In professional fishing, choosing the weight for your gear should not come down to habit alone. If the weight is too light, the line drifts and the gear does not fish where it should. If it is too heavy, handling becomes harder, the gear wears out sooner and contact with the seabed increases for no benefit.
Between those two extremes there is a balance point. Every crew fine-tunes it through experience, but understanding what determines it helps you get it right sooner, with less trial and error.
Two opposing forces
When gear is working underwater, two forces are at play. On one side, the weight pulls downwards. On the other, the current pushes the line horizontally.
The result of that tug-of-war is the angle of the line. The stronger the current relative to the weight, the more the line leans and the further the gear moves away from where it was set. The right weight is simply the one that keeps that angle within an acceptable range for the gear type and the fishing ground.
Current: the biggest factor
The force exerted by the current does not increase in proportion to its speed but with its square. This has a very practical consequence:
- At 2 knots, the drag is 4 times greater than at 1 knot.
- At 3 knots, it is 9 times greater.
In other words, doubling the current does not mean doubling the weight, but multiplying it by roughly four to keep the same angle. That is why a set that works well on a neap tide can fall short on a spring tide.
Depth: more line exposed
Depth works more linearly. The deeper you go, the more line is exposed to the current and the greater the total drag. If the depth doubles, the drag on the line roughly doubles too.
What is more, at the same angle, the deeper the set, the greater the horizontal displacement of the gear. A few metres of drift at 50 metres can become tens of metres at 400 metres, which matters when fishing specific grounds or close to other gear.
Weight on land is not the same as weight underwater
Seawater pushes up on any submerged object, so a fishing weight performs less underwater than it does on the scales. How much less depends on the density of the material.
Lead retains around 91% of its weight underwater. Zinc alloy retains around 86%. In practice, a zinc weight needs roughly 6% more mass to deliver the same performance as a lead one. It is a small difference and easy to compensate for in the design, but it is worth bearing in mind when switching from one material to another, rather than swapping gram for gram.
An indicative example
Take a 1.8 mm line that needs to reach a depth of 200 metres in a 1-knot current, without exceeding an angle of around 30°. A simple estimate puts the required weight at around 12 kg in zinc (about 11.3 kg if it were lead).
In a 2-knot current, that figure would increase roughly fourfold. The example greatly simplifies reality, but it helps show the order of magnitude and compare scenarios.
Each gear type has its own logic
Bottom longlining (SARE). As well as the end weights, the mainline needs intermediate weights to keep it on the seabed. In strong currents, reducing the spacing between weights tends to work better than making each one much heavier.
Net fishing (HARI). The weight is spread along the footrope, and its job is to keep the net taut and upright against the current. Without enough weight, the net lies over and loses catching area.
Trawling (SOKA). Here towing speed matters as well as the current. The weight must ensure contact with the seabed without creating excessive friction that drives up fuel consumption and impact on the seabed.
What no formula captures
At sea, more variables come into play: the current often changes with depth, tides shift in strength and direction during the set, the vessel drifts while shooting the gear, and the type of seabed affects how much weight is needed for the gear to settle. In waters such as the Cantabrian Sea or the Bay of Biscay, these changes can be significant within a single tide.
That is why the best tool is still to combine these principles with a simple log: depth, estimated current, weight used and how the gear behaved. Over time, that data lets you fine-tune sizing and avoid both losses from too little weight and unnecessary excess.
Want to know more about how seabed type affects your choice of weight? Read our article [internal link].
About GreenCastSea
GreenCastSea is a Basque Country company that manufactures lead-free fishing weights from 100% recycled zinc alloy, designed for industrial and professional fishing. Its Itsaspe range includes solutions for bottom longlining (SARE), net fishing (HARI) and trawling (SOKA), built to perform at sea with a lower environmental impact.