"at the helm" seems like common usage to me. Maybe not in the top 100 idioms, but everyone knows what you mean.
"tiller" is just the thing that controls the position of the rudder, which I think most people know about. (That said, nobody in aviation calls their rudder pedals the "tiller", so it might be a strictly nautical term.)
Yup, you're exactly right. I totally forgot about that. Thinking about it more, the rudder and tiller might be different things on ships too; I think a rudder you turn right to go right, and a tiller you turn left to go right, because of where the pivot point is?
On a boat the rudder is technically the blade that cuts through the water and the tiller is the thing that gives you enough leverage to apply force to control the direction of the rudder as it passes through the water; sometimes quite a lot of force is needed! Only quite small boats have tillers that you can directly manipulate. Larger boats typically have a wheel which controls hydraulic rams which manipulate the ‘tiller’
I hear people use "at the helm" and "change tack" to mean being in charge of something and changing direction or approach, respectively. People also "hitch themselves" to things, although I'm not sure that comes directly from the nautical usage.
A "hitch" knot is typically used to fasten around a fixed object. So you might use a hitch to tie the boat to the shore, or tie a line around a wooden (or these days aluminium or carbon fibre) boom. I'm not sure if it's purely nautical.
Boats are noisy and dangerous places at times, I wonder if the lexicon arose organically or intentionally on the ability to hear them clearly to avoid misunderstanding. Any pirates care to comment?
