RACING AND MARINE
Info on choosing a prop
Traditionally, propeller selection has been approached with apprehension, a "black
art" of sorts. We've tried to take away some of the mystery surrounding propellers
and replace it with facts you can use. By reading this information and referring to
the application guide*, you too, will be able to make intelligent propeller choices.
Best all-around performance is achieved when wide open throttle (WOT) engine
operation occurs near the top of (but within) the wide-open-throttle RPM operating
range designated by the manufacturer for that specific engine. Selecting the wrong
propeller not only reduces performance, but, in fact, can damage the engine.
An engine that does not reach the recommended RPM at wide-open-throttle is in
an "over-propped" condition, resulting in "lugging." This high-torque operation puts
a tremendous load on the pistons, crankshaft, and bearings. This may cause
On the other hand, an engine that revs past the recommended RPM will have
higher than normal wear and can also be damaged by fatigued parts breaking and
passing through the engine.
That's why it's essential that your engine is propped correctly for your boat/engine
combination and the type of boating you want to do. To make this selection,
propeller charts are published as guidelines for general applications of Mercury
Propellers. They are not intended, however, to be an absolute recommendation, as
boats and operating conditions vary. Use the guidelines suggested here, but
remember, the best propeller for your boating needs can be determined only by
You really should have more than one propeller. If you use your boat for more
than one type of activity, such as cruising, fishing, or skiing, you may well need
different propellers for the best performance in each type of activity. In any event,
you should keep a spare propeller on board at all times, along with a wrench that
will fit the propeller nut, pliers, a spare nut, spare drive sleeve, and related
How to read a propeller selection chart
Mercury Propeller charts include a boat speed range that is directly related to gross
weight and boat length. The interrelation of these factors has a marked effect on
the speed and performance of boats that fall within any given category. Generally,
gross weight is the major factor (total weight of the entire package -- boat, motor,
fuel, passengers, and miscellaneous equipment). When choosing a propeller, this
should be the primary consideration.
Propeller lines normally are designed so that each inch of pitch will change engine
RPM by 150 to 200. So, if the engine RPM falls too low on your first propeller
selection, try a lower pitched propeller to bring the RPM up. Higher-pitched
propellers reduce the engine RPM.
Pitch Change Calculation
There's an easy calculation to determine how much pitch change may be required.
Just follow these steps:
1. Check the specifications in your operator's manual for the recommended
operating range at wide open throttle (WOT) for your engine. A tachometer
is necessary to determine the WOT RPM.
2. Run the boat/motor combination at WOT to determine the maximum RPM.
Adjust engine trim angle for optimum performance.
3. If the WOT RPM is below the recommended RPM range of the engine, note
the reading of the tachometer. Take that reading and subtract it from the
top end of the operating range.
Example: Operating range = 5000-5600 RPM
Top End of operating range = 5600 RPM
Tachometer reading = 4800 RPM
Difference = 800 RPM
4. For every 1 inch of pitch change, the effect will be approximately 200 RPM.
Knowing this, take the difference in the above example at 800 and divide it
by 200. The result is 4. The next propeller to try will be 4 in. in pitch less
than the propeller that was first used. You should now either have the right
propeller or be only one size off.
The type of hull determines what kind of propeller is required to move the boat
through the water.
There are two basic hull types: displacement and planing. Displacement boats are
comparatively slow. They are supported by the weight of the water they displace
(static water pressure). They are generally designed with a somewhat pointed or
rounded stern as well as a pointed or rounded bow. They ride low in the water like
large ocean-going ships. The propellers on these boats run totally submerged and
will generally be of lower pitch.
Planing boats, which include most recreational boats, operate like a displacement
hull at low speed (below 10 MPH). With sufficient power, these boats rise to the
water's surface and operate in a skimming or planing mode, supported primarily by
the water's velocity pressure. Planing boats are generally faster and more efficient
than displacement boats. Their design is distinguished from a displacement design
by having a broad transom that meets the bottom with a basically sharp corner.
The propellers on these boats often are not fully submerged and thus need to
provide holding ability as well as higher pitch and rake because of the higher
The following is a description of typical hull designs for outboard and sterndrive
When a boat's bottom has no "V", but is simply flat from side to side, it is
generally referred to as a flat bottom. Where the bottom meets the side is called
the "chine." It may have angled corners (called square, sharp, or hard chines), or
rounded corners (called round or soft chines).
This is currently the most common bottom design,
offering good speed with a softer ride that depends
on the angle of the vee (called "deadrise"), the
radius or shape of the keel line, and the use of
strakes. To increase top speed with only a little loss
of softness in the ride, some boats are made with a small flat area at the very
bottom, called a "pad".
Each manufacturer chooses how many strakes, and how far back (toward the
transom) to bring each strake, in order to achieve the desired performance
characteristics. A greater use of strakes tends to make the boat run higher and
flatter, but a little harder.
Tri-Hull or Cathedral Hull
These hulls usually are vee bottoms with some
degree of added outside hull, often most
predominant near the bow. The benefit is a boat that
is more stable, particularly at rest. The penalty is a
rough ride in choppy water.
This design is most popular in racing circles. It differs
from the older catamaran bottom in that the inside
corners (between the bottom and the tunnel) are
quite sharp. This allows incredibly sharp high-speed
turns and a very soft ride. Some of these hulls have
experienced handling problems at low speeds.
This combines a shallow vee bottom with twin
tunnels, one on either side of a center pad. Top-end
performance is usually superior to a true vee bottom,
but this generally comes with a rougher ride in
choppy water and does not carry a load as well as a conventional vee.
A pontoon is basically a flat surface floating on two or
three pontoons usually made of aluminum. This
design maximizes a boat's usable space. Pontoons
can easily be run at planing speeds, but the front
ends must be properly shaped for planing.
Condition of the boat bottom
For maximum speed, a boat bottom should be as flat as possible in a fore and aft
direction (longitudinally) for approximately the last five feet (1.5 m). For best
speed and minimum spray, the comer between the bottom and the transom
should be sharp.
The bottom is referred to as having a "hook" if it is concave in the fore and aft
direction. A hook causes more lift on the bottom near the transom and forces the
bow to drop. This increases wetted surface and reduces boat speed, but it helps
planing and reduces any porpoising (rhythmical bouncing) tendency. The
manufacturer often builds in a slight hook. A hook can also be caused by not
trailering or storing the boat with support directly under the transom.
A "rocker" is the reverse of a hook. The bottom is convex or bulged in the fore and
aft direction. It can cause the boat to porpoise.
Any hook, rocker, or surface roughness on the bottom, particularly in the
all-important center-aft portion (critical bottom area), will have a negative effect on
speed, often costing several miles per hour on a fast boat.
Single- and Multiple-Engine Application
In single-engine installations, right-hand rotation propellers are almost always
used. This is merely the result of tradition, but is perpetuated by nearly all
recreational boat manufacturers placing their operator's position on the right-hand
side to compensate for the boat roll resulting from a right-hand rotation propeller.
When a boat is equipped with dual engines, counter-rotating propellers are
preferred. This balances any steering pull when the outboards or drives are
trimmed evenly. Most boat manufacturers prefer to rotate the propellers "out."
That is, right-hand rotation on the right side, left-hand rotation on the left side,
believing that overall handling is slightly better. This can also afford more balanced
steering when forced to operate with only one engine. Many older twin engine boats
use propellers of the same rotation. The disadvantages are that steering torque is
greater when trimmed well in or well out (assuming no power steering) and, in very
rough water, if a hull goes airborne, a pair of right-hand propellers (for example)
can walk the stern to the right a little.
Propeller Selection by Horsepower Range and Application
For Outboards up to 25 HP
Smaller engines are shipped with an aluminum propeller of moderate pitch and of
a sweptback, weed-shedding blade shape. Each engine has at least one other
propeller available of a different pitch for other boating applications, generally
slower boat speeds/heavier loads, high reverse thrust, higher speeds/light loads,
All "normal" propellers are rather "weedless," thick for durability, and of a flat
pitch/rake design that is easy to repair. They are intended for submerged
operation with all the blades rotating well below the water's surface. For improved
durability and performance, several stainless steel propellers are available.
Several high-reverse thrust propellers are available for engines from 4 to 25 HP.
These propellers are not of the weedshedding shape, but have rounded
symmetrical blades and pitch distribution such that the propeller has the same
performance characteristics operating in either direction. These propellers are
designed for submerged operation on displacement-type, slow, heavy boats such
as sailboats and work boats. For increased maneuvering, they provide about the
same amount of thrust in forward and reverse. They are designed to cope with the
exhaust gas discharge when in reverse.
For 30-60 HP Outboards
The 30-60 hp engine family shares a common propeller group. The type of
operation varies widely for this family. All types of boats, from slow moving
houseboats to very light runabouts, must be covered. The slower speed and
displacement boats will use the large diameter/large blade area lower pitches, such
as 12.5 in. diameter x 8 in. pitch, 12.25 in. x 9 in., and 12 in. x 10.5 in.. Most
common runabouts will use the higher-rake middle pitches of 12 - 16 in. If extra
durability is required (or desired), stainless steel (Vengeance, QSS) counterparts
exist for all pitches from 12 in. to 16 in. These propellers may turn a little lower in
RPM, but will generally hold better in turns because of their slightly higher cup. The
higher-pitch 17 in. and 19 in. aluminum props are available for boats that are
lighter and faster.
For 75-115 HP Outboards, Workboats, Barges, and Other Large Boats
These boats use the very lowest-pitch aluminum propellers available, 11 in. and 13
in.. These are large diameter/large blade area, thick, heavy-duty props. There are
stainless steel (Vengeance, QSS) counterparts in low pitches also -- 10 in. and 12
in. pitch -- which are more durable. Most suitable for heavy planing situations
would be the 13 in. pitch or 15 in. pitch in aluminum and the 14 in. pitch or 16 in.
pitch in stainless steel. In general, these propellers exhibit a smaller diameter and
an increase in rake over propellers of lower pitch. A heavy ski boat or boats
operated at higher altitudes may also call for these lower-pitch general-purpose
The 17 - 21 in. pitch aluminum and the 16 - 22 in. pitch stainless steel
(Vengeance, QSS) are the most commonly used propellers. Generally, runabouts,
family ski boats, and fishing boats that may range in size from 15 to 19 ft. and
weigh 1000-2500 pounds will use these propellers with the specific pitch being
determined by the boating activity, actual total boat weight, and horsepower
installed. These propellers provide a wide range of performance.
Bass Boats and Lightweight Fishing Boats
These boats will use propellers of higher pitch and may need a propeller with high
rake for holding and bow lift, (Trophy Plus, Tempest Plus, Laser II, QS 2000).
Boats in the 45 MPH+ speed range and of 90 hp or more can generally use this
extra bow lift to reduce hydrodynamic hull drag.
For V-6 Outboards and MerCruiser Sterndrives
Two major factors that determine propeller application for these engines are
submergence and function. Propellers operate fully submerged when their total
diameter is below the water surface. They are partially submerged when some
portion of their diameter is piercing the water surface (also referred to as
The amount of submergence is established by transom height or "X" dimension
(sterndrive). Generally, fully submerged propellers are considered the norm,
providing good all-around performance at standard transom height or "X"
dimension. Partially submerged propellers at raised transom heights or "X"
dimensions are more specialized, tending to favor top speed over "time-to-plane"
Work and Utility Boats, (27 ft.+ Cruiser)
Outboard and V-6 sterndrive powered work and utility boats and 27 ft. plus cruisers
whose propellers should be operating fully submerged will best be served by low
odd-numbered pitches (11 in., 13 in. & 15 in.) available in the standard aluminum
propeller line. If more durability is desired, a stainless steel (Silverado,
Vengeance) propeller of corresponding pitch can be used where available.
Single engine, Alpha One V-6 and small block V-8 powered sterndrives and V-6
outboard powered large, heavy boats will benefit from using the large
diameter/large blade area, low even-numbered pitch (12 in., 14 in. & 16 in.)
standard aluminum propellers. Acceleration, cruising performance, and reverse
thrust are markedly improved with these propellers.
Smaller, Lighter, Faster Utility, Offshore Fishing &
Small, light sterndrive and V-6 outboard powered boats with standard "X"
dimension/transom heights can use mid-pitch, (17 - 21 in.), aluminum propellers
for acceptable all-around performance. Comparably pitched stainless steel
propellers offer additional durability, improved holding ability and slightly better
top speed due to thinner blade sections. The stainless steel propellers usually
allow an increase in "X" dimension/transom height.
Laser II, High Five, Mirage Plus, Tempest Plus, Trophy Plus, and Revolution 4
propellers can be used to increase top speed further by increasing transom height
on outboard powered boats or by raising the "X" dimension on sterndrive boats.
Boats utilizing the Alpha One drive system generally perform well with small
diameter Laser II, High Five or Alpha One Performance series propellers.
Big-block V-8 sterndrives as well as V-6 outboard powered fishing boats should
perform well with the Bravo One Performance series 4-blade propellers. These
applications generally have favorable power-to-weight ratios and elevated transom
heights. Mirage Plus 3-blade propellers are also available for big-block V-8
sterndrives and V-6 outboards.
Bass boats and faster outboard and sterndrive-powered sport boats can use the
higher pitches (23 in. plus) in all stainless steel propeller lines. Boats in this speed
range will tend to have transom heights/"X" dimensions such that their propellers
are surface piercing, necessitating the use of Trophy Plus, Tempest Plus, Laser II,
and Mirage Plus propellers where top speed is the main consideration.
The Revolution 4, Mirage Plus, Tempest Plus, Bravo One Performance, and the
Offshore series of propellers are available for the high performance enthusiast.
The Mirage Plus and the Bravo One Performance propellers are designed for
big-block V-8 Bravo One sterndrive applications. The larger diameter and blade
area, when coupled with a higher horsepower engine, produce excellent
acceleration, holding and superior top-end performance.
For twin V-6 outboard-powered boats, use a Mirage Plus 3-blade, or a 4-blade
propeller such as the Alpha One Performance or Offshore series propellers. They
offer outstanding lift for carrying heavy loads. Many twin V-6 boaters report better
mid-range cruising speeds, reduced steering torque, and rough water handling
when using 4-blade propellers.
The Tempest Plus, the small tube Trophy Plus (4.25 in. tube), and the Laser II
offer great planing abilities and top speed for many other V-6 outboard packages.
Some lighter tunnel bottom shaped hulls require very high pitched (above 25 in.)
propellers. This type of hull generates bow lift aerodynamically and does not
require the propeller to assist in this function. Cleaver propellers, which provide
very little bow lift, are most effective on this type of boat.
Propeller applications for outboard and sterndrive-powered boats that have speed
capabilities above 90 MPH fall into the very high-performance or racing categories
and go beyond the scope of consumer-oriented high-performance pleasure
boating. If you have question about props for this speed range, call Glenn Reynolds
Switching Among Propeller Types/Families
When changing from 3- to 4-blade, are the pitches the same?
If you're changing to propellers manufactured by Mercury, the answer is
"yes!" For example, a 4-blade propeller is designed to run at elevated
transom heights (normally 2 to 3 inches above the transom). The 4-blade's
aggressive blade design and larger diameter help gain a performance
advantage. When compared to a 3-blade propeller at the same transom
height, the 4-blade will exhibit a lower engine RPM but, when the engine is
raised to the proper operating height, the RPMs of both propellers are
When upgrading from an aluminum propeller to stainless steel . . .
The most important concern to be aware of is the correct RPM range for your
engine. The propeller pitch regulates the engine RPM. Each inch of pitch is
worth about 200 RPM. Lowering the propeller pitch (going from a 23 pitch to
a 21 pitch) will increase the engine RPM (about 400 RPM). Increasing the
pitch (23 pitch to a 25 pitch) will decrease engine RPM. Most applications will
match up pitch to pitch and this is the best starting point.
In some instances, Mercury offers propellers in even pitches (20, 22, 24, etc.). In
these situations, go to the next higher pitch (24 pitch stainless to replace a 23
pitch aluminum). If your aluminum was already operating at the top of the RPM
range, the higher pitch propeller will allow you to see where you fall in the RPM
range with the new propeller without over-revving.
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Reynolds Racing and Marine, Inc.
1019 South Roane Street
Harriman, TN 37748
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