Crossing the estuary from Oeiras — scenario test

Question: would it be possible to keep a La Mare Apartboat XL at the Oeiras marina and, on a weekend, cross the Tagus estuary and anchor it near the opposite bank?

Short answer: the three elements of the scenario (Apartboat XL + Oeiras home berth + weekend crossings) are valid two at a time, but the three together don’t coexist. This is a requirements conflict, not a matter of willpower.

Facts

Entering and berthing at Oeiras: yes, it fits

• Porto de Recreio de Oeiras limits: 25 m length, 6 m beam, 3 m draft. A 12×5 m Apartboat XL with ~0.5 m draft clears with room to spare.
• A berth for 5 m of beam is a direct conversation with the marina — expect a multihull surcharge.
• The problem isn’t the pontoon, it’s the water outside: the port sits at the mouth of the Tagus, with prevailing N/NW winds (the nortada). Oeiras is a boat marina facing the Atlantic, with real sea at its doorstep.

The crossing is where the scenario breaks

• From Oeiras, “the opposite bank” is Trafaria/Costa da Caparica — and what lies in between is the throat of the estuary: the main shipping channel with commercial traffic, tidal currents that easily exceed 2–3 knots on spring tides, and Atlantic swell rolling in through the bar.
• A CE category D boat is certified for waves up to 0.3 m; on that stretch, that exists for a handful of glassy dawns per month in summer.
• A 12-metre box with walls that act as sails, two small outboards and a 5-knot top speed has zero margin when the wind shifts at 11 a.m.
• Outside its design category, insurance also lands in swampy territory.
• Even delivering the boat to Oeiras by water would need a weather window; by road, 5 m of beam means an abnormal load permit.

Verdict: with an Apartboat XL, that specific crossing is technically possible in a few hand-picked windows and imprudent in all the others.

The same dream in the right geography

Base at Parque das Nações and the same weekend becomes a viable classic:

• Leave on a calm morning with the tide in your favour, cross the Mar da Palha — the inner half of the estuary: wide, sheltered, no bar.
• Anchor in Seixal bay (mud, excellent holding, protected all around) or on the Alcochete/Montijo flats — 1h–1h30 from home at 5 knots.
• Wake up at anchor in front of the flamingos; return with the tide.
• Oeiras becomes 20 minutes by train instead of 20 minutes by boat.

Requirements generated by this scenario (→ Phase 1.3 matrix)

• Oversized anchor for a houseboat’s windage (sail-walls).
• Anchor light.
• Tides studied before every departure — the Alcochete/Montijo flats dry out.
• Daytime return — the Marinheiro licence is daylight-only navigation.
• Dinghy — Marley needs to get ashore, and nobody rows a house.

The decision this test opened

See plans/decisions.md, DEC-004: Oeiras as home berth (Must) forces a CE-C boat (trawler, motorsailer, liveable power cat à la Overblue — trading m² for seaworthiness); anchoring getaways in home mode (Must) keep the houseboat and move the base to Parque das Nações.