Sheer Revelation

Michael,

Notice the stbd half of the hull in plate 5. If you took a straightedge and held it along the sheer in that view, it would be fairly straight. When you look at the vessel in profile, the sheer appears curved. Another way to look at it is if you took the hull, flipped it over then laid the sheer on a flat surface, the sheer would not have any gaps. The effect of a flat sheer is most noticeable when the boat heels. It will have a very smooth arc when viewing at any angle of heel as opposed to having weird bulges or cuves that contradict the overall shape of the sheer.

There can be many subjective observations about whether or not this particular sheer looks good. The sheer is probably the most subjective feature on any boat and also probably the biggest money maker. I'm not saying this is the RIGHT way to make a sheer, it is simply an observation and if I'm totally wrong on this, feel free to point it out.

Might even be helpful to dig up some examples.

 

Believe it or not, a planar sheer could be one way of making sure the sheer is fair.

With my CAD I draw any sheer I want as well. But in this case I'm discussing planar sheers.

 

What is a "planar sheer". I have never heard or read the term before.

 

I've heard it from other designers.

See above for a graphic description. The starboard half of plate 5 pretty much describes what I'm talking about.

 

The sheer is a line, how can it be planar?

 

Seems Dreamer is going for a sheer which is close to a planar curve, i.e. it lies in the same two dimensional spaces. I think it is true that these often look good. It is also true that a lot of other good looking sheer curves are not planar. Gonzo's idea of hanging a piece of cardboard produces a curve which is guaranteed to be conically developable. I’ve also heard other designer desiring this characteristic of a sheer curve. I think that the planar sheer that most designers would draw would also be conically developable. The criteria of being conically developable seems to be more general.

What do you all think, is that a criterion for a good looking sheer curve?

 

I Use that for ply designs a lot , It just seams to work out well sometimes.
But a lot of it seams to depend on the amount of flair or tumble home.

 

A planar sheer is one where every point on th sheerline lies in one plane. It's been practiced for years. When you do this it is almost impossible to have the sheer look "powderhorned" from any angle you look at it. I've used it on occasion but sometimes the bow will look hooked in the upward direction from some angles and that needs to be tweaked out.

 

So you intersect the hull with a plane, is that right?

 

I don't think you will find many designers who have actually worked out their sheerlines as planar by design. It may be some work out that way.

 

It is a pretty simple concept. Joe P's post just before yours seems a nice explanation for people who have not heard the term before.

 

It would limit the design enourmously. I can't see the advantage.

 

It can be limiting but if it works it looks good. The advantage as stated before is that the sheer doesn't look powderhorned or "S" shaped from any angle.

The fwd and aft points of the sheerline define 2 points of the plane and the third point will lie near the lowest point of the sheer.you can tweak the lowest point back and forth about the lowest point and the widest point of the sheer until you get it where you like it.

Bob Perry among others has used planar sheers in the past, though not always.

It's just another tool in the yacht designer's tool box.

 

It is nice to see more people who can help set the record straight. As you say, a computer is just a different tool, and can produce any result that can be done by hand. It just lets you try more iterations faster.

Designing hull and foil shapes with computer has been around for more than 20 years. I think the people who say you can't get the shape you want, you have to accept the shape the computer wants you to have, have never seen an IOR shape, or a 6 Meter shape, or an ACC boat shape done on a computer. The designers of those types of boats would not use a tool that did not allow them to get precisely the shape they wanted.

In other disciplines most design work these days is done on a computer. Even organic shapes that are far more complicated than a boat hull can be modeled quite accurately on the screen.

If you can't get the shape you want and have to accept the shape the program gives you it is more likely due to your incompetence than an issue with the software.

 

Your belief has been proven wrong by thousands of successful designs.