How Big An Anchor Do I Need For My Boat? (Explained)

That well-known adage ‘size matters’ aptly applies to selecting the right anchor for your boat.

The problem, though, is that there isn’t a one-size-fits-all answer.

Inexperienced boaters may think they should select the size based upon the boat space allotted to store their ground gear.

But what it really boils down to is that when choosing the best anchor and line for your vessel, albeit a canoe, Jon boat, day cruiser, or open fisherman, several factors beyond size should be considered:

What Determines How Big an Anchor You Need?

Oddly enough, your choice of the anchor is not solely determined by the size of your boat but more so the displacement and structure.

For instance, boats with larger structures, particularly ones that may resist wind conditions more easily, would typically need a heavier ground tackle. Heavier ground tackle equates to bigger anchors.

On the other hand, boats that are more affected by winds and currents, such as flats boats, would be well advised to stick with something smaller and lighter.

Now, we use the term heavier here to describe holding power rather than the weight of the actual anchor. The same-sized galvanized steel anchor weighs more than its equivalent in anodized aluminum; hence, the type of metal the anchor is constructed can also determine the anchor’s holding power.

The largest problem with having an undersized anchor comes into play during those unforeseen emergencies that will, without fail, rear their ugly heads at the most inopportune time.

So, although you may never arrange to take your boat out in adverse conditions, it’s that one unplanned-for squall that whips up or the broken needle on the gas gauge that led you to misread your fuel level when out on the open water that will be the true test of your anchor’s worth and fortitude.

Things to Remember When Choosing a Good Anchor:

There are three considerations that a captain should take into account when selecting the right sized anchor.

It is important to keep in mind that you’ll need to choose the best anchor and the line that goes with it.

The combination of these two things is what you’ve heard us refer to as your ground gear or the equipment, including your anchor and rope (a combination of rope and chain that connects the anchor to your boat).

Weight:

Just as size is essential, remember that bigger is better.

Although you may not need the heaviest anchor for securing your vessel along the mangrove, you will, however, be thankful for the added weight when encountering an emergency like unexpected winds.

Holding Power:

Although weight is important, anchors are best evaluated by their holding power or the amount of force an anchor can withstand while securing the boat in place.

Environmental factors such as winds, currents, and waves greatly affect how holding power is formulated.

That being said, the general rule of thumb is that 100 pounds of holding power are usually necessary for safely anchoring a 20′ boat in 20 mph winds.

Bottom Conditions:

Anchors must develop enough resistance in the ocean floor to endure the environmental load.

So, whether it’s mud, grass, sand, or rock, what is under the water matters most when selecting the right size anchor. By taking expected bottom conditions into account, you increase your anchor’s ability to develop resistance, engage and penetrate the seafloor.

For instance, if the seafloor is rocky, you’ll need an anchor that can snag on large rocks or protruding objects to be safely set.

On the other hand, if you’re boating on a grass bed, an anchor that will dig deep into the mud is your way to go.

Suction:

Suction is created when your anchor pierces the surface of the ocean floor.

This, in turn, creates resistance. As the weight of the boat pulls on the rope, the anchor then ‘sets’ or digs in deeper. This is where the combination of both bottom conditions and weight will be important to the equation.

Although it may not seem important to the average Joe, one other unmentioned consideration in the purchase of the right anchor may have to do with who is doing the heavy lifting?

Are you using a windlass or manually pulling up your anchor? Keep in mind, it won’t only be the weight of the anchor that you’re lifting out of the water but the anchor chain and line that accompanies it.

Unearthing a heavy anchor is not for the weary and may need to be taken into consideration.

All in all, different types of boats require distinctive styles of anchors, but again, it’s what lies beneath that is critical.

Manufacturer Recommendations and Bottom Conditions:

If you find it difficult to determine the perfect anchor, you should always refer to the anchor manufacturer’s recommendations and specs before buying.

It’s also a terrific idea to consider more than one anchor, the main one at the bow and a smaller one to set the stern.

It’s also important to note, if you’re unfamiliar with the ground conditions in the area you are boating in, it will be important to do a little research and map out the area before settling on one anchor or another.

Anchor Types for Expected Bottom Conditions:

Let’s take a look at a few different types of anchors and the ground conditions they are commonly suited for:

Anchor Type	How it Works	Why This is Best
Danforth (Fluke)	The design of the two large triangular flukes (flaps) joined to the stock enables the flukes to position themselves to the sea bottom at an angle for maximum holding power.	Lightweight Stows flat  Excellent holding power-to-weight ratio Holds well in mud or sand A great choice for a secondary stern anchor Some disassemble for easy storage
Plow	When deployed, it first lands on its side and then rights itself while it, like the name indicates, plows into the seafloor.	Effective in grass, mud, sand, and other soft surfaces Best all-around holding ability in varying bottom conditions.  Resets easily in changing winds or currents
Mushroom	The silt from the bottom builds up over the anchor, resulting in extreme holding power.	Secures to silt, soft mud, and unpacked sand seafloor Best suited for permanent mooring Well suited for canoes or kayaks
Grapnel	Holding power comes from hooking onto another object. Although not the most reliable, it can create immense holding power.  This can also make retrieving the anchor difficult.	Secures to rocky bottoms Compact Folding prongs prevent punctures in inflatables Lightweight

What Anchor Line Should I Choose?

Equally as vital as the size of the anchor you select is the type of anchor line you choose, and it’s not quite as simple as picking a rope with a pretty little colored pattern that matches the pinstriping on the hull.

Esthetics has absolutely no place in the security and holding power of your boat.

When choosing your line, the size and type of your boat will determine what is best. Your decision will be, “Do I want a combination rope and chain, or am I better off using all chain?”

The rope you select should be made of three-strand or braided nylon since it serves as a shock absorber for abrupt pulls caused by weather conditions.

However, if you are using a windlass, be sure to avoid softer braided nylon rope, which can separate and get caught up in the equipment. Your safer bet is to opt for a three-strand line for durability.

Although braided polyester line lends itself to easy docking, remember that what lies beneath the seafloor can easily damage the rope, causing it to deteriorate and become unreliable.

At that point, it almost doesn’t matter what size anchor you buy if it’s left on the bottom of the ocean.

Unfortunately, with time, due to salt saturation, your line will become difficult to handle as it will harden and stiffen. Avoid working with old and worn-out line and instead replace it.

Why Do We Recommend Using Both Chain and Rope?

The primary reason for adding a chain to your anchor line is that it keeps the nylon from deteriorating when dragging along the seafloor when setting the anchor or as the boat swings.

The more technical reason for recommending chains is because of the weight.

Just as weight is important to the anchor, added weight on the line serves as additional security. It holds the rope to allow for a horizontal pull on the anchor, reducing the chances of unsetting.

Final Thoughts:

In essence, whether you need to throw out a line along the mangroves, raft up with friends on the sandbar, or hunker down during an intermittent squall, remember to keep these points in mind:

• Bigger is better
• Expected bottom conditions
• Holding power
• Manufacturer’s recommendations

Sources:

How to Choose an Anchor

How to Pick an Anchor Rope Size, Type, Length, and More

Selecting the Right Anchor