Water Craft Essentials

Dalan Zartman CRS, FF II, FI II, EMTP Rescue Methods 2014 ©

Outboard MotorsThe outboard motor

selection must begin with the required hp to effectively operate in the prescribed environment with the designated payload.

This is typically driven by the boat manufacturer based on load capacity, application, and boat design.

2 Stroke Enhanced

performance power to weight ratio

SimplisticAcceleration and

TorqueEmission

equivalent to 4 Stroke technology

Fuel economy comparable to 4 stroke technology

4 Stroke More ComplexCleaner

emissions in most cases

QuieterGreater fuel

economy in most cases

Decreased power to weight ratio

Jet drives vs Prop drivesJet Drives

FastShallowResilientInefficientRequire

propulsion to steer

Prop DrivesSlowerEasily

damaged in shallow water

EfficientCan steer

without propulsion due to rudder control

Additional ConsiderationsTiller controlTrimFuelMaintenanceReliabilityShaft Length

ConclusionTypically speaking, 2

stroke outboard motors with prop drives are optimal for swift water applications due to their weight to power ratio resulting in greater acceleration and reactive maneuverability in all water conditions.

PropsDurability

Affordability

Performance

3 BladesFaster top end

speedLess Torque

and acceleration

Material and Design impact performance

4 BladesGreater torqueSlower tope end

speedGreater

acceleration and maneuverability in all water conditions

Material and design impact performance

Prop featuresDiameterPitchRakeCup

DiameterDistance from tip to

tipThe greater the

blade, the greater the water displacement and thrust

The greater the blade, the greater the demand for torque and hp

PitchPitch results in the

theoretical distance traveled per revolution

Low pitch equals fast acceleration but lower top end speed

High pitch equals slow acceleration but greater top end speed

RakeRake impacts the

bow of the craft and its ability to plane out and reduce drag

Increased rake brings the bow up but may result in instability

Decreased rake brings the bow down but may result in increased drag

CupCup maximizes

the efficiency of the blades ability to “grab” the water.

Reduces ventilation and slipping

Increases RakeIncreases Pitch

Trouble ShootingVentilation (often confused with

cavitation) is the result of air pockets which make a prop lose its thrust. This manifests as free climbing RPM’s without gaining speed.

Typically caused by lack of power, high transom mountings, extreme trim, or sharp turns.

Corrective actions:Anti ventilation plate orientationProp Configuration – Increase CupIncrease hp

Trouble ShootingCavitation occurs when a

combination of decreased pressure and increased temperature cause gas bubbles to form which in theory “boil” and vaporize or implode causing pitting and scorching to the prop.

Typically caused by flaws or imperfections and damage to the lower unit or anti cavitation plane. Can also be caused by continuous or excessive ventilation.

Corrective Action: Eliminate ventilation problem Insure lower unit is undamaged

Trouble ShootingInability to plane out will

typically occur when the boat is underpowered, overloaded, or trimmed out inappropriately.

Corrective Actions:Insure weight and load

to power ratio is correctAdjust trimProp Configuration –

Increase Rake

2 Tinga SpecificationsRecommendations

Minimum 40 hp 2 stroke outboard

Maximum 60 hp 2 stroke outboard

4 blade prop with maximum diameter and minimum pitch and rake, moderate cup

Transom SpacerHands on training