Practice: Use these little helpers to inspect your underwater hull with ease

If you are travelling in crystal-clear and warm water, you will simply grab your swimming trunks and diving goggles if there is a problem below the waterline and have a quick look for yourself. But what if the temperatures are low and visibility is poor or the water is teeming with fire jellyfish? There are many reasons to look for a solution without getting your feet wet. But what can be inspected from a ship or dinghy, and what equipment is needed? And if nothing helps: how can the necessary dive be made easier and the possible time under water extended?

Checking the underwater hull with an action cam

For the first attempt, we attached a waterproof action cam to the boat hook and tried to take a look under the hull from the boat. If necessary, the camera can be attached to the pole with adhesive tape or a strap. However, it is much easier and safer to use a clamp mount with a ball head, also known as a crab clamp. This not only holds the camera reliably on the tube, it can also hopefully be fixed at the right angle. "Hopefully" because no wireless connection between the camera and smartphone is possible under water and therefore no viewfinder image is available. The camera therefore has to be aligned by chance.

It's amazing what can be captured even with an old GoPro in the murky waters of the Baltic Sea. A visual inspection of the log transmitter was easily possible from the foredeck. If you have a saltwater-proof smartphone, you can also use it. Depending on the model, it may be more favourable to use the so-called timelapse mode and take a photo or record a video every few seconds. In any case, the camera should be panned slowly and the angle of the boat hook should be varied. This gives you the best chance of catching the right section of the picture in between. The camera's wide-angle lens also helps here. However, in order to recognise details in the photos or videos, they must be transferred to a smartphone, tablet or laptop so that you can zoom in.

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However, the images also made it clear that the propeller or even the keel cannot be inspected from the boat using this technique, because despite the light-coloured antifouling and bright sun, the saildrive was at best only vaguely visible during our tests, and the same applied to the keel. It was not even possible to determine whether there was algae growth on the underwater hull, as the greenish colour of the water made the antifouling appear algae-green just a few decimetres in front of the camera.

To get further under the hull and thus closer to the propeller and keel, we used the SUP moored next to the boat. From the swimming platform, the boat hook, which was extended to 1.80 metres, reached so far under the hull that it was possible to take usable pictures of the saildrive. Again, the problem here is aiming. To avoid missing the subject, you should take a close look at where the drive is located in the ship. It is also advisable to use the video mode. This makes it easier to recognise the movement of the camera and guide it in the right direction for the next attempt. It may be possible to mount an endoscope camera parallel to the action cam on the boat hook. These cameras transmit the image via a long cable to the transmitter, which sends a viewfinder image to the smartphone. This may allow the action cam to be navigated to the right place on the underwater hull.

Next level: Underwater drone

Speaking of navigating: You should also be able to inspect the hull with the Dory underwater drone from Chasing. It is offered by Compass24, for example, and costs around 550 euros. The mini submarine is connected to a radio buoy by cable and is controlled by smartphone. It has a range of up to 50 metres and can dive to a depth of 15 metres. In practice, it can actually be used for underwater expeditions to the keel or propeller.

With the help of five small propellers, the submarine can automatically maintain its diving depth and orientation, but controlling it using a smartphone still requires practice, as does navigating underwater using the camera image. Once you have managed to navigate to the spot of interest, you can take photos and videos. The camera has HD resolution, but is blurred at close range. You should not expect macro shots. However, you can see much more than with the action cam on the boat hook, and it's a lot of fun to zoom through the water and explore the seabed and underwater ship.

Diving or snorkelling equipment

Hardly any sailing crew is likely to carry a complete set of diving equipment in the forecastle. But no yacht should be without a mask and snorkel. A wetsuit is also recommended on the North Sea and Baltic Sea, as even in summer there is a risk of hypothermia during longer underwater missions. Disadvantage of the rubber skin: It creates additional buoyancy, which makes it more difficult to reach the propeller. If you often go diving, you should therefore consider a set of diving weights to reduce the buoyancy of the Neo. The amount you need depends on your body size and the thickness of your suit. As a guideline, you can calculate around five per cent of your body weight in ballast. Important: there should always be residual buoyancy.

With the bulky underwater hulls of modern yachts, it is a long way from catching your breath to the place of use under the hull. Fins make it easier to descend quickly. However, the physical exertion also increases oxygen consumption and the deployment time is reduced. It is therefore advisable to pull a guide line across under the ship. Preferably at the right height, for example behind the saildrive. The line should be made of a material that sinks and should not be too tightly tensioned so that it is not too close to the hull and can be gripped easily. A polyester mooring line or sheet is well suited. Dyneema cordage is less suitable as its core is buoyant and does not sink as well. It is easy to pull yourself to the underwater hull using the guide line and hold on to it there. This saves air and extends the time available for inspection.

Use of a mini scuba tank on the underwater hull

The mini scuba tanks on offer in many places are also designed to extend the time spent underwater. This is basically diving equipment in which the pressure tank and regulator form a single unit that is held to the body by the mouthpiece without any additional straps or the like. Instead of the usual diving cylinders with a capacity of ten to 15 litres, the mini cylinder has a volume of between 0.2 and 0.5 litres, depending on the model. Although this is compact and lightweight, it means very short operating times. The mini cylinders are usually operated at a pressure of 200 bar. This results in a maximum air supply of 100 litres at atmospheric pressure. Consumption when diving depends on the breathing technique and depth. The further down you go, the higher the ambient pressure and the more air is required per breath, and physical exertion also plays a role. The more calmly the diver breathes, the lower the consumption.

An estimate for inexperienced divers working on the underwater hull is 30 litres per minute. The mini bottle will then last for just three minutes. On the other hand, hardly anyone can hold their breath that long and work on the ship at the same time. In addition, the mini diving systems can be refilled directly on board using a hand pump or compact compressor.

How helpful the mini bottle actually is can only be assessed in practice, so we tried out the system from Mini-Dive. The French manufacturer has branches in Germany and the USA and offers various versions. The system we tested consists of a 0.5-litre carbon fibre cylinder with regulator and a 12-volt compressor. It is available for 750 euros. The advantage of the carbon fibre cylinder is that it can be filled to 250 bar and therefore offers 25 percent more air supply than a 200 bar aluminium cylinder.

Filling the scuba tank

For filling, the bottle is connected to the filling hose of the compressor and this is activated. To supply the compressor with power, a cable with pole clamps is included, which should be connected directly to the battery - an understandable solution in view of the power consumption. At 250 bar, the compressor requires around 27 amps, whereby it heats up to around 45 degrees despite the very loud cooling fan. After 25 minutes, 250 bar operating pressure is reached and the dive can begin. The filling process must be monitored and stopped manually. Mini-Dive only offers a compressor with automatic switch-off for 230 volts; it costs 800 euros. To ensure that the breathing air in the cylinder is dry and oil-free, it is passed through an activated carbon filter with desiccant. This filter should be replaced after each complete filling.

Once the cylinder has reached the operating pressure, the compressor is stopped, the excess pressure is released from the filling hose and the compressed air coupling is disconnected. The system is then ready for use and you can dive. Once in the water, the small cylinder is hardly noticeable, the feeling is only slightly different from having a conventional snorkel in your mouth, except that you can continue breathing on the underwater hull. How long the air lasts depends heavily on your breathing technique; in the first few attempts, the red zone of the pressure gauge was reached after around 2.5 minutes.

With increasing familiarisation and correspondingly calmer breathing, the diving time could be increased to around five minutes

As you cannot see the pressure gauge during operation, the cylinder must be set down to check the supply. With increasing familiarisation and correspondingly calmer breathing, the diving time could be increased to around five minutes before the pressure gauge turned red. Even then, breathing air was still coming out of the regulator. The guide line is also very useful for saving air when diving with the mini cylinder.

A typical application for the system is likely to be clearing a tamp that has become entangled in the screw. The technique used to cut efficiently under water depends on the tool used. Because as supple as the line looks when dry, when it is wrapped tightly around the propeller and shaft several times, it stubbornly resists the knife attacks. It becomes particularly difficult when the cordage melts due to the frictional heat generated during wrapping. The fine nylon fibres of fishing nets in particular tend to pull themselves tightly into the gap between the propeller and the shaft bearing, where they bind together to form a plastic ring.

In the test, we tackled the cordage with an on-board knife, cutter, bread knife and, at the suggestion of a reader, also with garden shears.

Looking from above or diving down? The best equipment for both is on board

Everyone has to decide for themselves whether the investment in the Mini-Dive system is worth it for the short period of use. The air is enough to scrape pocks off the propeller, and Mini-Dive is a real benefit. However, if the underwater hull is to be cleaned over a large area, the system is unsuitable.

You should also remember that this is a diving system and that the same dangers lurk as when using normal diving equipment. You should therefore familiarise yourself with the subject matter in advance and at least master the basics. This is especially true if you want to go deeper than the keel.

The action cam on the boat hook, on the other hand, is a small financial outlay, and it doesn't have to be the latest model. It can also be used safely to check the situation with dry feet.