What size anchor for a 21 foot aluminum boat?
| Recommended anchor | 14 lb fluke (danforth) |
|---|---|
| Chain sizeworking load about 2,600 lb | 1/4″ |
| Chain lengthone boat length minimum, 30 ft floor | 30 ft |
| Rope sizenylon three-strand — the stretch is the shock absorber | 3/8″ |
| Bow height above wateradd this to the depth before working out scope | 4.1 ft |
| — Every anchor type at this length — | |
| Fluke (Danforth)best in sand and mud, poor in weed and rock | 14 lb |
| Plow (CQR)resets well, needs weight to dig in | 20 lb |
| Claw (Bruce)sets easily in most bottoms, lower ultimate hold | 20 lb |
| Modern spadehighest hold per pound, sets fastest | 16 lb |
| — How much rode to carry — | |
| 10 ft of water, 7:1 | 99 ft of rode |
| 20 ft of water, 7:1 | 169 ft of rode |
| 30 ft of water, 7:1 | 239 ft of rode |
| 40 ft of water, 7:1 | 309 ft of rode |
Notes
- Boat type changes the number, and it is windage that does it. A 21 ft aluminum boat works out at 14 lb against 16 lb for a conventional powerboat of the same length — lighter, because light displacement and low profile, and usually anchored in a river or a small lake rather than exposed water.
- Scope matters more than weight. A 14 lb anchor on 7:1 scope outholds a 22 lb anchor on 3:1, every time. The angle of pull at the shank is what sets the flukes; short scope pulls up instead of along. If the anchorage is too crowded for 7:1, the honest answer is to move, not to let out less.
- Measure depth from the bow roller, not the water line. On a 21 ft boat the bow sits about 4.1 ft up, so 20 ft of water is really 24.1 ft of rode geometry. Forgetting the freeboard is the most common reason a scope calculation comes out short.
- Chain is not optional. The chain holds the shank down so the pull stays horizontal, and it takes the abrasion on rock and coral that would cut nylon. One boat length is the working minimum, 30 ft here.
- In current, the anchor type matters more than the weight. River anchors and grapnels hold in rock and brush where a fluke will not set at all, and they break out when you pull straight up. On a small aluminum boat in moving water, that is usually the right tool rather than a heavier Danforth.
- Manufacturer tables differ, and they differ for good reasons — hull form, displacement and superstructure all matter and none of them are in a length-only table. Treat this as the size to start from, then round up if your boat is heavy for its length, carries a tall bimini, or lives on an exposed mooring.
A 21 ft aluminum boat
14 lb is the working anchor for this boat — overnight holding in normal weather, not a storm anchor.
Weight is the easy half
Anchor weight comes off a manufacturer table and every table is a length-only approximation. The half that actually decides whether the boat stays put is scope — the ratio of rode paid out to the depth measured from the bow.
At 7:1 the pull at the shank is nearly horizontal and the flukes bury. At 3:1 the pull is upward and the same anchor skates across the bottom. No amount of extra weight fixes a bad angle.
Measure the depth from the bow, not the water
The bow of a 21 ft boat sits roughly 4.1 ft above the water, and that height is part of the triangle. Twenty feet of water under the keel is 169 ft of rode at 7:1 once the freeboard is counted — a difference big enough to drag on.
Chain earns its weight
Chain does two jobs rope cannot. It holds the shank flat so the pull stays horizontal, and it takes the abrasion on rock and coral. One boat length is the working minimum, 30 ft here, in 1/4″.
The rope beyond it should be nylon three-strand. The stretch is a shock absorber, and it is the reason nylon is used rather than anything stronger and stiffer.
Nearby sizes
Read more
- New-Generation Anchors Against the Classics: What the Concave Fluke ChangedThe difference is not marketing. A concave fluke digs deeper under load where a plow tends to plane, and re-setting after a wind shift is where the gap is widest.
- "Anchor Weight per Foot: The Rule of Thumb, and Where It Breaks"A pound per foot is a rule that works in the middle of its range and fails at both ends, for two different reasons.
- Reading a Modern Scoop Anchor Chart, and What Its Small Print AssumesModern anchor charts look interchangeable and are not. The difference between them is usually one line of small print about assumed wind speed.
- Fluke Anchors: Where They Excel, and the Two Bottoms That Defeat ThemThe fluke anchor has the best holding-to-weight ratio in common use and two well-known failure modes. Both are about the bottom, not the size.