Bridge Clearance (BETA)* Tamaki Drive · Outboard Boating Club
i
LINZ data Offline copy saved
OBC bridge gap table (PDF) Bridge location

Safe passage times under the Tamaki Drive bridge using LINZ tide predictions and your boat's height above the waterline. Use at your own caution — read the conditions.

Current bridge gap — Loading tide data…
Date
—  
Time
— Departure
Select bridge span
Safety margin
0.50 m

Clearance you want left above the boat. Steps of 0.05 m; set 0 to see the bare gap. Set less than 0.50 m and anything under 0.50 m of clearance still reads as caution rather than safe — why.

Boat height saved on this device
—

Waterline to your highest point — mast, aerial, radar — with the boat unladen. Remembered next visit.

Passage windows

LINZ tide predictions · Auckland
Loading…

Next 7 days

* Beta — conditions and caveats

Check the OBC table before passage

This page is a convenience, not an authority. Always cross-check against the OBC Bridge Gap Calculation Chart before you commit to a passage. Use at your own risk.

How the tide height is worked out

LINZ publishes four tide times a day, not a continuous curve. Every height between them is interpolated using the traditional maritime Rule of Twelfths: across the interval between one tide and the next, the water moves one twelfth of the range in the first sixth of the interval, then two, three, three, two and one. The OBC chart states that it uses the same method, and this page is checked against all 407 values of that chart.

The Rule of Twelfths is an approximation of a real tide, and it assumes a smooth interval between two published points. Treat any figure between the published tide times as an estimate, and the closer you are to the margin the more that matters.

What the figures do and do not include

The gap is the span height above chart datum less the predicted tide: 6.2 m for the IN/OUT span, 6.5 m for the HIGH span. Spare is that gap less your boat height and your safety margin. Nothing here accounts for your boat's trim, its load, or anything that moves in a swell.

A window is shown as safe where the boat has the clearance described below, caution where it fits with less, and danger where it does not fit at all.

What “safe” means, and how your margin sets it

Your margin is your decision, and the page does not add to it. A moment is called safe once the clearance above your boat reaches whichever is greater: the margin you set, or 0.50 m.

So a 0.50 m margin, or any larger one, has no caution band at all — if the boat fits by your margin, it is safe. A 0.30 m margin fits at 0.30 m of clearance and is safe at 0.50 m, leaving 0.20 m of caution between the two. A margin of 0 fits at the bare gap and is safe at 0.50 m.

The 0.50 m floor exists because the tide figures are interpolated estimates, not measurements, and weather moves the water. It is a warning that you are close to the limit of what these numbers can tell you — not a second margin stacked on yours.

Weather moves the water

Predictions are astronomical. Barometric pressure, wind and storm surge routinely shift the real water level by a tenth of a metre or more, and a low pressure system raises it. Add margin in adverse conditions.

Where the data comes from

Tide predictions are published by Land Information New Zealand for the Port of Auckland. They ship with this page, so it works without a connection once loaded. Times are New Zealand local time, including daylight saving.