Engines, Drives and Propellers

Prerequisites

You should hold the RYA Powerboat Level 2 certificate and have the experience expected at Intermediate Powerboat level. You should have completed the High-Speed Handling lesson in this course, which introduces engine trim, trim tabs, ventilation, cavitation and twin-engine handling at speed. This lesson goes further into the machinery itself: how each type of drive behaves, how to choose and look after a propeller, how to manage fuel, and how to find and fix faults at sea. Basic engine checks are also covered in the Course and Skipper lesson; here we build those into a full routine.

Learning Objectives

By the end of this lesson you should be able to:

Compare outboards, sterndrives, shaft drives and waterjets, and explain how each affects handling at speed and in close quarters (RYA syllabus: boat handling, characteristics of different drive types).
Set engine trim and trim tabs together for different loads and sea states (RYA syllabus: high-speed manoeuvres, use of trim and trim tabs).
Handle a twin-engine boat using the engines rather than the wheel, and cope with one engine out of action (RYA syllabus: boat handling, twin engines).
Explain propeller diameter, pitch, slip and blade count, and recognise ventilation and cavitation from their symptoms (RYA syllabus: boat handling, propeller effects).
Carry out a thorough engine check before and after a passage (RYA syllabus: preparation for sea, engine checks).
Plan and monitor fuel use, including reserves and contamination risks (RYA syllabus: passage planning, fuel management).
Fault-find the common causes of engine failure at sea in a logical order (RYA syllabus: emergency situations, engine failure).

RYA Syllabus Map

Syllabus item	Section
Boat handling: drive types and characteristics	Drive Types Compared
High-speed manoeuvres: trim and trim tabs	Engine Trim and Trim Tabs Together
Boat handling: twin engines	Twin-Engine Handling
Boat handling: propeller effects	Propellers
Preparation for sea: engine checks	Engine Checks
Passage planning: fuel	Fuel Management
Emergency situations: engine failure	Fault Finding at Sea

Drive Types Compared

The way the power reaches the water decides how the boat steers, how it behaves astern, how it copes with shallow water, and what goes wrong with it. An Advanced Powerboat skipper may be handed any of these boats, so you need to know the character of each before you leave the berth.

Outboard engines

An outboard is a self-contained unit: powerhead, gearbox, leg and propeller, all clamped or bolted to the transom and steered as one. Modern four-stroke outboards range from small tender engines to units over 400 hp, and many fast RIBs and sports boats carry two or three of them.

Steering: the whole unit turns, so the propeller thrust is directed where you point it. This gives strong steering both ahead and astern, even at low revs.
Trim: the whole unit tilts, giving a wide trim range and the ability to tilt right up for beaching, shallow water or clearing a fouled propeller.
Maintenance: the engine can be lifted out of the water when moored, which reduces fouling and corrosion.
Weaknesses: heavy weight right at the transom, exposed to knocks, and the leg is vulnerable to grounding. A single outboard on a small boat can be damaged or swamped if the transom is pooped by a following sea.

Sterndrives (inboard-outboard)

A sterndrive has the engine inside the hull, just forward of the transom, driving through the transom to an outdrive leg that looks like the lower half of an outboard. The leg steers and trims in the same way as an outboard.

Steering and trim: similar to an outboard, with good thrust astern and a usable trim range.
Weight: the engine is inboard, so weight is better distributed and the cockpit is cleaner.
Weaknesses: the transom bellows (rubber gaiters) that seal the drive to the hull perish with age and can let water in; a failed bellows can sink a boat on its mooring. The leg stays in the water when moored, so anodes and antifouling need attention. Engine bays need ventilation before starting a petrol sterndrive, because petrol vapour is heavier than air and collects in the bilge.

Shaft drives

On a shaft-drive boat, the engine drives a fixed propeller shaft running out through the bottom of the hull, with a separate rudder behind each propeller. This is the common arrangement on larger displacement and semi-displacement motor cruisers and on many workboats.

Steering: the rudder only works well when water is flowing past it, either from the boat moving or from propeller wash. At low speed and astern, steering by the wheel is weak. Twin-shaft boats are steered in close quarters almost entirely on the engines.
Prop walk: because the propeller is angled down slightly and cannot be turned, prop walk (the sideways kick of the stern, strongest astern) is pronounced. On a single-shaft boat, learn which way the stern kicks and use it.
Protection: the propeller is under the hull, behind a skeg or keel, so it is better protected from grounding than an outdrive leg.
Trim: no engine trim, so trim tabs or interceptors do all the work of adjusting running attitude.

Waterjets

A waterjet draws water through an intake in the bottom of the hull, accelerates it through an impeller and pumps it out through a steerable nozzle at the transom. A reversing bucket drops over the nozzle to deflect the jet forward for astern thrust.

Advantages: no propeller below the hull, so jets can run in very shallow water and are safer near swimmers. They are used on rescue craft, some RIBs and personal watercraft.
Steering: you steer the jet stream, not a rudder, so no thrust means no steering. Throttle right back at speed and the boat may carry straight on regardless of the wheel. Always keep some power on to turn.
Astern and neutral: the bucket position sets ahead, neutral and astern. Neutral is a balance point rather than a disengaged gear, so the boat may creep.
Weaknesses: weed, plastic bags and stones can be sucked into the intake and reduce thrust. Many jets have a clearing hatch or a reverse flush. Efficiency at low speed is poor.

Summary of drive types

Drive	Steers by	Astern steering	Shallow water	Typical weak point
Outboard	Turning the whole unit	Good	Tilt up	Exposed leg, transom weight
Sterndrive	Turning the leg	Good	Trim up a little	Bellows, corrosion
Shaft	Rudder	Poor at low speed	Protected prop	Strong prop walk
Waterjet	Nozzle	Needs thrust	Very good	Intake blockage

Engine Trim and Trim Tabs Together

The High-Speed Handling lesson explains what engine trim and trim tabs do on their own. At Advanced level you are expected to use them together, and to set the boat up for the conditions without thinking about it.

What each control does

Engine trim (outboards and sterndrives) changes the angle of the propeller thrust. Trim in (down) pushes the bow down; trim out (up) lifts it. It changes the running angle of the whole boat fore and aft.
Trim tabs are flat plates on the bottom of the transom. Pushing a tab down lifts the stern on that side, which lowers the bow and lowers the opposite side. Used together, they lower the bow; used separately, they correct a list.
Interceptors are vertical blades that drop down at the transom. They work like tabs but respond faster and are common on newer boats.

A setting-up routine

Start trimmed fully in with tabs up, to get on the plane quickly with the bow down.
Once planing, trim out in small steps. The revs rise slightly and the speed increases as the wetted area drops. Stop when the boat begins to porpoise, then trim back in a touch.
Use tabs to level the boat if wind or crew weight is giving a list. Small adjustments of a second or two on the button at a time.
Retrim for every change of sea, load or course. Head sea: trim in, tabs down a little. Following sea: trim out, tabs up. Beam wind: tab down on the windward side to stop the boat leaning into the wind.

Signs of mistrim

Symptom	Likely cause	Remedy
Porpoising (bow bouncing rhythmically)	Trimmed too far out	Trim in, or tabs down slightly
Bow steer, boat darts sideways	Trimmed too far in at speed	Trim out
Heavy spray and slow speed	Bow too low, too much wetted area	Trim out, tabs up
Persistent list	Wind or uneven load	Tab down on the low side
Revs rise sharply, speed falls	Ventilation from over-trim	Trim in, reduce throttle, recover

Note the tab rule that confuses people: to lift the low side, put the tab on that same side down.

Twin-Engine Handling

The High-Speed Handling lesson introduces counter-rotating propellers and basic twin-engine control. This section develops the technique you need for the close-quarters tasks in the Close-Quarters Handling in Wind and Tide lesson.

Why the engines steer

On a twin-engine boat, the two propellers sit either side of the centreline. Running one ahead and one astern produces a turning couple: the boat rotates around a point near its middle. Running one engine ahead alone turns the boat away from that side (port engine ahead turns the bow to starboard). On shaft-drive boats with weak rudders, this is the main way to steer at low speed. On outboards and sterndrives, the legs also steer, which gives you even more control but also more ways to get it wrong.

Counter-rotation

Most twin installations have outward-turning propellers when going ahead: the starboard propeller turns clockwise and the port propeller anticlockwise, viewed from astern. Their prop walk cancels when both run together, and when used alone each one helps the boat turn the way you want. Check which you have before you need it.

Basic twin-engine controls

Straight ahead or astern: both engines in the same gear at the same revs, wheel amidships.
Turn on the spot: one engine ahead, the other astern. With legs or outboards, centre the wheel first so you know where the thrust goes.
Walk sideways: possible with outdrives by combining opposed gears with the legs turned (the principle of joystick systems). On shafts, it needs bow thruster help.
Hold position: small bursts on alternate engines against the wind.

Wheel amidships

Many skippers with outboard or sterndrive twins steer on the engines with the wheel locked amidships. This removes one variable: the gears do the steering. With the wheel hard over, a burst ahead on one engine and astern on the other can move the stern far more than you expect. Pick one method and use it consistently.

One engine out of action

At sea: run the good engine at moderate revs and correct the steady turning moment with the wheel. On outboards and sterndrives, tilt the dead engine clear of the water if possible to reduce drag. A boat that planed on two engines may only reach displacement or semi-planing speed on one, which changes the fuel picture completely (see Fuel Management below).
In close quarters: a single engine on a twin boat is offset from the centreline, so it turns the boat strongly. Make the approach slower and plan to use the good engine's kick. For example, with only the starboard engine working, astern pulls the stern to starboard; approach a berth port side to so that going astern brings you alongside.

Propellers

Diameter, pitch and slip

Diameter: the width of the circle swept by the blade tips. Larger diameters suit heavier boats and lower speeds.
Pitch: the distance the propeller would move forward in one turn if it were screwing through a solid. A 19-inch pitch prop would, in theory, advance 19 inches per revolution.
Slip: the difference between the theoretical distance and the actual distance. A planing boat typically has 10 to 15 percent slip; a heavy displacement boat much more.

A quick estimate of theoretical speed in knots is: pitch (inches) × propeller rpm ÷ 1215. Propeller rpm is engine rpm divided by the gear ratio. For example, a 19-inch pitch, engine at 5500 rpm, gear ratio 1.86:1: propeller rpm about 2960, theoretical speed about 19 × 2960 ÷ 1215 = 46 knots. With 12 percent slip, real speed is about 41 knots.

Choosing the pitch

Each engine has a rated wide-open-throttle (WOT) rpm range, given by the manufacturer, for example 5000 to 6000 rpm. The propeller should let the engine reach this range at full throttle with a normal load.

Too much pitch: the engine cannot reach its rated rpm. It labours, is slow out of the hole, and can be damaged by running overloaded.
Too little pitch: the engine over-revs and hits its rev limiter before the boat reaches its potential speed.
As a rule of thumb, each inch of pitch changes WOT rpm by about 150 to 200 rpm.
A boat used fully loaded with passengers and kit may need a lower pitch than the same boat used light.

Blade count and material

Three blades: the common choice, good top speed and efficiency.
Four or five blades: better grip in rough water and when trimmed high, smoother running, better hole shot, slightly lower top speed. Popular on RIBs that work in waves.
Aluminium: cheap, bends and sacrifices itself on impact (which can save the gearbox).
Stainless steel: stiffer, thinner blades, more efficient, holds its shape, but transfers more shock to the drive if it hits something. Most propellers have a rubber or plastic hub designed to slip under a severe impact.

Ventilation and cavitation

Both cause a loss of grip and are often confused. The High-Speed Handling lesson explains the mechanics; here is how to tell them apart and what to do.

	Ventilation	Cavitation
Cause	Surface air or exhaust drawn into the blades	Water boils at low pressure on the blade face
When	Tight turns, over-trim, leaving the water in waves, engine mounted too high	Damaged or bent blade, wrong prop, debris, very high blade loading
Symptom	Sudden rev rise, loss of thrust, then grip returns	Steady loss of performance, vibration, noise
Damage	Little in itself, but control loss can be dangerous	Pitting and erosion of the blades over time
Remedy	Throttle back, trim in, then reapply power	Inspect, repair or replace the propeller

Inspecting the propeller

At every check, with the engine off and kill cord removed, look for:

Bent, chipped or missing blade tips (a chip of a few millimetres can cause vibration).
Fishing line wrapped behind the propeller. Monofilament can melt into the propeller shaft seal and let water into the gearbox; this is a common cause of gearbox failure.
A spun hub (the propeller turns on the shaft under load). Mark the hub and shaft with a pen line; if the line no longer matches after a run, the hub has slipped.
Secure propeller nut and split pin or tab washer.

Engine Checks

The Course and Skipper lesson lists the basic checks. At Advanced level, you should carry them out in a fixed order so nothing is missed, and know what each is looking for.

Before starting: the WOBBLE routine plus fuel

Many instructors use the WOBBLE memory aid (Water, Oil, Belts, Bilge, Leaks, Electrics), with fuel checked alongside it:

Water: bilge dry or normal level? Bilge pumps working? Cooling water intake clear of weed?
Oil: engine oil level (inboards and sterndrives on the dipstick; four-stroke outboards on their dipstick); gearbox oil not milky (milky oil means water ingress, often from fishing line); power trim and steering fluid levels.
Belts: alternator and water pump belts tight and undamaged on inboards.
Bilge and fuel: bilge dry and free of fuel smell; enough fuel for the plan plus reserve, fuel lines and primer bulb sound, no smell of fuel, filter bowl free of water.
Leaks: look under the engine and around hoses, clips, the stern gland or bellows.
Electrics: battery isolator on, batteries charged, connections tight, kill cord and spare available, navigation lights working.

Also check the engine is in neutral, the leg is trimmed down, and the propeller area is clear of swimmers and lines.

After starting

Tell-tale: outboards should show a steady stream of cooling water from the tell-tale outlet within a few seconds. No stream means stop the engine and check the intake. A blocked tell-tale tube can be cleared with a piece of wire; a weak stream may mean a worn impeller.
Inboards: water coming out of the exhaust with the exhaust gases.
Gauges: oil pressure up, no warning alarms, voltage showing charge (13.5 to 14.5 V on a 12 V system).
Gears and steering: check ahead and astern on each engine while still secured, and full lock to lock.

After the run

Flush outboards and jets with fresh water if the manufacturer recommends it, especially after use in salt or silty water.
Check again for leaks, fuel smell and bilge water.
Note the engine hours and fuel used; this gives you real consumption figures for next time.
Trim outboards up when moored to reduce fouling, unless the boat dries and the leg could be damaged.

Fuel Management

The Weather and Planning lesson introduces fuel consumption and the rule of thirds. This section covers how to manage fuel during the day.

Consumption and speed

Fuel use rises sharply with throttle. A typical petrol outboard uses roughly 0.4 litres per hour per horsepower at wide-open throttle, and much less at cruising revs. So a 250 hp outboard flat out burns about 100 litres per hour; at a cruising 4000 rpm it might use 45 to 55 litres per hour. Diesel engines are more economical, roughly 0.2 litres per hour per horsepower at full power.

What matters is litres per mile, not litres per hour. Most planing boats have a sweet spot just above planing speed where the miles per litre are best. Below planing speed, in the hump, consumption per mile is at its worst: the engine is working hard and the boat is moving slowly. This matters if you lose an engine on a twin boat, or slow down for a rough sea.

Measuring it on your boat

Fuel flow meters on modern engines show live consumption. Without one, fill to the brim, run a known distance and refill. Record speed, rpm, load and sea state. Build a table you can trust, such as:

Rpm	Speed (kn)	Litres per hour	Litres per mile
2500	9	22	2.4
3500	22	35	1.6
4500	30	58	1.9
5500	37	92	2.5

In this example, the best range is at about 3500 rpm; slowing to 9 knots in waves actually uses more fuel per mile.

Reserves and monitoring

Plan to use no more than one third of your fuel outbound, one third returning, and keep one third reserve, as described in the Weather and Planning lesson. Rough weather, a head sea or towing can double consumption.
Check the fuel at fixed points on the passage, such as each waypoint, and compare with your plan. If you have used more than planned at the halfway point, turn back or divert early.
Do not rely on the fuel gauge alone. Sender units in tanks can stick or read wrongly when the boat is trimmed bow-up. Know your dipped or flow-meter figure.

Fuel quality

Water in fuel, from condensation in a part-empty tank or a leaking filler cap, is a common cause of engine failure. Water-separating filters should be checked and drained before use.
Ethanol blended petrol (E10 in the UK) absorbs water and can separate into a layer at the bottom of the tank if left for months, which engines will not run on. Use fresh fuel, keep tanks full when laid up, and use a stabiliser if recommended by the manufacturer.
Diesel bug (microbial growth at the water and fuel interface) blocks filters on diesel boats. Keep tanks full, drain water and carry spare filters.
Portable tanks: check the vent is open, the connector clicks fully home, and the primer bulb firms up when squeezed.

Fault Finding at Sea

The Emergencies and Rescue lesson covers keeping the boat safe after an engine failure: anchoring, calling for help and preparing a tow. This section covers working out why it stopped, and getting it going again where you safely can.

Make the boat safe first

Before you lift the engine cover: is the boat drifting towards danger? If so, anchor or call for help now. Then put the crew to work: a lookout, someone on the radio watch, and the most competent person on the engine.

The logical order

Most engines need three things: fuel, air and spark (petrol) or fuel, air and compression (diesel). Most failures at sea are simple and fuel related. Work from the simplest cause outwards.

Kill cord. Is it attached? Has the clip pulled off the switch? This is the most common reason an engine will not start after a stop.
Gear lever in neutral? Most engines will not crank in gear.
Battery and isolator. If the engine will not turn over, check the isolator switch, battery connections and main fuse.
Fuel supply. Is there fuel? Is the tank vent open? Is the fuel line kinked or the connector loose? Squeeze the primer bulb: it should go firm. A bulb that stays soft is drawing air or no fuel.
Fuel filter. Is there water or dirt in the bowl? Drain it.
Overheating. Did an alarm sound before it stopped? Check the tell-tale or exhaust water. A plastic bag over the intake is a common culprit. Stop the engine, tilt it up and clear the intake.
Propeller. Is something wrapped round it? A rope round the prop can stall the engine or stop the boat moving even though the engine runs. Engine off and kill cord out before you reach in.
Electrics and sensors. On modern engines, a fault code may put the engine into limp-home mode with reduced revs. Read the display and the manual.

Common faults and fixes

Symptom	Likely cause	Action at sea
Will not crank	Kill cord, gear, battery isolator	Check each in turn
Cranks but will not fire	No fuel, empty tank, vent shut, kill cord switch	Prime, check fuel and vent
Runs then dies, especially at high revs	Fuel starvation, blocked filter, water	Drain filter, open vent, change tank
Alarm and reduced power	Overheating or low oil	Stop, check intake and tell-tale
Revs fine, little thrust	Spun hub, damaged prop, ventilation, weed in jet	Inspect prop or clear jet intake
Vibration	Damaged prop or something wrapped	Stop and inspect

Spares and tools to carry

A basic kit for a day boat: spare kill cord, spare fuel filter and impeller (if you can fit them at sea), spare propeller with nut and washer, the tool to fit it, spare fuses, insulating tape, cable ties, a sharp knife for cutting rope off the prop, spark plugs and plug spanner for petrol engines, and the engine manual. Practise changing the propeller ashore; doing it for the first time at sea is a good way to lose the nut over the side.

Worked Example: Losing an Engine on a Twin RIB

You are skippering an 8 m twin-outboard RIB, two 150 hp engines, four crew, returning from a 20-mile passage. You have 120 litres of fuel left from 300. The plan used 75 litres for the return at 28 knots.

The failure. Ten miles from home, the starboard engine alarm sounds and it slows to idle. You throttle both engines back to neutral, and the boat settles off the plane.

Make safe. You are 3 miles offshore with no hazards downwind, in a Force 3. No immediate danger. You brief the crew: one lookout, one ready on the VHF.

Fault finding. The display shows an overheat warning. The tell-tale on the starboard engine has no stream. You stop the engine, remove the kill cord, tilt the engine up and find a plastic bag over the lower unit intake. You clear it, lower the engine and restart. The tell-tale runs strongly. You let it idle for two minutes and watch the temperature fall back to normal.

Decision. The engine may have suffered from the overheat. You choose to run both engines at reduced revs, about 3500 rpm, and watch the gauges. If the alarm returns, you will continue on the port engine alone.

Fuel on one engine. If you had to run on one engine, the boat would probably not plane with four crew. At 8 knots, consumption might be 18 litres per hour, giving 1.25 hours for 10 miles, about 23 litres. Your 120 litres would be ample. A key point: a single engine at hump speed can burn more fuel per mile than two engines planing, so always check the figures before deciding.

Report. After landing, you log the overheat, check the impeller at the next service, and note that the cause was debris, not a pump fault.

Common Mistakes

Forgetting that a waterjet needs thrust to steer, and throttling off when trying to avoid something.
Not checking the tell-tale after starting, and cooking an impeller within minutes.
Fishing line round the propeller shaft left unchecked until the gearbox fills with water.
Using the fuel gauge as a fact. Measure consumption, and check the fuel at each waypoint.
Wrong propeller pitch for the load, so the engine labours and never reaches its rated rpm.
Confusing trim tab directions, and making a list worse.
Wheel hard over while using twin engines to manoeuvre, so the stern swings much further than planned.
Diving into the engine before making the boat safe when it stops.
Spare propeller with no nut, washer or tool, or never having practised the change.

Summary

Outboards and sterndrives steer by turning the drive and trim by tilting it; shafts steer by rudder and have strong prop walk; jets steer the water stream and need thrust to turn.
Use engine trim for fore and aft running angle, and tabs or interceptors for levelling and fine trim. Retrim for every change of sea and load.
Twin engines turn the boat by opposing thrust. Know whether your props are outward turning, and choose a consistent wheel position for close quarters.
Pitch should let the engine reach its rated WOT rpm with a normal load; one inch of pitch changes rpm by about 150 to 200.
Ventilation is air drawn in from the surface; cavitation is water boiling on the blade. Treat ventilation with throttle and trim; treat cavitation by fixing the prop.
Check engines in a fixed order before starting, check the tell-tale after starting, and record fuel and hours afterwards.
Plan fuel in litres per mile, keep a one-third reserve, and check fuel against the plan at fixed points.
When the engine stops, make the boat safe, then check kill cord, gear, battery, fuel, filter, cooling and propeller in that order.

Check Your Understanding

Why can a waterjet boat fail to turn when you throttle back hard to avoid an obstacle?

Answer: A jet steers by directing its water stream. With no thrust there is nothing to steer with, so the boat carries on in a straight line. Keep some power on while turning.

Your RIB develops a list to port in a beam wind from starboard. Which tab do you lower?

Answer: The port tab. Lowering a tab lifts the stern on that side, which raises the low side.

Your engine reaches only 4600 rpm at full throttle; the rated WOT range is 5000 to 6000 rpm. What does this suggest and what would you change?

Answer: The propeller has too much pitch for the load (or the hull is fouled or overloaded). Reduce pitch by about two to three inches to bring the WOT rpm into the rated range.

What is the difference between ventilation and cavitation, and how do you deal with each?

Answer: Ventilation is air from the surface or exhaust drawn into the propeller, giving a sudden rev rise and loss of grip; throttle back, trim in, then reapply power. Cavitation is water boiling at low pressure on the blade, often from a damaged prop, giving steady loss of performance and blade erosion; inspect and repair or replace the propeller.

After starting an outboard, there is no stream from the tell-tale. What should you do?

Answer: Stop the engine at once. Check the cooling water intake for weed or a bag and clear it, and clear the tell-tale tube. If there is still no stream, suspect the impeller and do not run the engine.

Why should you check behind the propeller for fishing line?

Answer: Monofilament line wrapped behind the propeller can cut or melt into the propeller shaft seal, letting water into the gearbox and causing failure. Milky gearbox oil is a sign this has happened.

Your engine stops at sea. List the first five things you check, in order, after making the boat safe.

Answer: Kill cord attached and switch fitted; gear in neutral; battery isolator and connections; fuel in the tank with vent open and connector secure (primer bulb firm); fuel filter for water or dirt.

Why might running home on one engine of a twin boat use more fuel per mile than running on two?

Answer: On one engine the boat may not plane and may run at hump speed, where the engine works hard but the boat moves slowly. Litres per mile can then be higher than when planing efficiently on two engines.