Handmade fishing lures - Hydrodynamics

I don't have the required funding for the wind tunnel I really need for this post, so at great personal expense, I'll be using crayon.

In air, a wing generates lift by creating a low pressure system above it by forcing the air to stretch out as it passes over the curve of the wing. The curve over the top of the wing means the air has to travel over a longer distance than the air going over the straighter bottom section. Stretched air equals less air per lump of space, equals low pressure. When you have less pressure above something, compared to that which is below it, that something gets sucked up.

A sail sucks a boat along in the same way, using the low pressure created by a wing shape to suck the boat forward. If you look down on a sail from above as in this realistic diagram, it has a cross section like that of a wing (thats the wing bit sticking out to the left). With a sail, the force generated is roughly at right angles to the bulging side of the sail. The wing shape is made by the bulging bit on one side, and on the other side by the tendency for the air to take a short cut straight across from one side of the sail to the other.


In a wing shape moving along level with the horizon, the force is roughly upwards.

I'm pretty sure the forces acting underwater are similar, or at least behave similarly, to those that work in air.

The force acting on a lure is also roughly upwards.

those diagonal purple wind lines are from a different diagram

A lure is often designed as a wing shape, but strangely, this helps the lure dive deeper. As far as I can tell, it does this by lifting the tail, and forcing the nose to pitch down.

It also has a bib. A bib doesn't act as a wing, but rather, acts more like a rudder. The bib adds length to the lure without adding lift at the front of the lure. The bib also encourages the lure to follow the direction to which the bib points, because it makes it too hard for the lure to swim directly towards the fishing rod. Point the bib down, and the lure will go down. The bib is set so that it presents as a flat plate resisting being pulled along. Picture pushing a flat dinner plate through the water. If you turn it edge on, its much easier to push. If you drop the plate flat into water, it wont just sink straight down, it will try its hardest to get some sideways to add to it's downwards. In much the same way, the bib wants to move in any direction other than flat up or flat down. Because the lure is being pulled along from roughly the front, the bib is limited in just how stubborn it can be. It negotiates a compromise and moves generally forwards, a bit down, and has a go at moving a bit to each side. The attached line gives it a bias to forwards.

A dropped dinner plate might also go to one side, spill a bit of pressure, then rock back to the other side. This may repeat so the path of a dropped plate may well be a zig zag all the way to the bottom. I generally encourage experimentation, but if you must drop plates into the bath, I suggest waiting until you find yourself bathing at someone else's place, and use their plates. Interestingly, and not without an incredible amount of forward planing on my part, this dinner plate zig zag might also go some way to explaining how a bibbed lure gets it's swimming action.

A sail boat achieves a similar compromise. If the wind is coming from one side, it wants to blow sideways with the wind, but the shape of the boat, and the sail set so that it sucks the boat forward, make the boat track roughly forwards.

That means a sailboat can sail into the wind, but not directly. In fact there is around 45 degrees each side of dead into the wind where you can't point your sail boat (pictured here scribbled in red). Whilst that last point is perhaps the most interesting, it isn't really relevant.

So, to reuse a previous crayon graphic. A long bib set at an angle pointing just slightly lower than flat, can force the lure to pivot at the tow point, and point down. This will force it to rotate about the tow point.

Some kind of vague approximation of that effect is depicted here by green arrows pivoting around a drawing of a bow, tied in imaginary string.





A long flat bib encourages the lure to swim to the bottom, a short, sharply downwards angled bib makes for a shallow diving lure.

Exactly why a short, steeply angled bib makes for a shallow diving lure remains a mystery to me, so for the time being, will remain unexplored.