RIB Engine Size and Fuel Consumption Guide
Quick answer: Choose an outboard that places your normal loaded RIB comfortably within the builder’s approved power and weight limits—not simply the largest number on the transom plate. Fuel economy depends on the complete boat, propeller, load, trim, sea state and speed, so the only dependable figure is measured litres per nautical mile from a realistic sea trial.
How much horsepower does a RIB need?
The right engine must lift the boat onto the plane without strain, operate inside its recommended rpm range and deliver the control required for the boat’s real use. A tender carrying two people has a different requirement from the same hull carrying dive cylinders. A family boat cruising locally has a different duty cycle from an offshore RIB expected to maintain progress into head seas.
Begin with the manufacturer’s minimum and maximum permitted power and maximum transom weight. Then calculate the likely operating load: crew, fuel, batteries, anchor and chain, electronics, seating, safety kit and activity equipment. Use our RIB size guide before power selection if the basic platform is not yet decided.
Engine-size decisions at a glance
| Priority | What to favour | What to check |
|---|---|---|
| Easy planing with load | Strong mid-range torque and correct propeller | Loaded acceleration and full-throttle rpm |
| Fuel range | Efficient cruise at the boat’s normal load | Litres per nautical mile, not litres per hour alone |
| Low-speed work | Predictable throttle and steering control | Marina handling, trolling and charging output |
| Offshore resilience | Appropriate installation, fuel reserve and possibly twins | Weight, drag, servicing and redundancy plan |
| Trailerability | Lowest complete weight that meets the mission | Boat, engine, fuel and trailer plated limits |
Why maximum horsepower is not automatically best
More power can improve acceleration, load carrying and throttle authority, but it also adds purchase cost, transom weight and potentially fuel demand. A heavy engine can alter static trim and reduce payload available for people and equipment. Selecting only by top speed can produce a boat that is less balanced in normal use.
Under-powering creates its own problems. An engine that must work near full throttle to hold an ordinary cruise may be noisy, inefficient and unable to recover speed in waves or with a heavier load. The target is usable reserve within the approved envelope.
Match the propeller to the loaded boat
Horsepower and propeller choice cannot be separated. Diameter, pitch, blade area and material affect acceleration, engine rpm, grip and efficiency. Mercury’s owner guidance states that propeller selection should allow the engine to operate within its specified full-throttle rpm range under normal load. An incorrect propeller can reduce performance and potentially harm the engine.
Complete propeller testing with the fuel, crew and equipment normally carried. Record maximum rpm, acceleration, cruise speed and fuel use. If the engine cannot reach the prescribed band, do not assume the answer is simply more horsepower; propeller, loading, hull condition, mounting height and engine health all need checking.
How to compare RIB fuel consumption
Litres per hour tells you how quickly fuel leaves the tank. Litres per nautical mile tells you how much fuel is required to cover distance. For passage planning, the second number is usually more useful:
Litres per nautical mile = litres per hour ÷ speed in knots.
Example: if a boat uses 24 litres per hour at 24 knots, that test point equals 1 litre per nautical mile. This is an illustration, not a prediction for any Storm model.
Build a simple test table at displacement speed and several planing speeds. Mercury’s guidance on finding an efficient cruise recommends repeating measured runs at increasing rpm and comparing fuel economy. Modern engine displays or networked instruments can make this straightforward.
What changes fuel economy?
- Speed: pushing beyond an efficient cruise can increase consumption sharply.
- Load: people, fuel and equipment increase displacement and planing demand.
- Trim: poor engine or weight trim creates unnecessary hull drag.
- Sea state and wind: head seas require different speed and throttle choices from flat water.
- Tube and hull condition: marine growth, damage or low tube pressure can affect drag and attitude.
- Propeller: damage or unsuitable pitch prevents the engine working efficiently.
- Engine installation: mounting height and rigging influence drag and water flow.
- Driving: repeated hard acceleration and unstable throttle use more fuel.
Single versus twin outboards
A single installation is lighter, simpler and normally cheaper to buy and service. Twin engines can add manoeuvrability and propulsion redundancy, which may be valuable for longer offshore work, but they also add weight, drag, maintenance and complexity.
Two engines do not automatically create full redundancy. Shared fuel, batteries, controls or contaminated fuel can affect both. The decision should reflect operating area, recovery plan, payload and the design of the specific boat. Larger Storm/Charge offshore RIBs can be assessed for appropriate single- or twin-engine configurations.
Plan fuel range conservatively
Do not multiply a calm-water brochure figure by tank capacity and call it safe range. Establish usable fuel capacity, measured consumption at realistic cruise, expected sea and wind, diversion distance and a deliberate reserve. Recalculate when load or conditions change.
A commonly used planning discipline divides fuel between the outward passage, return and reserve, but it is not a substitute for a passage-specific calculation. Navigation errors, weather, fouling, repeated acceleration and searching for a berth can all consume the margin.
Engine weight matters as much as horsepower
Compare dry engine weight, oils, steering, battery and rigging. Two engines with similar rated power may place different loads on the transom. The builder must approve the installation and assess its effect on capacity, trim and handling.
For trailered boats, engine and fuel weight also count toward the trailer’s actual load and the tow vehicle’s limits. The RIB ownership-cost guide helps compare the wider financial effect of different power packages.
Sea-trial checklist for engine selection
- Load the boat as it will normally operate.
- Confirm the propeller and engine mounting specification.
- Record time to plane and visibility through acceleration.
- Record rpm, speed and fuel rate at repeatable test points.
- Calculate litres per nautical mile.
- Check full-throttle rpm only where safe and permitted.
- Test trim response, turning, low-speed control and reverse.
- Review engine temperature, charging and warning systems.
- Repeat in more than one direction to reduce wind and tide distortion.
Choose the complete propulsion package
Compare Storm models and approved power ranges on the RIB comparison page. Use the Storm RIB configurator to define the intended boat and equipment; the final engine, propeller and rigging package should then be checked against load, use and delivery requirements.
RIB engine and fuel FAQs
Does a bigger outboard always use more fuel?
No. Consumption depends on how hard the engine works and how efficiently the boat travels at a given speed. A larger engine at moderate load can sometimes match or beat an undersized engine working hard, but weight and installation still matter.
What is the best cruising speed for a RIB?
There is no universal speed. Test several stable planing speeds and compare litres per nautical mile. The efficient point changes with load, propeller, trim and conditions.
Should I compare litres per hour or litres per mile?
Use both. Litres per hour helps monitor the engine and time-based use; litres per nautical mile is more useful for comparing passage range at different speeds.
Are twin engines safer offshore?
They can provide propulsion redundancy, but only if the installation and operating plan avoid shared failure points. Twins also add weight, cost and maintenance, so assess the whole system rather than counting engines.