Wave Resistance on Submarine vs Surface

[rnlock]

I would suggest that you check your math.
And, FWIW, an arbitrary sectional area to wetted perimeter ratio has very little to do with the overall resistance of a submarine on the surface or submerged. Some (and not even most) moderns submarines are round but that has to do with many other issues than resistance.

There's an easier way to say it. If an axisymmetric "submarine" is half submerged, the displacement is just half of what it is submerged, and half the area is submerged. When it's fully submerged, you have the full displacement and the full area. So the ratio between the two is the same. Not that I can remember just why I wrote this in the first place, but it's not wrong. I suppose things vary a bit in the intermediate states.

If these two states had the same low velocity, where drag from making waves was insignificant, the resistance per displacement would be the same, too. But that would have to be very slow, I think.

 

[rnlock]

I think if you had a towing tank setup that could pull a deeply submerged, 1/10 scale model of a sub at 10 times the speed of the real sub, you could get some pretty good data. So all you need is almost infinite money. I suppose if you had a slow sub, you could take off the props and just have a fast sub tow it. That could solve the problem with the resistance near the surface if you used a really long tow cable. The deeply submerged resistance might be determined by dropping a weighted sub model in a very deep body of water until it went fast enough to get the Reynolds number right. Bruce Carmichael did this when studying laminar flow on bodies of revolution at high Reynolds numbers. As far as getting the Froude and Reynolds number on a model right at the same time, I guess you'd have to set up a towing tank on Jupiter. ;-)

 

[zstine]

A submerged submarine with a round cross section has the same wetted surface to displacement ratio as the same sub floating with its axis at the surface. The submerged sub has twice the wetted area AND twice the displacement. If the sub is capable of floating with half its hull out of the water, that hull is too large for the load it's carrying. Or else it's really a boat.

Consider a semi circular cross section 1 foot wide. The circumference is pi/2 feet. The area is (pi/2) r^2. For a submerged craft with a circular cross section, it's pi and pi(r^2)

There is a misunderstanding of what I'm investigating. Essentially, I was interested in the difference of resistance on a SWATH hull with infinitely small strut (eg submarine) compared to a Catamaran hull of equal Disp. I know the catamaran is much better at higher speed because of less surface area, but what about speeds right at the hump fn=5? Since a submarine (running deepish > 3x diameter) does not experience a hump (surface effects), I thought it may have a speed at which it has less resistance than the catamaran hull (given equal displacement and length). But I believe now that the catamaran always has lower resistance than a submarine/Swath despite surface effects.

 

[AJB]

Zstine
A full feaso for medium speed cat ferry for Sydney Harbour suggested there could be benefits for a Swath configuration, but then decided that the operational risks and other added costs would negate the gains...

So back to 14:1 length to beam cat hulls...

If you could post the surface area maths, that would be interesting..

 

[gonzo]

If an axisymmetric "submarine" is half submerged, the displacement is just half of what it is submerged, and half the area is submerged. When it's fully submerged, you have the full displacement and the full area.

The displacement is the weight of the displaced water, whether it is half submerged, totally submerged, or any fraction thereof.

 

[jehardiman]

If an axisymmetric "submarine" is half submerged, the displacement is just half of what it is submerged, and half the area is submerged. When it's fully submerged, you have the full displacement and the full area.

LoL; You understand you can't build a submarine like that...a physical impossibility....think about it. Zstine's concept is little better...once the demihull is submerged, with no waterplane in the strut you are going to need a really big, heavy, and expensive control system that most likely won't work in most circumstances.

 

[jehardiman]

But I believe now that the catamaran always has lower resistance than a submarine/Swath despite surface effects.

If you believe that for all catamarans you are incorrect; however overspecified you need the environment. Based on you experience stated in other threads, it is my belief that you are forming your mental models without a full understanding of all the quirky things in the ancient interface, i.e. all the snarky hydrostatic, hydrodynamic, and oceanographic issues. No one hull form is always better than another.

Horses for courses....

 

[jehardiman]

LoL; You understand you can't build a submarine like that...a physical impossibility....think about it.

Actually, on second inspection it could be done if you want to get into the more snarky aspects of compartmentation related to hydrostatics. But even if you could physically build it, it would be a pain to outfit.