Question on Drag

If I take a one foot square piece of plywood, and could stand it on end (vertically) in the water... (ignore for the purpose of this question that the plywood will not remain in this vertical position, and will fall over) ...

Beside that I lay an identical piece of plywood floating flat (horizontally) on the water.

Now I pull both forward at the same speed, and with the same power input to both panels...

Is the drag equal in both cases?

 

No fair! The square leading edge of the flat paiece will be wider than the vertical piece, therefore more wave drag. I know, that is what you are trying ot show, but.....
At a certain speed the flat piece will start to plane, and then the drag difference will tip the other way. ;-)

Steve

 

Let's make it fair

OK, for purposes of discussion we will round off the leading edge of both panels to the same profile.

 

Foxx - that's gonna be difficult....... ;-)

 

Hi guys,

Sorry... I don't understand how the rounding of the edge will make the wave drag issue go away... I'd imagine that you'd get quite different results due to the difference in wave pattern... I'd think that there would be papers on this... Froude's plank expts and all...

I guess if you tow them slowly enough and they have equal surface area, the drag should be close to the same, but you'd have to have pretty fancy gear to measure such small drag...

Neat topic... what got you thinking about it?

Dave

 

"Neat topic... what got you thinking about it?"

I suspect we're going to find out - it may have something to do with the "long skinny boats" thread.....

Steve

 

"Now I drag both forward at the same speed, and with the same power input to both panels"

If you can do this, the drags are equal.

I would point out that one piece uf plywood has 2 sides dragging and the other has 1 side dragging.

Since water depth has some effect on drag, if the shore is closer than the bottom or the shore is more distant than the bottom, one has more drage than the other.

One panel has a positive angle of attack the other has a zero angle of attack.

Where is this going?

 

I assume you're wanting to ignore wave drag, too, and only consider skin friction.

The answer is, "No." The vertical plate has both sides wetted while the floating plate has one side wetted. The vertical plate therefore has twice the skin friction of the horizontal plate.

 

Guest and Tspeer - friction drag should be equal, if the plywood isn't made of Lignum vitae........ Think about it. ;-)

To be more accurate - the wetted surfaces will be equal, although the Rn may have changed on some faces.

Steve

 

Steve, surely not, if say the panel is made of balsa wood (specific gravity 0.1 say) the panel floating flat will have approximately 1 square foot wetted surface and that floating vertically will have only 0.2 square foot wetted. In the original question what did you mean by equal speed and power input, this does not sound consistent?
John

 

For only the wetted surface area of a plate one can use the ITTC formula
cf = 0.075/(logRn - 2)**2where rf = 0.5 rho v**2 S Cf

yipster

 

sorry guys ... the original question I posed was asked from the perspective of ignorance - not to demonstrate 'anything' in particular. There were no hidden agendas or lead-ins that the question was meant to spring ... it was just a simple question.

Not being a naval architect myself, and having VERY little understanding of the math and physics related to such a question, I thought I would ask it here. I had previously concluded that the drag would be equal until such time as the flat (horizontal) panel began to plane, but suspected that other parameters might affect the answer.

I understand that if the panels were made from materials having different weights this would affect drag related to each, but I'm thinking that if both the vertical and horizontal panels are made from the same material (same weight) it wouldn't make a difference - or am I missing something here?

The drag produced by the flat (horizontal) panel can be reduced by diminishing the wetted area through planing ... is there any way to reduce the drag from the vertical panel?

 

I'm not offering this as an answer but adding it as a question for those who know more than I. Would surface ventillation reduce drag on the vertical panel?

I assume that if it were acting as a foil there would be arguments against ventillation.

Hmmm...... another question: Whether ventillation has a positive or negative effect on surface drag (I'm guessing positive), would differential ventillation of a fixed foil induce differential lift?

I'm sorry if I'm straying from the thread. Tell me and I'll move it.

Doug Carlson

 

sorry that was a "of the shelf formula" i came acros before, apropriate but not intended to impres anyone or giving a answer to your question. neat topic tho, and weight does matter, speed too.

yipster

 

Yipster

Good to hear your input. Cheers! I have to admit, ya lost me there in the technical jargon, but I'm sure there are others here who understand completely. I will try to keep up.