High-Speed Handling

Prerequisites

You should be confident at the helm of a planing powerboat from Powerboat Level 2 and Intermediate: getting on the plane, steering at speed in calm water, slow-speed manoeuvring, coming alongside, and the basic effects of propeller walk and engine trim. You should also know your boat's controls: throttle and gear lever, power trim and tilt, trim tabs if fitted, and the kill cord. This lesson builds on the previous lesson's briefing and kill cord routine; nothing here should be practised without the kill cord attached.

Learning Objectives

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

Compare deep-V, RIB and catamaran hulls and explain how each behaves at speed.
Explain how a hull moves from displacement through the transition hump to the plane.
Use engine trim and trim tabs to set the best running attitude, and recognise and correct porpoising, chine walk and bow steer.
Turn safely at speed, and explain why turns can eject crew or cause loss of grip.
Distinguish propeller ventilation from cavitation, and know what causes each.
Handle a boat with twin engines, at speed and at close quarters, including with one engine out of action.

Hull Types

The shape of the hull decides how a boat rides in waves, how it turns and how much power it needs. Most fast boats you will drive on the Advanced course are one of three types, seen from astern in the diagram below.

Deep-V

The deadrise is the angle between the bottom of the hull and the horizontal, measured at the transom. A deep-V hull has a deadrise of about 20 to 24 degrees. The sharp V cuts into waves rather than slamming on them, so the ride is softer in a head sea.

Advantages: good rough-water ride, directional stability at speed, banks into turns like a motorbike so crew are pressed into their seats.
Disadvantages: needs more power to plane than a flatter hull, rolls more at rest, and can be prone to chine walk at high speed.

Chines are the outer edges of the bottom where it meets the topsides. Many fast hulls have a small flat ledge (a chine flat) there, and several spray rails along the bottom. Spray rails throw water sideways, add lift and help the hull release from the water to plane.

RIB (rigid inflatable boat)

A RIB is a deep-V GRP or aluminium hull with buoyant tubes around the sides. The tubes sit just clear of the water at rest and when level, and touch it when the boat heels or is heavily loaded.

Advantages: very stable at rest even with people on the tubes; tubes add reserve buoyancy (a RIB that is flooded still floats); a forgiving, soft-edged boat for coming alongside other boats; tubes knock down spray.
Disadvantages: open, wet and cold; tubes must be kept at correct pressure; the light bow can lift in strong wind, which is a real risk of a flip in big waves at speed.

Catamaran

A powered catamaran has two narrow hulls with a tunnel between them. At speed, air compressed in the tunnel adds lift and cushions the ride.

Advantages: wide, stable platform for work and fishing, little roll at rest, and a smooth ride in a short chop.
Disadvantages: some cats bank outward in tight turns, which feels alarming and can throw crew towards the outside; a following sea can bury a bow, and the tunnel can slam in big seas. Handling differs a lot between designs, so learn your own boat's habits in calm water first.

Getting on the Plane

Displacement, hump and planing

At slow speed, every boat is in displacement mode: it pushes water aside and is limited to roughly its hull speed. As speed rises, the boat starts to climb its own bow wave. The bow rises steeply, the stern squats, and visibility ahead gets worse. This is the transition or hump. The resistance is at its highest here, and the boat burns a lot of fuel for little speed.

Once over the hump, the hull rises and runs on the surface, supported by water pressure on the aft part of the bottom (planing). The bow drops, the wake flattens, resistance falls relative to speed, and the boat accelerates. A typical 7 m RIB planes cleanly at about 15 to 18 knots.

How to plane quickly and safely

Check all round, especially ahead (the bow will block your view for a few seconds).
Trim the engine fully in (leg tucked under), and put trim tabs down if fitted. This holds the bow down over the hump.
Warn the crew: "Going up."
Open the throttle smoothly and fully. Do not stop on the hump.
As the boat comes onto the plane, ease the throttle back to cruising revs, then trim out gradually until you find the best running attitude.

If the boat will not climb over the hump, move crew weight forward, check for weed or a damaged propeller, and check you are not overloaded.

Trim

Trim is the fore-and-aft attitude of the boat when it is running. It is the single most important adjustment you make at speed, and you should keep adjusting it as the sea and the load change.

Engine trim

On an outboard or sterndrive, the power trim moves the leg in or out:

Trim in (leg tucked towards the transom): the propeller pushes the stern up, the bow goes down.
Trim out (leg pushed away from the transom): the propeller pushes the bow up.

The diagram below shows three running attitudes.

Three running attitudes: trimmed in with bow down, correctly trimmed, and trimmed out too far causing porpoising

How to find the right trim. On the plane in calm water, trim out a little at a time while watching speed (GPS speed over ground) and engine revs. Speed will rise as wetted surface falls. When the boat starts to bounce (porpoise) or the revs rise sharply without more speed, trim back in a touch. That is the best trim for those conditions. Most planing boats run best at about 3 to 5 degrees bow up.

Trim tabs

Trim tabs are small plates at the bottom of the transom, operated by hydraulic or electric rams. Lowering a tab creates lift at the stern on that side.

Both tabs down: bow down. Useful to get on the plane, to see over the bow, or to drive into a short head sea.
One tab down: raises that side. To level a boat that is listing to port (for example, from a crosswind or crew sitting on one side), lower the port tab.

Use tabs in short presses and wait a few seconds for the effect before pressing again. Over-correcting is the common mistake. Return tabs to neutral before slowing down or turning hard, or the boat may dive or turn unexpectedly.

Trim problems at speed

Problem	What you feel and see	Cause	Correction
Porpoising	Bow rises and falls rhythmically, faster with speed	Trimmed out too far; bow too light	Trim in, tabs down a little, or reduce speed
Chine walk	Boat rocks quickly from one chine to the other at high speed	Deep-V running on its narrow keel area, trimmed out too far at high speed	Trim in, reduce speed, adjust tabs to level
Bow steer	Bow digs in; boat veers or darts off course without warning	Trimmed in too far at speed, especially in a following sea	Trim out, reduce speed
Listing	Boat runs with one side low	Crosswind, crew weight, engine torque	Move crew, use one trim tab

Bow steer is dangerous. When the bow is trimmed down at speed, the forefoot (front of the keel) grips the water and acts like a rudder at the wrong end. The boat can suddenly turn hard, sometimes enough to throw crew out. If the steering feels light and "nervous" at speed, trim out at once.

Turning at Speed

What happens in a turn

A planing boat turns by banking and sliding. A deep-V or RIB banks inward, pressing crew into their seats. As the bank increases, the hull loses some lift and grip, the propeller rises nearer the surface, and the boat slows.

The risks in a fast turn are:

Crew ejection. Unwarned crew can be thrown across or out of the boat, particularly on a cat that banks outward or if the hull trips.
Hooking. On some hulls, a sudden tight turn causes the chine to dig in and the boat to "hook" violently, with a risk of capsize or throwing everyone out.
Propeller ventilation. The propeller draws in air and loses grip (see below), and the stern slides.
Own wake. A tight turn brings you back across your own wake at speed, which can launch the boat.

How to turn safely

Look round, and especially behind, before turning. Fast boats often have other boats close astern.
Warn the crew: "Turning to port."
Reduce speed before a tight turn. A good rule is to come off the plane for any turn of more than 90 degrees in a short space.
Trim in a little so the propeller keeps its grip.
Turn progressively, not by spinning the wheel.
Watch for your own wake and other boats' wakes.

Never use a crash stop (snapping the throttle to neutral at speed) unless you have to. The stern wave following the boat catches up and can flood over the transom, and crew are thrown forward. Slow down progressively.

Propeller Ventilation and Cavitation

These two terms are often confused. Both cause a sudden loss of thrust, but the causes and cures are different.

Ventilation

Ventilation happens when the propeller draws air from the surface or from the exhaust. The blades lose grip, revs rise suddenly, and the boat slows or slides.

Causes:

Engine trimmed out too far, so the propeller is too close to the surface.
Engine mounted too high on the transom.
A tight turn at speed, when the propeller rises in the water.
Driving in heavy, aerated water, such as when jumping waves or crossing wakes.
Damage to the anti-ventilation plate (the flat plate above the propeller).

Cure: throttle back until the propeller grips, trim in, and then accelerate again smoothly.

Cavitation

Cavitation is different. At high blade speed, water pressure on the back of the blades drops so low that the water boils, even when cold, forming vapour bubbles. When these bubbles collapse on the blade surface, they cause erosion pitting of the propeller.

Causes:

A damaged, bent or dirty propeller (nicks on the blade edges are common).
Wrong propeller for the boat (too much pitch or diameter for the engine).
Very high revs, or objects such as fishing line or weed near the propeller.

Cure: inspect and repair or replace the propeller, and check it is suitable for the boat and engine.

Twin-Engine Boats

Why twins

Many fast boats over about 7 m have two engines. Twins give redundancy (you can get home on one), more power for a given engine size, and much better control at close quarters.

Counter-rotating propellers

On a twin installation, the propellers normally turn in opposite directions. Seen from astern, the starboard propeller turns clockwise (right-handed) and the port one turns anticlockwise (left-handed). Both propellers then turn outward at the top. The paddle-wheel effect (propeller walk) of one cancels the other, so the boat goes straight in reverse, and the steering is balanced at speed.

Handling at speed

At speed, twins are driven together: both throttles matched, both engines trimmed the same, and steering with the wheel. Most twin installations have a sync gauge or automatic synchroniser to match revs. Unmatched revs cause vibration, a "beat" in the engine note, and a slight pull to one side.

Close-quarters handling with twins

At low speed, twins let you turn the boat on the spot without moving forward or back. The diagram below shows the principle.

Plan view of a twin-engine boat: both ahead goes straight; one engine ahead and one astern spins the boat on the spot

Both ahead: the boat goes straight.
Port ahead, starboard astern: the port side is pushed forward and the starboard side pulled back, so the bow swings to starboard.
Port astern, starboard ahead: the bow swings to port.

Rules for close-quarters work:

Keep the throttles at tickover. Use the gear levers to control the boat, one or both at a time, in short bursts.
On shaft-drive boats, leave the wheel amidships and steer with the gears. On outboard and sterndrive boats the leg angle also steers the thrust; learn in open water whether your boat spins better with the wheel amidships or turned.
Because astern thrust is usually weaker than ahead thrust, an equal ahead/astern spin may creep forward. Add a short burst of astern on both to stop it.
Use the wind: a fast boat with a high bow and little underwater area blows downwind quickly when stopped.

Coming alongside with twins. Approach at a shallow angle, slowly. As the bow comes near the pontoon, put the outer engine astern to stop the boat and swing the stern in. For example, coming alongside port side to, the starboard engine astern pulls the stern towards the pontoon. Then neutral, and step off with lines.

Leaving a berth with twins. To spring the stern off when pinned on a pontoon, rig a spring from the bow leading aft to the pontoon and put a fender at the bow. Put the outer engine gently ahead: the spring stops the boat moving forward, the bow pivots against the fender and the stern swings out. Once the stern is well clear, slip the spring and go astern on both engines.

One engine out of action

If one engine fails at speed:

Throttle back on the good engine, stop the boat and find out why the other has failed. Check the kill cord and fuel first.
Tilt the dead engine clear of the water if it is an outboard, to reduce drag.
Run on one engine at moderate speed. The boat will pull towards the side of the dead engine; correct with the wheel and, if fitted, with the trim tab on the side of the working engine.
You may not be able to plane with a full load. Plan for a much slower, more fuel-hungry passage home.
Tell the coastguard or a shore contact if your return will be late.

Worked Example: Setting Up a RIB for a Fast Passage

You are skippering a 7.5 m RIB with a single 250 hp outboard, four crew and a full tank, in a short northerly chop of about 0.5 m. Your course is 180 degrees.

Getting on the plane. You trim fully in, tabs down, warn the crew and open up smoothly. The RIB climbs the hump in about four seconds and planes at 17 knots. You ease to 4,200 rpm and tabs back to neutral.

Trimming. You trim out gradually. GPS speed rises from 24 to 27 knots; then the bow starts to bounce slightly. You trim in a touch, and the bouncing stops at 26.5 knots. That is your trim for this sea.

Listing. A steady breeze from the west, with two crew sitting to port, gives a list to port. You ask one to move to starboard and lower the port tab slightly; the boat levels.

Turning. A sailing dinghy fleet ahead forces you to turn 90 degrees to port. You look behind (a fast motor cruiser is 200 m astern), call "turning port", come off the plane, and turn at 10 knots. Back on course, you check the crew are holding on before accelerating.

Changing sea state. Later, the sea turns to a following sea from the north. The boat starts to feel nervous and darts a few degrees off course: early bow steer. You trim out by a couple of degrees and slow down by 3 knots; the steering settles.

Common Mistakes

Leaving trim fully in once on the plane. The boat ploughs, uses more fuel and risks bow steer. Trim out once planing.
Trimming out too far, then wondering why the boat porpoises or the propeller ventilates in turns.
Over-correcting with trim tabs. Use short presses and wait.
Turning hard at full speed without warning the crew or looking behind.
Crash stops that bring the stern wave over the transom.
Confusing ventilation and cavitation. Ventilation is air from the surface (trim and turning); cavitation is vapour and blade damage (propeller condition and selection).
Using the throttles, not the gears, for close-quarters twin handling. Tickover and gear shifts give much finer control.
Unsynchronised twin engines at speed, causing vibration and a pull to one side.

Summary

Deep-V hulls ride well in waves but need power; RIBs add tubes for stability and buoyancy; cats are stable but may bank outward in turns.
Plane by trimming in, accelerating smoothly over the hump, then easing back and trimming out.
Find the best trim by trimming out until the boat porpoises, then back in a touch. Typical running attitude is 3 to 5 degrees bow up.
Porpoising and chine walk mean trimmed out too far; bow steer means trimmed in too far at speed.
Lowering a trim tab raises that side; use tabs in short presses.
Before turning: look behind, warn the crew, slow down, trim in a little, turn progressively.
Ventilation is air reaching the propeller; cavitation is low-pressure boiling that pits the blades.
Twins turn outward at the top; one ahead and one astern spins the boat on the spot. Use the gears at tickover for close quarters.

Check Your Understanding

What is deadrise, and how does a deep-V's deadrise affect its ride?

Answer: Deadrise is the angle between the bottom of the hull and the horizontal, measured at the transom. A deep-V has around 20 to 24 degrees. The steep V cuts into waves instead of slamming, giving a softer ride in a head sea, but needs more power to plane and rolls more at rest.

Describe how to get a heavily loaded boat onto the plane.

Answer: Look all round, trim the engine fully in and put tabs down if fitted, warn the crew, then open the throttle smoothly and fully without stopping on the hump. Once planing, ease back to cruising revs and trim out gradually. If it still will not plane, move weight forward and check the propeller and load.

Your boat starts to bounce rhythmically at speed in calm water. What is happening and what do you do?

Answer: It is porpoising, caused by trimming out too far so the bow is too light. Trim in a little (or lower the tabs slightly) or reduce speed until it stops.

The boat suddenly darts off course in a following sea, and the steering feels light. What is the likely cause and correction?

Answer: Bow steer: the boat is trimmed in too far, so the forefoot grips the water and steers the bow. Trim out and slow down.

Your boat is listing to starboard because of crew weight. Which trim tab do you lower?

Answer: The starboard tab. Lowering a tab creates lift on that side of the stern, raising the low side. Better still, move crew weight first.

Explain the difference between ventilation and cavitation.

Answer: Ventilation is air drawn into the propeller from the surface or exhaust, usually because the engine is trimmed out too far, mounted too high, or the boat is turning hard. Cavitation is the water boiling on the back of the blades because of very low pressure, usually because the propeller is damaged or wrongly sized, and causes erosion pitting.

Why should you avoid a crash stop at speed?

Answer: When you cut the throttle suddenly, the boat stops but its stern wave keeps coming. It can flood over the transom, and crew are thrown forward.

In a twin-engine boat, which engine do you put ahead and which astern to turn the bow to port on the spot?

Answer: Starboard engine ahead and port engine astern. The starboard side is pushed forward and the port side pulled back, so the bow swings to port.

One outboard of a twin installation fails during a passage. List the main actions.

Answer: Slow down and stop; check kill cord and fuel to try to restart; tilt the dead outboard clear of the water; continue at moderate speed on the good engine, correcting the pull with the wheel and tabs; expect not to plane with a full load and allow more fuel and time; tell the coastguard or your shore contact if you will be late.