Your Knowledge: Super Tune your ride.

[Blackjack-sven]

Just wanted to add a thread where everyone can add thoughts and skills to talk about in regards to making "THE PERFECT BOAT"

First thing I'd like to add is a question of props and some theory behind them. I've read that a very sharp leading edge and a perfectly squared off trailing edge is the best type of prop. But if you look at the wind that comes off of a big rig, it "eddies" causing more power loss. An airplane wing utilizes a smooth trailing edge retaining a smooth wind exit. If this sounds right to you then why are we squaring off the props on the trailing edge? Why not sharpen the trailing edge so that the water that gets dumped off the end is nice and smooth for the leading edge of the next blade to encounter instead of turbulence.

[Meniscus]

Blackjack-sven, many do sharpen the trailing edge. I'm no prop guy, but maybe Egneg could provide some clarity for your inquiry.

[Meniscus]

This subject was most recently discussed here: http://forums.offshoreelectrics.com/...ead.php?t=7367 Post #15 & #16

[egneg]

The trailing edge should be flat, sharp edges, and 90 degrees to the face.

[Gerwin Brommer]

Which wing works best ?
-Birdwing

Do they have smooth, rounded , alumium trailing edges ??

[Meniscus]

This is along the same lines as a wedge rudder as opposed to a blade style?

[Gerwin Brommer]

Rounded and smooth : is sticky
Wedged/thinned/sharp edged : "disturbed" , let go more easy

pic : rudder : sharp edges, stepped in the back, tip of the blade : pointing up to the back to avoid lift

[Meniscus]

Rounded and smooth : is sticky
Wedged/thinned/sharp edged : "disturbed" , let go more easy

pic : rudder : sharp edges, stepped in the back, tip of the blade : pointing up to the back to avoid lift

Now that is a great picture!

[Blackjack-sven]

The trailing edge should be flat, sharp edges, and 90 degrees to the face.

Just trying to get the best info on rc boats in one place so anyone can find it.

Now, being the foremost "prop guy" to talk to, please explain as to why it's best to have the trailing edge be flat, sharp edged, and 90 degrees to the face? Obviously the symmetry is self explanatory, but the reason for the square back isn't quite so obvious (at least for me )
Seems like my theory has been tried with little success , so I'm interested in the theory and test results of your (and anyone who knows best) reasons for this style.

I did get this from Darin Jordan and I found this to be of great help.

http://rcboat.com/prop.htm

And while were on sharpening metal we should also talk about thickness' of the blades for all three items that hit the water. The rudder, turn fins, and prop(s). How thick (or thin) of a material should be used to keep the weight down but still keep the strength?

[egneg]

Water curls as it passes the trailing edge causing more thrust as it exerts pressure on the flat edge much like eddy currents in a stream or river. It is a good example of high pressure versus low pressure. It is the same principle used in racing - cars drafting behind the car in front of it.

[detox]

The backend of a late model Corvette is large and flat. Corvette racing has won many titles.

[Fluid]

High-speed surface-piercing props are not like ship or small outboard props. They operate in a cavitaing regime where the water does not even contact most of the front face of the blade. This creates the low pressure area that 'pulls' the prop forward much the same as with an airplane prop. Water does not behave like air - the squared off trailing edge promotes the formation/extension of the bubble past the edge of the blade, reducing drag. It really doesn't matter if it seems intuitive or not - the fact is the flatter the trailing edge is the better.

Weight matters not at all for rudder, prop or fins. What matters is drag and stiffness. Thin a prop blade too much and the blade leaves the hub - bad idea. Ditto for fins and rudder. The rudder needs to be thick enough to house the water pickup if you use one. It too needs a flat trailing edge like the prop, for some of the same reasons. One additional reason is that the water behind the blade needs to fill in the void as the rudder passes - air fills it first because it moves a lot faster than water. With the gap temporarilly filled with air the water can take its time, reducing overall drag.

When an object like a fin or rudder pierces water it moves the water aside; any time the object's cross section changes dramatically the water separates from the object's surface, causing a 'bubble' to form. This air bubble pulls the object right and left randomly, causing wandering and poor rudder control. I've seen the results of a "streamlined" rudder shaped like a 'wing' - the boat wandered badly and turned poorly. A wedge rudder was substituted and the boat ran great. This was not your grandfather's FE boat either.

Learn more here: http://www.rcboat.com/527_3.pdf





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[Blackjack-sven]

Weight matters not at all for rudder, prop or fins. What matters is drag and stiffness.

I'm not debating that going too thin isn't bad, but the weight loss for the boat in general should help with acceleration and cornering as the reduced weight will reduce over all boat weight increasing acceleration and decrease the boats centrifugal force around corners keeping it planted better. NO???

[paulwilliams]

Aerofoil section blades for rudders or propellers are intended to operate fully submerged without any cavitation or ventilation. Surface drive requires ventilation or it fails to work effectively.

Look again at the picture Gerwin posted of the rear end of that full size powerboat. Note the ducts above each propeller. These are to ventilate the prop. Without these ducts, the boat may not plane, and the load imposed on the drive system as it tries to turn fully submerged props of this size would be huge.

If you still think aerofoil (ie., teardrop shaped) sections are lower drag than a wedge in water, consider this: the X15 hypersonic (mach 5+) research plane had a wedge section stabiliser. At the speeds the X15 was capable of, even at the outer limits of the atmosphere, the air behaved more like a fluid than a gas.



Paul

[Blackjack-sven]

I know where to bend a prop but how far do you bend it to improve performance and shape the cone of water exiting, and how much is too much? What tool(s) do you use to do this?