I suddenly realised, other than the guys with actual steam experience, there's probably a lot of guys scratching their heads and saying "What IS that idiot taking about?"
Priming, or drawing water is when large amounts of liquid water is carried over with the steam. There are a variety of causes, but the two most common are: too much water in the boiler, and bad water (stuff with impurities that change the surface tension causing it to form a foam.
Priming is not good for a few reasons: 1.the water washes away the oil film from the cylinder walls and internal moving parts. 2.Liquid water does not compress, so if too much is left trapped in the cylinder it will cause the engine to "pound"... extreme cases can result in bent or broken parts (A launched cylinder head can be pretty spectacular, but hazardous to anybody in its path)
3. The syphon action that results in priming also causes large steam bubbles (called pockets) to form below the normal water level. Two problems here:a.it gives you a false high water level reading (definitely not good)and b. wherever the pockets touch heated metal, the surface is essentially bare (very, very, bad). This isn't a complete or very scientific explaination, but I hope it helps.
"Mik"
See, I ain't as dumb as I look.