[Dan Evans]

Hi everyone,
There is something about hull design that has been bugging for a while and if anyone can help me with it I would really appreciate it!

I know that a larger WL length to beam ratio creates a hull that can slip through the water easier because the hull does not need to push the water away as rapidly as a short and wide hull.

My question is this, does the WL length to draft ratio have this same effect?

For example, if my question isnt clear enough, two displacement yachts of equal displacement have the same WL but one has wide beam/shallow draft and the other has narrow beam/deeper draft. which will encounter less resistance or will it be the same?

I know that in real life it would be more complicated than this because the shape of the hulls would be completely different but im just trying to keep it simple.

Thanks,
Dan

[LLC]

The ship's hull resistance has two major components, one is the wavemaking resistance the other the frictional (or viscous) resistance. Other components are the entry angle etcetera but to answer your question we'll leave these out of the consideration for now.

a wide shallow ship will have more wavemaking resistance than the deep narrow version since wavemaking resistance depends very much on the shape at the freesurface. The faster a ship will (have to) go the more important the wavemaking resistance becomes as a percentage of the total drag and as such the powering requirements etcetera. The pressure increase deeper down will increase the viscous drag slightly but this usually can be ignored mostly as the effect is minute compared to wavemaking drag.

At slower speeds the wavemaking resistance is much less important part of the overall resistance and thus for low Froude number operating vessels a wider body is a good option.

To answer your question: D/L has not the same effect on drag as B/L because the B/L ratio is directly related to the wavemaking resistance whereas D/L is not directly related to any of the resistance components.
Hope this helps.

[Dan Evans]

Thanks LLC, that cleared it up for me!

Dan

[U.Petricich]

There are many factors effecting the quality of a Hull .The purpose in doing a good hull design is mooving it with less energy as possible.The factors effecting this for example are the L/B ratio, the entry angle on the water of the Bow the wetted surface area, the quality of the water lines (should be smooth as possible and without concavity specially in the first part of it) the diagram called curve of areas is extremely important to define the quality of a hull design.You can notice when a hull flow in the water is causing a wave with nodes one on Bow and one at the end of the Boat this normally have a lenght that define the critical speed of the boat. The purpose of a good project is to incease that lengh to increase the speed with same amount of energy, this is called dinamyc lenght. The second node is far away from the end of boat and is increasing the virtual lengh of a Boat. In the atached example you can see an 85' long boat with an engine of 350HP cruising at 15 Knots speed displacement in that case 50Tons.
regards U.Petricich