PSModel a look inside

HOME

Introduction

Quick Start

Quick Series
Facts

PSModel
Program

Data Input

Data Output

Importing a
Data file into
MS Excel

Main Menu

Main Screen

Printing
Graphs
and Data

Screen Colors
and Fonts

Screen
Graphs

Shortcuts

Working
with Data

Testing:

Test Sheet

Examples

19' Ski Boat

34' Sailboat

41' Utility Boat

Technical

Blade Area
Ratio

Calculating
the Cavitation
Number

Horsepower
Losses

Hull Speed

Kt Breakdown

Propeller
Geometry

Propeller Law

Wake Factors

Glossary

References

Wake Factors:

The flow into the propeller is disturbed by the presence of the boat hull and appendages as the boat moves through the water.  As a result, the mean water velocity or Advance Velocity into the propeller is usually less than the boat velocity.  This is expressed mathematically as:

Va = V × (1-w)

Va -- propeller Advance Velocity, (ft/sec)
V -- Boat velocity or speed, (ft/sec)
(1-w)  -- is called the Wake Factor
w -- is called the Wake Fraction and is usually less than 1

Possible sources of wake factor (1-w) information for a specific boat are:

1) the boat manufacture's technical department

2) Internet forums on marine propellers or boat design

3) references 8, 9, and 12 -- Reference 8 concerns auxiliary sailboats; reference 9 has information on wake factors for displacement, planing, and sailboat hulls; and reference 12 has a great deal of information on wake factors for ships, but is a technical book for Naval Architects and Marine Engineers.

Approximate Wake factors (1-w) for various boat types are discussed below:  

Sailboats:

Type of sailboat Wake Factor
(1-w)
full keel, single propeller in an aperture, minimally inclined shaft 0.85
full keel, single off-center propeller 0.90
fin keel, single propeller on an inclined shaft ahead of a spade rudder 0.90

Outboard motor or I/O powered boat:

1) Contact the technical department of the outboard motor manufacturer to see what wake factor they recommend.

2) If unable to obtain information from the manufacturer, assume that (1-w) = 0.95.

Flat or V bottom boats:

The graph below is based, in part, on information from reference 14, and applies to boats with the following characteristics:

1) single or twin propellers on inclined shafts

2) propeller(s) set mostly outside wake shadow of hull

3) displacement or planing boats

V-bottom boat

Wake factor for flat or V-bottom hulls onr or two propellers

Small trawler like hulls:

The graph below is based on information from reference 13, and applies to boats with the following characteristics:

1) single propeller on a level or slightly inclined shaft

2) propeller is usually in an aperture between the hull and rudder

3) propeller sets well within the wake shadow of the hull

The Block Coefficient, Cb, is calculated as follows:

Cb = Boat weight / (Lwl × Bwl  × T × Q)

where:

Boat weight = boat weight or displacement in pounds (lbs)

Lwl = length of the boat on the waterline (ft)

Bwl = beam of the boat at the waterline (ft)

T = mean draft of the molded hull, the molded hull does not include keel, propellers, or rudders (ft)

Q = weight of one cubic foot of water = 62.4 lbs/ft3 if boat is used in fresh water or 64.2 lbs/ft3 if used in salt water

Wake factor for small trawler like hulls, single screw

Full bodied ship like hull forms:

The graph below is based on information from reference 12, and applies to boats with the following characteristics:

1) single or twin propellers

2) propeller(s) set well within the wake shadow of the hull

3) hull has a relatively large Block Coefficient

The Block Coefficient, Cb, is discussed under Small trawler like hulls above. 

Wake factors for full bodied hulls, single and twin propellers