What is a mushroom anchor, and how does it hold a mooring?

What it looks like

A mushroom anchor is a heavy cast-iron bowl, shaped like an upside-down mushroom cap, with a short stem rising from the center. The chain attaches to an eye at the top of the stem. It is simple, with no moving parts, which is part of why it has lasted.

How it holds

A mushroom doesn't grab the bottom the way a boat's anchor does. It is lowered onto soft mud or sand and sinks in under its own weight. Over the following weeks the sediment settles around the bowl, and a suction builds up that resists being pulled out. That is why a set mushroom can hold many times what it weighs, and why it tends to hold better the longer it has been down.

Two things follow from that. A mushroom needs a bottom soft enough to sink into, so it suits muddy harbors and not rock or hard gravel. And its holding power is not a fixed number at the moment it is set. It grows as the anchor beds in.

Why moorings use them

• Simple to set. A mooring barge or service boat lowers it into place. There is nothing to dig or drive.
• Familiar. Any mooring service knows how to size and set one.
• Strong for its weight. Burial and suction do much of the work, so a mushroom holds well without needing to be as heavy as a block.

The one well-known weakness

The stem is a vertical post. As tides, current and wind swing a moored boat through a full circle, the chain can wrap around the stem, and each swing can pull it tighter. That is chain wrap. It shortens the effective scope and raises the peak loads on the rest of the mooring, and it is hard to see from the surface. (The full explanation is here.)

Where MoorSafe fits

MoorSafe is a mushroom anchor with one change: three steel bars rise from the bowl to a single point above the stem, forming a cone, so the chain has nothing to catch on. The bowl and the cast-iron base are the same, and the gear and setting practice stay the same. We tested it side by side against a traditional mushroom, and the results are here, including where the traditional anchor did better.

Choosing an anchor depends on your bottom, your boat and your harbor. Here is how the main types compare, and your mooring service can advise on what suits your mooring.