Determining the Optimum Propeller for Your Boat
Here at E-NATION Magazine, the question we are asked most frequently is “What is the best prop for my boat?” Most of the time the person does not include any of the important information that is needed to even suggest a starting point for propeller selection. To illustrate the differences in performance and economy among various props, we had the opportunity to try nine different propellers on a 1983 25′ Pursuit cuddy-cabin boat repowered with new 150 hp E-TEC outboards.
Our testing consisted of three styles of Evinrude propellers in 17, 18, and 19 inch pitch sizes. The Viper and the Rebel series were 3-bladed props while the Cyclone models had 4 blades. Two propellers, the 19″ Rebel and the 19″ Cyclone, were eliminated during initial testing due to performing outside of the 5300 to 6000rpm full-throttle operating band. There is also an optimum rpm range, 5500 to 5750 rpm, for these engines that provides for the best overall performance according to Evinrude Engineering. Dealers have quick-reference charts showing both the factory operating rpm range along with the optimum recommended WOT range for the newer model E-TEC outboards.
Twin 150 hp E-TEC engines replaced 2002 FICHT 175 hp outboards that originally pushed the boat to about 49 mph with a light load. Amazingly, the E-TEC engines with 50 fewer ponies reached a similar speed with a normal load aboard. With less horsepower providing the same speed demonstrates the improved technology and design found in newer Evinrude propellers compared to props from years ago. The 25′ Pursuit cuddy cabin came equipped with an Armstrong transom bracket and a heavy fiberglass T-top with a radar dome. This resulted in a weight-forward center-of-gravity condition making both trim position and bow-lift important for good performance.
The 25′ Pursuit included fly-by-wire ICON electronic throttle and shift and the digital display I-Command gauges for accurate data readings. A handy feature of the ICON controls is a synchronization button which electronically matches both engines to identical rpm and trim settings. Another advantage of ICON is the ability to “nudge” the engine rpm up or down in small increments with the press of a button. This gives a fine adjustment of the boat speed for trolling, changing sea conditions, and to achieve best fuel economy. I-Command digital gauges, known for their accuracy, display rpm, speed, and fuel consumption on the screens while computing the miles per gallon. Results shown in the following tables are the average of two runs, 180° apart to cancel out any effects of wind and current.
Choosing a propeller is more than just reviewing top speed and rpm results. Other aspects have to be considered such as the type of boating and what kind of load is aboard. Is the owner only looking for high speed or perhaps is fuel economy the over-riding factor? For offshore operation quick acceleration to power through a following sea may be more important than going fast.
Another concern relates to the weather conditions during testing. On a hot summer afternoon outboards develop less power and may lose several hundred rpm compared to a cooler day with less humidity. Our tests were performed on a dry brisk 60° day so we had to keep in mind that rpm will be lost during warmer weather and to consider props with a higher rpm to compensate for the speed loss during the hotter time of year.
We added one item to the data gathering that should be a part of every engine and propeller test. Typically performance reports only show rpm, speed, and fuel consumption with no standard information for a boat owner to reference and compare fuel consumption. The automotive industry publishes an EPA required fuel economy rating for 30 mph and the marine industry should also record fuel mileage at a constant speed. Engine rpm is not a good data point as different gear ratios and various size propellers affect the boat speed at the same throttle setting. Because fuel economy is measured in miles per gallon, the boat’s fuel economy testing speed must always be the same to keep things standardized for ease of comparison. We decided on 30 mph as that is a comfortable and popular cruising speed on calm water. In addition it is a speed with which everyone can associate and most boats can easily attain. Notice in the chart below that the 17″ Rebel uses less fuel and gets better mileage than the others. Three of the propellers are responsible for increased fuel consumption of more than a gallon per hour to achieve the same 30 MPH cruise. The recorded fuel data in gallons per hour (GPH) is the total for both outboards.
The Rebel, Cyclone, and Viper props delivered acceptable performance considering the boat loading and weather conditions at the time of the tests. Also we were surprised to see that fuel consumption at full throttle was almost identical with all the props even though engine rpm and speed varied. Cruising fuel economy, as expected, fluctuated with the different types and sizes of props.
– accelerated very strongly on the initial punch but then lagged as the boat came up on plane. The increased transom lift and reduced bow lift was noticeable by other boaters from the Pursuit’s flatter running angle.
– the additional transom lift also caused a loose feeling at high speeds requiring minor steering corrections to maintain a steady course.
– Delivered satisfactory performance and bow lift but the cuddy cabin was light without normal cargo aboard. The Viper 18″ and Viper 19″ performance was adequate for the present boat configuration and weather conditions but would possibly drop the WOT rpm below the acceptable threshold with more weight onboard and in warmer weather. Moreover, the 30 mph cruise economy was not on par with the Rebel props.
We concluded the Rebel 17″ to be the propeller of choice as it can remain within the WOT rpm range for both heavy and light loads. In addition it had the best fuel economy at 30mph, the best acceleration, and it delivered better bow lift and handling characteristics. The 17″ Viper could be considered as an alternative selection for most boating conditions. Keep in mind that every propeller is a compromise of sorts and it is important to consider all the data and conditions before determining the best choice for an application. Depending on your type of boating it is not unusual to carry two sizes or styles of propellers to accommodate various situations such as long-distance cruising, water skiing, top speed running, or hauling heavy gear.
Remember to always boat safe.
For more propeller information, click on the following links for a booklet or video about props and propping your boat correctly.
The following tables are comparisons of speed, fuel consumption, and fuel economy for the seven propellers that we evaluated.
John Greviskis, host and founder of the Ship Shape TV show, refers his viewers to the following data in E-NATION during the episode about choosing a propeller for the E-TEC powered 25′ Pursuit .