One end fixed, one end floating
Everything awkward about a gangway comes from that sentence. The hinge is on the pier at a fixed elevation. The bottom lands on a float, and the float rides on the water, so the drop the ramp has to cover is the pier deck minus the water level minus the freeboard of the float. When the water goes down, the drop goes up, and since the gangway is a fixed length, the only thing that can absorb it is the angle.
The defaults on this page: a pier deck at eight feet, a float with 1.2 feet of freeboard, water swinging between one and five. At high water the drop is 8 minus 5 minus 1.2, or 1.8 feet. At low water it is 8 minus 1 minus 1.2, or 5.8 feet. The same ramp, the same site, and the rise it covers has more than tripled.
The slope, and why the quoted number is usually the flattering one
On a twenty-four foot gangway the drop is the vertical side of a right triangle whose hypotenuse is the ramp. At high water: run equals the square root of 24 squared minus 1.8 squared, which is 23.93 feet, so the slope is 1.8 over 23.93 — 7.5 percent, or one in 13.3, or four and a quarter degrees. Nobody would blink at that.
At low water: run is the square root of 576 minus 33.64, which is 23.29 feet, and 5.8 over 23.29 is 24.9 percent, one in 4.0, just under fourteen degrees. That is a steep, wet, cleated slope that nothing rolls up, and it is the same ramp on the same day of the year that the seven and a half percent figure came from.
Run the target the other way and the size of the problem is obvious. Holding one in twelve over a 5.8 foot rise needs a run of 69.6 feet and a ramp of 5.8 times the square root of 145, which is 69.8 feet. Against the twenty-four foot gangway that fits on the site, that is the whole design problem of gangways in one comparison, and it is why long gangways, intermediate floats and multi-section arrangements exist.
The bottom end slides, and by how much
Since the ramp length is fixed and the rise changes, the horizontal reach changes too. In the example the run is 23.93 feet at high water and 23.29 at low, so the toe of the gangway travels about 7.7 inches along the float deck between the two stages. That is why the bottom end sits on rollers or a wheeled shoe instead of being pinned, and it is why anything bolted to the float in that band — a cleat, a bumper, a hose, a power pedestal — ends up in the way.
Note that the travel is small compared with the change in angle. Steepening a ramp costs a lot of slope and very little length, because the ramp is mostly horizontal and the trigonometry is unkind at the flat end. That is also why adding two feet to a short gangway does much less for the low water slope than people expect.
Freeboard is not a constant either
The float deck sits above the water by whatever its own loading leaves it, and that changes as people walk onto it. A section that reads fourteen inches of freeboard empty may read eight with a crowd on it, and those six inches are added directly to the rise the gangway covers, on top of whatever the water is doing. Working the numbers at the empty freeboard gives the friendly answer twice over. The freeboard at load comes out of the dock float calculator.
Anything built in, on or over water is permitted work in most places, and the permission usually comes from more than one desk. The state environmental or natural resources agency, the Army Corps of Engineers district for the waterway, the local building department, and on many lakes the utility or authority that holds the shoreline licence all have a say, and what each one wants is different from one state and one waterbody to the next. None of it is on this page. Start with the state agency and the Corps district for the water you are on, and get a marine contractor who works that lake or that stretch of river involved before the arithmetic turns into an order.
Questions people ask
How long should a dock gangway be?
Long enough to hold the slope you are designing to at the worst water level, which is almost always low water. The arithmetic is short: take the rise at that stage, which is the pier deck elevation minus the water level minus the float freeboard, then multiply it by the square root of one plus the square of your slope ratio. A 5.8 foot rise at one in twelve gives 5.8 times the square root of 145, which is 69.8 feet of ramp. What slope figure applies to your project comes from the standard your jurisdiction has adopted or from whoever specified the gangway, and this page does not supply one.
Why is my gangway so steep at low water?
Because the rise it covers is the pier deck minus the water level minus the float freeboard, and the water level is the only term that moves. Take a pier deck at eight feet, a float with 1.2 feet of freeboard, and water that runs between one and five feet on the same datum: the drop is 1.8 feet at high water and 5.8 at low, more than three times as much, over a ramp whose length never changed. On a twenty-four foot gangway that is the difference between 7.5 percent and 24.9 percent. The high water figure is the one people quote and the low water figure is the one that decides whether the ramp is usable.
Does the bottom of the gangway move, and how much?
Yes, and that is by design. The ramp is a fixed length with a fixed hinge, so when the rise increases the horizontal reach shortens and the bottom end slides toward the pier along the float deck. On a twenty-four foot gangway over a four foot water level range the travel works out at roughly eight inches. It is why the float end rides on rollers or a wheeled shoe rather than being pinned to the float, and why the band of deck it sweeps has to stay clear of cleats, bumpers, hoses and pedestals.
Should I use the float freeboard empty or loaded?
Loaded, if you want the honest answer, because the freeboard goes straight into the rise. A floating section rides higher with nobody on it and settles when people walk aboard, and every inch it settles is an inch added to the drop the gangway has to cover at that moment. A section that reads fourteen inches empty and eight with a crowd has moved the gangway rise by half a foot on its own, and the moment the ramp is steepest is the moment there are people on the float. The loaded freeboard comes out of a buoyancy calculation rather than a tape measure on a quiet morning.
Is a gangway slope regulated?
Frequently, and not by anything this page could tell you. Slope limits for a walkway, an accessible route or a marine gangway come from the standard your project is held to, which varies by jurisdiction, by whether the facility is public, and by what the structure is classed as, and marine gangways are often treated differently from ramps on land precisely because the lower end moves. Take the figure from the authority having jurisdiction or from whoever specified the gangway, enter it on this page, and let the arithmetic tell you what length it implies. The page reports differences between numbers you supplied and issues no compliance verdict.