Information for Crossing Cook Strait
Cook Strait is a natural wind funnel bordered by high geographic features on the North and South Islands, and at times creates a formidable gauntlet for boats sailing to and from the South Island. The following information is provided to help you cross the Strait safely.
Wind direction in Cook Strait is normally NW or S to SE. Storm force winds occur on average about 25 times a year. Gale force NW winds are usually very localised, but S gales affect the whole Strait. SE gales are more pronounced on the western side of the Strait.
Tidal streams in Cook Strait along the South Coast of the North Island generally flow towards the NW for 5 hours and SE for 7 hours. However, they are unreliable and the surface currents are affected by prolonged periods of strong winds and the shoreline configuration which can cause counter currents if close inshore.
High water occurs at Tory Channel about 4 hours later than Wellington at spring tides and slightly less at neap tides. This difference causes the strong streams. The streams are stronger during spring tides, which occur at two weekly intervals, when high water Wellington is about 0600 and 1800.
Tidal stream predications for Tory Channel have generally been found to be very accurate.
Broken water is experienced in different parts of the Strait. This is caused by the cold north-setting bottom current being forced to the surface on meeting the steep slopes of submarine canyons, which will also cause modification of the tidal stream. Broken water is also caused when the wind flow is against the tidal stream, especially between Sinclair Head and Cape Terawhiti, off the eastern entrance of Tory Channel, and in the vicinity of the Brothers Islands.
Fog is rarely found in the Cook Strait area as the wind funnel effect sweeps it away. Late summer is usually the only time of year fog is experienced. Low cloud and rain though can severely restrict visibility.
Karori Rock stands in the centre of a considerable area of underwater hazards. The rock is approximately half a mile offshore and surmounted by a lighthouse. A sectored light is on the Southern end of Tongue Point and shows red to the south east, covering various hazards including Thoms Rock, approximately 1 mile offshore ,and 1 mile southeast of Karori Rock. Thoms Rock has about 1.5 meters of water over it. About one mile northwest of Karori Rock is Luna Rock which has 1.8 meters of water over it. Luna Rock is close to the shore and not covered by the red sector of Karori Light.
The only offshore hazard in the middle of Cook Strait is Fisherman's Rock. Lying almost midway between Cape Koamaru and Mana Island it has a clearance of 10 metres. However the area surrounding the rock can be hazardous in strong tide or wind conditions. Large overfalls and seas can develop in these conditions.
The combination of broken water and strong wind gusts between Sinclair Head and Cape Terawhiti make it advisable to transit this section of the passage (commonly known as The Karori Rip) at slack water. This section of your voyage is generally the most uncomfortable and should not be attempted by small craft or the inexperienced except in favourable conditions. Strong southerly winds with a SE setting tidal stream can make this area a real horror show, and in the interests of safe boating and harmonious crew relations, should be avoided. The times given are assuming a speed to 6-7 knots.
Leave Wellington to arrive at Sinclair Head 1 hour before HW Wellington. You will then have a favourable tidal stream across Cook Strait. Seaview to Sinclair Head is approx 12 nautical miles.
Alternatively, leave Wellington to arrive at Sinclair Head 1 hour before LW Wellington. You will then have an adverse tidal stream across Cook Strait, but this may be preferable in northerly winds. Tory Channel will be reached at slack water.
Leave Tory Channel 2 hours after the tidal stream commences setting E. You will have a favourable tidal stream across Cook Strait - Cape Terawhiti - Sinclair Head area.
Alternatively, in southerly winds leave Tory Channel as the tidal stream starts setting W. You will have an adverse tidal stream across Cook Strait, but this may be preferable in southerly winds. Cape Terawhiti-Sinclair Head area will be reached at slack water.
Tory Channel Eastern Entrance
Ships make frequent use of Tory Channel and you must keep clear of the mid-channel at the eastern entrance when large ships are present. All vessels transiting the entrance are required to give a warning of this on VHF Channel 18 (Port Operations Channel). The instructions for Tory Channel radio broadcast were significantly changed in September 2010. Notice to Mariners NZ160(P)/10 outlines the reporting procedure that should be followed.
Whether a warning is heard or not, the entrance should be navigated with caution. Ferry and shipping timetables are subject to seasonal changes, and skippers are reminded that they must keep clear of all shipping lanes at all times.
Ships maintain a listening watch on the following frequencies:
Within Wellington Harbour limits: VHF Channel 14
Within Marlborough Harbour limits: VHF Channel 18
Cook Strait: VHF Channel 16
This is situated between Karori Rock and Sinclair Head and lies 0.35 miles (650 metres) outside a line joining these points. Rock hopping in this area is not advisable.
Day: Keep Pencarrow Lighthouse visible through Sinclair Head until Karori Rock is open from Cape Koamaru.
Night: Keep Pencarrow Light visible (red or white) until Tongue Point Light changes from red to white.
Day: To pass about one mile off Sinclair Head, keep Pencarrow Lighthouse (lower) and Old Lighthouse (upper) in line.
Night: Keep in the white sector of Pencarrow Light to pass 0.7 miles off Sinclair Head.
Checking on Tidal Set
Day: Observe Karori Rock relative to Sinclair Head or Turakirae Head. Observe Tory Channel entrance relative to Mount Stokes.
Night: Observe Tory Channel Entrance Leading Lights.