Reduced buoyancy in the Bow V (Hydrostatics)

I think it has clicked.

Where is the common preferable location for the LCB on a power boat?

Would it be like the shoe box method, basically trying to get the LCB as central as possible from bow to stern?

 

I think the chances you will be able to design something that is up to scratch, is not good, judging by your questions. Which is probably what could be said about anyone coming into a field they are unfamiliar with. They has been an immensity of trial and error that has gone into the design of boats, to arrive at what is available today. Which is not to say they can't be improved upon, they are still evolving, but rarely or never by people unfamiliar with what is already around. You probably should read a book or two about boat design before proceeding further.

 

HobbyBoatBuilder, if you intend to make the boat reasonably comfortable to ride, then it is well to embrace some basic physics. There is some point in the boat where the center of buoyancy resides. For simplicity let us say that it is in the middle of a 20 foot boat. Think of the boat as a playground see- saw. Think of the boat as if a loaf of sliced bread. The first section, or bread slice, of the boat will have some degree of support that is a function of the distance from the pivot point....in this case the center of buoyancy of the boat. The first section might be nine feet from the CB and the first section might have a potential flotation of one ninth the flotation of the middle bread slice. It is a lever. nine times one ninth is equal to one. Therefore the forward section will have as much potential to support the boat longitudinally as the middle section. The boat is not a shoe box.

To do the calculations just multiply the distance from the CB times the potential buoyancy of the section being examined....................I hasten to confess that boat design is not as cut and dried as the simple lever moment of force that is implied. Never mind. That is a good starting point for the understanding of fore and aft stability.

Designing for rough water fore/aft stability gets a little bit sticky. In rough water the bow might plunge making the potential for the forward section volume much higher which will tend to raise the aft sections. ...or conversely if the bow rises over the back side of a a steep wave, the lever force might be diminished and the aft section disproportionately buoyant. That sort of stuff is what informed design principals are faced with. For what it is worth the area distribution of the sections (bread slices) will usually resemble the bell curve. Changing the area distribution in an extreme fashion will result in a boat that will not only loosen your dentures but may even be dangerous.

 

Then of course, introducing the physics of planing, adds a whole nother dimension. The best reminder of that is probably the old board-surfer's trick of "hanging ten" , which illustrates that a suitably shaped bottom moving at speed, alters the dynamics considerably.

 

Very illustrative and perfectly correct if we change the center of rotation of the boat, which is not the CB but the centroid of the waterplane area.

 

Tansl, I do not wish to start a controversy. But there is this......The centroid of the waterplane area is a two dimensional number. The boat does not pivot about a two dimensional area. Almost surely it pivots about a three dimensional location....we can have the "centroid" of a three dimensional body if we use the term loosely. For a boat or other floating object, it seems to me that it is the center of buoyancy. That concept would apply to hot air balloons or Blimps as well to boats. I am merely an aspiring student so I will be pleased to learn more of the technical aspects of this subject. (that last sentence is not to be interpreted as sarcasm. I am as old as dirt but I am still interested in learning about things that I have yet to grasp)

 

You have a part of reason because I have expressed myself wrong. The boat, when floating freely in water, rotates around axes that pass through the centroid of the waterplane area. The center of buoyancy can never be the reference of the rotations of the boat. That is a very common mistake from what I have seen in this forum.
Neither do I want to start a controversy. Things are as they are, even if I express myself wrong. But it is very praiseworthy that you have a youthful spirit and want to continue studying (this is not sarcasm).

 

If you are interested in really calculated examples of locus of the centroid of waterplane area (CF) and the locus of the center of buoancy (CB) it is graphically displayed on this website:
Berechnung der Schwimmlage von Booten http://www.bootsphysik.de/
In the position of rest the longitudinal and transversal co-ordinates of CF and CB are nearly the same. If you relocate weights (physically spoken: masses) you can watch an increasing difference.
(First click button top right: "English", CF is marked by a green point, CB is blue. Input box "mass to relocate": 800, next line "longitudinal": 800, then click button "apply relocation".
The only reaction at first step is a moved red dot (CG), then click "find equilibrium" (right column, near bottom). You will see a significant shift of CB forward while the CF is (nearly) unmoved.
Buttons marked with " i " provide information.)

 

Thank you Heimfried. That is a useful resource.