Theoretical question regarding Hull design

[Dragoon]

There are two hulls. One built light the other heavey. Once launched, however, they both displace 10,000 lbs.

Given identical frictional coefficients, what is the major factor to cause one hull to require more force to move than the other.

Of these two hulls, one long and narrow and the other fuller, which requires less force to move initially and sustain momentum?

Thanks. for your assistance. It's been a while since I've actually had to do this sort of stuff since I've been doing non-engineering for 12 years.

 

[GregLocock]

Wave resistance.

That is, the power that is needed to generate the wave pattern. Typically longer more slender hulls have less wave resistance at a given speed. At low speed this is offset by their greater surface are which creates more frictional drag.

There's a few papers and so on around, names to look for are Holtrop and michlet.

For a brief and nasty introduction to wave resistance you might want to look at

http://www.geocities.com/greglocock/index.html

where, on the Battleships page there is an article on this very subject.

Cheers

Greg Locock

 

[raj71]

hi,
i think you are looking for a factor called as the wetted surface area which more than others would determine the resistance required to be overcome in this case.

 

[sunesismo]

The biggest factor will be the wetted surface area - the Reynolds' number calculation will determine the most efficient design relative to propulsion requirements.
However, ship design also needs consideration of the operating conditions of the vessel. Where will the vessel be normally transiting? What sea states and operational criteria? How much roll and pitch is acceptable? Depth and width requirements for ports and berthing? And so forth....

 

[satitk]

Tje lightly built vessel will have more wetted surface,so more resistance and therefore more power reqirement.
The slender built ship will reqire less power to start
and maintain.

 

[colinp]

For displacement hulls either the residual resistance or the frictional resistance may be dominant. It depends on the hull geometry and how fast the hull is moved though the water.

In general, very small ships and yachts (you said about 5 tonns?) have higher residual resistance because they have higher installed power for their displacement and are driven at higher speed to length ratios (V/(g*L)^.5 or Froude number). Wave making resistance increases rapidly at higher Froude number for typical displacement hulls.

Larger ships, especially commercial ships are carefully optimized to minimize fuel consumption at the design speed. As a result of this optimization process the residual resistance and the frictional resistance become approximately equal. Just below the design speed the frictional resistance is higher (relative the residual), just above design speed the residual resistance becomes higher.

There are notable exceptions. Motorized barges, dredges and very shallow draft vessels have higer frictional resistance due to their large wetted surface for their displacement. Even in small sizes they might have higher frictional resistance. Also, very fine ships, that is ships with a very high length to beam ratio have lower wave making resistance which keeps the residual resistance lower than the frictional resistance even at unusually high speed to length ratio.

You also asked about starting and stopping. The finer hull will require less force to accelerate and decelerate. Hulls with a higher prizmatic coefficient have higher liquid added mass coefficients. In a vessel of the size you described this difference would be hard to detect. In large vessels (like VLCCs) with very low installed power for their displacement liqid added mass matters. When they try to accelerate or decelerate the mass of water circulating around the hull also has to accelerate and decelerate. This is experienced by the hull as a transient increased resistance.

Did I answer the right question?
talk to you later
Colin Pitts