Couple of daft questions about tides which I should probably know the answer to by now, but this is the fist time I have come across these particular scenarios.

All the textbooks and examples we were ever given give nice numbers in between MLWN and MLWS which is nice and straightforward and easy to calculate. Unfortunately that doesn?t help when you get given something like this where you have no examples to refer back to and none of the books seem to cover it. Trying to work out the heights of tides for arrival and departure at Tanjung Pelepas.

First question is regarding the arrival at 0500 on the 17th of November. The standard port is Singapore. The values are as follows:

HW 0148 3.0m
LW 0711 1.5m
Seasonal correction +0.1m

Tanjung Pelepas changes
MHWS 2.8 +0.3
MHWN 2.2 +0.1
MLWN 1.2 +0.2
MLWS 0.6 -0.1
Seasonal correction +0.1

So when you take into account the seasonal change for Singapore then the mean levels you are interpolating for to get the secondary port corrections are HW 2.9m and LW 1.4m. HW is not a problem as it is just a straightforward case of extrapolation but I am unsure about how to approach the LW interpolation.

Should I interpolate between the values for MHWN and MLWN (as this is between 2.2m and 1.2m, which is where 1.5m lies)?

Or extrapolate out for the values of MLWN and MLWS (because these are the low water values)?

I have tried both and they give slightly different answers so I am not sure which is the correct way to go about it?


Second question is the departure:

Height of tide at Tanjung Pelepas 2300LT 17th Nov 2012. The standard port is Singapore and the following values apply:

LW 17th 1935 0.2m
HW 18th 0241 2.9m
Seasonal correction +0.1m

Tanjung Pelepas changes


HW -0010
LW -0010

MHWS 2.8 +0.3
MHWN 2.2 + 0.1
MLWN 1.2 + 0.2
MLWS 0.6 - 0.1

Now obviously we need to interpolate to find the times for HW & LW at Tanjung Pelepas. After applying the seasonal change for the standard port of 0.1m the mean levels that we need to interpolate to are 2.8m for HW (which is fine because it corresponds exactly to the correction of +0.3 for MHWS) and 0.1m for LW. The LW is causing problems because it lies outside the given values for MLWN (1.2) and MLWS (0.6). When I try to extrapolate this to get the correction I am getting the following:

[ (0.2-(-0.1)) / (1.2/06) ] x (0.6-0.1) = 0.25 => -0.1-0.25 = -0.35m

Applying this:
Standard port LW: 0.2m
Seasonal change: -0.1m
Standard port mean level: 0.1m
Seasonal change: + 0.1m
Correction: - 0.35m
Secondary port prediction: -0.15m

Correct me if I?m wrong, but I don?t think I should really be getting negative values for tides, should I?

So either I have made a mistake somewhere or this tide can?t be calculated using this method.

Anyone able to give me some guidance on these? I have asked the officers on my ship but they don?t know how to do it either.