Minecraft – Ghast farm geometry

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It's my second try to get the ghast farm attachment to my gold farm working, and it's still bad.

The farm is located halfway up the Nether Top. The first try was a 4-tall chamber below the first floor of the gold farm. The gold farm uses turtle eggs to lure zombie pigmen to a central drop shaft and it works like charm. The first attempt had rails running down the middle of the lower spawning floor, and it failed badly as pigmen would crowd by the rails unable to cross them, and stop the minecart or get grabbed by it. The second try is pictured below:

Now the pigmen don't clog it up anymore, happily jumping to their death in pursuit of the eggs (the bottom of which had to be protected by trapdoors from all sides, otherwise baby pigmen managed to land on it). But ghast spawn and don't move – I guess 4 blocks is too low for them. There's a 3-block gap to the rail for the minecart running around the farm (to a kill zone with a cactus and hoppers; tested in creative, works), and another 3 blocks to the outer wall. I'm hoping it's sufficient to keep the ghasts from flying out of the farm and still let the large magma cubes drop down. Now I'm not sure – will dropping the floor by a block suffice to get the ghasts to move, or do I need more? Will I need to move the rail lower too, or can it stay at the same level? Also, should I prevent the rails from magma cubes, and if so, how? Also, endermen started spawning and it's now a matter of time until they steal the sand block from under the cactus unless I protect it somehow… will a glass roof 2 blocks above suffice? How far should it reach?

I've sought designs on Youtube but they are divided between "showcases" that usually show extremely complex super-efficient ghast farms and really gloss over the mechanics, often completely avoiding angles that would show the geometry of the spawning/catching area, or they are just small manual farms where the ghasts getting stuck is a boon.

Best Answer

Not sure this is a full answer, but have you considered using a flying machine to push the ghasts to where you want them? Then you don't have to change the geometry of your system.

Related Solutions

Minecraft – Why can’t I get Zombie Pigmen to spawn

The answer you are looking for is hidden inside you own question: "there are a lot outside". The game randomly checks places to put mobs and since the outside is much larger than your trap, most of the zombie pigmen spawn there. The mob limit is exhausted pretty much instantly and the game doesn't spawn any more, unless some of the existing mobs de-spawn or get killed.

You will get zombie pigmen to spawn in your trap eventually, but you may have to wait hours for just a few spawns. You need to prevent spawning outside your trap to improve rates of your trap. This is very difficult to do in the nether, since you can't use light or water to prevent spawning. That is why you don't see zombie pigmen traps, building one is a monumental project. Notice for example here a huge area is covered with half-slabs to prevent spawning.

The spawnable area is +/- 8 chunks from each chunk where there is a player (minus the chunks on the edges of that area). If a single player is online, mobs spawn in 240x240 blocks area centered on the player chunk, that is roughly +/- 120 blocks from the player.

Minecraft – How to deal with magma cubes in the Nether

There's an oddity in monster spawning you can exploit. As is well known, mobs can spawn on top of any block with a flat, opaque top surface, even if there is a non-solid block such as a torch or rail on top of that block.

However, the selection of center points for groups (“packs” as Minecraft Wiki describes them) of mobs is slightly different: it requires that there be empty space which is an air block, nothing else (not even a torch). Only once such a point has been found will mobs spawn with their feet at that level in the area around that point.

Therefore you can prevent spawning entirely as follows: make sure that in the vicinity (up to 20 blocks horizontally) of your rail station, at any particular y-level, either there are no empty air blocks, or at the y-level below it there are no regular valid spawning surfaces.

For example, consider this cross-section of an underground rail tunnel.

5  #########     _ slab
4  ###   ###     # stone/netherrack
3  ###   ###     R rail
2  ###R_R###     
1  #########

No mobs can be spawned in the middle of a tunnel of this form:

No mobs will spawn with their feet at levels 4 or 3 because there are no solid blocks below those levels (all individual spawn locations are invalid).
No mobs will spawn at level 2 because there are no air blocks at that level (all pack spawn locations are invalid).

However, if there are any air blocks at level 2 within 20 blocks of the tunnel, then they may cause spawning. So, if the tunnel is in midair as such things often are in the Nether, you'd have to build a 20-block-wide flange beside the tunnel at level 2 to prevent spawns (preferably made of slabs so as not to itself be a spawning platform).

Disclaimer: I've never tried to engineer spawns in this way; I'm just going by others' documentation of the way it works. (And yes, this was already mentioned in the comments of this question; but answers should be posted as answers, and I felt a fuller explanation would be useful.)

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