Propeller Geometry and Dimensions

Optimizing the performance of Boston Whaler boats
Acseatsri
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Propeller Geometry and Dimensions

Postby Acseatsri » Tue Jan 07, 2020 9:08 pm

jimh wrote: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.


I don't buy this. The thrust would tend to fall off the end of the blades soooner on the smaller diameter. That's why I believe the VOLUME of water displaced is more efficient than the VELOCITY of the thrust. No amount cupping or rake will make up for diameter, as the aforementioned characteristics only affect velocity of the water column, not the size.

The Mercury REV4 prop mentioned is 14 5/8" diameter vs your 14.5" Cyclone vs my Powertech OFS4 15 1/4". I suspect the REV4 prop would compare nearly identically to the Cyclone due to the like diameters. Also, if you look at the props, the Powertech seems to have an "elephant ear " shape featuring larger blades.

jimh
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Re: Propeller Geometry and Dimensions

Postby jimh » Wed Jan 08, 2020 10:19 am

Acseatsri wrote:
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.


I don't buy this.


I interpret your reply as meaning you do not believe in geometry. In contrast, I do believe in geometry. We will have to come to some agreement about geometry before we can proceed to discuss this disagreement. However, I am quite certain that the manufacturers of propellers specify the diameter dimension of their propellers using the same geometry that I do, that is, they use the distance from the center of the propeller shaft to the tip of the blade, measured at a 90-degree angle to the propeller shaft. This means that propellers with raked blades have a smaller diameter dimension than if you were to measure the distance from the blade tip to the propeller shaft center along the angle that the raked blade makes with the propeller shaft.

Acseatsri
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Re: Propeller Geometry and Dimensions

Postby Acseatsri » Fri Jan 10, 2020 5:43 pm

I measure diameter as twice the distance from the centerline of the hub to the tip of the prop. This distance never changes whether the blades are swept back or not. Hence, the volume of water displaced will never be larger than the footprint of the diameter of the prop. The velocity will change with the pitch and rake, but the diameter of the water column will never change.
Just by my own experience and others says that a larger diameter will almost always get better fuel mileage than a larger pitch. This has been true on outboards right up to an 86' aluminum catamaran that we built almost 20 years ago. 34" diameter x 34 pitch props turned to nearly the same max rpm as a 36" diameter x 32 pitch, but the larger diameter was 2 mph faster. Did the exact same thing on a friend's boat when he replaced a blown Yamaha 300hp hpdi with an F250 Yamaha. Marina sold him a 14.5 x 17p prop, hit 6000 rpm at 36 mph, tested aa 15.5" x15 prop, hit the same rpm at 39 mph. I recently bought an OFS4 x 15 prop on EBay to test on the same boat. I'll get you the results this spring.

jimh
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Re: Basic Geometry and Dimensions of Propellers

Postby jimh » Fri Jan 10, 2020 6:22 pm

Acseatsri wrote:I measure diameter as twice the distance from the centerline of the hub to the tip of the prop. This distance never changes whether the blades are swept back or not..


You have a strange sense of distance. The normal measurement of propeller radius is the distance the blade tips project from the propeller center line, and the diameter twice that distance. But that dimension has NO RELATION to the blade area if the blades are angled back or racked. This was the POINT I was making when I told you that the four-bladed propeller had raked blades. Even though the distance that the blade tips project from the propeller shaft might be a smaller dimension that the three-blade propeller, the ACTUAL length of the blades was longer than that "diameter" dimension because the blades are raked back.

Until we can agree this basic principle of Geometry, we cannot go further in this discussion.

Would a diagram help you to understand the difference between the length of raked blade measured along its surface from tip to propeller shaft and the projection of that blade tip as measured at right angles to the propeller?

Acseatsri
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Re: Propeller Geometry and Dimensions

Postby Acseatsri » Fri Jan 10, 2020 6:42 pm

What difference does rake matter to the diameter? It's still going to cast the same diameter shadow whether the blades are raked or not. There's no "implied diameter " based on the geometry, it's either 15" or whatever the dimension is or isn't, I have no clue what you're talking about.


BTW, what's the difference if I measure the radius and double it to get the diameter? It's a little tough to check a 3 blade prop, but a 4 blade is direct measurement.

jimh
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Re: Propeller Geometry and Dimensions

Postby jimh » Sat Jan 11, 2020 1:11 pm

The reason for this discussion stems from your initial comment--"I don't buy this"--regarding my completely true statement about propellers with raked blades having a dimension measurement of being smaller, but if the blades were not raked aft the dimension measurement would be bigger.

I cannot believe we are still have a discussion about geometry. I will have to make a sketch to explain this to you.

networkSketch.png
Fig. 1. The same propeller blade, with and without rake, results in different values for the propeller dimension "diameter." For a raked blade, the diameter is 2A. For a nonraked blade, the diameter is 2B, which is clearly going to be larger. Yet the blades are the same length, and have the same blade area.
networkSketch.png (24.51 KiB) Viewed 4341 times


You cannot discard the notion of a propeller having blade rake when you want to compare blade area by just comparing the diameter dimension. As my sketch shows above, the same blade with a raked mounting produces a smaller diameter dimension. This was exactly what I replied to your comment regarding my comparison of two propellers with different diameters in which I pointed out that the smaller diameter propeller had raked blades, so there was no certainty that it also had less blade area on the basis of a smaller diameter.

Do you "buy" this explanation?

jimh
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Re: Propeller Geometry and Dimensions

Postby jimh » Sat Jan 11, 2020 1:15 pm

Acseatsri wrote:What difference does rake matter to the diameter? It's still going to cast the same diameter shadow whether the blades are raked or not.


In my world, raked makes a difference. In my world raked blades cast a smaller "shadow."

jimh
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Re: Propeller Geometry and Dimensions

Postby jimh » Sat Jan 11, 2020 1:17 pm

Acseatsri wrote:...what's the difference if I measure the radius and double it to get the diameter?


I don't know what other method you are making a comparison to. For any propeller the "diameter" dimension given is the diameter of a circle that the blade tips inscribe when rotating. It does not matter how many blades there are. Blades are assumed to be identical.

Acseatsri
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Re: Propeller Geometry and Dimensions

Postby Acseatsri » Sun Jan 12, 2020 8:18 am

I'll buy that rake can result in a larger blade surface area, but the inscribed circle or DIAMETER is still the same. Your drawing makes no sense.

jimh
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Re: Propeller Geometry and Dimensions

Postby jimh » Sun Jan 12, 2020 10:28 am

The discussion has ended, because we cannot agree on a fundamental element: geometry. It is silly to try to discuss propeller performance when the participants have incompatible views about geometry. If you don't believe my fundamental premise that is based on geometry, we cannot have any further discussion about "this."

Acseatsri
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Re: Propeller Geometry and Dimensions

Postby Acseatsri » Sun Jan 12, 2020 4:06 pm

I agree [that no further discussion is possible].

I really can't see what effect geometry has on diameter.

Maybe someone else can jump in and explain it better.