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CDMA
03-13-2002, 11:30 AM
Why surface drives and raised x dimensions work:

Imagine a screw as you are screwing it into a solid uniform media. Every rotation the screw moves a fixed distance determined by the size of the threads. Now you over tighten the screw and strip out the hole. Now no matter how much you turn the screw there is no inward, or forward motion. Now relate this to a propeller. The aft end of the deep vee cuts the water and creates a disturbance or lack of media similar to the stripped out screw. Now the bottom of the propeller is in clean water cause the prop is deep enough down to be in unobstructed water. However now the top of the prop is in this disturbed area of air and water. The prop can get very little if any forward thrust in this non-uniform media. However do to the fact that there is substance there ( a mix of water and air) a tremendous amount of drag is put upon the prop. The type of drag that does not aid in forward thrust. Causing the engine to do work that is not advantageous to out cause, forward motion.

So now the object is to get this prop in complete clean water. Ideally the best bet would be to drop the prop far aft and deep in the water away from the disturbance the hull creates. However doing this causes even more drag and is self defeating. So the next best bet is to reduce the drag on the top half of the propeller because it is not doing any thing of any valve. So we set the prop way far back (arneson) where the surface of the water in clean and undisturbed by the hull. Then we raise the prop up to the surface and actually lift the top half of the prop out of the water. Now the top half is doing absolutely nothing to aid forward movement but it at least has near zero drag now that it is in complete air which is about 26 times less dense then water. We then can also increase prop diameter and not worry that dragging that massive top blade around the top half of the circle.

Now a raised x dimension is the same idea but slightly different. Instead of lifting the prop out of the water we are trying to use that disturbance at the top of the water to our advantage. The closer to the surface we get the more air there is and the less water hence less drag on the top half of the prop. By cautiously getting the top half of the prop as close to the surface as we can and still keeping the bottom half of the prop under the boundary layer in laminar (undisturbed flow) we can gain some of the advantages of the surface drive. However go too high and the hub starts to ventilate causing the bottom half of the prop to grab air (lack of media) and cavitate. This totally defeats the purpose of our goal. Keep in mind that even if in a straight line the prop grabs enough water on the lower part as you turn and the boat leans the prop is actually lifted causing air to be caught. So if you only want to go straight and never turn your x can be a lot higher then if you like to chose the direction you are going.

Now the only thing I can’t completely explain at this point is the Blackhawk. While I have a few ideas and theories none are at the “ willing to stand behind them point”. Technically it should just cavitate all day long but that is another whole post….

So that is a little bit of basic propeller hydrodynamics for you.

Chris

Trivia- The center of Gravity of a boat is above the waterline. This is a un- natural place for the CG to be. What keeps the boat from just rolling over??? Pat on the back to the first right answer.

Digger
03-13-2002, 11:45 AM
center of bouyancy (CB). As the CG moves, CB shifts to stay in line with it, "cancelling" the rolling or pitching moment (up to a point)

great discussion on prop dynamics!!

Woodsy
03-13-2002, 11:49 AM
Way to go Chris...

It was great to see someone finally attempt to put propeller hydrodynamics in easy to understand terms... Lord knows I haven't been able to. About the BlackHawk... same principal as your Arneson, except it runs in the somewhat laminar flow of water returning to its original origin after being displaced by the hull. The reason for the 2 counter-rotating props is to make up for the lack of propeller diameter. The two counter-rotating propellers are much more efficient than a single cleaver would ever be.

Woodsy :D :D

PS: I know the answer... but I'll let a few of the others guess...

CDMA
03-13-2002, 11:54 AM
Digger,

Ding Ding. More or less at least. Basically a moment it created by what you said causing the boat to right. I will make that another post one day...but that will require diagrams and that takes time.

Woodsy,

That was along the lines of what I was thinking. Now the real question is where can I find someone to make this...

http://www.donzi.net/photos/doc1.jpg

Chris

Forrest
03-13-2002, 04:42 PM
Chris, way to go! I knew some of that skoolin' would pay off sooner or later. Next thing you know, you'll be throwin' some rif-ma-tic our way that will put a big hurtin' on all of our heads! Anyway, I agree with what you said above, and you said it well; however, in the third paragraph shouldn't the word ventilate be used rather than cavitate? Cavitation implies vacuum bubbles forming and violently collapsing.

CDMA
03-13-2002, 05:39 PM
Forrest,

Note : Looking at the back of a boat you are looking at the face of the prop. The back of the prop actually is the forward side.

Hmmmmm very good. I had to think about that one for a few minutes and even read a few things. I think you are right to a point but technically what it comes down to is that ventilation is a form of cavitation. Cavitation is a generic word used to describe various different hydrodynamic princibles. I mean there literally are entire books on cavitaion. I mean there is sheet, bubble, cloud, tip vortex, hub vortex and supercaviation ( the propeller never actually touches water!!! when it is rotating) I think ventilation is a better word however and I do stand corrected.

For those of you that don't know traditional caviation is a really interesting subject. Most think of a propeller as pushing a boat through the water and while they are right the boat is also pulled through the water. What comes out must have gone in right??? So what happens is that there is high pressure in the face of the prop and low pressure on the back of this prop. What happens is that the pressure decreases so much that it causes the vapor pressure of water to decrese. From basic chemistry we know that as pressure decreases the boiling point of that fluid also decreases. Thefore under extreeme situations where pressure becomes extreemly low ( including a negative pressure)the boilng point of water goes below the temperature of the ocean. When this happens, like on the back of a propellor blade, the water flash boils. This water now is a gas in the form of tiny bubbles. Well as the propellor keeps going pressure increase again and all of the sudden that gas no longer wants to be a gas. So then the gas bubbles colapse and turn back into water. The real problem is that when the bubble collapse water crashes into the back of the propellor blade at rediculously high speeds. Enough to actually eat or erode the blade away over time.

So Forrest is correct that ventilation is a better word. However keep in mind that no matter how the air gets back there, wether due to reduced pressure or picking up air from the surface, that cavitation will occur. Even if the air comes from the surface as the pressure increases on that air now behind the blade the same thing as above happens and the water crashes into the back of the propellor at high speeds.

Chris

seano
03-13-2002, 06:19 PM
Very good work Chris, I think you'll have a bright career ahead of you and I think we'll all benefit! It's great to see that some of today's youth is interested in more than just Limp Bizkit! And for the record, I'm more of a Linkin Park and Saliva fan! Click Click Boom!

Hope to see you around the east end this summer.

HP 600SC
03-13-2002, 06:43 PM
Chris, is the center of gravity always above the
waterline?

CDMA
03-13-2002, 06:48 PM
Ted,

Not always but usually. In powerboats almost always. The most common exception are big racing sailboats with a few tons down at the keel.

Chris

3 homeworks
2 papers
1 test tommorow...I am growing tired of this stupid computer

Oh goody goody webmaster fixed my blueprint...anyone want to make me one now????

ClassicRazorback
03-13-2002, 07:37 PM
I did'nt think the center of gravity could change?

Chris

Rootsy
03-13-2002, 09:53 PM
Gosh and the frightening part is i understand all of this stuff... yeeeeeesh... i'm gonna have nightmares now!

Formula Jr
03-14-2002, 02:06 AM
Excellent stuff Chris! Keep it comeing. I also don't know yet how a Blackhawk ever worked. You would think it just would just spin in the slop. There is another consideration, that being that in a open water boat, everytime you are air borne, there is no forward thrust. So how did surface drives ever beat out drive systems that set the prop in deeper water? Was that just what the throttle guy did? Keep it "grounded?"

Woodsy
03-14-2002, 05:42 AM
Although there is no forward thrust, the less boat in the water, the less drag = faster. The less prop in the water, the less drag = faster. Chris is right about only about 1/2 of your propeller is actually working. Think in percentages, not actual blades... each blade is doing some work, mostly on the bottom of the rotation, but 1/2 of the blade is ineffecient because of disturbed water, rotation speed at the tip (cavitation) etc... The key to speed is to turn the biggest prop you can at a speed where the prop is most effecient. Usually the lower the rpm the prop turns, the more hydrodynamically effecient it is. That why you see all of these gear ratios for your drive which in effect slow down the rotation speed of the propeller. Of course, then you need big power to spin these steep propellers. That is why torque, and not horsepower is more important in a marine application.

The Blackhawk uses 2 counter-rotating propellers to push the water. Its just a more effecient means of propulsion, and it makes up for the lack of a large diameter propeller. It also runs in the smooth water that was displaced by the hull. Think Russian "Bear" aircraft.... 650mph on 4 counter-rotating propeller driven engines... very effecient. As fast or faster than most jets at the time it was designed.... still in use today!

Chris, a counter-rotating Arneson type setup?? Hmmmmm..... It could be doable.....

Woodsy :D :D

ALLAN BROWN
03-14-2002, 07:14 AM
Arneson has a drop-box drive. Air is about 812 times less dense than water. The propuslsive effort is NOT like a screw in wood, it is the differential pressure on the front and back of the blade, or so I thought.

AVickers
03-14-2002, 09:17 AM
The best way to think about cavitation is that rather than air, it is steam! Cavitation, in all its forms, is a result of the application of Boyle's Law -- which has to do with pressure and vaporization of liquids. In short, at sea level, water boils at 212 degrees F. As you go up in the mountains, the boiling point is lower. If you pressurize the process (like in a pressure-cooker), the boiling temperature goes up.

A propeller works by creating low pressure on its leading face (and, conversely, a higher pressure on the trailing face). And here is usually NOT where cavitation occurs (a prop that loses it's "bite" along its face is usually "aerating" -- not "cavitating"). [Cleavers tend to be less susceptible to aeration...but that's another discussion.]

Anyway, where actual cavitation does occur is on the leading and trailing edges of the prop as it rotates... Here, the velocity through the medium is so high that VERY low pressures or even vacuum occurs. Cavitation off the trailing edge is not a big problem because it falls off in the "swirl" behind the prop (it does, however, eat up some energy as it converts water to steam...) The cure here is to make sure the trailing edge is shaped such that water flows well off the prop and no vacuum can be caused.

The big problem w/ cavitation is when it occurs along the leading edge of the prop -- usually in the area of a ding or in an area where the leading edge or cup is improperly formed. If water turns to steam in this area, the gas bubbles, as they move across areas of low pressure on the prop face expand and displace the water on the prop surface -- the stuff that the prop is able to "bite" to get forward traction. Not only does your prop lose some of its traction as a result of these bubbles, but energy is expended in the formation of the steam...at the expense of forward motion.

I'd even speculate that sometimes the temperatures in these areas are very high (...even creating small pockets of superheated steam). When you look at the faces of an aluminum prop, you can actually see what are called "burn marks" in the areas where these bubbles of superheated steam form and roll off. (BTW, superheated steam has a higher temperature than the melting point of aluminum...).

CDMA
03-14-2002, 09:55 AM
Chris,

You are right. It is the center of bouyancy that moves not the CG. However it is possible for the CG of a boat to move. Anyone know how??? I give a hint...free surface correction.

Brownie,

Now we are getting into how and why the prop works. The first thing they teach you in prop design is that a prop is a wing. However I was just using the screw analogy to expain what happens when there is nothing for the prop to be in contact with. However there is what we call the "paddle wheel effect". Basically that even if it wasn't a wing the prop would still 'throw" enough water to make you go.

Chris

ALLAN BROWN
03-14-2002, 10:31 AM
I'LL BUY THAT.

HP 600SC
03-14-2002, 06:24 PM
I thought water was 800 X more dense than air.........

CDMA
03-14-2002, 06:43 PM
Yeah you and brownie are right on that...sorry about the mistake.

Chris

CharlieTuna
03-14-2002, 07:18 PM
I think I read all that somewhere else.......
is that article from Popular Mechanics about a year ago? Very good stuff! Hey cool picture of my
namesake.....Thank you HarborMaster!!

CDMA
03-14-2002, 07:27 PM
Nope that isn't an article. Just stuff floating around in my head.

Chris

Dr. Dan
03-14-2002, 10:37 PM
This is scary, I'm remembering my :rolleyes: Physics/Chemistry Classes...............I think I am learning something again! Good Stuff Chris! wink ...Doc

Formula Jr
03-14-2002, 10:59 PM
Chris. You wrote an article. Only available here on Donzi net and published on the World Wide Web. Great stuff! I like the Allard Drive. Careful though, some of your ideals may be future patents. You can Patent an "Improvement" of a previous design. Start a Patent Log. Your professiors will know what I'm saying.

Gary S.
03-16-2002, 02:54 PM
Avickers,,,,I believe we could have a few interesting discussions on steam properties. My knowlage of superheated steam is anything over 212 Degrees F that is saturated, And the definition if Superheat as in Thermodynamics. water vapor that has been heated, at constant pressure and out of contact with the water from which it was formed, to a temperature significantly above its saturation point, so that it does not recondense to water as readily as ordinary steam; used to provide increased efficiency in steam engines. " I'm not picking,,,it has just raised an eyebrow as to what causes the burn marks,, My question,,,do you think it's possible ? I'm curious, could a prop do that ?????

Donzified
03-16-2002, 05:55 PM
would a cuped prop cavatate more because of it's trailing edge?

Boatless
03-17-2002, 06:32 PM
Chris, Arneson patented a 'Counter Rotating Propeller" design back in the 80's.

What happened to it? I guess the market was not there for the concept then. Maybe they will re-introduce it in the future..