How Do Tides Affect Boats?

Of all the variables that affect boating, one of the greatest is the tide.

There are obvious impacts such as the amount of water that you can expect to find under your keel, but there are also much more subtle impacts such as which way you can expect to lay when at anchor.

Let’s take a look at them all in turn.

Under-keel Clearance

Under-keel clearance is the distance between the keel of your boat and the seabed.

In deep water, under-keel clearance is not really an issue because you know that there is always plenty of water underneath you. As you approach land, however, it will start to reduce.

Sometimes, your under-keel clearance will reduce to such an extent that you need to consider the amount of tide as well.

This is particularly common in small boat harbours, that might even dry out completely at low tide.

You’ll need to check the tide for the day that you are boating, and then remember the time of high water so that you can safely navigate the shallow harbour.

If you get it wrong, and the tide drops away too far, you could be left high and dry.

Currents (& Their Impact on Manoeuvring)

Predominantly caused by the tide, currents are the flow of water from one area to another.

Along an open coastline, the impact of the tide on currents can be minimal. When you get to an area where the flow of water is restricted, however, it can be very different.

For example, in the Pentland Firth, the tide causes currents in excess of 15kn.

Any boats trying to navigate through a current need to take account of the movement of the water as well as the boat’s movement.

Most places will not experience tidal flows anything like those in the Pentland Firth, but even a small current have a big impact.

If you are manoeuvring onto a berth, you will be travelling very slowly.

It only takes a small current to change your approach vector, making things go wrong very quickly.

If you are handling a boat in a tidal area, make sure you know the state of the tide and the impact that it has on the local currents.

If you are unsure, just remember that after high water, the currents will generally be flowing out of a harbour.

Slack Water

To avoid currents completely, you should consider going in your boat at slack water.

Usually occurring at high water and low water, slack tide is often considered the best tide for boating.

The reason that slack water is considered the best is that all the effects of the tide are minimal. Currents are eliminated, fewer waves and generated, and less force is placed on moorings and anchor cables.

To determine when slack water will be, you can look at the tidal curve for your local area.

Tidal curve in an area with a large tidal range
Sample tidal curve.

Slack water occurs when the tidal curve is the flattest, which usually happens at either low tide or high tide.

I have written a complete article about slack water, which you can find here: When Is Slack Tide?

Which Way Will Your Boats Face?

When at anchor, or moored using a rope over the bow, a boat will face into the tide.

The reason that a boat has its anchor on the bow is that it keeps it facing into the flow of water.

Boats are designed to move forward, with the least resistance to the water when moving in that direction.

By anchoring over the bow, the boat settles in a position where the least amount of force is placed on the anchor cable, reducing its chance of dragging.

If you tried to drop the anchor over the side of the boat instead, it would tend to settle side-on to the current, applying a massive force to the anchor cable, increasing the chance of pulling it out of the seabed.

Generation of Waves

The ocean is subject to different forces, all pulling and pushing in different directions. Aside from the tide itself, the next biggest force is the wind.

As the wind can’t drag the entire oceanic body in the same way that the tide does, instead it acts only on the surface, resulting in waves.

Over a relatively short area, wind waves form in only the top part of the sea. Across a greater distance, the wind has a greater effect, generating a swell instead.

When waves generated by the wind interact with the tide, you can get some interesting effects depending on the relative directions.

Wind against tide

With wind waves running in the opposite direction to the tidal flow, you get a “wind against tide” situation.

When the wind is set against the tide, waves become steeper, often breaking at the top.

It is a similar effect to the waves breaking as they run up a beach. The tide is creating drag against the bottom of the wave in the same way as a shallowing seabed does.

Knowing that steeper, breaking waves are likely to form when the wind runs against the tide means that you can account for it more accurately when planning your boating trips.

You can time your passing of a critical point to coincide with the wind and tide in a favourable direction instead.

Wind with tide

With wind waves running in the same direction as the tidal flow, you get a “wind with tide” situation.

When the wind is set with the tide, waves become longer and shallower, leading to a more comfortable boating experience.

The effect of spring tides and neap tides on boating

Although they do not have a direct effect, spring tides and neap tides will either magnify or reduce the tidal effects that we have already discussed.

During a spring tide, currents will generally be stronger, leading to rougher waves generated against the wind and additional strain being placed on anchor cables.

Conversely, during a neap tide, currents will be reduced, with all other corresponding effects reduced accordingly.

You can read up more about spring and neap tides in my article: What Is The Difference Between Spring & Neap Tides?