Optimum finess ratio for given internal volume

[aerohead]

Originally Posted by miket

Im not talking about the Cd as much as the CdA. A longer vehicle will have more internal volume with a smaller cross section. Will the skin friction offset the smaller cross section?

miket,CdA is the ultimate criteria for air drag,so you could definitely play around with dimensions.For example:
The 'Template' would constitute a 40%-Thickness fuselage,Cd 0.04-free-air,Cd0.08-in ground-effect,Cd 0.12-with 'skinny' tires.
An 8.33:1 streamline body would constitute a 12%-thickness fuselage,Cd 0.066-free-air,Cd 0.132-in ground-effect,Cd 0.172-with 'skinny' tires.
So,for instance,a Honda Civic CRX at 18.5 square-foot frontal area and the 'Template's 2.5:1 full tail @ Cd 0.12 would have the same drag as a 8.33:1,12%-thickness fuselage-shaped body @ Cd 0.172 and 12.9 square-feet frontal area ( a 1960s Lotus Elan with the windshield removed ).

[dmac257]

Originally Posted by miket

My guess is that submarines are close the the optimum shape.

In 1980 there was a design proposal to make a fast attack submarine that was 1/3 the diameter and change the propulsion from steam turbine inside the pressure hull to water lubricated/cooled electric motor outside the pressure hull to eliminate the shaft seal issue. Fluid dynamics predicted that it would go faster and deeper but the idea was rejected because of other requirements .. submarine isnt optimum shape at all.

Don

[miket]

Well why dont they make the submarine longer and narrower?

[aerohead]

Originally Posted by miket

Well why dont they make the submarine longer and narrower?

miket,in the thread 'FLOW-IMAGES' I posted some submarine hulls.With the sails removed it's hard to distinguish one from another even though they represent subs from around the world.It appears that when naval architects run their numbers for hull design,they're all basically coming up with the same solution.It seems that for the contained equipment and crew,the envelope the engineers are choosing to enclose it all already offers the best solution.
If you had the inclination,you could contact U.S.NAVY designers at the David Taylor Tow Basin in Maryland and bounce that stuff off of them.