Here is a link for Fletcher (if he is interested)
https://en.wikipedia.org/wiki/Ocean_pow ... idal_power
From the above link:
Tides are controlled mainly by the gravitational pull of the moon. About once a day the moon rotates about the earth, attracting as it travels the bulge of water called the high tide that also travels round the earth. There are actually two high tides, because the earth and moon, as a system, both rotate about a common centre of mass. This centre is two-thirds out from the centre of the earth, not at the centre of the earth. The effect of the earth spinning about this centre is that it behaves as a centrifuge, resulting in a second high tide bulge in the ocean most distant from the moon.[5]
A second influence on the tides occurs because of gravitation from the sun. Gravitation from the sun has less influence than the moon, because it is so much further from earth. However, the sun influences the tidal range. When the sun, earth and moon are aligned in a straight line (at new and full moon), their tidal effects combine, producing the particularly high and low tides called spring tides. When the sun is at right angles to the moon, the effects are partially cancelled, producing the small tides called neap tides.[5]
New Zealand has a relatively small tidal range, usually less than two meters. However, some of the larger harbors on the west coast of the North Island, in particular the Kaipara, experience significant currents as the tides rise and fall.
Altogether there are sixty-two recognized natural influences on the tides, though only some will be significant at a given location.
The gravitation of the moon and sun are the most important.
