🎣 How to Plumb the Depth When Fishing

Why Plumb the Depth?

There are a few reasons to plumb the depth.

  1. To find out how deep the water is. This may sound obvious but the general rule of thumb is that if the water is deeper than your rod then it’s too deep to fish with a float.
  2. To get your bait onto the bottom. This is where most fish will be feeding, some fish will feed higher in the water but its nearly always good practice to start on the bottom and move up if necessary.
  3. To find out what the bottom is like. Whether it’s weedy, silted or rocky, whether there are any slopes, drop offs, or hollows etc. This is to get an overall picture of what the bottom is like and to help you decide where to fish.

Types of Plummet

Clip Plummet

The clip plummet does exactly what the name suggests, clipping onto the hook. The two tabs are squeezed together to open the mouth of the clip and the hook is placed inside before releasing the tabs, the spring inside the plummet holds it closed with the hook inside.

Cork Plummet

The cork plummet is quite distinctive as it looks like a bell with an eye on the top and a strip of cork set into the bottom. The hook and line are threaded through the eye and then the hook is set into the cork, giving you a very securely attached plummet.

Over sized Split Shot

Place a large split shot (large enough to sink the float) just above the hook. This is a quick way of plumbing the depth but it does not give the same feel for the bottom as either of the above methods. This method can be adapted / changed for over depth fishing with the use of the over sized split shot, which we will cover later.


There are two slightly different methods depending on whether the float is self cocking or not. A self cocking float automatically rights itself in the water whereas a normal float will lie flat on the surface.

Normal Float

Let’s deal with a normal float to begin with. Set up your rig with the float set at an approximate depth and locked in place with two small split shots, make sure these are small enough not to cock the float, and then place your chosen plummet on (or near if using the split shot method) your hook. Now cast to the area you want to fish and watch to see how the float behaves.

The principle of this is very simple. The plummet will sink to the bottom while the float tries to rise to the surface. If the amount of line between hook and float is too short then the float will not be able to reach the surface, while if the amount of line is too long then it will rise to the top and lie flat.

If the float disappears below the waters surface you are set too shallow and need to move the float away from the hook / plummet. Move the float away from the hook by six to ten inches and re-cast to the same area. If the float disappears below the surface again then move it away from the hook by another six inches. Keep doing this until the float becomes visible. If the float is now laying on the surface then adjust it by moving it towards the hook by three inches. If it’s still on the surface then continue by moving it towards the hook an inch at a time until you have found the right depth. If however it has once again sank beneath the surface then adjust it away from the hook an inch at a time. Keep adjusting it up and down until it is set to the correct depth.

If the float begins by laying on the surface then you are set too deep and need to move the float towards the hook / plummet. Use the same technique as above except begin by moving the float towards the hook, and keep adjusting the float until you find the correct depth.

If the tip of the float is visible then you are at the right depth. Check around you swim to ensure that it is all at the same depth. You can now take the plummet off and begin fishing, or set your rig a little deeper to fish over depth, or shallower to fish off the bottom. Don’t forget to add more shot to set the float correctly in the water.

Tip: Ensure you leave slack line between your rod tip and float when plumbing the depth to ensure you are not influencing the float in any way.

Self Cocking Float

The only real difference between a normal float and self cocking float is that the self cocking float will not lie on the surface when set too deep, thus you have no indication if you are set correctly, or too deep as they will both look the same.

To overcome this it is best to start with the float too shallow so it disappears below the waters surface, and then keep adjusting it up (away from the hook) by six inches until it becomes visible. Once the float is visible then adjust it down (move float towards the hook) by three inches. If it is still visible then move it down again by an inch, or if it has gone beneath the surface then adjust it up by an inch. Keep adjusting the float, whilst reducing the amount you move it, until you have found the correct depth. As before you need to check the rest of your swim to ensure it is all at the same depth, and then add shot to get the float sitting correctly in the water.

Fishing Over Depth

This is a technique that many anglers use The process is to place an oversized shot a distance away from the hook, the distance between the shot and the hook can vary but for this instance we will assume that it’s three inches, leaving loose line between the shot and hook. Lock the float in place with two small split shot, a normal float that does not cock itself works best. Then plumb the depth like before, using the over sized shot as the plumb, until the tip of the float is visible.

The advantage of this technique is that if you change swims it is easy to re-plumb the depth as you are basically confirming the depth every time you cast in, and the shot makes a much smaller splash than a larger plummet.

When the fish picks up the bait it will not feel the resistance from the float, but it will often lift the shot off the bottom, causing the float to lift up. This is a bite and you should strike!