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Why is a minehunter made of wood?

Send a steel ship and then a wooden minehunter over an old mine, one step at a time, and watch what the Earth's magnetic field does. The minehunter is the U.S. Navy's Avenger class[1][2], and the sources are at the bottom.

Press button 1 to start. Each button plays one step in this slice of harbor. An old mine lies on the sea floor, left behind long ago, and it is still waiting.
Not real timeNot real time

How strongly each draws a magnetic field in

Steeldraws it in99
Woodfar too small0.00000043
Waterpushes it out−0.000008

Each figure says how much more strongly than empty space it draws a field in. From one published table[5], steel does it about 230 million times as strongly as wood. Another source found wood pushes a field out a little instead, as water does[6]. Either way, wood and water barely touch it.

What the old mine feels

steel shipminehuntercable onwhat it waits for

the needle points along the Earth's field

Earth's field[4]– nT
On the water–
Its hull[1][2]–
How hard the hull's steel or wood draws the field in[5]–
Length[1]– ft
Weight, fully loaded[1]– tons
The mine's needle–
Degaussing cable[1][8]–

The big idea: steel draws the Earth's magnetic field into itself and bends it, and an old magnetic mine is made to feel that bend. Wood hardly touches the field, so a wooden ship can pass over almost as if it were not there.

How the numbers were worked outOpen it to see every sum and all 8 sources.Click here again to fold it away.

Where "about 230 million times" comes from: The Engineering ToolBox gives carbon steel a relative permeability of 100 and wood 1.00000043[5]. Take away 1, the value for empty space, and you get how strongly each draws a field in: 100 − 1 = 99 for steel and 1.00000043 − 1 = 0.00000043 for wood. 99 ÷ 0.00000043 = 230,232,558, about 230 million. Steels differ: HyperPhysics gives quenched steel 50 at first and 100 at most[7], and from 50 the same sum gives about 114 million. The page uses the ToolBox's carbon steel.

Wood and water: the same table gives water 0.999992, so 0.999992 − 1 = −0.000008, which means water pushes a field out a little[5]. The sources do not agree on which way wood goes. The ToolBox gives it a tiny pull in, while a 1938 study measured wood and its cellulose as diamagnetic, pushing a field out a little, as water does[6]. Either way it is tiny: HyperPhysics notes that being paramagnetic or diamagnetic "represents a very small magnetic response" next to ferromagnetic materials such as iron and steel[7]. The 230 million uses the ToolBox's figure for wood.

The minehunter: the Avenger class is 224 feet long (68.28 meters), 39 feet wide and 1,312 tons (1,333.06 metric tons) at full load, with four Isotta Fraschini diesels of 600 horsepower each (the first two ships had Waukesha diesels) and a hull of "wood covered by glass reinforced plastic"[1], which the Navy's fact file calls "fiberglass sheathed, wooden hull construction"[2]. Naval Technology names the woods (oak, Douglas fir and Alaskan cedar), says the wooden hull covered with fiberglass gives "flexibility, strength and a low magnetic signature", that this "allows the craft to withstand the blast from naval mines", and that the engines "have very low magnetic and acoustic signatures"[3]. The Navy lists a degaussing system among its gear[1], and the Navy Museum of New Zealand explains how one works: cables around the hull carry "a measured electrical current" to "cancel out the ship's magnetic field"[8].

The Earth's field is roughly 25,000 to 65,000 nanoteslas (0.25 to 0.65 gauss), depending on where you are[4]. A steel ship's field "adds to or subtracts from the Earth's magnetic field" as it moves, and that change is what a magnetic mine's detector is made to sense[8].

Where the numbers come from

  1. The Avenger class: "Hull: Wood covered by glass reinforced plastic"; 224 feet (68.28 meters) long, 39 feet (11.89 meters) wide, "1,312 tons (1,333.06 metric tons) full load"; four diesels, Waukesha on the first two ships and Isotta Fraschini of 600 horsepower each on the rest; its equipment includes a magnetometer degaussing system: U.S. Navy, Surface Force Atlantic, Mine Countermeasures Ships (MCM) info page
  2. "The ships are of fiberglass sheathed, wooden hull construction", designed for "finding, classifying and destroying moored and bottom mines": U.S. Navy fact file, Mine Countermeasures Ships - MCM (updated 21 August 2020)
  3. "Oak, Douglas Fir and Alaskan Cedar woods were selected for the hull", with an outer coat of fiberglass, giving it "flexibility, strength and a low magnetic signature"; this "allows the craft to withstand the blast from naval mines"; its four Isotta Fraschini diesels "have very low magnetic and acoustic signatures": Naval Technology, Avenger Class Mine Countermeasures Ship
  4. "Earth's magnetic field intensity is roughly between 25,000 - 65,000 nT (.25 - .65 gauss)": NOAA National Centers for Environmental Information, Geomagnetism Frequently Asked Questions
  5. Relative permeability: carbon steel 100, wood 1.00000043, water 0.999992, air 1.00000037: The Engineering ToolBox, Permeability
  6. A study of wood and its parts: "The diamagnetic anisotropy and absolute susceptibilities of wood, lignin and wood cellulose have been determined", that is, it measured them as diamagnetic, pushing a field out a little: P. Nilakantan, Magnetic anisotropy of naturally occurring substances. III. Wood and its constituents, Proceedings of the Indian Academy of Sciences A 7, 38 to 49 (1938)
  7. Steel with 0.9% carbon, quenched: a relative permeability of 50 at first and 100 at most; being paramagnetic or diamagnetic "represents a very small magnetic response" next to ferromagnetic materials such as iron and steel: HyperPhysics (Georgia State University), Magnetic Properties of Solids
  8. "A steel-hulled ship is like a huge floating magnet"; building it in the Earth's field gives it magnetism of its own; as it moves, its field "adds to or subtracts from the Earth's magnetic field", which a magnetic mine's detector is made to sense; degaussing cables around the hull carry "a measured electrical current" to "cancel out the ship's magnetic field": National Museum of the Royal New Zealand Navy, Degaussing

The drawing is not to scale. The harbor is drawn shallow next to the ships, and the steel ship is drawn the same size as the minehunter so the two can be compared. The slant of the field lines, how far they bend, how far the needle swings, the shape of the trace in the mine's panel, where its dashed line sits, and how much the minehunter's own metal moves the trace before the cable cancels it are this drawing's own choices, not measurements. No step is timed to real life. The mine is drawn plainly and stands for no particular make, and its panel, with a needle and a trace, is a way of picturing the change it senses, not a drawing of any real mine's instrument.