Question on Drag

John - I assumed that the panels were not balsa ;-) Especially since the OP stated "plywood" so I assumed (I know, dangerous game) that the SG would be in the 0.50 region.
Steve

 

FYI The ITTC formula yipster indicated is the basis for tank testing and scaling between model and full size using the relation CT=CR+CF where CR (Residual drag ie wavemaking etc) is the constant unkown that we measure for in the tank and CF (frictional component) is from above and is calculated for both model and ship. Of course this is all coefficient form. Most basic NA books have some description of the process even the "Dummies guide" types.
Brett

 

Apparently "boundary area bleeding" would have been a better terminology for what I was trying to express above. Could this also be referred to as "induced cavitation"?

ref:
http://www.usc.edu/CSSF/History/2003/Projects/S0101.pdf
http://www.curj.caltech.edu/archives/vol1/1004005.pdf

Doug Carlson

 

Doug, thats very high speed! 230 mile hr torp's and supercavitating passenger submarines that could cross the atlantic in less than an hour! from caltech! makes me wonder where they plan that crossing. reading edmund pope's story i wonder if i dear mentioning a new water repelant boat paint that couses cavitation and should reduce resistance dramaticly?

yipster

 

foxxaero -

Allow me to set up your experiment just a bit differently:

1) Let's assume as SailDesign has suggested your block has specific gravity .50 relative to water; length is always 12 inches; and displacement is always 144 cubic inches (so your full block has a volume of 288 cubic inches).

2) We get to do 4 variations on your block that maintain the length and displacement:
A) immersed dimensions are 12" l x 12" w x 1" d.
B) immersed dimensions are 12" l x 6" w x 2" d.
C) immersed dimensions are 12" l x 3" w x 4" d.
D) immersed dimensions are 12" l x 1" w x 12" d.

3) We get to pull the block at 3 different speeds:
A) so slow it's all about friction drag
B)

 

Sorry, I posted accidently, so I'll try again

Allow me to set up your experiment just a bit differently and throw it open again for more discussion:

1) Let's assume as SailDesign has suggested your block has specific gravity .50 relative to water; length is always 12 inches; displacement is always 144 cubic inches (so your full block has a volume of 288 cubic inches); block surface is highly polished.

2) We get to do 4 variations on your block that maintain the length and displacement:
A) immersed dimensions are 12" l x 12" w x 1" d.
B) immersed dimensions are 12" l x 6" w x 2" d.
C) immersed dimensions are 12" l x 3" w x 4" d.
D) immersed dimensions are 12" l x 1" w x 12" d.

3) We get to pull the block at 3 different speeds:
A) so slow it's all about skin friction drag
B) moderately slow so we start considering the proportions of skin friction, wave drag, and viscous resistance (drag due to turbulent vs laminar flow). Giving blocks B, C, D a foil shape of constant section get discussed here
C) Fast so we can talk about planing in block A and maybe B. Location and and angle of pull on our towline becomes critical here - your boat/board won't plane if it's being towed forward and down - think poorly trimmed powerboat or sailboat!

Let's assume our tow tank has infinite depth and width; we're going to stay away from cavitation issues; and forming our block into a sculpted hull with a wing keel is off limits.

The question is now "what is the difference in towing force for the 4 blocks when pulled at the same speed for each of the 3 suggested speeds?" And the drag is the same for all 4 blocks only when pulled so slow all we are really talking about is friction drag (because they all have equal wetted area). The other permutations is where it gets more complicated...

 

I think Foxxaero's real question is, "What are the components of drag and what are the major influences to each one?"

There's not a single way of accounting for drag. Being consistent with your bookkeeping is as important as how you classify the different contributions to drag. How you set up your bookkeeping will depend on your ultimate objectives and what data you have available.

Froude divided the drag into viscous drag and residual drag, but took the viscous drag as being that of a flat plate (uniform pressure gradient) with the same wetted surface area as the hull. So any differences in skin friction from a flat plate get lumped into the residual drag. Residual drag is usually divided into wave drag and form drag, with form drag being what's left when you subtract out the wave drag; it's intended to account for the effects of the viscous boundary layer that show up as pressure forces due to the boundary layer effectively distorting the shape of the hull. When you have dynamic lift on the hull (planing) or hydrofoils of any kind (keel, rudder, flying foils), you have to add the induced drag due to hydrodynamic lift to the list. Each of these drag components has a different origin and varies with speed in a different way.

The skin friction is not a function of the materials or their orientation. The flow in the boundary layer that is against the surface does not move relative to the surface - it's dragged along by the boat. The skin friction is a result of the shearing in the boundary layer that takes place between the surface and the freestream flow. It is also significantly affected by how the exterior pressure is changing in both the streamwise and crossflow directions, and by the history of the boundary layer's development as it's swept past a given location. When you have separation (where the flow does not follow the surface shape), it's also affected by what happens downstream, too, because you can have flow moving backward along the surface.

On a boat, it's very difficult to estimate the dynamic pressure distribution over the hull, and for a long narrow hull (like a ship) there are long stretches where the pressure isn't changing very much. So the flat plate approximation isn't such a bad one. And since it's as important to be consistent as to be accurate, the International Tow Tank Conference (ITTC) has defined a standardized variation of skin friction vs velocity to be used in reducing the data from tow tank tests and extrapolating them to full scale.

I think the ITTC skin friction line is what you were getting at in your original question. It would apply equally to surfaces that were horizontal and those that were vertical.

 

If we submerge both pieces in deep water to the point where surface effect no longer 'exists', with both pieces at a zero angle of attack, then drag will surely be the same.