Re: First sailboat, project Hobie cat 16
In general terms, without dwelling on the finer points....The boom runs along the foot (bottom) of the main sail. The main sheet (rope in a series of pulleys to multiply you mechanical advantage) attaches between the boom and the traveler and acts generally as the accelerator in a sail boat. Pull the boom in closer (tighter), the boat goes faster.
The traveler can be (if it's working) set out toward either side along the rear spar. When running down wind (wind at your back) this allows the boom and sail to swing wider to capture more air. When pointing across the wind (perpendicular to it, i.e. sailing N. with an E. wind) the traveler is generally pulled in closer to the center line of the boat. And for most purposes leaving the traveler in the center position will do you just fine.
The boom is pushed out to the leeward side of the boat when underway. In a traditional turn (coming about) into the wind, it swings to the opposite side of the boat pretty gently like a weather vane with the mast at the leading edge. If the boat is turned down wind (called a jibe, and generally avoided in heavy wind) there is a point where the wind will switch from one side of the sail to the other. Depending on how far the main sheet is out at the time, the swing of the boom can be fast and furious. Knock you out, knock you into the drink.
I'm sure you will have a fun time with your BIL. You can quickly get out in more exciting wind conditions.