Overnight anchoring under a thunderstorm risk on Lake Lucerne
At 17:30 in mid-August you anchor a 9.5 m, 4.5 t motor cruiser in a bay on the north shore of Lake Lucerne to swim and stay the night. Your echo sounder reads 14 m under the bow; the chart shows mud and weed with a small 6 m shoal 60 m to the north-east. You carry 40 m of 8 mm chain, 30 m of 16 mm nylon warp and a 10 kg anchor, and there is no windlass. The forecast is 5 kn southerly overnight, but the afternoon convective outlook gives a 60% chance of thunderstorms after 22:00 with gusts to 40 kn and a wind shift of about 120° to north-west. One 12 m sailing yacht is already anchored 60 m to your east. Your GPS anchor alarm is set to a 40 m radius. Your crew would prefer to stay; the nearest alternative anchorage is 40 minutes away.
1. Work out the scope you need and the swinging circle it creates. What does that tell you about staying 60 m from the other yacht?
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Scope. The standard in sheltered water with mud and weed is 5:1 on chain-and-rope mixed rode: 14 m of depth plus about 2 m of bow height gives roughly 16 m from the bow roller to the bottom, so 5:1 is around 70–80 m of rode. You carry exactly 70 m. That means all of it, with nothing in reserve to veer if the wind gets up - which is precisely the wrong configuration for a night with a 40 kn gust risk.
The swing circle. Radius ≈ rode + boat length. At 5:1 that is about 70 + 9.5 = 80 m. The yacht to the east has roughly 40 m of rode and a 12 m hull, so her circle is about 52 m. To swing without contact you need 80 + 52 = 132 m of separation, not 60 m. Even dropping to 4:1 (about 56 m of rode, 65 m radius) gives 117 m of required separation. Both are far more than 60 m.
Conclusion. The berth is wrong, not the scope. Anchoring here and hoping the wind stays south is a bet on a 60% thunderstorm risk with no spare rode and a neighbour who will swing through the same 120° shift. Options: move to a berth at least 130–150 m clear of her and preferably into 8–10 m of water where 5:1 needs only 40–50 m of rode; or take the 40 minutes and go to the alternative anchorage.
Also reset the alarm. A 40 m radius is smaller than your own swing circle - it would stay silent while you dragged onto the neighbour.
2. How do you set and verify the anchor, and what exactly do you do at 23:15 when the gust front arrives, the alarm sounds and your transits open up?
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Setting. Lower the anchor, do not throw it. Pay out rode as the boat drifts back at 0.5–1 kn, keeping the chain straight, until you reach the chosen scope. Then take a turn and set it with slow astern at about 800–1,000 rpm: the bow should dip, the rode should come up bar-tight and the boat should stop. Ease back to idle and watch whether she holds or creeps.
Verifying. Take two transit bearings on shore features roughly 90° apart and write them down with the time, the depth and the GPS position. Then let the boat settle and take them again. A GPS alarm is a backup, not the check.
At 23:15. Start the engine first - before anything else. Life jackets on, crew on deck with a torch, transits re-checked, depth noted. If the bearings are opening and the depth is increasing, you are dragging.
Recovery. Motor slowly upwind to take the load off the rode, then weigh the anchor - do not cut it away unless it is fouled and you are in danger. Do not try to power against 40 kn with the anchor still down.
Then decide. The shift to north-west puts the neighbour downwind of you, so the immediate danger is drifting onto her. Either re-anchor well clear with 6:1 in shallower water, or run for harbour. With an unforecast 40 kn in an open bay at night, leaving is the defensible call.
3. Why does 8 mm chain on a 4.5 t boat become a liability at 40 kn, and what would you change about the ground tackle if you had to sit it out?
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Catenary disappears. Chain holds the shank down and absorbs snatch because of its weight. At 40 kn of wind on a 9.5 m boat the load is well over 200 kg, and 8 mm chain at 70 m of scope hangs dead straight - the catenary that cushions the surge is gone. What reaches the anchor is a series of shock loads, and shock loads are what break anchors out and pull them across the bottom.
The rope is the snubber. Nylon stretches, chain does not. In a chain-and-warp mix the nylon section should be doing the energy absorption, and the chain-to-warp splice should be long and well made, ideally backed by a separate snubber line with a chain hook taking the load off the splice at the bow roller. If the nylon is fully out and bar-tight, it has nothing left to give.
What to change. More scope - 7:1 if there is room, which is another reason to anchor in 8–10 m rather than 14 m. A second anchor (a kedge laid out at 30–45° to the main rode) or a stern anchor to keep the bow into the gusts during the shift. A weighted sentinel on the rode to restore some catenary. And a watch: engine running, anchor alarm set to a realistic radius, crew briefed on what dragging feels like and who does what.
The real answer, though, is that ground tackle sized for a summer night on a lake is not sized for 40 kn. When the forecast says 60% thunderstorm, choose the anchorage for the storm you might get, not the one you hope for.