Understanding Working Load Limits for Your Boat's Ground Tackle
Equipping your boat with adequate ground tackle is an essential part of owning a boat. Before anchoring, or even before shopping for ground tackle, it's important to determine what type of load will be on the ground tackle. Understanding your holding needs as well as your equipment's working load limits play a vital role in keeping you safe on the water. If your ground tackle is inadequate for the boat or conditions, it will leave you at serious risk of equipment failure.
How to Determine Your Holding Power Needs
Holding power is the ability of an anchor to hold a given weight. Be certain that your anchor and ground tackle can deliver the performance you will need for your boat type and the wind conditions. The size of your boat and the wind conditions are two main factors that determine how much holding power your boat requires.
The table above uses data published by the American Boat and Yacht Council (ABYC) and illustrates the potential loads for various boat sizes at different wind speeds. A good rule of thumb to remember is as the wind speed doubles, the holding requirement quadruples.
Note: this table assumes boats of average beam and windage. If your boat has above average beam or windage, this will increase the load requirement.
Scope is the length of anchor line relative to the distance from the deck of your boat to the sea bottom. In order to mitigate shock loads as the boat pitches up and down, it's very important to make sure the boat is anchored with plenty of scope. More scope means less vertical strain on the boat anchor thereby decreasing the chances of unsetting the boat anchor. Most anchoring articles and anchor manufacturers agree that a scope of 7:1 achieves the anchor's designed holding power.
Total Holding Power in 10 Feet of Water
|Anchor Rode Length||Scope||Total Holding Power|
Matching Your Anchor Rode Components
Now that we have an understanding of what type of loads we might see, we need to match anchor rode components to meet these loads. The three main components we will be discussing are anchor line, chain, and shackles. However, if additional components are added to the anchoring system, such as anchor swivels or snubbers, it's equally as important to check that the breaking strength and working load limit of all components can handle the loads.
Breaking Strength vs Working Load limits
Breaking strengths of ground tackle are fairly straightforward to understand - these are the absolute limits that components can endure before failure. Working load limits (WLL) are a percentage of the breaking strength and are safe operating limits for that particular component.
- Nylon rope can have wide ranges of WLL depending on their quality and intended purpose:
- 3-strand high-quality nylon generally has a range of 20% - 25% WLL of breaking strength.
- Double braided high-quality nylon is a bit stronger with a WLL range of 25% - 35% of breaking strength.
- Proof Coil and BBB chain have a WLL that is 25% of its breaking strength.
- High Test chain (HT) has a WLL that is 33% of its breaking strength.
- Shackles generally have a WLL of 20% of their breaking strength.
The table shown above illustrates the safe working load limits of common anchor rode styles and sizes. In simple terms, the working load limits of each component of your rode should match or exceed the load your boat will endure based on the figures gathered from our first chart.
While the figures shown in this article generally apply for the average casual boater, the more serious boater should be aware that extra variables, such as current impact, shock loads, and the extent that the boat shears around at anchor should be taken into consideration to account for additional holding power requirements.