Weekly quiz: anchoring scope rule of thumb
Anchoring scope is always a hot topic. We hope it will make a good quiz topic.
So, on to our question of the day:
🔹 What is a good rule of thumb when determining how much anchor rode to pay out?
And bonus question:
🔹 What are some of the (many) factors this may depend on?
👉 We know this topic can lead to hot discussions. So please reply courteously and constructively on one of our social media below. We’ll post the solution right here next Sunday!
Rule of thumb for anchoring
More than 500 of you, hailing from all over the world, replied, liked, commented and reshared this quiz. A huge thanks to all who took the time to answer and share a wealth of insights and practical experiences!
The most commonly recommended minimum scope is 3 times the water depth. This is considered a safe starting point for a short stay in calm conditions. For example, in 10 meters of water, a minimum of 30 meters of chain should be paid out.
In moderate winds or when the anchorage is less protected, increase the scope to 5 times the water depth. For strong winds or storm conditions, a scope of 7 times the water depth or more is advised, with some simply letting out all available chain if conditions are severe.
There were simple alternative rules suggested by the community for determining anchoring scope:
- Fixed length plus depth: Several sailors recommend using a fixed length of chain plus a multiple of the water depth. For example, 15 meters (50 feet) + 2 times the water depth. This method ensures a minimum amount of chain is always on the bottom, providing a safety margin.
- Wind speed multiplier: Others suggest multiplying the water depth by the expected wind speed in Beaufort. For example, for 35 knot winds (7 beaufort), pay out 7 times the water depth. This rule adjusts the scope based on anticipated wind conditions.
- Minimum chain length: some propose a minimum chain length based on the boat’s length, such as 3 to 6 times the boat’s length. This rule ensures a sufficient amount of chain is used, regardless of water depth.
Lastly, in foul weather, anchoring in very little water is not necessarily the best option. The shallow water reduces the recoil the boat can take from a wave or gust before the chain is fully taut.
Factors Influencing Anchoring Scope
Apart from water depth, many other factors should be taken into account. As some of you pointed out, this is why there is no definitive answer on the “right” way to anchor, because there are too many variables. Here are some of them:
- Wind strength: Stronger winds increase the load on the anchor, requiring more scope.
- Wind direction: Changes in wind direction can affect the swing room needed and the load on the anchor.
- Wave height: Higher waves increase the dynamic load on the anchor, requiring more scope.
- Current strength and direction: Stronger currents can reduce holding power, necessitating more scope. Furthermore, if tidal streams will reverse, sufficient scope is needed to ensure the anchor will reset in the new direction.
- Type of seabed: The holding power of the anchor varies significantly with the nature of the seabed (sand, mud, rock, etc.). Grass or weeds can foul the anchor, reducing holding power and requiring more scope.
- Tidal range: Accounting for tidal changes ensures the scope is sufficient at all times.
- Boat displacement: Heavier boats require more scope to hold securely.
- Boat windage: Boats with more windage (surface area exposed to wind) experience greater forces, requiring more scope.
- Freeboard: The height of the bow roller above the water affects the angle of the chain and the load on the anchor.
- Anchor type: Modern anchors like Rocna, Spade, and Bugel have superior holding power, reducing the scope needed.
- Chain weight and size: Heavier chains provide more holding power and help keep the chain on the bottom.
- Proximity to other boats: In crowded anchorages, shorter scope may be necessary to avoid entanglement with other boats’ anchors.
- Swing room: The available space for the boat to swing around the anchor can limit the scope used. This can render some anchorages unsuitable in some conditions.
- Duration of stay: Staying longer or overnight requires more scope to account for changing conditions.
- Weather forecast: Anticipated changes in weather conditions can influence the scope needed.
- Anchor set: Properly setting the anchor by backing down can improve holding power, affecting the scope needed.
- Use of bridle or snubber: These can reduce load on the anchor and chain, improving holding power and affecting scope.
- Crew experience: To end this list on a slight joke, it seems there might be some correlation between the anchor chain length paid out and the captain’s age 😉
Finally, a useful resource was mentioned: the Anchor Chain Calculator app.
👉 Our next quiz on aligning markers is now live and awaiting your answer!
