Fixed Docks, Pilings and Tidal Range

Prerequisites

• The names of the dock lines (bow, stern, forward and after springs) and the basics of coming alongside. These are in Docking Under Power (ASA 118).
• Cleats, fenders and making fast, from Mooring, Anchoring, and Berthing.
• Reading a NOAA tide prediction for a time and height. See NOAA Charts, Tides & Coast Pilot.

Learning Objectives

This lesson covers parts of the ASA 104 Bareboat Cruising standard (tying to a fixed dock in a big tidal range, the Bahamian moor, rafting, engine failure) and ASA 118 Docking Endorsement (fixed vs floating docks, bow thrusters). By the end of it you will be able to:

• Explain why a fixed dock needs different lines from a floating one.
• Use NOAA tide predictions to find how far the boat will rise and fall.
• Set long spring lines so the boat stays safe through a full tide.
• Tie up to pilings, alone or in a slip, and protect lines from chafe.
• Describe a Bahamian moor and how to raft up safely.
• Plan for docking or leaving with no engine, and use a bow thruster sensibly.

Floating Docks vs Fixed Docks

A floating dock rises and falls with the tide. Your boat and the dock move together, so once your lines are right they stay right. Most modern marinas use them.

A fixed dock stands on pilings driven into the bottom. It does not move. Your boat does. Fixed docks are common on older waterfronts, fuel docks, town landings, and anywhere ice or storms make floating docks hard to keep.

The difference matters most when the tide changes:

A boat tied with short, tight lines to a fixed dock at high water will be left hanging from them as the tide falls. Cleats can tear out and the boat can heel against the dock. At low water, tight lines will hold the boat down as the tide rises.

How Big Is the Tide Here?

Before tying to a fixed dock, find the range of tide: the difference between high and low water. Get it from NOAA Tides & Currents for the nearest tide station, and look at the predictions for the time you will be there, not just the average.

US ranges vary enormously. These are NOAA mean ranges (mean high water minus mean low water):

Two things to remember:

• These are averages. Spring tides, around new and full moon, have a bigger range than the mean. Use the actual predictions for your dates.
• Wind and pressure change the real level. A strong onshore wind or a storm surge can lift the water well above the prediction.

In Key West a fixed dock behaves almost like a floating one. In Down East Maine it needs careful planning.

Lines for a Big Tidal Range

The trick is to use long lines at shallow angles. A long line changes its length very little when the boat goes up or down. A short line changes a lot.

Here is the geometry. Tie up at mid-tide and the boat will rise or fall about half the range either way. At Eastport, with a mean range of about 18.4 ft, that is roughly 9 ft each way.

• A spring line 40 ft long, running fore and aft along the dock: when the boat drops 9 ft, the line needs to be the square root of (40 squared plus 9 squared) long, which is about 41 ft. Only 1 ft of slack covers it.
• A breast line running straight across 8 ft of gap: the same 9 ft drop needs the square root of (8 squared plus 9 squared), about 12 ft. That is 4 ft more, half again its length.

So in big tides:

1. Rely on long springs. Lead the forward spring from the bow well aft to the dock, and the after spring from the stern well forward. They do most of the work of holding the boat in position.
2. Make bow and stern lines long too. Lead them well ahead and astern to a far cleat or piling, not straight across.
3. Avoid breast lines, or leave them slack enough for the whole range. Take them off if you are leaving the boat.
4. Set the lines at mid-tide if you can, then check them at high and low water. If you arrive at high water, leave the slack you will need for the fall.

Fenders on a fixed dock

Fenders on a fixed dock hang against pilings, not a flat face, and they ride up and down as the boat moves. A fender board (a plank hung outside two fenders) bridges between pilings and keeps the fenders from rolling off. Hang fenders so they still protect the hull at both high and low water.

Pilings and Slips

Tying to a piling

A piling is round, and lines slide down it as the tide falls. Ways to deal with it:

• Use a clove hitch or round turn and two half hitches at a height that suits mid-tide, and expect it to slide. On a big range, lead the line to a cleat on the dock instead.
• Drop a loop over the top of a piling where the line can slide freely. This only works if the piling is tall enough that the line can never ride over the top at high water.
• Make the line long enough to reach from the boat to the piling at any state of tide.

Slips with pilings

Many US marinas, especially in the Chesapeake and the South, have slips with a fixed finger pier on one side and pilings at the outer end. Typical setup:

• Bow (or stern) lines go to the dock at the inner end.
• Lines from the outer quarters or bow go to the outer pilings, often crossed so each runs to the far side. Crossing them makes them longer, which helps with tide.
• Springs run along the finger pier.

Pick up the outer piling lines first as you come in, so the boat cannot be blown onto the next slip. Have a crew member ready with a pre-made loop and a boathook. Backing into a slip is covered in Docking Under Power.

Chafe

Chafe destroys lines quietly. Wherever a line crosses a rough dock edge, a piling or a fairlead, it wears with every wave and every tide.

• Slip chafe guards (lengths of hose or purpose-made sleeves) over the line where it touches.
• Check rough, splintered or barnacled pilings.
• Re-check after a blow. A line can wear right through in one stormy night.
• Use nylon for dock lines: its stretch absorbs shock loads.

The Bahamian Moor

A Bahamian moor uses two anchors from the bow, laid roughly 180 degrees apart along the line of the current. As the tide turns, the boat swings around a small circle instead of sweeping across the whole anchorage.

It is useful in narrow channels and rivers with reversing currents, or crowded anchorages where swinging room is tight. Its weakness is a wind or current from the side, which loads both anchors at an awkward angle. The full method, and other two-anchor setups, are in Anchoring in Challenging Conditions.

Rafting Up

Rafting means tying boats side by side, either at anchor or to a dock. It is a sociable part of cruising, but it needs care:

• Biggest boat anchors. The heaviest boat sets a good anchor and the others come alongside it. In a big raft, extra anchors may be set from the outer boats.
• Fenders and lines. Plenty of fenders between boats, plus bow and stern lines and springs on each pair.
• Stagger the masts. Line boats up so their masts and spreaders are not side by side. When boats roll, spreaders can tangle and break.
• Courtesy. Cross another boat by its foredeck, not through its cockpit, and ask first.
• Have an exit plan. If the wind rises or swings, the raft loads its anchor heavily. Agree in advance who leaves first and how, and break up before dark if the forecast is poor.

No Engine: Docking and Leaving

The ASA 104 standard asks you to think about engine failure in a busy channel or a crowded anchorage. A few principles:

Get out of danger first. If the engine dies in a channel, keep steerage way if you can, get out of the channel, and have an anchor ready to drop. Call for help on VHF early if you are drifting towards trouble.

Docking under sail. Choose a berth you can approach with the wind on the beam or forward of it, so you can let the sails flap to slow down. Approach slowly, under a small sail if possible, and plan a way out. If the wind blows you onto the dock, you can drop sail well upwind and drift down onto the dock with fenders out.

Use your dinghy as a tug. A dinghy with an outboard tied alongside the boat (a hip tow) can push it slowly into a berth.

Warping. Run a long line from the boat to the dock or a piling and pull the boat in by hand. To leave, run a line to a piling or another boat and pull yourself out.

Leaving without an engine. If the wind blows off the dock, you may be able to simply let go and drift clear before raising sail. If it blows onto the dock, warp off, use the anchor or get a tow.

Bow Thruster Basics

A bow thruster is a small propeller in a tunnel through the bow, low on the hull. It pushes the bow sideways without the boat moving ahead.

• Use it in short bursts. Many electric thrusters are made for brief use only and can overheat or flatten the batteries if run continuously. Check the manual for your boat.
• It works best at low speed. Once the boat is moving, the water flow past the tunnel makes it much less effective.
• Strong wind can beat it. On a windy day the bow may blow off despite the thruster.
• Remember the pivot point. Pushing the bow sideways swings the stern the opposite way.
• Practise docking without it. Thrusters fail, and many boats do not have one.

Worked Example

You arrive at a fixed town dock in Portland, Maine, at 1400, about half-tide on a rising tide. Suppose the NOAA prediction for the day shows high water of 10.3 ft at 1700 and low water of 0.2 ft at 2310 (made-up figures for this example). The dock edge is rough and the pilings are 12 ft apart.

1. Range. 10.3 minus 0.2 is 10.1 ft. From now to high water the boat rises about 5 ft; it then falls about 10 ft to low water.
2. Lines. Rig a long forward spring from the bow to a cleat or piling about 30 ft aft, and an after spring from the stern to one about 30 ft forward. Lead bow and stern lines well ahead and astern.
3. Slack. Leave just enough slack for the next 5 ft rise, then plan to check at 1700. At high water, let out enough to cover the 10 ft fall before you go to bed.
4. Fenders. Hang a fender board between two fenders to bridge the pilings. Check the fenders will still be against the board at low water.
5. Chafe. Fit chafe guards where every line crosses the dock edge.
6. Leaving. At 2310 the boat will be about 10 ft below dock level. Know where the ladder is.

Common Mistakes

• Short lines at a fixed dock. The boat hangs from them at low water, or is pulled down at high water.
• Relying on breast lines in big tides. Their length changes most.
• Using average range instead of the day's prediction. Spring tides and onshore winds give more.
• Fenders that float up or drop off the pilings. Use a fender board.
• Ignoring chafe. Unprotected lines on rough pilings can part overnight.
• Holding the thruster on. Short bursts only.

Summary

• Floating docks move with the tide; fixed docks do not, so your lines must allow for the full range.
• Get the range from NOAA predictions for your dates. It runs from about 18 ft at Eastport to just over 1 ft at Key West.
• Long springs at shallow angles change length very little. Short breast lines change a lot.
• On pilings, expect lines to slide, use long lines and a fender board, and stop chafe.
• A Bahamian moor limits swinging with two anchors laid along the current.
• Raft up behind the biggest boat's anchor, stagger the masts and agree an exit plan.
• Without an engine, get clear of danger, then sail, tow with the dinghy or warp.
• Use a bow thruster in short bursts at low speed, and practise without it.

Check Your Understanding

1. Why does a fixed dock need longer lines than a floating dock? The boat rises and falls with the tide but the dock does not, so lines must allow for the whole range.
2. Where do you find the tidal range for your dates in US waters? In the NOAA tide predictions for the nearest station on Tides & Currents.
3. Why are long spring lines better than breast lines in a big tide? A long line at a shallow angle changes length very little as the boat goes up and down; a short breast line changes a lot.
4. What is a Bahamian moor used for? Limiting swinging in a narrow channel or crowded anchorage with reversing current, using two bow anchors laid about 180 degrees apart along the current.
5. Give two rules for using a bow thruster. Use short bursts, and use it at low speed. Also: expect strong wind to overpower it, and practise without it.

Sources

• https://tidesandcurrents.noaa.gov/stationhome.html?id=8410140
• https://tidesandcurrents.noaa.gov/stationhome.html?id=8418150
• https://tidesandcurrents.noaa.gov/stationhome.html?id=8665530
• https://tidesandcurrents.noaa.gov/stationhome.html?id=8724580
• https://tidesandcurrents.noaa.gov/datum_options.html
• https://tidesandcurrents.noaa.gov/tide_predictions.html