Comparing Four-bladed and Three-bladed Propellers

Optimizing the performance of Boston Whaler boats
Acseatsri
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Comparing Four-bladed and Three-bladed Propellers

Postby Acseatsri » Tue Jan 07, 2020 11:04 am

[Separated from a discussion about test data I posted from using a three-bladed and a four-bladed propeller of different diameters.--jimh]

[To see test data resulting for a test in which] the four-bladed propeller tested was the same or close to the same diameter as the three-bladed propeller would be interesting.

It's been my experience that diameter almost always seems to trump pitch when it comes to efficiency. When I changed to four-blade propeller from three-blade propeller, the Powertech OFS4 was 15.25-inch diameter and the three-blade was 15.5-inch diameter. I DID see a difference in the lower planing speed and efficiency, and I don't believe it was torque related but rather less slippage due to greater blade area.

jimh
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Re: Comparing Four-bladed and Three-bladed Propellers

Postby jimh » Tue Jan 07, 2020 4:06 pm

ACSEATRSI--thanks for you comments. They got me thinking:

The four-bladed propeller, even with smaller diameter dimension, has more blades than the three-bladed propeller, and on that basis there could easily be as much blade area or even more blade area in the four-bladed propeller as there is in the three-bladed. If you look at the blade design, the amount of blade area in the blade tip is much less than in the blade roots. On that basis, I suspect the CYCLONE 4 propeller has more blade area than the MIRAGEplus propeller.

There is a four-bladed propeller with a diameter about the same as the three-bladed MIRAGEplus : the REVOLUTION4. The total blade area of a REVOLUTION4 compared to a MIRAGEplus is certainly going to be much greater in the four-blade, probably one-third more blade area.

Note that the four-blade CYCLONE 4 propeller produced a lower engine speed at WOT, and that was probably because it has more blade area.

Also, the smaller diameter dimension for the CYCLONE 4 propeller is mostly due to the blades being raked aft. If the blades on the CYCLONE were not raked, then the diameter of the propeller would be larger.

jimh
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Re: Comparing Four-bladed and Three-bladed Propellers

Postby jimh » Wed Jan 08, 2020 10:22 am

Acseatsri wrote:... believe the VOLUME of water displaced is more efficient than the VELOCITY of the thrust.


I am not sure what exactly you are measuring with your term "more efficient." Perhaps you can explain a bit about exactly what aspect improves in efficiency by using more volume of water and moving it at a slower speed.

Also, you may find this prior discussion to have some interesting data:

PROPELLER DIAMETER AND FUEL ECONOMY
http://continuouswave.com/ubb/Forum4/HTML/007966.html

jimh
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Re: Comparing Four-bladed and Three-bladed Propellers

Postby jimh » Wed Jan 08, 2020 1:02 pm

A further problem for a four-bladed propeller compared to a three-bladed propeller with respect to the efficiency of producing thrust may occur with analysis as an airfoil. A marine propeller is often described as being an airfoil that flies through water. If we look at airplanes, we see that all modern aircraft use a single wing. Very early aircraft often used two wings or more. From the design evolution of aircraft, a reasonable interference may be drawn from the now universal use of a single wing to create lift: it works the best. The single-wing foil is perhaps analogous to a two-bladed propeller. Maybe it works the best, too.

In boat racing hydroplanes, we tend to see only two-bladed propellers used. Perhaps this is because they are more efficient overall, in part because they produce less drag. As boat speed increases, an important factor in determining how much power is needed to turn the propeller is drag. In racing hydroplanes, most of the boat hull is out of the water, so the propeller itself becomes a major contributor of drag.

Another concern may be tip vortex. This new and highly-promoted Sharrow propeller is based on the notion of eliminating blade tip vortex. Along that line of thinking, the more blades on a propeller the more blade tips to produce more blade tip vortices.

There is a general problem with outboard engines and propellers: the diameter of the propeller is limited by the space available in the propeller aperture of the outboard engine gear case. For many outboard engines, the propeller diameter cannot be more than 15.75-inches, which leaves just 0.125-inch clearance between the propeller blade tip and the anti-ventilation plate. Notably, SUZUKI pioneered the use of a gear case with a larger propeller aperture, and for SUZUKI engines it is common to see propeller diameters of 16-inches or a bit more. SUZUKI also used lower gear ratios to increase engine torque at the propeller shaft, and to turn larger propellers more slowly than generally had been used on outboard engines.