Question 1

On the bridge, you have a wet and dry bulb thermometer (hygrometer) mounted in a Stevenson screen. What is this instrument used for, and why is the reading significant to an OOW on watch?

Accepted Answer

It measures the wet-bulb and dry-bulb air temperatures; the difference between the two (the "depression") is used, together with tables, to determine the relative humidity and dew point of the air, which helps the OOW assess the likelihood of fog forming (fog is more likely when the air temperature is close to the dew point) and the general state of the atmosphere A wet and dry bulb thermometer (hygrometer) consists of two thermometers: the "dry bulb" reads the ordinary air temperature, while the "wet bulb" has its bulb covered by a wetted wick, so evaporative cooling lowers its reading below the dry bulb temperature (the lower the humidity, the greater this "depression"). Comparing the two readings against standard psychrometric tables allows the relative humidity and the dew point temperature to be determined. The Stevenson screen is a louvred, white-painted box that houses the instrument, shielding it from direct solar radiation and precipitation while allowing free air circulation, so the readings represent the true ambient air condition rather than being skewed by the screen itself. For an OOW, the dew point is particularly important: as the air temperature approaches the dew point, the relative humidity approaches 100% and fog or condensation becomes increasingly likely, so monitoring this trend supports decisions such as posting an additional lookout, reducing speed, or switching on navigation lights and sound signals for restricted visibility in good time.

Question 2

You're preparing a passage plan for a coastal leg in a yacht drawing 3.2m. At one point the chart shows a least depth of 3.8m at chart datum. What MUST you check before finalising this leg?

Accepted Answer

The tidal height at the time of transit to confirm sufficient under-keel clearance Charted depths are referenced to chart datum (often LAT), so you must add the predicted tidal height for your transit time to get the actual depth, then compare to your draft plus a safety margin to confirm UKC.

Question 3

During your watch the ECDIS gives a "safety contour crossing" alarm. What is the correct response?

Accepted Answer

Assess the situation against the planned route and charted depths, take action to maintain safe water, and inform the Master if there is any doubt A safety contour alarm indicates the vessel may be approaching water shallower than the set safety contour. It must be investigated against the route and charted information, with avoiding action taken and the Master informed as appropriate — never simply silenced.