By the Numbers: Gear Ratio Determination and Speed Calculations for Your Boat
Bass boaters and other high performance enthusiasts are always looking to obtain the maximum performance out of their rigs. Whether trying new props, changing the boat and motor set-up, or by re-distributing the load, a go-fast boater is forever reaching for that last mile-per-hour of speed.
Outboard motor gearcases come in all styles, sizes, and ratios depending on the motor’s attainable speed, horsepower, and type of usage. It is important for the savvy boat owner to know the gear ratio of his motor’s lower unit for propeller speed and efficiency calculations. The following article illustrates a simple procedure to determine the gear ratio of popular Johnson and Evinrude gearcases.
For safety purposes, it is recommended that you first disconnect the battery and remove the ignition key to avoid accidental starting which could result in damage or injury. Remove the sparkplugs, noting their condition and checking the gap on them if they have been in use for a while. On E-TEC and DI ( FICHT ) outboards, record the cylinder number location of each sparkplug to be certain that every one is reinstalled in the exact same hole from which it was removed. The positioning of the electrode gap is important to the proper operation of these high-tech low-emission outboards.
Have someone rotate the propeller while you shift the motor carefully into forward gear. Place a piece of masking tape on one of the blades and make a line on it with a Magic Marker. Turn the prop slightly counterclockwise until it stops to remove any normal free play inside the gearcase. Now slowly rotate the flywheel clockwise (while looking down at it) until the mark on the blade reaches the 12:00 position and is centered underneath the anti-ventilation plate and trim tab as illustrated in this first image.
NOTE: Do not turn the flywheel counter-clockwise as that could damage the water pump impeller.
Mark the flywheel for reference and rotate it EXACTLY three turns clockwise and observe the propeller-blade mark position. Compare the location with the subsequent images to determine the engine’s gear ratio and the number of teeth on the lower-unit’s pinion and driven gears.
If the mark ends up at about the 3:30 position, the gear ratio is 2.25:1 or .44 with 12-27 gears.
If the line indicates a 6:00 position, the gearcase has a .5 ratio or 2:1 with 13-26 gears.
A 1.85 or 1.86 ratio will place the mark close to the 7:00 position signifying 13-24 or 14-26 gears respectively . The “O,” “L,” and “L2” style gearcases have a 1.85 ratio (13-24) while the “M” or “M2” (magnum) lower-units contain 14-26 gears resulting in a 1.86 ratio.
The high-ratio “L2″gearcase ( 2008-2010 ) displays the line at the 9:00 position signifying that it is a 1.71 ratio or .58 with a 14 tooth pinion and a 24 tooth driven gear.
If the gear ratio is known it is easy to calculate a theoretical top speed along with propeller efficiency if the actual boat speed has been measured accurately.
The formula for top speed takes into account the rpm of the propeller itself which is the engine rpm divided by the gear ratio, and then multiplying the answer with the pitch of the propeller in inches divided by 1056. Converting miles per hour into inches per minute results in the number 1056 used in the formula.
SPEED = [ENGINE RPM ÷ GEAR RATIO] X [PROP PITCH ÷ 1056]
On an a boat and motor with a 1.85 gearcase ratio, a 17″ pitch propeller, and the engine turning 5600 rpm, calculating the theoretical speed is easy.
5600 rpm ÷ 1.85 = 3027.027 — [ENGINE RPM ÷ GEAR RATIO]
17″ ÷ 1056 = .016 — [PROP PITCH ÷ 1056]
Theoretical top speed — 3072.027 X .016 = 48.432 or 48.4 mph rounded off
If we have the actual speed of this boat, we can calculate propeller efficiency which is expressed in percentage of slippage. The less slip, the more efficient the propeller is for obtaining the highest speed and best fuel economy.
If the GPS measured top speed is 44.2 mph, divide that by the 48.4 mph theoretical speed and subtract that answer from 1.0 then multiply it by 100 to determine the percentage of slip.
44.5 mph ÷ 48.4 mph= .92 subtracted from 1.0 = .08 X 100% = 8% slip
An 8% slip designates an efficient prop. Many recreational propellers generate around 10% slip at full speed while high-performance propellers on very fast boats are closer to 6 or 7%.
For the mathematically challenged there are a number of programs to make these calculations easy. Just type “boat propeller calculator” into an online search engine and download your program of choice. One popular calculator may be found at http://www.rbbi.com/folders/prop/propcalc.htm .
By knowing your engine’s gear ratio and performing a few calculations it is simple to figure out the efficiency of your propeller. Compare several props this way to determine which is best for your application. To help choose a good all-around efficient prop, run the numbers at both full speed and at your cruising speed as some propellers are great for top end but are not very efficient at normal speeds.
We hope this information helps you become a more informed boater and assists you in obtaining the best overall performance from your boat and motor. Safe Boating!