What the Downstream Lock Keeper Wants to Know
You depart Allington Lock on a rising tide at 0920 and plan to reach Queenborough by early afternoon. The weather log at departure reads: Wind 2 (Light Breeze 4-6 kts), sea state Smooth, visibility Good, pressure 1018mb and steady. By the time you pass Sun Pier at Chatham, the pressure has dropped four points in two hours and the sky state has shifted from Partly Cloudy to Heavy Clouds. You are looking at a 22-mile run across the lower estuary with a building SW wind and no waypoint data showing where you were when conditions changed.
A ship's log that was filled in at departure and then abandoned until arrival does not tell you what the conditions were when the decision to continue was made. A running log does.
The Tidal Data That Changes the Plan Before You Leave
High Water at Destination and Low Water at Destination are the two fields that determine whether a passage plan is viable before the engine is started. The Medway and lower Thames run tidal systems where the difference between arriving at low water and arriving at high water can mean the difference between getting into Yalding — which requires passing under a bridge with limited air draft at anything near high water — or sitting at anchor in the reach for three hours waiting for the tide to fall.
These are not secondary planning considerations. They are the constraints around which the departure time is built. A passage plan for Allington Marina to Burnham-on-Crouch that does not reference the tidal window at Queenborough, which sits at the confluence of the Medway and the Swale, is incomplete. The log fields exist so the tidal calculation is recorded at planning time, not reconstructed from memory after the fact.
The three waypoint fields give you intermediate position logs mid-passage. A waypoint at the Medway buoy with a time stamp, a position fix at Stangate Creek entrance, and a final approach position — all with GPS coordinates — create a passage track that is usable for incident reporting if something goes wrong and for debriefing a leg that took longer than planned.
The Engine Log and the Fuel Arithmetic
Engine Hours, Trip Distance (nm), and Fuel Usage are the three fields that turn the log into maintenance and operational data. The fuel usage is calculated automatically from engine hours at a consumption rate of 7.72 litres per hour — a figure calibrated to the specific vessel. Over a season of 30 logged passages, the accumulated engine hours drive the service interval calculation. The 200-hour service for the Bukh or the Yanmar is not theoretical — it is a number that accumulates one logged trip at a time.
Fuel Remaining is calculated from Fuel Added minus the computed Fuel Usage for the trip. This is the field that prevents an embarrassing fuel stop at Chatham Marina because the last four passages were not logged and the total consumption was not tracked.
The Total Trip Time calculation — departure time subtracted from arrival time, converted to hours — gives you the passage rate in knots against the Trip Distance logged. A 22nm passage that took 3h15m at WOT minus tidal assist is 6.8 knots over ground. Compare that against the engine room's calculated hull speed and you know whether you had flood or ebb working for or against you.
The Weather Log section stands independent of the Cruise Log for a reason. Sky state, precipitation, wind speed on the Beaufort scale, wind direction, visibility category, sea state on the Douglas scale, temperature, and pressure are the meteorological record that an MAIB incident form or a damage claim with the insurers will require if a passage ends badly.