Sizing horsepower for a Lombok-built boat starts from three numbers: the boat’s loaded displacement, the speed the schedule genuinely requires, and the hull form’s behaviour between displacement and planing regimes. For a typical 12–15 m Gili-crossing hull carrying 20–30 passengers, that maths usually lands between 2 × 200 hp and 2 × 300 hp of outboard power; for a displacement wooden hull of the same length, a single 100–150 hp inboard diesel is often the honest answer. Everything else — brand loyalty, what the neighbouring operator bought, round numbers — is noise.
Start with displacement, not length
Length is the number everyone quotes; weight is the number the propeller feels. A 14 m fibreglass fast boat might leave the yard at 8 tonnes light and cross at 12 tonnes with passengers, crew, fuel and luggage. A 14 m solid-timber hull can weigh double that before anyone steps aboard. Power sizing that ignores loaded displacement produces the classic Lombok disappointment: a boat that hit the promised speed on sea trial — light, half fuel, six people — and never sees that speed again in service.
On desk-supervised projects the weight estimate is a living document from the first specification meeting: structure, machinery, tankage, outfit, passengers at 85 kg each with bags. The sea-trial protocol then tests the boat at representative load, not at its flattering minimum — the same discipline described on the build process and timeline page.
The speed question: what does the schedule actually need?
Speed costs power at a brutal rate. Pushing a planing hull from 22 to 28 knots does not add 27 percent power; depending on the hull it can add half again or more, with fuel burn to match. So the desk asks the operating question first: what crossing time does the timetable need, in what sea state, at what load?
- Bangsal–Gili Trawangan shuttle: short legs, many cycles per day. 20–24 knots loaded is usually the sweet spot; beyond that, fuel and engine-hours cost more than the minutes saved.
- South-coast dive runs: longer legs, exposed water. A boat trimmed for 18–22 knots with reserve power for head seas beats a nominal 30-knot boat that must throttle back anyway.
- Displacement cruisers and fishing hulls: hull speed rules. Past roughly 1.34 × √LWL (in knots and feet), extra horsepower buys wake, not speed. Size for hull speed plus a weather margin and stop.
The full specification logic for the crossing boats — hull form, deadrise, tank placement — is laid out in the post on Bangsal–Gili fast boat specifications.
A working power-estimate method
For planing and semi-planing hulls, a first-pass estimate the desk uses at concept stage is Crouch’s method: speed in knots = C ÷ √(displacement in lb ÷ shaft horsepower), with C between about 150 for average planing hulls and 190 for clean, light fast boats. Rearranged for power, a 12-tonne loaded hull targeting 24 knots with C = 165 needs roughly 560 shaft horsepower — which is why 2 × 300 hp outboards appear so often on that class of boat once real-world margins are added.
For displacement hulls the estimate is simpler: 3–5 hp per tonne of displacement delivers hull speed in calm water; 6–8 hp per tonne adds the margin to hold schedule against monsoon-season head seas. These are planning numbers, refined against propeller charts and, on larger commissions, a proper resistance calculation before any engine order is placed.
Outboard or inboard changes the whole equation
The power figure cannot be settled apart from the installation type. Outboards give the Gili trade quick swaps, shallow-water flexibility and transom-mounted simplicity; inboard diesels give displacement hulls torque, fuel economy and engine rooms that crews can service at sea. The running-cost comparison over a five-year horizon — purchase, fuel per crossing, service intervals, resale — is worked through in outboards versus inboards for a Gili passenger boat, and it changes the horsepower answer more often than owners expect: a fuel-efficient diesel at 80 percent load frequently beats a larger outboard pair at 60 percent on total cost.
The margin nobody should skip
Tropical service erodes power. Fouling on an unlifted hull can steal two or three knots inside a season; humid intake air, ageing propellers and accumulated weight do the rest. The desk specifies a 15–20 percent service margin: the boat must make contract speed at trial with the engines pulling no more than 85 percent of rated rpm at full load. An engine pair sized to hit the number flat-out on day one is undersized by definition.
Propeller selection is where that margin is actually delivered. The correct propeller lets the engine reach its rated rpm range at wide-open throttle, loaded. Over-pitched props — endemic on locally repowered boats — lug engines below their rated band, raise exhaust temperatures and shorten engine life while feeling deceptively fast at cruise.
What this means for your budget
Machinery is routinely the largest single line in a Lombok build quotation — often a quarter to a third of the whole project for fast boats. Right-sizing power is therefore the biggest single cost lever an owner controls: one engine-size step down, justified by an honest speed requirement, can fund the entire electronics and safety fit. The current USD bands and how machinery moves them are on the price and quote page, and the wider commissioning decisions sit in the fast boat section of the fast boat build programme.
Bring the desk your route, your timetable and your passenger count — not a horsepower figure. The right number falls out of the mission; the wrong number falls out of a brochure.
Frequently Asked Questions
How much horsepower does a Gili passenger fast boat need?
A typical 12–15 m hull carrying 20–30 passengers usually lands between 2 × 200 hp and 2 × 300 hp of outboard power for a loaded cruise of 20–24 knots. The exact figure follows loaded displacement and the timetable’s honest speed requirement, not the neighbouring operator’s brochure.
What horsepower suits a displacement wooden hull?
Plan 3–5 hp per tonne of displacement for hull speed in calm water, and 6–8 hp per tonne to hold schedule into monsoon-season head seas. Beyond hull speed, extra power buys wake rather than speed, so a single well-sized 100–150 hp diesel often serves a 15–20 m hull honestly.
What service margin should the engines carry?
The desk specifies 15–20 percent: the boat must make contract speed at sea trial with engines pulling no more than about 85 percent of rated rpm at representative load. Tropical fouling, humid intake air and accumulated weight erode performance, and a boat sized flat-out on day one is undersized.
Why does propeller selection matter as much as engine size?
The propeller decides whether the engine can reach its rated rpm range at full throttle, loaded. Over-pitched propellers lug the engine below its band, raise exhaust temperatures and shorten engine life while feeling deceptively strong at cruise. Correct propping is where the paper margin becomes a real one.
Related reading: quieter fast boats by design — how power choices feed straight into noise and vibration.