that's kind of a basic drawing showing the wall tubes,, the boat plans place the site glass and try-cocks piping coming off the boiler at 40 degrees aft of the beam center line (if it was further aft it would conflict with the engine piping and space),, letting the engineman keep an eye on the level,,(the pressure gauge is also in the area) and also keeping the space for movement clear on that side of the boiler.. also being a water-tube boiler and not a fire-tube the level is a little more less effected by rolling,, boats are always rolling or pitching a experienced operator would know when to trust the level..