Longines HydroConquest 39mm · Ref. L3.780 · 20mm lug · 39mm case
At a Glance
| # | Best For | Strap | Material | Price |
|---|---|---|---|---|
| 1 | Overall / First Purchase | Hunter Green Epsom Slim Strap | Epsom Calfskin | $110 |
| 2 | Dressy / Formal | Matte Purple Alligator Signature Strap | Alligator | $205 |
| 3 | Everyday / Casual | Pastel Blue Nubuck Slim Strap | Nubuck Calfskin | $110 |
| 4 | Minimalist / Refined | Cognac Shell Cordovan Slim Strap | Shell Cordovan | $165 |
| 5 | Casual / Relaxed | Colibri Rubberised Leather (Togo) Signature Strap | Togo Leather (Rubberised) | $110 |
| 6 | Active / Sporty | Navy Tropical CTS Rubber Strap | FKM Rubber (Tropical) | $145 |
| 7 | Comfort / Wildcard | Brown Alcantara Signature "Teddy Bear" Strap | Alcantara | $110 |
✓ No import fees — delivered duty-free✓ Lifetime warranty✓ Ships globally from USA, UK & Singapore
I have always believed that the right strap can fundamentally change how you experience a watch. The Longines HydroConquest 39mm (ref. L3.780) proves this better than most. The finishing and attention to detail speak for themselves. The 20mm lug width opens up a wide range of possibilities.
The case measures 39mm with a thickness of 12mm — proportions that sit comfortably on most wrists without dominating them. It strikes that balance between presence and wearability that makes it suited to almost any strap.
Here are my 7 picks — and why each one works.
The 20mm lug width means your options are curated but well-covered. Already know what you're looking for? Browse all Longines HydroConquest 39mm-compatible straps on the Strap Finder →
Swipe to browse all 7 picks →
1. Best Overall — Hunter Green Epsom Slim Strap
Your first strap upgrade should feel like an obvious improvement. This one does.
The Hunter Green Epsom Slim Strap features Epsom leather, tanned by Haas. The fine, uniform grain gives it a clean, contemporary look that resists scratches and moisture with ease. This is leather that looks as sharp six months from now as it does on day one. At 2.2mm thick, it balances presence and wearability nicely.
The Longines HydroConquest 39mm's 39mm case is ideally proportioned for this strap. Whether you are wearing it casually or dressing it up, the combination feels intentional. It is the kind of pairing where both the watch and strap look better together than apart.
A combination that's greater than the sum of its parts.
Worth a look in other colourways:
- Black Epsom Slim Strap
- Tinware Grey Epsom Slim Strap
- Izmir Blue Epsom Slim Strap
- Orange Epsom Slim Strap
2. Best Dressy — Matte Purple Alligator Signature Strap
When you need to dress the Longines HydroConquest 39mm up, this is the strap that delivers.
The Matte Purple Alligator Signature Strap is crafted from genuine alligator leather — not embossed calfskin, the real thing. Each scale pattern is unique, and the matte finish gives it a refined presence that catches light without being flashy. The leather is supple from the start with minimal break-in needed. At 4.5–2.2mm thick, it balances presence and wearability nicely.
On the Longines HydroConquest 39mm, this strap preserves the watch's go-anywhere character. The 39mm case at 12mm thick pairs naturally with the padded construction without creating any imbalance on the wrist.
Once you see it on the wrist, it just clicks.
The range goes deeper if you want to explore:
- Matte Black Alligator Signature Strap
- Matte Navy Alligator Signature Strap
- Matte Himalayan Alligator Signature Strap
- Matte Mahogany Alligator Signature Strap
3. Best Everyday — Pastel Blue Nubuck Slim Strap
The strap you reach for without thinking — that's what an everyday pick should be.
The Pastel Blue Nubuck Slim Strap is crafted from nubuck leather — soft, velvety to the touch, and comfortable from the first wear. No break-in period needed. The suede-like texture adds a casual warmth that takes the edge off a formal watch without losing sophistication. At 2.2mm thick, it balances presence and wearability nicely.
What makes this pairing work on the Longines HydroConquest 39mm is the contrast — the refined case finishing against the texture creates visual interest without conflict. With a 39mm case, the proportions stay balanced.
Once you see it on the wrist, it just clicks.
The range goes deeper if you want to explore:
- Natural Nubuck Slim Strap
- Navy Nubuck Slim Strap
- Light Grey Nubuck Slim Strap
- Dark Brown Nubuck Slim Strap
4. Best Minimalist — Cognac Shell Cordovan Slim Strap
Sometimes less is more. This strap proves it.
The Cognac Shell Cordovan Slim Strap is crafted from premium materials with careful attention to detail and construction quality. At 2.2mm thick, it balances presence and wearability nicely.
On the Longines HydroConquest 39mm, this strap preserves the watch's go-anywhere character. The 39mm case at 12mm thick pairs naturally with the padded construction without creating any imbalance on the wrist.
The kind of strap that earns a permanent spot in your rotation.
If this style appeals but you want a different tone:
- Black Shell Cordovan Slim Strap
- Navy Shell Cordovan Slim Strap
- Natural Shell Cordovan Slim Strap
- Green Shell Cordovan Slim Strap
5. Best Casual — Colibri Rubberised Leather (Togo) Signature Strap
Not every strap needs to make a statement. Sometimes relaxed is the right move.
The Colibri Rubberised Leather (Togo) Signature Strap uses rubberised leather — a material that blends the look and feel of leather with the water resistance and durability of rubber. The smooth finish is clean and modern, and the pastel colour options add a playful element to otherwise serious watches. At 4.5–2.2mm thick, it balances presence and wearability nicely.
What makes this pairing work on the Longines HydroConquest 39mm is the contrast — the refined case finishing against the texture creates visual interest without conflict. With a 39mm case, the proportions stay balanced.
Alternatives in the same family:
- Black Rubberised Leather (Togo) Signature Strap
- Navy Rubberised Leather (Togo) Signature Strap
- Hunter Green Rubberised Leather (Togo) Signature Strap
- Taupe Rubberised Leather (Togo) Signature Strap
6. Best Sporty — Navy Tropical CTS Rubber Strap
A sportier option that brings comfort and durability without sacrificing style.
The Navy Tropical CTS Rubber Strap is made from FKM rubber — a high-performance elastomer that resists chemicals, heat, and ageing. Unlike silicone, it does not attract dust or lint. Cut-to-size construction means you trim it to your exact wrist measurement — no leftover tail, no keepers. At 4.0–3.3mm thick, it balances presence and wearability nicely.
What makes this pairing work on the Longines HydroConquest 39mm is the contrast — the refined case finishing against the texture creates visual interest without conflict. With a 39mm case, the proportions stay balanced.
Simple, effective, and genuinely enjoyable to wear.
Alternatives in the same family:
7. The Wildcard — Brown Alcantara Signature "Teddy Bear" Strap
Not the obvious choice, but sometimes the unexpected pairing is the best one.
The Brown Alcantara Signature "Teddy Bear" Strap is crafted from Alcantara — the same Italian material used in luxury car interiors and high-end sportswear. It has a suede-like softness that is immediately comfortable, with a luxurious hand feel that belies its toughness. At 4.5–2.2mm thick, it balances presence and wearability nicely.
Paired with the Longines HydroConquest 39mm's distinctive dial, this strap adds a layer of personality that the factory option does not offer. The 12mm case thickness means a padded strap like this one feels proportional and substantial.
The kind of strap that earns a permanent spot in your rotation.
If this style appeals but you want a different tone:
- Dark Grey Alcantara Signature Strap
- Baby Blue Alcantara Signature Strap
- Dark Brown Alcantara Signature Strap
- Green Alcantara Signature Strap
A Note on Buckle Colour
Quick but important: always match your buckle to your case. The Longines HydroConquest 39mm's Stainless steel case calls for a silver (brushed or polished) buckle. All Delugs leather straps ship with a pin buckle and can be upgraded to a Spring Deployant Clasp for extra security and convenience. All Delugs straps come with quick-release spring bars pre-installed, so swapping straps takes seconds.
How to Change the Strap on Your Longines HydroConquest 39mm
Swapping straps on the Longines HydroConquest 39mm is one of the easiest upgrades you can make — and with Delugs straps, you don't even need a tool.
If your Longines HydroConquest 39mm came on a bracelet, you'll first need to remove it. The easiest way to do this at home is with a Bracelet Removal Tool, which lets you compress the spring bars without risking scratches to the lugs or caseback. Once the bracelet is off, you can slide your new Delugs strap straight in.
All Delugs straps come with quick-release spring bars already pre-installed, so changing straps is completely tool-free. Here's how:
- Prepare your workspace. Lay the watch face-down on a soft cloth or microfiber pad to protect the caseback from scratches.
- Release the spring bar. Locate the small lever on the quick-release spring bar. Pull it back with your fingernail or fingertip to compress the bar.
- Remove the strap. While holding the lever, angle the strap out from between the lugs. Repeat for the other side.
- Insert the new strap. Slide one end of the spring bar into the lug hole. Pull the lever on the quick-release spring bar knob, guide the entire spring bar into the opposite lug hole, and then release the knob. You should hear a small click when the spring bar extends and seats into the lug hole.
- Check the fit. Give the strap a gentle tug to make sure both spring bars are securely locked in. If there's any play, reseat the spring bar.
The entire process takes under a minute once you get the hang of it — no spring bar tool required. The Longines HydroConquest 39mm uses a standard 20mm lug width, and all Delugs straps in 20mm will fit perfectly.
Frequently Asked Questions
What is the lug width of the Longines L3.780?
The Longines L3.780 has a 20mm lug width. 20mm is one of the most common lug widths, so the selection is at its widest. All Delugs straps are available in 20mm.
What is the reference number for the Longines HydroConquest 39mm?
The reference number for the Longines HydroConquest 39mm is L3.780. You can find this on the caseback of your watch or on the original paperwork. Knowing your reference number helps ensure you get the right lug width when ordering a strap.
Can I use a dressy leather strap on the Longines HydroConquest 39mm?
The HydroConquest 39mm is one of the most versatile watches in the Longines lineup — it takes both dressy and casual straps equally well. That's part of what makes it such a rewarding watch for strap changes. Don't be afraid to experiment.
Do Delugs straps come with spring bars?
Yes — every Delugs strap comes with quick-release spring bars pre-installed. This means you can change straps by hand in seconds, without needing any tools. The quick-release mechanism uses a small lever that you pull with your fingertip to compress the spring bar.
What is the best strap material for the Longines HydroConquest 39mm?
The HydroConquest 39mm's versatile nature means it works with almost everything. Start with what matches your most common wearing occasion — rubber or sailcloth for active use, nubuck or waxy leather for everyday, and alligator or saffiano for dressier moments. The Strap Finder makes it easy to browse by material and style.
Does Longines use quick-release spring bars?
Most Longines watches use standard spring bars that require a tool to remove. All Delugs straps come with quick-release spring bars pre-installed, so once you make the switch, future changes are completely tool-free.
Where can I find straps compatible with the Longines L3.780?
The Longines Strap Finder shows every Delugs strap compatible with the L3.780, filtered by lug width and style. You can also use the general Strap Finder to browse by brand, collection, and material. If you need help choosing, our team is available 24/7 via live chat.
Finding Your Strap
The beauty of the Longines HydroConquest 39mm is that it rewards experimentation. A dressy alligator for the office, nubuck for the weekend — each strap brings out a different side of the watch.
Use the Longines HydroConquest 39mm Strap Finder to see every compatible strap in one place, filtered to your exact reference. Or if you'd rather explore by feel, the general Longines Strap Finder lets you browse the full lineup and find your match.
Not sure which strap is right for your specific HydroConquest 39mm? Our team is available 24/7 via live chat — drop us a message and we'll help you find the perfect match for your watch, wrist size, and style.
Quick Specs
| Case diameter | 39mm |
| Lug width | 20mm |
| Lug-to-lug | 46mm |
| Case thickness | 12mm |
| Water resistance | 300m |
| Crystal | Sapphire with multi-layer AR coating on both sides |
| Case material | Stainless steel |
| Movement | Automatic (72h power reserve) |
About Delugs
Founded in 2018 and headquartered in Singapore, Delugs is dedicated to elevating the experience of watch ownership through the simple yet powerful act of strap changing. We believe that straps are integral to how a timepiece is worn, perceived, and enjoyed. Our mission is to make strap changing a lifestyle — an everyday act of self-expression and versatility.
Delugs is an independent brand with a global community of clients, with a strong online presence and growing physical retail footprint. Our direct-to-consumer focus keeps us closely connected with the watch enthusiast community — fostering conversations, hosting events, and helping shape a more inclusive and expressive culture around watch collecting. We also work closely with watch brands to craft straps that complement their timepieces, bridging the gap between brand and collector, form and function.
Our catalogue spans leather, rubber, and fabric straps, with options for every watch and every occasion. We are committed to craftsmanship and quality, product design and innovation, and outstanding customer service. We offer watch owners the freedom to change straps as easily as changing outfits, and the opportunity to engage more deeply with the watches they love.
Visit our flagship boutique at Raffles Hotel, Singapore →
— Marcus
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