I fiddled with my minimal watergun design and reduced it to the smallest/fastest-repeating single-cart guns i could come up with.
Powerless:
.WF WF
╔▲W ╔═W WWW
▼▲W ▼╚W ~║W
W▲▲W W╗╚W WWW
╚╝ ╚╝
z+0 z+0,track z-1, track
W = Wall
F = Fortification, north of shot location
~ = constant water source
This setup just uses ramp oddities to load water and accelerate the cart. The NS ramp in the channel is enough for reliable loading - the cart enters at ~25k speed, gets slowed to loading speed (<10k) by maximum-depth water, then slowly crawls out and onto the adjacent impulse ramp. Since the loading ramp has no downward connection, the cart faces no ramp deceleration. All ramps on the upper level accelerate the cart normally. The three angled ramps give enough speed to enable a shot (~55k speed are needed to shotgun the cargo). The straight EW ramp re-accelerates the cart after the shot and re-cycles it onto the loading path.
Tested and proven, needs exactly one cart and delivers one shot per 35 steps. If there's no ceiling above the fortification, the water is expelled on the z-level above. If there's closed ceiling above, the water is pushed through the fortification itself.
Powered:
.WF WF WF
RR ╗═ ew
~▼▲W ~║╚W ~▼▲W
RR ╔╚ nw
W W
visible track rollers
Fairly tricky, because i optimised it for the fastest single-cart cadence i could conceive. It uses roller-on-corner features to shave off a few steps. Loading water happens just like in the powerless version, on a non-accelerating ramp in a water-filled trench, which is passed from north to south.
The cart then rolls onto the north-pushing roller on the SE track tile. That one turns the cart back north and "around the corner" immediately, at highest speed (50k). I.e. it leaves to the east on the very next turn.
On that tile, the west-pointing roller once again tries to turn it around instantly and, again, pushes it around the corner, this time to the north. That roller, too, works at maximum speed.
The cart now rolls onto an impulse ramp. That's required to get it to shot-worthy speed, and it must get three steps worth of acceleration there. One step will get neutralised by the checkpoint effect upon leaving the ramp and >55k speed are required for a shot. The chain of two opposing highest-speed rollers moving the cart onto the ramp means that it enters at a sub-coordinate where it has enough space to get the full three turns of acceleration.
The cart rolls off the ramp and onto a medium-speed west-pointing roller. On the next step, it will crash into the fortification and releases its load, in a northwesterly direction (~58k northward speed, 30k westward speed). It technically stops at the fortification but is instantly re-accelerated to the roller's speed, which now moves it off to the next tile west.
On that tile, there's yet another opposing-mode roller, pushing east to move the cart south via corner redirection. That roller works at "high" speed. That's the highest speed compatible with loading water onto a cart in a one-tile trench loader.
All this effort achieves a single-cart cadence of one shot per thirteen steps. And this is what it looks like in action:
http://mkv25.net/dfma/movie-2743-mini-watergunThat one's rotating in the opposite direction due to space constraints. The minimal powerless watergun can be seen a bit to the east and north, currently not active.
EDIT: In the powered design, time consumption breaks down as:
one step on each corner/roller tile - 3
two steps on the shot tile -> 5
three steps accelerating on the ramp -> 8
five steps loading in the trench -> 13
Loading is the biggest time sink, at five steps per cycle. This should also be the theoretical cadence limit for a cannon with multiple carts per barrel.
The powerless cannon has of necessity a longer cadence, since it can only accelerate through ramps, at 5k/step. And since in every cycle of a watercannon, the cart must be accelerated from <10k (slow enough to take on water) to >55k, that translates to 10+ steps of acceleration. The cart also needs to be re-accelerated (via ramps) after delivering the shot to send it to the loader, and the loader itself consumes time, too. I actually managed to accelerate the cadence of the powerless gun to 31 steps by _increasing_ the footprint slightly:
. WF WF
╔▲▲W ╔╝═W
~▼+▲W ~║+╚W
W▲+▲W W╗+╚W
╚▲╝ ╚╔╚
W W
visible track
After the shot, the cart not just rolls off a single ramp, it goes through another full impulse ramp, accelerating it to about the maximum speed legal for loading water. Loading at this speed takes only four steps (instead of over ten when coming off a single ramp). Due to the larger footprint, the circuit contains four acceleration ramps for the actual shot and not the minimum three. All corners are passed in single steps thanks to the checkpoint effect. The faster loading more than compensates for the longer time spent on ramps, so this cannon has a slightly faster cadence than the smallest design presented above. I re-tooled the powerless cannon for this design and tried it out (that's how i know of the improved cadence
).