KARLOW wrote:I'm looking for better performance which is better fuel mileage mainly between 18 and 22 knots. and good performance at slow troll speed.
Okay, now we can help you with a new propeller recommendation because we have an idea of your goal.
For best fuel economy you probably should try a three-bladed large-diameter propeller with considerable blade area, such as a Mercury MIRAGE
plus . Fuel efficiency is generally related to big blade area, very little blade rake, good blade design (with very sharp trailing edges), and thin blades.
KARLOW wrote:I tried a four four blade propeller, but it [was] too noisy at troll speeds. It makes the whole boat resonate.
The four-bladed configuration in a propeller on an outboard engine means that TWO blades will SIMULTANEOUSLY pass into the shadow of the gear case and the skeg. When a propeller blade is in the shadow of the gear case or the skeg, it does not get the same water flow as when not in the shadow. The result is an influence on the engine instantaneous propeller shaft velocity, because there is a REDUCTION in the loading on that blade. On four-bladed propellers there are only TWO shadow events per revolution, so the FREQUENCY of these instantaneous changes in the propeller shaft velocity is LOWER, but the amplitude is HIGHER because two blades are involved simultaneously.
With a three-bladed propeller each blade passes through the shadow of gear case and skeg separately, so there are SIX shadow events per revolution, and the amplitude is LOWER because only one blade is involved with each shadow event.
Exactly how the whole boat will be EXCITED by these vibrations depends on many factors, so in some instances a four-bladed propeller might REDUCE vibration and in other instances a four-bladed propeller might INCREASE vibration. Which effect occurs depends on the propeller rotation speed, the frequency of shadow events per revolution, the coupling of the engine to the transom, and any resonances in the boat itself.
WHAT EFFECTS FUEL EFFICIENCY
The engine "sweet spot" is when the Brake Specific Fuel Consumption (BSFC) peaks at a certain engine rotation speed and power delivered into the load. Real engines have curves of the engine BSFC, but recreational outboard engines do not provide any data. BSFC is measured in pounds of fuel consumed per horsepower delivered.
A best guess approximation for optimum BSFC is generally at an engine speed of about 75-percent of maximum to 85-percent of maximum. In the case of a BF150, if it can run at 6,000-RPM, then the optimum engine speed range for best BSFC might be 4500 to 5100, but that is just a wild guess. HONDA should really TELL you what the optimum BSFC engine speed range will be.
The other factor is the boat speed and the design of the propeller. The propeller efficiency is not constant; it varies with the speed of motion through the water. The rate of rotation of the propeller relative to the engine speed of rotation is a function of the gear ratio.
In summary, to get the best fuel economy the propeller efficiency and the engine efficiency should PEAK at the same boat speed and engine speed. There is NO WAY to calculate this for a particular boat with a random engine and random propeller.
Again, try a large-blade-area three-bladed steel propeller with thin blades and not much rake with good blade design with cup and sharp trailing edges, otherwise known as a Mercury MIRAGE
plus or similar design, and choose a pitch that is appropriate for the engine, gear ratio, boat, and loading.