Originally from the The SMAC Academy by buster
The new player often overlooks the power of the crawler. One reason is that the AI does not use them much and so only doing single player you won't see the awesome power of a huge crawler fleet unless you make one yourself.
Skillful usage off crawlers is one of the things needed to make you a serious contender in a multiplayer game.
Using a crawler is simple - you build it - move it to some spot, press o, and harvest one of the resources.
The magic of the crawler - I think - comes from the fact that Bryan Reynolds managed to balance out the various factors very well. Cities tend start growing slow when they gain in size. This is because drones start appearing making it necessary to reallocate workers to doctors or other specialists to prevent riots. This prevents the city not making enough nutrients to grow. Once you get a former, a few defensive units and maybe some extra units- support start slowing your production. The solution to both is the crawler and the problem is best avoided by getting them made before the problem appears.
It's power comes from the fact that it is cheap to build and that building one and setting it to harvest minerals makes it quicker to build the next one. If you keep the pattern up you will quickly be turning out a crawler in one turn. In the early stages just one or two harvesting minerals will often double the production output of your city.
Using crawlers harvesting nutrients it is possible to keep a pop boom going as only usually a few specialists are needed to quell riots and so the amount of extra nutrients needed to get the +2 to pop boom can usually be provided by a crawler or two. See the pop booming write up for more details on this.
It is more normal than not to see the top players actually have as their top priority in the early stages of the game to get the wealth technology and beelining their research towards it. Hopefully we will have a write-up on research beelines soon. Once you get the tech, then set all your cities to crank out a crawler or two, then maybe a colony pod or a former (depending on size and what is already built), then more crawlers, formers and colony pods. Maybe a recycling tank or a rec common if the city needs it, but otherwise facilities are low priority at this point. Getting crawlers and terraforming done (so they have something to crawl) and more cities built is the key.
It is a quick way of getting into high production gear and it is how to get rapid expansion done in a short time.
As crawlers are specialist units (only harvest one resource) it is common to terraform with that in mind. In the beginning (when resource limits are still in place) a forest square producing 2 minerals is perfectly acceptable for a crawler to work on. Bonus squares are the best targets though - as they don't suffer from the limits - thus even before you get gene splicing, a bonus nutrient square can be farmed with no penalty or a bonus mineral square can be mined and you get the full benefit of mines. Rocky - mineral bonus squares with a mine & road making up 7 minerals already while other squares are restricted to a max out put of two are my personal favorites. Realistically though, most crawlers will be crawling forest for minerals early on.
Later game use of crawlers
As restrictions gets lifted it gets viable to build mines on rocky ground and condensers or farms on rainy ones. As it is also now time to reallocate some crawlers to nutrients and having a crawler fleet is what will enable you to pop boom your city to size 14 and turn it into a production, research or energy-making monster (or all). Without crawlers for most cities your growth will stall & your production go sluggish. The crawlers give the boost needed for it to take off and after that are still great value.
Later in the game there are various strategies. Many have found it advantageous to turn some cities into specialist cities having most citizens to be say, thinkers or engineers and then having crawlers supply the nutrients and minerals needed to support the city. It is one way of creating a city with very high research and energy output. They then buy whatever they need rather than build it. The masters of this strategy can be awesome to watch and count many of the very top players.
Others use them to boost production. In any case you will be happy you built them and having them around is a great benefit when you need to quickly build a Special Project. Moving them to a project building city gives the opportunity to turn their production cost into minerals towards the SPSecret Project - making it possible to grab an SP in one turn. Just don't forget to rebuild your crawlers after using some in this way.
Later in the game sea crawlers (or trawlers) get into wider usage (they are made by going into unit workshop and fitting a harvester component unto a seagoing chassis (typically a foil chassis to minimize cost). They are quicker to send to places outside city limits and most are used to harvest energy and some to harvest nutrients. The reason being is that sea squares with kelp farms make good nutrient sources and tidal harnesses provide good energy sources compared to land squares with similar terraforming.
Some players make research cities - with the facilities (and preferably SPs) that boost research and a huge fleet of energy collecting crawlers or trawlers. The research output of such cities often rival that of a whole less well-organized faction.
Additionally crawlers provide some other benefits. They can act as early spotters and obstacles if enemies are coming your way. Trawlers for instance can be freely fitted with the radar special ability when it becomes available.
Finally - when you see that one of your opponents has 65 crawlers crowding his empire and you yourself have 5, it is not time to start laughing at this silly player. It is time to start forming alliances because chances you will make it on your own are slim. Make knocking out his crawler park (cheap choppers are great at this) a high priority. However, you can expect that your opponent has given defense of his crawler fleet high priority and has units in place aimed at stopping you from getting to his crawlers. He knows their value.