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 × (1w)
Va  propeller Advance Velocity, (ft/sec)
V  Boat velocity or speed, (ft/sec)
(1w)  is called the Wake Factor
w  is called the Wake Fraction and is usually less than 1
Possible sources of wake factor (1w) 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 (1w) for various boat types are
discussed below:
Sailboats:
Type of sailboat 
Wake Factor (1w)

full keel, single propeller in an aperture,
minimally inclined shaft 
0.85 
full keel, single offcenter 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 (1w) = 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
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 / (L_{wl} × B_{wl}
× T × Q)
where:
Boat weight = boat weight or displacement in
pounds (lbs)
L_{wl} = length of the boat on the
waterline (ft)
B_{wl} = 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/ft^{3} if boat is used in fresh water or 64.2 lbs/ft^{3}
if used in salt water
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.
