Bridge clearance (BC) is the distance from a boat's waterline to the highest point on the topsides. It indicates the height of a bridge under which a boat can pass. Bridge clearance is listed on marine charts, with the height above mean high water given. To estimate the bridge clearance for a Kittiwake 23 sailboat you need to calculate the "air draft", the distance from the surface of the water to the highest point on the vessel, by getting its "I" dimension, distance down the front of mast to the level of the main deck (the tricky bit is where is the main deck on a Kittiwake 23, is in in front of the mast?), is 27.5 ft per SailboatData.com - KITTIWAKE 23 Sailboat , then drop a line from the deck to the waterline and measure that dimension, and lets assume it is 3 ft, then add 1 ft at the top of the mast for a wind vane, you arrive at an estimated bridge clearance of 27.5 + 3.0 + 1.0 = 31.50 ft for air draft. Then get the bridge clearance from the chart and subtract air draft to arrive at the vessel's clearance, i.e., distance in excess of the air draft which allows a vessel to pass safely under a bridge or obstacle such as power lines, etc.
Welcome Dave to the SBO Forum... @sail sfbay did a great job of clarifying the way your measure the boat to get the boats "Air Draft".
Air Draft and an understanding of the tides in the area of the bridge is the second element of the equation. Navigations skills and the charts give you the Bridge clearance. If you know your tide level and are at low tide you get bonus space to slip your boat through under the bridge.
An additional element to consider is weather. Is the wind blowing from a direction that will pile up the water under the bridge? Wind blowing in the direction of the tide from a large body of water (i.e. Bay) can push water up under a bridge modifying the water level from the tide level.
Oh the fun of playing on boats. So many fun things to learn.