U-Boat Darkmoon 44 MM Gray SS

An oil-filled replica watches swiss that looks great and is within reach

Have you always been interested in oil filled watches? I was! My first contact with them dates back to the 1990s. At that time, Sinn, Bell & Ross and Kienzle launched their first oil-filled models almost simultaneously. Today U-Boat has a whole line of attractive oil filled watches and I had to try out the Darkmoon 44 MM Gray SS.

Sinn begrudgingly retrofitted an existing diver’s model to the 810Q and eventually rated it to 8,000 meters. SIOK, Kienzle’s Italian agency, presented The Deepest in titanium, rated for a depth of 9,000 metres. It was later even updated to 12,000 meters with a resin casing. Bell & Ross rated the Hydro Challenger (later Hydromax) at 11,100 meters in 1997 and was eager to claim that the brand’s oil-filled watches were unique. Obviously not, but we have to understand that this was almost before the internet, when things weren’t as transparent as they are now.

oil filled watch
The above examples are intended to address pressure and depth ratings for dive watches. Despite being rated for depths up to 12,000 meters, it is clear that the quartz crystal in the movement cannot withstand pressures of more than 500 bars. That equates to 5,000 meters, which is more than enough for any dive watch worn by a human. With the UX, Sinn still has an oil-filled dive watch in its catalog.

In addition to providing a relatively simple solution to increasing the pressure rating of a diver’s watch, oil filling also brings about another phenomenon-improved readability and appearance. Ressence Type 3 and Type 5 are excellent examples of how to use the enhanced readability provided by oiling. While the Type 5 may be marked as a diver’s watch, the Type 3 uses oil filling for enhanced readability and appearance for design purposes only.

U-boat Dark Moon
The looks lead me to the U-Boat. For aesthetic reasons, U-Boat uses oil injection in two of its model lines – Capsoil and Darkmoon. I’m glad U-Boat sent one of the latter to write this hands-on review. The model U-Boat provided me was a Darkmoon 44 MM Gray SS. The nomenclature says it all: the watch measures 44.8mm in diameter, has a gray dial, and a stainless steel case. It’s a good thing the model names are so descriptive, because U-Boat offers many variations of Darkmoon. Available in 40mm and 44mm sizes, there are at least six different dial colors, two case colors, and even a version with a red crystal. Don’t hesitate to explore all the variations here.

The Darkmoon is not a diver’s watch, so the reason for the lube can’t possibly be to increase the pressure rating. But another advantage that comes with oiling—improved readability and an attractive look—is well exploited.

oil swelling
When looking at a U-Boat Darkmoon, the first thing you’ll notice are air bubbles under the glass in the oil. Some might see this as a flaw, but it’s intentional and, in fact, necessary.

Oil expands and contracts as temperature changes. Unlike air, oil is incompressible. So if an oil-filled watch heats up, the oil will expand, and if the case is full of oil, the oil has nowhere to go. To solve this problem or situation, different fake watches swiss brands have found different solutions. Sinn uses a membrane-like bottom cover. As the oil inside the watch expands due to increased temperature, the inside of the watch back moves slightly outward. Ressence invented a neat, purpose-designed system in which seven air-filled or air-filled bellows are placed in the oil. These bellows expand or compress to compensate for changes in the internal volume of the oil between -5° and +55° Celsius. This range covers all temperature conditions the watch may encounter.

U-Boat Bubbles
However, simpler systems leave a small amount of air in the oil. The air bubbles can be compressed and retracted, acting like Ressence’s bellows. The biggest difference is that the air bubbles move freely in the oil and are visible on the dial. To me, the bubbles in the dark moon are not annoying at all. I’d even say I love it because it would make a great conversation starter. I do wish the air bubbles were slightly smaller, but I guess the size may have been calculated taking into account the expansion and contraction of the oil volume. For this watch, the smaller air bubbles most likely won’t do the job properly. As we can see in the picture of Kienzle The Deepest at the beginning of this article, the air bubbles are much smaller, but I’m sure so is the amount of oil in the watch.

What else is good?
With a battery-operated oil-filled watch, there’s always the worry of changing the battery. If you don’t pay attention, the oil will overflow when you open the watch to replace the battery. Unlike U-Boat Darkmoon. The brand has developed an innovative locking system (Locking Ring System) that allows quick and hassle-free battery changes. Another feature I welcome is the left crown. Specifically, with a watch of this size and the crown on the right side there is always a risk of irritating the back of the hand. However, I’m happy to report that despite its 44.8mm diameter, the Darkmoon is very comfortable to wear. The rubber strap is soft enough and I like that it has a regular pin buckle. For me, especially on stronger straps, this is often more comfortable than a folding clasp.

I love wearing the U-Boat Darkmoon Gray 44 MM Gray SS. I think I was drawn to it because of a picture on Instagram, and in real life, it didn’t disappoint. While the 44.8mm size fits my 18cm (7in) wrist, if I were to buy one, I think I’d go for the 40mm version. Of course, U-Boat is known for its large size watches, but I believe the 40mm version will be less flashy for its size and more attractive due to its extraordinary aesthetic.

Watch Specifications
Darkmoon 44 MM Gray SS
refer to
Gray to black, hands and numerals treated with white Super-LumiNova
case material
stainless steel AISI 316L
Case size
Diameter: 44.80mm
Superdome Sapphire
Bottom cover
Solid stainless steel with locking ring system for easy battery replacement
Rhonda 712.3 Quartz
50 meters/5 ATMs
Black rubber with stainless steel pin buckle
hours and minutes