Sheer Revelation

I put together some more plates to demonstrate a planar sheer (and I reduced the size of those monster images at the beginning of the thread).

The first image shows how the plane intersects the sheer of this half-hull.


plate p1

Looking at in in profile you can see how 'planar' does not mean 'flat'


plate p2


plate p3

You can adjust the plane in any axis to control the shape of the sheer


plate p4


plate p5


And without question, the sheer using this method will be perfectly fair (given that other lines such as sections, buttocks, waterlines, diagonals, etc are fair).

Hopefully this illustrates that you are definitely not limited by using a planar sheer and in fact it's possible that many designers have created planar sheers without consciously trying to do so. It just worked out that way.

This isn't necessarily a discussion of computers vs. hand drawings. That was not the intention of the OP (I would be more than happy to start a new thread so we could discuss that ). In fact, it's possible you may be able to use this same method drawing by hand. Would have to think about that...

 

Rick that is a nice presentation of the idea.

 

I'm still a little shaky on planar sheer. Are you saying that if you position a plane properly, every point on a planar sheer will touch it?

I have noticed that if I tilt my model away from me the sheer eventually becomes a straight line, or very close to it. And if I keep going, it turns into an interesting reverse sheer.

 

yep

notice plate p3 and plate p5 above. The first one has the long edge of the plane parallel to the WL. The other plane is 2° down from parallel. You can easily see the effect on the sheer in those two images. If you were to make the plane 2° above parallel, you would have a reverse sheer and THAT would just plane look weird. Actually not even a reverse sheer, The sheer would have the bow end lower than the stern end.

 

thanks

 

just for grins, I experimented (sorry for the mirror image). Here is the results of a plane that rises fore-aft. The sheer still has spring, but now the low point is foreward of midships and the stern is a bit higher than the bow.


plate p6



plate p7

 

Dreamer,I don't have the means right now (posting from my iPod) but since you have the ability to create a 3D PDF (see anther thread) you could post your model to anyone with the free Adobe Reader version 7 or higher. They could rotate it around to see how it looks.

 

good point Joe

Here's a 3D pdf

Right click on the view and set the render mode to solid outline. It will be easier to see the sheer.

 

hm...
good points there, dreamer...

i never had any thoughts about the sheer or the why one looks good and one looks just weird and ugly...
now i realised that on all my drawings (freeship) i tweaked the sheerline to simply 'look good' when viewed in 3D and ended up having the sheer in a perfect plane... although the plane is absolutely horizontal and not at an angle like in your models - it is in a plane nevertheless...
it is good to be pointed to subtle design criterias that way...

 

Rick,
You have a nice illistration for the set of sheer curves that fit in a plane. Similarly, i think there is a more general set of sheer curves that lie on the surface of a cone. Will your software help you find the cone which the sheer curve lies on? I suspect the apex of the cone is above the hull, outboard and forward of midship.

 

You will still have to develop the sheer to have it appear the same way in 3d.
It will give a good indication of how it might develop , but just because the
program looks good dose not mean it will in real life and optics .

 

Good question about the cone. That would be an interesting study. Will need to give that a shot. My suspicion is, however, that the curve you're thinking of is actually created by passing a plane through the cone.


As for the 'just because the program looks good dose not mean it will in real life and optics', you just singlehandedly disqualified decades of software development and millions (billions?) of dollars of boats that have been built in that time. I would guess that a great deal of trust is placed in how good it looks in 'the program'.

 

Except for being a geometric oddity, is there any advantage? I think that a if a curve is graceful and fulfills the purpose, there is nothing more needed.

 

Frank,

An advantage of 3D cad programs is that you do actually develop the lines in 3D. Also, you can rotate the model in all axes to make sure your sheer line looks fair, something you cannot do in 2D. In some ways it is like working with a physical model of the boat. I have over 30 years of experience in boat design and I would not go back to paper and pencil except to produce preliminary sketches.

 

I can see the boat before I draw it. Paper and pencil are pleasant compared to a computer. Either method can produce preliminary results. However, the computer has narrower limits that are set by the programmer.