improving performance - reverse bulbous bow principle, amplify wave near stern

Just slow down until the bow wave is under the stern.. No other tricks needed

 

I think you'll find that's the purpose of a canoe stern.

If you were to have a rounded stern, in the absence of viscosity it would do what you ask, as it would have a stagnation point at the stern. However, because of viscous effects, it is not possible to bring the flow to a halt in a finite distance next to a solid boundary without causing separation. So there's a need to have more wedge-shaped waterlines, which produces the classic canoe stern.

 

Hi Tom!
I read several papers a few years ago that described encouraging
experiments of boudary layer suction in towing tanks, but I have yet to see
systems implemented on real ships.

Have you seen any BL suction (or blowing) systems deployed successfully on
boats, wings, or anywhere else?

Leo.

 

I haven't seen any BL suction or blowing systems. We had a polymer injection system working on USA 17, but took it off before the regatta. (They needed the weight reduction more than they needed the extra speed.)

 

My understanding, though I don't recall what it's based on, is that typically the drag reduction due to boundary layer suction does not offset the power consumed by the suction system.

 

That's my conclusion too, David.
I had a long report from the US Navy that gathered together over 100 weird
and wonderful drag reduction systems, techniques and devices.
One of the more interesting was a way to simulate a bulbous bow by
squirting water through a thin nozzle low down at the stem. The main idea
was to create the effect of a bulb without the skin-friction penalty and to be
able to turn it off when not required. Lab experiments proved the concept,
but on a real vessel the power required to pump water was greater than the gains.

 

Was it really so?

But i have read air bubbles squirted all around the ship ,to reduce the wetted surface area hence the frictional drag ,and heard it is successul.

If it is possibe to work on the whole for the ship ,why not for the bulbous bow.
which wld have been a fantastic ,if it was succesful.

 

In the work I read it wasn't a mixture of air and water. It was just a jet of water released under water.

 

This one?

 

Or this one?

Cross-sectional area distribution along the body determines wave drag, largely independent of the actual shape. http://en.wikipedia.org/wiki/Area_rule

I see some progression of the rule from Coca cola body to SLICE.

 

Can Boundary layer suction principle can atleast be used in aft area ,where the boundary layer growth is predominant.Having a porous sideshell in the aft area.

Well,Not sure is it economically viable.But Definitly worth a research.

 

It was done a long time ago. See:
"Resistance studies of a ship model with boundary-layer suction"
Sander M. Calisal,
California University, Berkeley,
College of Engineering,
March 1969.

 

I'm skeptical of trying to understand water waves by analogy with shock waves in transonic and supersonic flow. There are fundamental differences in the physics of water/gravity waves compared to shock waves.

Shock waves are not dispersive. The speed of propogation of shock waves is independent of "wave length" and depends only on the fluid properties. The angle of shock waves from an aircraft/missle/etc/ varies with the speed of the body producing the shock wave. The speed of propogation of the "energy" in the waves is the same as the speed of the wave itself.

Water/gravity waves are dispersive. The speed of propogation of water waves depends on the wave length. The angle of the leading edge of the waves from a vessel moving over the free surface is independent of the speed of the vessel (in deep water). The speed of propogation of the "energy" in the waves is half the speed of propogation of the waves (in deep water).

Gravity waves in shallow waters behave similarly to shock waves. Shallow in this use means the water depth is much smaller than any lengths associated with the waves.

 

David, It does not have to be supersonic. It is just that I find many Aeronautical theories and mathematics are similar to Naval architecture.

For example, the Sears Haack shape is similar to a double ended canoe or the Mitchells thin ship theory. The aerodynamic behavior of a cigar shaped body is similar to Professor Inui's study on source and sink. The bump on the 747 was developed because it was found to reduce drag, much like the bulbous bow.

But I am not into serious study on these things. I am more interested in the shape of afterbody and how it increases efficiency, hence this thread caught my attention.

 

Aircraft speed does not need to be supersonic for shock waves to be present. The flow over a 747 and other jet transports at cruise is transonic. Even though the aircraft's speed is subsonic, the flow over the aircraft is locally supersonic and there are shock waves present. That's why the area rule is applicable to jet transport aircraft.

The area rule would suggest that the a sailboat with a keel should be have the cross-sectional area of the canoe hull reduced in the vicinity of the keel for minimum wave drag. But the shock wave physics which the area rule is based on have some fundametal differences with the physics of free surface waves.