snal-
I think the"notchy" feel is the little pistons that are in the front steering mechanism operating as you turn the wheel. With the system low on fluid, the response and feel are different. Air at the front end (helm) gives it that on/off feel to the compression and movement of the air/fluid combination. Solid fluid feels smooth.
So where is it going?
If you've checked everything out for leaks, it must be evaporating, or, more probably, leaking and evaporating back at the steering slave cylinders (shafts) and then evaporating. Air could enter the fill cap up front as fluid is lost out the back, causing the "notchy" feel, and causing the need for fluid replacement.
You can smell hydraulic fluid, so it has vapor pressure, and will evaporate, though slowly. You might not notice it, and some may drip overboard if your moored or in a marina.
Is the boat kept in a warm climate, in the sun, or someplace hot?
That's my guess...so far, at least!
Joe. :teeth :thup