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[billw]

I'm trying to power my magma pump stack with some water wheels. I dug a channel from the stream to near my stack, then routed it down into a cavern. Then I placed a large set of water wheels in it (2 wide, 4 sets), and hooked them all  together. I'm getting motion, but it only seems to fluctuate between 100 and 400 hundred. It actually seems better when I first turn the water on or when I turn if off (when its 2 - 5 deep), rather than when it is 7 deep and in full flow. What's the deal? Why am I not getting full power? How can I get a stronger flow? Is there some way to totally redirect the stream? I can't think of how to block it off to do so... I want my waterwheels safely underground and near to my pumps.

[Quatch]

When all the water is at 7/7 under the wheels there is no "flow". 7/7 water causes teleportation. Check the wiki for further details, but in short you need to decrease the volume of input water. A row of diagonal depressurizers will likely help immensely.

[Daetrin]

When all the water is at 7/7 under the wheels there is no "flow". 7/7 water causes teleportation. Check the wiki for further details, but in short you need to decrease the volume of input water. A row of diagonal depressurizers will likely help immensely.

A (properly filled) Dwarven Water Reactor is constantly at 7/7 for most of its squares..  If you stick wheels in a river they'll turn even at 7/7.  I thought that flow was...somewhat odd at best.  You might do better to put all your water wheels in a stream and route the axle a longer distance.  You can build lots and lots of wheels in a steam without much mining, after all.

[Kanddak]

Water wheels are black magic and even experts in DF physics/fluids don't always understand when they will or won't work. The "you have to avoid 7/7 water" thing is a myth. There's some poorly-understood process for deciding whether water is "flowing" for waterwheel purposes which has little or nothing to do with the processes that actually move water.
I recommend building your waterwheels directly in the river and running axles from there. Wall/roof them if you like.
You can also reliably make them work by digging a channel in an aquifer, then ponding one bucket of water into the channel from a level above to cause all the water in the channel to become "flowing". Don't ask me why that works but it always does.
In 40d I had a very reliably process of diverting underground rivers into snaking channels with waterwheels over them, and once I got the waterwheels to start turning, I could seal the input and output doors and they would keep running forever anyway.
I can also, for some reason, make a looping aqueduct fed by a pump from a natural source of water, and run waterwheels over the aqueduct; but when I make an artificial reservoir and try the same thing, it doesn't work and I don't know why.
The key element seems to be staying as close as possible to natural water sources/sinks.

[LemonFrosted]

When all the water is at 7/7 under the wheels there is no "flow". 7/7 water causes teleportation. Check the wiki for further details, but in short you need to decrease the volume of input water. A row of diagonal depressurizers will likely help immensely.

A (properly filled) Dwarven Water Reactor is constantly at 7/7 for most of its squares..  If you stick wheels in a river they'll turn even at 7/7.  I thought that flow was...somewhat odd at best.  You might do better to put all your water wheels in a stream and route the axle a longer distance.  You can build lots and lots of wheels in a steam without much mining, after all.

The issue is that rivers and streams are both "full" and have "flow." Their "flow" component is specifically an attribute of being river tiles. 7/7 "still" water tiles use teleportation and only have flow when behavior causes orthogonal tiles to swap a unit of water.

Basically it's 20x easier to build giant axels. Even a terribly inefficient path can be easily powered since it's easy to fit a lot of water wheels in a small space. I pumped magma 15 z-levels with a stack and only used ~350 power out of the 600 worth of water wheels I'd built.

[billw]

Hmm okay, that seems fairly consistent with my experience. I definitely get better power when the flow is ebbing. So maybe I will try a two wide channel from the river that widens to three wide before the wheels to get the depth down a bit. Anyway, even with only fluctuating power between 100 and 400 I still managed to pump enough magma for my forges. Now I need to make it good enough for my siege traps.
Thanks for the tips guys, glad to know its not just me with the problems.
As for putting the wheels out-side in streams: that doesn't seem very dwarfy to me...

[Grimlocke]

I build perpetual motion machines that are completely filled with 7/7 water, and which constantly provide their maximum power.

On the other hand a large reservoir filled with an avarage of 3,5/7 water that was just waving up and down a bit was also enough to constantly power waterwheels. Filling it with 7/7 water made it powerless.

Deciding factor here could be that the first one uses pumps. Pump-pressured flows seem to behave differentely then normal diffusing flows, perhaps being marked as flow as long the pump moves water is part of that.

[billw]

Yeah, the channels I use to get water from the stream to my wheels are v. long and full 7/7 all the way, except right at the end where they drop down to empty into the cavern beneath. My guess is that I am only getting power from the wheels that are near the drop, where the differences in depth are causing flow. Okay, so that offers some solutions, but I might just go with water reactors instead. I didn't want to originally as they are cheating the laws of physics. But then if the waterwheels already cheat the laws of physics then what the hell.

[Kanddak]

The issue is that rivers and streams are both "full" and have "flow." Their "flow" component is specifically an attribute of being river tiles. 7/7 "still" water tiles use teleportation and only have flow when behavior causes orthogonal tiles to swap a unit of water.

This is only half-true. 7/7 water tiles are indeed either "flowing" or "still" for waterwheel purposes, but the former is not specific to river tiles and can occur in other circumstances, including artificial channels built in some ways but not others. It's an unsolved mystery.

[Quatch]

For my perpetual motion, I find they will randomly jam if it is mostly 7/7. The trick is the details (IMO), if you have exactly enough for 7/7 in the cistern at all times, and 1 cube is moved up by the pump and allowed to _flow_ into the remainder, then it will be full but flowing. If it teleports or skittles around the top, then its no good.

Much easier to leave the system at 5/7 or so and not worry.