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I'm working out the back story of a flying castle run by a king who has a squadron of 200 air cavalry. All the knights ride hippogriffs: head and wings of large eagles, bodies of horses.

Horses eat oats. Eagles eat meat.

How do I calculate how large the grounds surrounding the flying castle need to be, to grow enough oats and to supply enough meat (lamb? pork? goat?) to keep the mounts fed?

My intent for the flying castle is not that it is a battleship. My intent is that there is a hemisphere of land which flies, and the castle sits in the center. The land supports the castle inhabitants (a thousand people) with crops. So, oats could be grown on the flying piece of rock around the castle proper.

Technology level and tone is Age of Enlightenment with some magic. So, roughly 1730 to 1750.

Edit: I'm asking for help calculating the resources needed to feed the mounts. I have an idea of how much land surrounds the castle to feed the thousand inhabitants inside: a radius of two miles, mostly fields with some small forests and hills. The lands around the castle have enough resources (fields) to feed the inhabitants. The castle can send parties down to trade for resources and to forage. I'm trying to work out what can be provided locally and what will be a problem. The castle is not a military team; it's a small town with a normal population distribution of a thousand people.

Second Edit: while the castle might be five hundred feet or less from the surface, and be able to forage on the planet in times of peace, I'm trying to put together a plan for feeding the hippogriffs in times of war, when forage is not possible and the castle is over the ocean or the frozen north.

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    $\begingroup$ How much does a horse eat? How much does an eagle eat? Add together, x 200. Maybe add 50% to be sure. How do the dietary requirements of your hippogriffs differ? $\endgroup$ – Wolfgang Aug 5 '17 at 0:07
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    $\begingroup$ In modern times, a cow of 1000 pounds eats every day 100 pounds of fodder (which require 500 square feet of land) to produce 0.60 pounds of meat. Porks are 4 times more efficient and chicken 6 times. Goats are 10% less efficient than cows. But I don't know how much food an acre of land could produce in 1730. Or how much an hippogriff eats. $\endgroup$ – Alberto Yagos Aug 5 '17 at 5:26
  • $\begingroup$ Are you asking for just the hippogriffs, or the thousand people as well? Does the castle retrieve building materials from the ground, or do there need to be forests? How regimented are the inhabitants with their diets? Can they be supplemented by foraging on the ground below the floating castle? $\endgroup$ – jdunlop Aug 5 '17 at 6:07
  • $\begingroup$ Another interesting question you should ask yourself is: how far is this castle of yours by the mainland (let's assume there is a mainland, common non-flying ground) travelling by air? If the answer is "not much", you could send hunt parties of air-knights to gather preys for the hippogriffs. This way you wouldn't need to grow at least some of your food on the castle's grounds. Of course it would not be a good idea (if somehow you go under siege) but it's a source of venison. $\endgroup$ – Liquid - Reinstate Monica Aug 5 '17 at 20:09
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    $\begingroup$ The answer can be calculated by counting the number of ferrets hanging from Haggrid's neck during the introduction to BuckBeak scene in HP Prisoner of Azkaban, then multiplying that by the number of mounts in your squadron. That would give you a rough daily dietary need. If you can trade rats for ferrets, then you won't need any additional resources. Most castles come with an infinite supply of rats. I would offer this as an answer, but I've gotten this far without quoting HP and with all due respect for a wonderful set of books, I don't want to start now. $\endgroup$ – Henry Taylor Aug 6 '17 at 4:38
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Food

Since your hippogriffs have head and wings of large eagles and bodies of horses, their digestive system is one of a horse. Thus before assessing how much land do you need to feed the cavalry, we need to address the problem of suitable food.

Avoid Meat

Horses are herbivores, their digestive system does not work with meat that well. They can consume it in small quantities, but a diet rich in meat would kill a horse.

This is actually good news. It is easier to grow and store plant-based food.

Oats Are Only a Concentrate

Horses also do not do that well if their diet is based on grains. Oats are useful for giving a horse some energy or gaining a bit of weight. However, a grain-rich feed may cause colic, which is frequently lethal. Under normal conditions no more than 40% of fodder can be replaced with concentrates. Oats can be given as the main food before the battle or when the cavalry is on the march. The riders should monitor their hippogriffs carefully to avoid overeating and colic.

Grass and Hay — The Horse's Best Friends

Grass and hay are the staples of the horse's diet. Grazing is the best option for a horse. Hay is the second-best option. They help to keep a horse healthy since they are rich in fibre.

Salt and Minerals

A horse needs a small amount of salt and minerals. They can be mixed into feed. Another option is to have a block of salt that a horse can help itself to when needed. There is no need to worry about overeating salt.

Anatomy

I am going to discuss beaks only. Eagle beaks are great for tearing flesh, but they are terrible at grazing and chewing grass and hay. Since hippogriffs are chimaeras, they can have beaks with teeth. (Interestingly, birds have genes responsible for teeth but they are deactivated.) You will need to experiment with actual anatomical placement, but teeth are absolutely necessary to keep hippogriffs healthy and happy.

How Much Food Does a Hippogriff Need?

A healthy horse eats about 2-2.5% of its own weight daily. On average, birds eat need 25-50% of their body weight. Hippogriff will probably have lower body temperature and will be flying less than an average bird since it can use legs to walk and run. So, I would suggest taking 5% of body weight as the lower limit (when hippogriffs are in a 'stable' during peaceful times) and 15% as the upper limit ('action mode': a lot of flying, fighting, pregnancy, etc.).

I will use heavy war horses as a reference point for hippogriffs since you want them to carry knights. Medieval war horses weighted from 1,500 to 2,000 pounds (680 to 910 kg). Let's assume that an average hippogriff is about 1750 pounds (about 790 kg). Therefore, it will need from 87.5 to 262.5 pounds (39.6 to 119 kg) of feed daily. You can work out the yearly consumption depending on your political climate.

Food Production

You should use crop rotation system in combination with the hippogriff husbandry. Grow grass for hay and grazing and use manure to fertilise the land.

It would be ideal to also have dedicated pastures where hippogriffs can graze freely. The pastures do not have to be on the same rock as the castle. However, in this case, enough feed should be stored in event of war.

Crop Yields (see this paper for detailed information)

NB: these are back-of-the-envelope calculations based on data for the Great Britain. Your climate and soil might produce different results.

  • Oats: approximately 12.3 bushels per acre (net yield; about 775 kg per hectare)
  • Hay: approximately 540 pounds per acre (600 kg per hectare; based on Medieval Inner Asia yields)

The average storage losses due to spoilage were about 10%.

The final calculations will depend on your decisions regarding the size of hippogriffs, their appetites, and lifestyles.

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  • $\begingroup$ Your description fits a pegasus squad much better than a hippogriff squad which are supposedly (at least partially) carnivores based on the beak which as you argue is meant for tearing meat but not for chewing plants. $\endgroup$ – Alex2006 Sep 28 '17 at 13:25
  • $\begingroup$ @alex2006, Strictly speaking hippogriffs cannot exist at all ;) As I explained, it’s more probable that they are herbivores because horses have an extremely specialised digestive system. I also think it would be a nice touch to make hippogriffs more horse-like temper-wise. It might create an interesting contrast with our preconceptions. $\endgroup$ – Olga Sep 29 '17 at 7:29
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A horse eats 1/2 a pound of grain per 100 pounds of animal per day. For the ease of calculations, let's say your hippogriff weighs 2,000 pounds (a large horse's size). That's 20 pounds of grain per day. An acre of land typically yields about 33 bushels of grain per year. Each bushel is about 60 pounds. So you can feed three hippogriffs on one acre of land per day. To feed 200, you need 4,000 pounds of grain per day. That would be about 24,000 acres (or 37.5 square miles).

Now, let's assume each hippogriff will eat a 50/50 split of grain and meat so cut those numbers in half (so 12,000 acres or about 19 square miles).

Most big predators eat 4 to 7% of their body weight per day in meat. For ease, let's say 5%. 5% of 2,000 pounds is 100 pounds a day. Since we are assuming a 50/50 split works, that's 50 pounds of meat per day per animal so 10,000 pounds of meat per day. Cows could be convenient meat, but then you have to have more land to grow the grain to feed the cows. Some sort of roaming herds of large animals like elk that the hippogriffs could hunt would probably be easiest (I assume they'd have to live on the ground below the flying castle).

Per Day 10,000 pounds of meat and 4,000 pounds of grain for your entire hippogriff population

(Eating all grain or all meat would make this scenario a lot easier.)

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  • $\begingroup$ If you need any additional information, OP, let me know. $\endgroup$ – A.G. Weyland Sep 28 '17 at 16:03
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Type of food

Based on my image of a hippogriff, I would rather go towards them eating meat only. If you plan to have a vicious, bloodthirsty king, he could feed his beloved animals with human meat, mainly of the killed enemies and peasants who refuse to pay taxes. If, what I guess is more probable given the fact your castle is surrounded with fields and peaceful farmers, you plan to have calm and kind king, who uses his army only to defend his valuable castle, animal meat would be sufficient. I would guess rodents and small animals in natural habitat. However, if you need to feed an army, you would have to have a big farm for meat animals so something like chickens or rabbits is most probable. However, supplementing the diet with some kind of crop might be needed for economical purposes.

Calorie needs

Single, average working horse needs about 25 000 kcal a day. If we imagine a horse with wings, which is used to fly around the castle by men in armor, I think we can call it "average work". However, maintenance of a single horse requires only 15 000 kcal. So if all 200 hippogriffs don't fly around on daily basis, I guess we can set average need at about 20 000 kcal.

Energy in food

Finding info about calories in animal meat wasn't easy as I assumed that hippogriffs can eats whole chicken/rabbit.

Some forums claimed that a whole chicken (with skin) is about 1300 kcal. I am not sure if the number is including bones and other stuff humans don't usually eat - the answer may need some improvement.

As the rabbits are generally less fat than chickens (about 220 kcal to 300 kcal) we can assume you would need 3 rabbits for every 2 chickens we need.

If we would like to add some crops into the diet, corns seems like the best option with 1500 kcal per pound.

The math

Assuming average calorie need of 20k kcal per hippogriff, we need around 4 million kcal a day to feed whole army. That would translate to about 300 000 chickens. The number seems enormous, so that needs verification.

The crops might be better documented (link below) so the math should be OK. An acre of corn provides 15 million of calories per acre per year. That means that corn that could feed your army for a year - 1,25 billion kcal - would take about 85 000 acres.

Summary

Letting your mounts feed in the wild may be a smart idea as feeding 200 horses is a costly task. Hippogriffs may easily hunt for the birds,rodents and even deer or eat grass in the meadows beneath the flying castle. It will make your life much, much easier.

Links: http://www.dayvillesupply.com/hay-and-horse-feed/calorie-needs.html https://www.washingtonpost.com/lifestyle/food/in-defense-of-corn-the-worlds-most-important-food-crop/2015/07/12/78d86530-25a8-11e5-b77f-eb13a215f593_story.html?utm_term=.dd22a1f16b76

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    $\begingroup$ I think you are off by a factor 1000 for the corn calories. Other than that, thank you for actually using kcal and not cal in this answer. $\endgroup$ – a CVn Aug 9 '17 at 13:02
  • $\begingroup$ You are right. I am used to having all data provided in kcal :) Will edit it right away. $\endgroup$ – Jakub Tustanowski Aug 9 '17 at 13:08
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What is a Hippogriff?

You appear to be taking the half-eagle/half-horse description literally. In this case, it's an omnivore where the front half and back half have different digestive needs. However, that's really unlikely to be the case. My answer will be from the point of view of horses because once the math's established you can apply it to any dietary requirements you wish. I bring this up because many a good story has fallen upon deaf ears for the lack of originality. Therefore, what, really, is a Hippogriff in your world?

The Dietary Requirements of Horses Hippogriffs

There is an issue, but I'll deal with that a bit later.

And before we begin: remember that I'm oversimplifying a remarkably complex process.

Merck has an excellent paper on the subject of horse nutrition that covers everything from water to lactation. There's more to keeping a large animal alive than just the proverbial bread and water. Dietary requirements change for the different phases of the animal's life and (in your case) the different kinds of combat the animals may experience.

Water

A sedentary animal needs 5L/100Kg/day of water. Everything will increase this amount. Merck recommends unlimited access to clean water. That's a bigger issue for your flying island than food because water will be needed for everything: humans, animals, agriculture, driving the engines of manufacturing, sewer/cleaning... just to name a few.

Most of the believability of your story will not revolve around the science of keeping Hippogriffs alive. It will revolve around the believability of your water sources.

According to the USGS an inch of rainfall on one square mile results in 65.78 million litres of water. This sounds like an enormous amount until you realize how much water is needed for farming. An answer to this question suggests 3.5 acre-feet/acre of water per year. 640 acres/square mile: that's 2.76 billion litres annually or 42 inches of rain. (Remember that rainfall requirements depending on soil and crops will vary wildly from this figure.)

This, of course, depends on your soil foundation, top soil quality, what food you're actually growing, where your island is (hot? cold? desert? jungle?), and your altitude (closer to the sun, faster evaporation, though it has little to do with the sun and a lot to do with air pressure). If you need to store the water, you need 2.76 million cubic meters of space. Granted, that sounds bigger than it really is. It's a fraction of the size of a city resevoir (on the order of 1/30th - 1/100th). But, you stil need to make the space, and I don't know how thick your island is. Lakes and ponds might have to be bigger than usual because you don't have the depth to work with.

Food

Merck has tables that indicate your average weight animal needs 16.7 million calories a day if idle and twice that for heavy labor (like combat). That sounds like a lot, but as it turns out, there's a difference between calories, Calories, and megacalories. From the previous link we find:

Once the hay test is completed, the results will show the digestible energy in the hay. It will appear on the test as "DE, equine Mcal/lb." For example, if the test reports a reading of 0.78 DE, equine Mcal/lb it means each pound of hay will offer 0.78 megacalories.

Using the adult 1,100 pound horse that is doing moderate work how much of this hay would need to be consumed? He needs 24.6 Mcals per day. Divide 24.6 by 0.78 which equals 31.5 pounds of hay. This horse is going to be in bad shape - there is no way he can eat that much. Better hay needs to be found (the best thing to do) or a concentrate needs to be fed.

And from this site we discover how much hay/alfalfa/etc we need. For that 1,100 pound horse and assuming we get but one crop per year (REALLY IMPORTANT ISSUE) we get 0.03*1100*365/2000 = 6 tons of feed per animal or 1,200 tons for your cavalry. Reducing this paper from Purdue University to the basic facts (and ignoring the complexities of soil quality, rain, sunlight, etc.) you get 3.4 tons of feed per acre per harvest.

Putting It Together

Assuming one harvest per year, you need 353 acres or 0.55 square miles just for animal feed and the ability to aquire or store 1.52 million cubic meters of water. (It's the same amount of rainfall, though, because of the change in acreage: You're still looking at 42 inches of rain, unless you allow space to lie fallow as a rain shed, which is a good idea. Farm an acre, use 3 for rain shed, now you need only 10-12 inches per year.).

One More Thing

But, there's that issue I mentioned. Hippogriffs fly. That means the high-energy requirements of birds. This is a pull-it-out-of-thin-air guess, but you'll likely need to double the food intake to sustain regular flight. That doubles the requirements of the previous paragraph.

But It's More Than Just Land — it's transport

The second biggest problem you face (next to water) is transport. What happens in a drought, or if some disease hits your crops, or you choose to reduce your landmass so that you're only self-sustaining for short periods of time during war? You need to move mass (water and food) from the surface to your island.

Ugh.

Some gut reactions suggest a horse can pull up to 2X its weight, but really shouldn't carry more than about 15% of its weight (150# for a 1,000# horse) and generally can pull about 1.5X its weight.

The problem is that you need to fly everything to that island. That means we're dealing with "carry" figures, not "pull" figures. Unlike a cart, which distributes the weight on the wheels thereby reducing the actual force load on the horse. Your Hippogriffs must carry the load as if it were on their back.

If we assume you only use your Hippogriffs for transport, and that one rider can wrangle four cargo carriers, then you can move (on average) 120# per Hippogriff or 9.6 tons if the whole fleet is used. You need 125 trips (of the entire fleet) if you need to fully stock for a year. Take you, what, at lest a month... maybe two?

Another chunk of believability in your story will be emergency preparedness, which boils down to transportation. How do you haul cargo to your island? You will need resources for animal maintenance, building maintenance, human maintenance, news & intelligence, and a host of other things. Yes, you can make most of it yourself (assuming nothing goes wrong), but you're just one natural disaster away from everybody dying if you only have 200 fighting birds. You'll need a reasonably comprehensive transport system. If you're only using hippogriffs, you'll need another 100 for regular transport and probably another 100 to swap out animals who are sick, injured, pregnant, tired, etc.

Note that I just doubled your landmass.

Your Last Problems: Metal, Wood, and Coal

Finally, a floating island will have little in the way of mining or logging. You'd need a floating state to have enough forest to be self-sustaining with wood, and even if you created a big enough floating island to have mining, it's a finite resource.

You need to move metal and wood to your island, exacerbating the transport problem because they're both heavy.

On top of that is the need for heat for everything from comfort to cooking to metalworking. I'm assuming coal for convenience.

And that's assuming you're not building your fortress/castle out of stone. Granted, after it's built all you need is maintenance... but rocks are heavy.

Water and transport... they're the real problems.

TL;DR

That was a book. If you read it and all the supporting links, you're well educated! If not, here's the short and skinny:

  • You need about 4 square miles dedicated to food for your Hippogriffs. One for actual production, three for rain shed and rotation.

  • You need about 0.33 square miles dedicated to water storage... assuming your island is funnel-shaped and every drop of rain water is channeled to the lake. If not, it'll need to be bigger. And while I'm thinking about it, attacking your island from underneath to punch a hole in the lake would be a great tactic for your enemies....

  • You need 100–200 Hippogriffs in addition to your cavalry for supporting functions including transportation and down-time replacements.

  • You need regular transport (as in nearly daily transport) to keep non-renewables in stock including metal, wood, stone, and burnables like coal.

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