Disk Area?

[esdbass]

Can someone please explain the details of "disk area" when in reference to a propeller in relation to it's function on a boat?

[rock]

Disc-area Ratio is the ratio of blade area to diameter area. A 15" diameter prop would have a disc area of 176.6", pi x r x r ( 3.14 x 7.5 x 7.5 ). The blades have what is called an "expanded area" which is determined by taking the mean (average) width of the blade times the length of the blade from the root to the tip. Multiply that by the number of blades and you have the total expanded area. Now divide the total expanded area by the disc area and you have the Disc Area Ratio. In other words, how much of the diameter of the prop is blade. Typically the higher the ratio the slower the prop must turn and the heavier the boat can be.
This is incredibly over simplified and there are many other factors that will determine a props suitablilty for a specific use. Now you should be really confused.
Rock

[esdbass]

Rock: Oh yes now I am even more confused but let me ask you something based on what you said. Does an "Over the Hub" prop usually have inherently more disk area than a "Through Hub" prop because the length of the root to the tip is greater for an Over the hub? Also am I right in that the more blades a prop has the greater the "disk area" Ie 4 or 5 blades @ a given pitch would have greater disk area than a 3 blade prop of the same pitch?

[rock]

The disc area has nothing to do with the amount of blades. The disc area ratio, however, does. The disc area is the area of a circle the same diameter as the prop. The ratio is the relationship between the actual disc area and the total area of blade. An over hub prop will have the same disc area as a thru hub prop but the ratio will be different. Think of it this way. You are looking at your prop from the rear. Picture that circle with the three or four blades in it. Now color in the blades. The DAR is the colored area compared to the uncolored area. Look for a book by Dave Gerr called the Propeller Handbook. I am kinda like the guy that stayed at a Holiday Inn last night. I pick up just enough info to be dangerous and in a lot of cases wrong. There is so much involved in prop work and design that even reading and studying this book will leave you scratching your head. Good luck.
Rock

[esdbass]

Rock: Thanks for the info and that way of visualizing the disk area ratio was very easy to understand!

I will check out that book as well.