Just to confuse things. I don't use a water basin in my Brinkman to smoke a pork butt. I think there is enough fat to keep it moist. I do mop it about every hour.
I agree the fat cap is crucial and you should always cook with it on the top but so is the thermal mass in a vertical water smoker. Without gives you no benefit of the steadying out the cooking temperatures which is important when cooking with natural fuels (wood, charcoal) when we're dealing with a small area cubic inch wise in a vertical compared to a horizontal which has much more area and therefore doesn't require the thermal mass for temp regulation as much. wink: It also helps keep the temps down and from running away. In other words, it helps smooth out the peaks and valleys in temp. Of course, that it adds moisture to the cooking environment is just bonus. Sorry but I'm quite adamant about the water pan, especially since this guy sounds like a beginner and will need the benefit of having as steady a temperature as possible.
Not doubting your skills without it at all ThrRoff, I just think he's going to do much better using it, especially at first.
If you want to know more about vertical water smokers, this is a good site although it's focus is on the Weber Smokey Mountain aka bullet (my baby), you can glean a good bit of knowledge anyway:
Be sure and check out the modifications people do to these things. It's like Monster Garage except for grills!
Also, you guys with WSM, take the tip from the virtual bullet and buy yourself a Brinkman water pan since it holds much more water and doesn't require refills as the stock water pan does.