By Mike Gridley
For many years now, boaters have debated the question of four-blade versus three-blade propellers. The traditional argument is that four blades are slow and three blades are fast – end of discussion. But higher fuel prices have now entered into the mix and speed has taken a back seat to efficiency for many boaters.
That’s not to say four-blade propellers cannot be designed for speed. New designs have come a long way and a four-bladed propeller can be manufactured to deliver speed as the top result or to combine speed with its more traditional attributes of increased water displacement, grip and lift.
The goal with this My Boat Project was first to come up with a four-bladed propeller set that would deliver enhanced cruising efficiency for a 36-foot twin inboard-powered express cruiser. The secondary goal was to enhance the acceleration curve to minimize the time and drag to get the boat onto plane to reduce fuel burn.
United Propeller and Machine in Orillia, Ont., provided us with forms to complete on the current setup. The data we gathered included the boat’s specifications, as well as the engine and transmission model and serial numbers so accurate details for horsepower, torque and gear ratios could be sourced.
Next, we headed out on the water with the boat’s typical load configuration to record speed and acceleration numbers, along with fuel consumption data at various RPMs. This established the baseline performance of the original factory-specified three-bladed props.
With the data in hand, it was off to meet up with Amanda Holloway at United Propeller and Machine. At the shop, Amanda entered all of the boat’s specifications and performance data into an advanced propeller selection program by Prop Scan. This program gave Amanda some valued direction to combine with her years of experience so she could formulate a recommendation for the boat.
Amanda recommended a Michigan X-Series propeller for the job. The new X-series wheels are CNC machined as opposed to the old cast method of manufacturing. This delivers a superior product with consistent blade attributes of thickness, rake, pitch and cupping from the root to the tip of the blade. Her recommendation was for a cupped propeller with a 19-inch diameter and 19-inch pitch. A small curvature or lip located on the trailing edge of the blades, cup allows the propeller to get a better bite in the water. This results in reduced ventilation and slipping, while providing a quicker hole shot in many cases.
With the new props fresh out of the box, United Propeller analyzed the new propellers with their Prop Scan system – a part of their quality control process. This highly accurate computer system, along with a skilled technician, is used to inspect the accuracy and hydrodynamic form of a propeller. Each section of the propeller blades are scanned and recorded for inaccuracies. All blades are compared and propeller sets can be accurately matched.
The scan of these props pointed to some minor cupping and pitch tweaking that would ensure a vibration-free and efficient set of propellers. After some machining and work on the block, the props measured up perfectly, exceeding the top ISO 484 Class S standard.
With the boat on shore back at Queen’s Cove Marina, Yves Quinelle helped with removing the old props. A good propeller puller and a big hammer made quick work of the task. If your boat has not had its props off in a while or you boat in salt water, you will probably have to add some heat to the mix.
Installing the new four-bladed props was quite straightforward. First, the key was inserted into the keyway and slid onto the shaft and as far over the key as it would go. Using the first prop nut and a block of wood to lock the propeller in place, we tightened the nut, ensuring the propeller hub went all the way back on the shaft’s taper. Then we installed the lock nut and secured it with a stainless cotter pin.
Back out on the water, it was time put the theoretical work to the test. I was impressed. The new four-bladed set delivered performance improvements across the board. Mid-range cruise at 3500 RPM increased by just over 2 knots to 23 knots. At the same time fuel consumption was improved by an average of 1 gph per engine. Over the course of a season, this will translate into significant savings at the fuel dock. Another win for the new props were their ability to keep the boat on plane at a lower RPM – again, saving fuel and engine wear. As bonuses, top speed increased by 1 knot and time to plane improved by 1.6 seconds.
Overall, vibration was all but eliminated, cruising RPM went down, speed went up and the big 454s burned less fuel.