Passage Planning with Cold Front and Limited Visibility
You are planning a 40 nm coastal passage from Port Elizabeth to Knysna along the South African coast. Your vessel is a 10 m sailing yacht with an auxiliary engine, cruising speed 6 knots under power, 5 knots under sail. The forecast is for a cold front to pass in the afternoon, bringing SW 25-30 knots and moderate to rough seas, with visibility reducing to 2 nm in rain. You have paper charts, a hand-bearing compass, a GPS, and a depth sounder. The coastline is rocky with few safe havens. You plan to depart at 06:00 to arrive before dark. The chart shows a prominent headland at 20 nm from Port Elizabeth, and the approach to Knysna requires crossing a shallow bar with a depth of 3 m at CD; your draught is 1.5 m. The tide at Knysna: HW at 14:00, height 1.5 m; LW at 20:00, height 0.5 m. Use the rule of twelfths for tidal heights.
1. Assess the feasibility of the passage given the weather forecast and vessel performance. What are the key risks?
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The passage is 40 nm. At 6 knots, it will take about 6.7 hours, arriving at 12:40. However, with a cold front bringing SW 25-30 knots, you will be heading into or across the wind and sea, which will reduce your speed and make conditions uncomfortable. The risk of heavy weather, poor visibility (2 nm in rain), and the need to cross the Knysna bar are significant. The bar crossing requires a tide height of at least 1.5 m (your draught) plus under-keel clearance of say 0.5 m, so you need a depth of 2.0 m over the bar. Charted depth is 3 m at CD, so even at LW (0.5 m) the depth is 3.5 m, which is sufficient. But the bar can be dangerous in rough seas. The main risks are: (1) encountering the cold front before reaching Knysna, with strong SW winds and rough seas; (2) reduced visibility making navigation and hazard avoidance difficult; (3) fatigue and seasickness; (4) the bar crossing in deteriorating conditions. You should consider delaying departure until the front passes, or diverting to a closer safe haven (e.g., Plettenberg Bay) if conditions worsen. Monitor the forecast closely and have a plan B.
2. Calculate the tidal height at Knysna at 16:00 and determine if you can cross the bar at that time with a minimum under-keel clearance of 0.5 m.
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Tide at Knysna: HW at 14:00, height 1.5 m; LW at 20:00, height 0.5 m. Range = 1.0 m. At 16:00, it is 2 hours after HW. The duration of fall is 6 hours. Using the rule of twelfths: first hour: 1/12 of range = 0.083 m; second hour: 2/12 = 0.167 m; total fall after 2 hours = 3/12 = 0.25 m. Height at 16:00 = 1.5 - 0.25 = 1.25 m. Charted depth at bar = 3.0 m. Actual depth = 3.0 + 1.25 = 4.25 m. Your draught = 1.5 m. Under-keel clearance = 4.25 - 1.5 = 2.75 m, which is well above the required 0.5 m. So you can safely cross the bar at 16:00 from a depth perspective. However, you must also consider sea conditions: with SW 25-30 knots and rough seas, the bar may be breaking. You should assess the actual conditions on arrival and be prepared to abort if it is unsafe.