SED lets UK seafarers deduct 100% of foreign earnings from income tax IF they spend ≥365 qualifying days outside the UK across an eligible period. The HMRC rules are unforgiving — one missed day can sink a year's deduction.
What counts as a qualifying day
- Full days at sea (00:00 to 23:59) on a vessel registered for international voyages
- Travel to/from joining a vessel (with proof of itinerary)
- Shore leave abroad outside the UK during a qualifying voyage
What kills it
- Day 0 / Day 365 calculation mistakes — count days with HMRC's Form HS205 worksheet
- More than 183 days in any rolling 365-day window in the UK
- Unpaid yard periods on UK soil
Keep a running spreadsheet from day one. We've seen four-year cases unwound by a missed shore-leave entry from year two.