Magnet Pull Force Explained: What Affects Real-World Holding Power Underwater

How Pull Force is Measured

Pull force is the perpendicular force required to detach a magnet from a ferromagnetic surface under controlled conditions. The standard test protocol for our magnets involves:

• Test surface: 10 mm thick polished low-carbon steel plate
• Surface finish: Smooth, clean, degreased — no paint, rust, or coatings
• Temperature: 20°C (68°F)
• Contact: 100% face contact — magnet sits perfectly flat on the surface
• Pull direction: Perpendicular (straight away from the surface)

Under these conditions, our Diver Magnet - 250 delivers a rated pull force of 250 kg. But no underwater surface matches these laboratory conditions.

Pull Force Ratings Across Our Range

The Diver Magnet range is rated from 115 kg to 500 kg across ten models, each with full specifications on its product page.

• Diver Magnet - 115 — 115 kg rated pull force
• Diver Magnet - 140 — 140 kg rated pull force
• Diver Magnet - 160 — 160 kg rated pull force
• Diver Magnet - 200 — 200 kg rated pull force
• Diver Magnet - 230 — 230 kg rated pull force
• Diver Magnet - 250 — 250 kg rated pull force
• Diver Magnet - 280 — 280 kg rated pull force
• Diver Magnet - 320 — 320 kg rated pull force
• Diver Magnet - 400 — 400 kg rated pull force
• Diver Magnet - 500 — 500 kg rated pull force

Each product page includes the full technical specification — magnet grade (N42–N48), coating type, housing material, eye nut rating, and dimensions. Visit any product page to see detailed specs.

Factor 1: Surface Condition

Surface condition is the largest single derating factor. The condition of the metal surface determines how much of the magnet's face makes effective contact:

• Paint and coatings: Each layer of paint creates a non-magnetic gap between the magnet and the steel
• Rust and corrosion: Iron oxide is significantly less magnetically permeable than clean steel. Heavy corrosion creates an air gap and can reduce pull force substantially
• Marine growth: Barnacles, mussels, and algae create physical barriers. Even thin biofilm reduces surface contact
• Surface roughness: Rough or pitted surfaces reduce the contact area, creating effective air gaps across the face

Practical implication: Paint, marine growth and rust all reduce pull force, often a lot. There are too many variables for a fixed figure: size the magnet with the margin under Choosing the Right Capacity, below.

Factor 2: Air Gaps

Even a tiny gap between the magnet face and the steel surface reduces pull force, often a lot. The rating assumes full, flat contact on clean steel.

Air gaps are created by surface irregularities, paint buildup, trapped debris, and curved surfaces where the flat magnet face cannot achieve full contact.

Factor 3: Material Thickness

Rated pull is measured on thick (10 mm) steel. If the steel plate or structure is too thin, the magnetic field cannot fully close through the material, and pull force drops, often a lot.

Ship hulls (typically 12–25 mm) and heavy structural steel provide more than adequate thickness. Thin-walled piping, sheet metal, and lightweight structures may not.

Factor 4: Temperature

Our N42–N48 magnets are rated for a maximum operating temperature of 80°C (176°F).

• Normal water temperatures: No change you would notice in practice.
• Above 80°C: The magnet starts to lose strength permanently, so keep it away from hot work.

For most underwater operations, temperature works in the diver's favour — cold seawater keeps the magnet below its rated temperature.

Factor 5: Pull Direction and Angle

The rating is for a straight, perpendicular pull. Pulling at an angle, or applying lateral (sliding) force, detaches or slides the magnet with much less force, often a lot less.

This is why current direction matters. A diver holding position on a hull in a strong cross-current is experiencing lateral force on the magnet, which is much easier to overcome than a perpendicular load.

Choosing the Right Capacity

Choose a magnet whose rated pull force is at least 2–3× the holding load you need — more for dynamic, angled or vibrating loads. Rated pull is measured under ideal conditions; actual holding force depends on surface condition, contact area, coating and paint, and the direction of the load. Diving magnets are positioning and holding tools, not lifting equipment.

Use our comparison page to evaluate all models and their specifications side by side, or read our buying guide for application-specific recommendations. For comprehensive product specifications including dimensions, weight, and material details, browse our full Diver Magnet range.