Ground tackle
Boat anchor types
Danforth, plow, claw, and mushroom do different jobs. Enter the boat length for a starting size from the Fortress fluke chart and Lewmar's Delta plow chart. The right pound is not a promise the anchor will hold.
Danforth-style fluke
FX-7
4 lb on the Fortress chart
Fortress pairs that model with 3/16 in proof-coil chain and 3/8 in three-strand nylon, and says it replaces a steel fluke of 6-9 lb. Fortress sizes these models for a boat of average windage, in about 30 knots of wind, on an average bottom, with moderate protection from open water. The maker says a storm anchor should be one or two sizes larger.
- Plow, Lewmar Delta
- 14 lb
- Claw
- Maker's chart
- 5:1 scope
- 70 ft
This reading of Lewmar's Delta chart uses 14 lb (6 kg) for boats up to 30 feet. It is a Delta weight, not a CQR weight. If the boat is heavy for its length, move up a model.
Lewmar's claw sizes are bars, not a table this page can quote as a single pound.
14 feet from the bow to the bottom, times 5. Fortress calls 5:1 the minimum.
Which anchor is which
Danforth (fluke)
Better in: Sand and mud. Weaker in: Weed, rock, and hard clay.
A Danforth is a fluke anchor. Two flat flukes dig in. The stock sits down at the crown, so it is an awkward fit on a bow roller. It is a strong lunch hook in sand and mud, and a poor one where the bottom is weedy or hard. Fortress is a modern aluminum anchor in this family. A 4-pound Fortress is not the same recommendation as a 4-pound steel fluke. The chart below uses Fortress's own length bands, and the steel weight Fortress says that model replaces.
Plow and CQR
Better in: Mixed bottoms, and a bow roller. Weaker in: Very soft mud, unless you go up in size.
A plow is shaped like a farmer's plow and lives happily on a bow roller. The older CQR has a hinged shank. Lewmar's Delta is a fixed plow with a weighted tip. The pounds in the calculator are Delta's chart, not a universal plow weight and not a CQR weight. A plow for the same boat is usually much heavier than an aluminum fluke.
Claw (Bruce pattern)
Better in: A quick set in a mix of bottoms. Weaker in: Reading a single pound off this page.
A claw is one piece. The original pattern is the Bruce anchor, built for oil rigs and then scaled down for boats. It sets quickly, holds in a variety of bottoms, and stows on a roller. Lewmar prints a claw weight as a bar against boat length. Those bars are not a simple table, so this page does not turn them into a pound. Use the maker's chart for the claw in your hand.
Mushroom
Better in: A permanent mooring in mud. Weaker in: A lunch hook or an overnight stop.
A mushroom is a mooring anchor. It gains holding after it sinks into mud and stays there. It does not dig on a short set the way a fluke or a plow does. This page has no mushroom size by boat length, because a mooring mushroom is chosen for the mooring, the bottom, and the storm load, not from a day-anchor length chart.
Danforth-style size by boat length
Fortress sizes these models for a boat of average windage, in about 30 knots of wind, on an average bottom, with moderate protection from open water. The maker says a storm anchor should be one or two sizes larger. The pounds are the aluminum Fortress model. The steel column is the fluke weight Fortress says that model replaces, not a separate Danforth company price list.
| Boat length | Model | Aluminum | Replaces steel fluke | Chain | Nylon |
|---|---|---|---|---|---|
| 16 to 27 ft | FX-7 | 4 lb | 6-9 lb | 3/16 in | 3/8 in |
| 28 to 32 ft | FX-11 | 7 lb | 10-13 lb | 1/4 in | 3/8 in |
| 33 to 38 ft | FX-16 | 10 lb | 14-18 lb | 5/16 in | 1/2 in |
| 39 to 45 ft | FX-23 | 15 lb | 19-28 lb | 3/8 in | 5/8 in |
| 46 to 51 ft | FX-37 | 21 lb | 33-50 lb | 3/8 in | 3/4 in |
| 52 to 58 ft | FX-55 | 32 lb | 50-65 lb | 1/2 in | 7/8 in |
| 59 to 68 ft | FX-85 | 47 lb | 70-90 lb | 1/2 in | 1 in |
| 69 to 89 ft | FX-115 | 60 lb | 90-100 lb | 1/2 in | 1 1/4 in |
| 90 to 150 ft | FX-125 | 69 lb | 100-170 lb | 1/2 in | 1 1/4 in |
Plow size by boat length
Lewmar prints Delta weights as bars against length marks at 20, 30, 40, 50, 60, 70, 80, and 90 feet. This table reads each published weight as the size for boats up to the next mark: 9 lb through 20 feet, 14 lb through 30 feet, and so on. That is a reading of the bar chart, not a second table Lewmar typed out. A heavy boat, or a boat at the light-shaded end of a bar, should move up a model. Heavier Deltas continue past 90 feet.
| Boat length | Delta weight |
|---|---|
| 1 to 20 ft | 9 lb (4 kg) |
| 21 to 30 ft | 14 lb (6 kg) |
| 31 to 40 ft | 22 lb (10 kg) |
| 41 to 50 ft | 35 lb (16 kg) |
| 51 to 60 ft | 44 lb (20 kg) |
| 61 to 70 ft | 55 lb (25 kg) |
| 71 to 80 ft | 70 lb (32 kg) |
| 81 to 90 ft | 88 lb (40 kg) |
CQR plows and claw anchors have their own charts in Lewmar's catalogue. This page does not convert those bars into pounds. A mushroom is a mooring anchor and is not on either chart.
Scope, then the harbor
Fortress says to use at least 5 feet of rode for every foot from the bow down to the bottom, and at least 6 feet of chain for every 25 feet of water depth. Short scope is a common reason a well sized anchor drags. Add the height of the bow roller to the water depth before you trust the number.
Wind is the other half. Thirty knots, the assumption on the Fortress chart, is already a lot of wind for a small open boat. The Beaufort scale and the harbor call are how you decide whether this is an anchoring day at all.
Check today's call for a harborBeaufort scaleKnots to mphHow much wind is too much
Anchor questions
What is a Danforth anchor?
A Danforth is a fluke anchor with two flat flukes that dig into sand and mud. It is a common lunch hook. It tends to skate on weed, rock, and hard clay, and the stock at the crown makes it clumsy on a bow roller. Fortress is an aluminum anchor in the same family. Size it from the Fortress chart, not by matching the pounds of a steel Danforth.
What is a plow anchor?
A plow anchor is shaped like a farmer's plow. The CQR is the older hinged plow. Lewmar's Delta is a fixed-shank plow with a weighted tip, and it is the plow chart on this page. Plows stow on a bow roller and are a common cruising anchor in mixed bottoms. The Delta pounds here are Lewmar's, not a weight that fits every plow brand.
How do I size a boat anchor by length?
Enter the boat's length overall. The tool reads Fortress's Danforth-style chart and a length reading of Lewmar's Delta plow chart. Fortress assumes about 30 knots, an average boat, and an average bottom, and says to go one or two sizes larger for a storm. Length is only the start. A heavy boat, a high cabin, short scope, or a weedy bottom can make a correctly charted anchor drag.
Is a mushroom anchor good for anchoring overnight?
No. A mushroom is for a permanent mooring in mud, where it can bury over time. It is a poor choice for a lunch hook or one overnight stop. Use a fluke, plow, or claw sized from the maker's chart, with enough scope.
Does the right anchor weight mean it will hold?
No. Weight is a starting size from a maker's chart. Holding depends on the bottom, the scope, how well the anchor set, and how much wind the boat catches. Fortress's minimum scope is 5 feet of rode for every foot from the bow down to the bottom. Check the harbor forecast before you trust a new hook in a new cove.