See your claim, quickly and easily.

The Seafarers' Earnings Deduction (SED) is a UK tax relief that lets qualifying seafarers deduct 100% of their sea-going earnings from their taxable income. For a seafarer earning, say, £80,000 a year, a successful SED claim typically recovers £15,000–£25,000 of income tax that was deducted through PAYE.

It exists because the UK wants to keep its merchant fleet and offshore industry crewed by UK residents. The relief has been on the books since 1988 and is set out in HMRC helpsheet HS205. It's the single largest tax break available to working seafarers.

It's complicated. That's the catch. The rules are unforgiving and one wrong day can invalidate an entire year's claim.

Three things trip people up most often:

• The 50% test. Across your eligible period, no more than half of your days can be UK days. Get this wrong and the whole period collapses.
• The 183-day cap. No single return to the UK can last more than 183 consecutive days. One overrun and the period ends on the day you exceeded it.
• The midnight rule. What counts as a UK day or a non-UK day is decided by where you are at midnight — not where you worked, where you flew, or where you slept.

Most people who lose claims aren't trying to game the system. They mis-count, they assume a travel day works one way when it works the other, or they don't realise they were over the 183-day limit until months later. We built 183 so the maths runs in real time and you see the consequence of every voyage the moment you log it.

Broadly, you qualify if all of the following are true:

• You're a UK resident (or resident in an EU/EEA country) for tax purposes during the claim year.
• You're employed as a seafarer — most of your duties are performed on a ship.
• You've built up an "eligible period" of at least 365 days that's mainly made up of days when you were absent from the UK.
• Across that eligible period, your UK days are no more than half the total, and no single return to the UK lasts more than 183 consecutive days.

"On a ship" matters. Offshore installations like fixed platforms don't count, and the UK sector of the North Sea is treated as part of the UK for SED purposes. Mobile offshore vessels — drillships, semi-submersibles, FPSOs in some cases — generally do qualify, but the precise vessel and your role on it matter. We check this for every customer at sign-up.

Your fail date is the date your current eligible period would break if you stayed in the UK from today onwards. It's the single most important number for an active seafarer.

If your fail date is six months out, you've got runway. If your fail date is next Tuesday, a delayed crew change matters very much. The app shows it on the home screen so you can plan your rotations, your shore leave, and your time at home around it.

The HMRC working sheet doesn't give you a fail date. You have to calculate one yourself by simulating "what if I stayed in the UK from today?" — which most people don't, which is why claims fail.

HMRC's rule: you are absent from the UK on a day if you are outside the UK at midnight at the end of that day. The day you leave the UK counts as a non-UK day if you're outside UK waters by midnight; the day you return counts as a UK day if you're back inside UK waters by midnight.

This is where most informal claims go wrong. People count the calendar days of a trip, or the days they were "working abroad," when the only thing that matters is the midnight position. Travel days in particular are ambiguous and need to be checked carefully.

For voyages, the test is whether the voyage (or the relevant part of it) begins or ends outside the UK. A coastal trip that never crosses an international boundary doesn't count, even if it took two weeks.

The claim itself is short — a single box on the Additional Information pages of your Self Assessment return. Almost all the work is in the supporting evidence, and HMRC can ask for it at any time, sometimes years after the claim is filed.

What we keep on file for every customer:

• Discharge book entries and crew lists
• Payslips for each employer in the period
• Flight tickets, boarding passes, and travel itineraries
• Hotel receipts for any non-UK stays
• Vessel midnight positions for each voyage day
• Voyage logs showing departure and arrival ports

The app captures this as you go. If HMRC asks, we produce the full evidence pack — usually within a working day — and most enquiries close without further questions.

183 is the number that decides almost every SED claim. No single return to the UK can last more than 183 consecutive days without breaking your eligible period. Half of 365 is roughly 183. It's the line between a successful claim and a failed one.

Naming the company after it is on the nose, intentionally. We want you watching that number.

Our partner is. We work with a chartered accountant who has filed SED claims for offshore workers for years. Every claim we file goes out under their signature. The app, the data, and customer operations are run by our own team.

Existing providers are spreadsheet-and-email services. You send in your evidence by email, somebody enters it into a spreadsheet, and you find out at year-end whether your claim worked. Between claims, you have no live view of your own position.

183 gives you a real app that calculates your fail date in real time and shows the impact of every voyage as you log it. The claim itself is still filed under a chartered accountant's signature, same as the alternatives. We expect to charge a premium over the existing spreadsheet-and-email services because the visibility you get is fundamentally different — and because our security posture is built around the SK Tax breach of customer passwords being a thing that should not happen again.

Yes — up to four years back, which is HMRC's general time limit on overpayment claims. If you've been eligible but never claimed SED, that can be £20–30k recovered in one go, sometimes more for senior offshore workers. Our backdating fee will be announced with the rest of pricing before launch — and you pay nothing if HMRC doesn't pay out.

We hold every piece of supporting evidence — payslips, midnight positions, flight logs, discharge book pages, hotel receipts — securely on file and ready to produce. The SED claim itself is short; the work is in the evidence. We make sure that's solid.

Once your first SED claim has been processed, you can apply for an NT (No Tax) code. With NT in place, your employer stops deducting income tax at source — so instead of paying tax through PAYE and reclaiming it a year later, you keep the money in your account from day one.

HMRC will only issue NT after your first SED claim, so it's a year-two move, not a year-one one. We handle the application as a paid add-on once you're eligible.

Stripe is the easy path — annual billing, card on file with Stripe (not us). We can also handle bank transfer or PayPal if Stripe is awkward for you. Mention it when you sign up and we'll sort it.

It's designed phone-first because that's what offshore workers actually carry. It works on tablets and laptops too. Self-service migration is best on a laptop. If you'd rather skip that step, the done-for-you migration covers it (priced as a paid add-on, with a discount on Tier 2).

Feel free to get in touch before signing up. Telegram us at @admin183tax or email [email protected]. No commitment, no obligation. We'd rather you ask the awkward questions before paying than after.