If you remember the equation for the area of a circle (which of course you do), you’ll remember that:

Area = πr2

Where r is the radius of the circle.

So if the area of a circle changes with the square of the radius, and the tail and neck have a radius one-third of the body, the weight would be one-third squared—one-ninth—the weight of the body. So the neck and tail weigh:

2.4 tonnes/m x 1/9 = 0.3 tonnes/m

Multiplying that weight by our estimated lengths of 13.3 m for neck and tail, we get:

Tail: 13.3 m x 0.3 tonnes/m = 4 tonnes

Neck: 13.3 m x 0.3 tonnes/m = 4 tonnes

There is still the issue of the head, which will add a not-insignificant amount to the weight of the neck third of the dragon.

Sick of maths at this point, and pretty sure I was close, I simply added another tonne to my total weight, to arrive at the total dragon weight:

31.9 tonnes (body) + 4 tonnes (tail) + 4 tonnes (neck) + 1 tonne (head) = 40.9 tonnes

The New Zealand Air Force’s largest helicopter is the HH90, with a slung load capacity of 2.4 tonnes.

They’re not helicoptering this dragon off the mountain (which, of course, they were never meant to do--I could have saved them the maths and just told them that, but that would have been no fun at all).;)
I can't wait to see how they get him down! Even a triple-tray truck (which wouldn't get up into steep mountain territory) would be dicey, due to the articulation/gaps between trays ...
FYI if you post this on your blog, I'd happily share it. It was fascinating!
I was thinking, actually, of chatting with Dan Koboldt to see about putting on his Science in SFF blog. My blog is fundamentally not a writing blog (though I might put it up there anyway, just because it's fun) ... I'll let you know if and when it shows up elsewhere.
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