Why uncertainty in buoy drag coefficient puts oceanographic data quality at risk

In World Wars 1 and 2, Allied ships were at significant risk to U-boat torpedo attacks. These U-boat torpedo attacks were sinking thousands of vessels. To protect their fleet, the allies desperately needed new ideas and technologies.

One concept was called dazzle camouflage. It consisted of wild stripes, colours, and shapes painted on the ship hulls. Usually, camouflage is about making something harder to spot, but this was the exact opposite. In fact, the crazy patterns made the ships even easier to spot from a U-boat periscope. So how was this protecting the vessels?

USS West Mahomet in dazzle camouflage 1918

World War 1 cargo ship USS West Mahomet with dazzle camouflage in 1918

Normally, U-boats were used to looking at typical ship features, like a mast, or some part of the superstructure. All those wild shapes and colours would make all these features unrecognizable. By making those features unrecognizable, the dazzle camouflage threw off the torpedo attack by introducing uncertainty.

Similarly, it’s uncertainty in the drag coefficient of an oceanographic buoy that can throw off data quality. Data quality is affected by the amount of oceanographic mooring deflection. And mooring deflection is very often driven by the effects of drag forces. So what exactly do we mean by uncertainty in the drag coefficient?

The total drag force is affected by several factors

The drag coefficient is only one of these factors. The other factors include water density, structure size and shape, and flow speed. Now, all these other factors are directly measurable quantities. You already know the dimensions of the buoy you’re working with. The water density and flow speeds are usually known reasonably well, too. So the drag coefficient is the odd one out.

It’s just not easy to directly measure the drag coefficient

It can also change dramatically depending on the structure size and shape. Usually, these two aspects are what we mean by uncertainty when working with the drag coefficient.

It’s not always obvious what the drag coefficient value is

If there’s a big range in the drag coefficient, well, that means there’s going to be a big range in the total drag on the oceanographic buoy itself, too. Oceanographic buoys can be substantial structures. And that means there can be substantial total drag forces.

StableMoor ADCP Buoy

Buoys can be substantial structures: the DeepWater Buoyancy StableMoor can be several meters long and weigh hundreds of kilograms

The higher the drag force is, the more an oceanographic mooring will deflect in water currents. There’s most often some kind of water current present. So it’s not a question of if there is mooring deflection, but really, how much mooring deflection.

Why is mooring deflection significant?

The sensors on an oceanographic mooring are there to measure something at a specific point in space. If there’s mooring deflection, they are measuring at a lot of other spots in the ocean. So this mooring deflection starts to introduce a lot of error in the measurements. Even worse, when the mooring deflects, it can also start tilting at an angle. This tilt can actually stop some sensors from working entirely.

So if you want to anticipate your data quality, you really have to understand what the mooring deflection is. The mooring deflection can be heavily influenced by the drag on the buoys. But it does depend on how many and where the oceanographic buoys are on the mooring. You do really have to be careful if there’s a large buoy mounted at the top of a mooring. Drag forces at the top of have a lot of leverage for deflection.

But what about the drag from the rest of the mooring?

Indeed, oceanographic moorings can be almost any length, from a few meters in shallow water to several kilometers long at full ocean depth. There can be a lot of drag on the mooring line itself. If the buoys are only a fraction of the mooring length, the drag on the mooring itself can dominate the total deflection.

It’s when the mooring isn’t so long, and large buoys are used at or near the top that you have to start paying close attention to the drag coefficient. The uncertainty in the drag coefficient can result in a significant impact.

We covered a few nuances of the drag coefficient for a buoy

Now it’s time to review. The drag coefficient of an oceanographic buoy can be difficult to measure directly. It can change dramatically and depend on the shape of the buoy. These are usually the two primary sources of uncertainty in the drag coefficient. The reason to be wary of this uncertainty is when it starts to affect the deflection of the mooring. The greater the potential for drag forces, the greater the possibility is for mooring deflection. If the mooring deflection is all over the place, the sensors on the mooring will be moving all over the place, too.

The real goal of the dazzle camouflage on ships in the allied fleet in World Wars 1 and 2 was to introduce uncertainty to throw off torpedo attacks. In this case, the more uncertainty, the better! But we have the opposite goal here in that we want to drive the uncertainty in the drag coefficient lower to reduce the impact on data quality. We showed why this was important, but we didn’t cover how to do it.

Next step

We covered why buoy drag coefficient uncertainty is important, but we didn’t cover what can be done about it. Read part 2 here on how to control the uncertainty in the drag coefficient.