Marine Propellers
Arthur Waite, Ph.D.
news group who want to know about the different style propellers, how to
size them, and how the parameters can affect the sizing decision. As
someone who specializes in sizing marine props for all kinds of
applications, I can tell you that it's a complex problem, especially when
you have to meet a performance guarantee.
When sizing a propeller, the goal is to match the thrust requirements of
your boat to the thrust produced by a propeller. This is done by altering
the propeller characteristics (diameter, dar, p/d, blade number and prop
rpm for a selected speed ) for that particular blade style until
equilibrium is reached. This sounds simple, but it requires a detailed
resistance calculation of your boat (perferably sea trial data, if
available), to determine boat thrust requirements, as well as computer
software to perform the itteration. Once this has been done, the prop can
be sized so that you get either "good" performance when operating under
loaded conditions, or it can be sized so that you can go like hell at wot.
Unfortunetly you can't have both, but there is a point somewhere in between
that makes everyone happy (sometimes!).
Here is some usefull info that everyone should know about props...
There are several propeller types that most commercially available stock
propellers are based on. The Wageningin B-Series (air foil blade sections)
and the Gawn series (flat face segmental sections). The B-series props are
manufactured with a rounded leading edge, and produce higher efficiency
than the Gawn series. The B-series has a slighly more complicated shape,
and is prone to cavitation developing at an earlier stage than the Gawn
series. The Gawn series blade is a popular blade section, and is commonly
used for props on pleasure craft and high speed boats. It also finds use
in towing applications. The ogival section is slightly less efficient than
the air-foil shape, but because of it's evenly distributed lift it is less
prone to cavitation. Other props exist as well, the Newton-Rader and Tulin
blade styles are designed to operate efficiently in highly cavitating
applications, and of cource we can't forget about surface piercing
propellers (Rolla series). Ducted or shrouded propellers serve no useful
purpose for pleasure craft and high speed boats. They are mainly used in
towing applications where thrust developed at static pull needs to be as
high as possible. Nozzles can increase static pull thrust by as much as
50% as compare to an open wheel operating in the same conditions. Nozzles
also require a different blade design, commonly refered to as a kaplan
blade, it is designed to give excellent propeller characteristics when
operating within the nozzle.
Diameter - in theory, the larger the diameter, the higher the efficiency,
except on boats where speed is in excess of 30 knots. In the region of
higher speeds, the extra wetted surface of things like the propeller
itself, shafting, struts, bearings etc cause more drag and hence the
decrease in efficiency. Larger diameter props also generate more thrust,
and are also a way for the designer to keep cavitation levels of the blade
below 10%l. The larger the diameter, the slower the shaft speed must be,
this is due to torque loading. Some negative points about diameter, the
larger it is, the more power to turn it, and there may be diameter
restrictions due to geometrical constraints of your hull.
Blade area - Blade area generally doesn't affect boat performance, but the
trend is that less blade area increases efficiency. Blade area is one of
the tools used by naval architects to control cavitation. The greater the
area, the less cavitation that will occur on the back face of the blade.
The greater blade area distributes the generated pressures so that the lift
in any particular spot stays below a certian cavitation level (the standard
is 10%, and a good designer can meet it almost every time). Some other
concerns about blade area are if the area is to low, structural concerns
will require that the blade thickness be increase regardless of the
material being used. There are guide lines as to blade thicknesses set out
by orginizations like ABS, DNV Loyds etc...
Number of blades - As with blade area, the fewer the blades the greater the
efficiency and the greater the thrust. The reason for increasing blade
number is to reduce blade loading, and hence cavitation (hopefully) and to
reduce propeller induced vibrations. If you're looking for good
acceleration (boaters call it "hole shot") then a four bladed open wheel is
probably a good choice. If you're looking for wot (wide open throttle)
then a three bladed is probably the way to go.
Pitch - is the theoretical distance that the propeller will travel in one
revolution. For example, a square wheel (one where the diameter and pitch
are equal) having a pitch of 26 inches would travel 26 inches/revolution.
Reality, however, dictates that the prop actually travels alittle less than
theory would have it travel. This is known as slip and it has everything
to due with the medium that the prop is traveling through, and nothing to
due with "hull geomerty" as some people seem to think. Slip's behaviour
also follows well developed trends. To calculate slip, first calculate the
geometrical speed of advance, GSA = pitch[feet] x RPM/101.3. Slip is
defined as - Slip = 1 - speed/GSA where speed is in knots. Pitch and
blade angle are different. For a fixed pitch propeller, the pitch is
constant throughout the entire blade. The blade sections that make up the
blade have their angles of attack set so that each section is advancing the
same distance as the others. Some points about pitch are the more pitch,
the more power to turn your wheel. The higher the rpm, the less pitch is
needed to drive your boat through some distance as compared to lots of
pitch and low rpm. High pitch will cause back face cavitation, and too low
may cause face cavitation (rare though - mainly seen in tug boat
applications).
Skew - is another way for the designer to reduce propeller induced
vibrations. The skewed design allows the blade sections to pass through a
region of water at different times (starting from root to tip) and hence
reducing the large periodic pressure changes that a non-skewed blade would
see. As for an increase in performance figures, there will be no noticeble
increases in thrust in the ahead direction. There will be a significant
decrease in backing performance. Skew blades are also more expensive to
manufacture because of the complexity of the design.
Rake - is another one of those tools used by the naval architect to steal
alittle extra prop diameter where geometric constraints need to be
enforced. As an example, a calculation has been done and it has been found
that a 17 inch prop will develop 9% cavitation, but I only have room for
16.5 inches and this prop produces 15% cavitation. One solution would be
to rake the blades, thus giving me my 17 inches, and keeping cavitation in
check. Comparing two props having the same design parameters, one raked
and the other not, there is no increase in thrust. In the event that
ventilation becomes a problem, raking the blade aft my solve it.
Cavitation - is the point where the pressure on the blade falls below that
of water vapour pressure. At this point, the liquid changes into a vapour
under a vacuum and produces bubbles or cavities. As the bubble travels
across the blade, it collapses in the form of a shock wave on the surface
of the blade producing pitting, erosion, and even structural damage (bent
blade tips). Cavitation also produces noise and vibration. Cavitation and
Ventilation are not the same thing. In some designs, ventilation is
induced as a means to control cavitation, but aside from that, the most
common methods of cavitation control are, more blade area, more diameter
and adding blade cup over the trailing edge of blade.
hope this helps
Arthur
Arthur Waite, Ph.D.
marine engineer/naval architect
(propeller design consultant)
OSG Design Group
M DeMetz
Mike