How to Work Out Tidal Stream Rate from a Tidal Diamond

Read the spring and neap rates for the right hour from the tidal diamond table, then interpolate using today's range. With rates of 2.4 knots at springs and 1.2 at neaps, and a range 88% of the way to springs, the stream runs at about 2.3 knots.

The question

The table for tidal diamond A gives, at HW+2: direction 075°T, spring rate 2.4 kn, neap rate 1.2 kn. Today's range at the standard port is 4.4 m; mean ranges are 4.7 m (springs) and 2.2 m (neaps). What is the stream at HW+2?

Working, step by step

1. Interpolation factor = (4.4 m − 2.2 m) ÷ (4.7 m − 2.2 m) = 0.88.
2. Difference between spring and neap rates = 2.4 − 1.2 = 1.2 kn.
3. Rate = neap rate + factor × difference = 1.2 + 0.88 × 1.2 = 2.26 kn.
4. The direction comes straight from the table: 075°T, the direction the stream flows towards.

The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.

Try it yourself

Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.

Learn the method

• Course to steerGuide
• Spring and neap tidesGuide

More worked tide examples

• Height of tide at a given time
• Time the tide reaches a height
• Depth from charted depth
• Under-keel clearance
• Crossing a drying height
• Bridge clearance
• Anchoring depth and scope
• Springs or neaps from the range
• Secondary port: time of HW
• Secondary port: height of HW

Frequently asked questions