Sheer Revelation

as a novice (begginner, noob, whatever) boat designer, I've recently had a revelation. In my first design (Grand Portage Great Lakes 40), I received some comments that the low point in the sheer was too far forward. It was too subtle for me to really notice and every attempt I made to move it aft, did not give me the results I was looking for. I decided to investigate why I was having so much trouble.

This first image shows my adjustment to the sheer in profile simply by pulling the sheer down.


plate 1

Note that the low point is in the area of sta 7. Looks good, right?

but, in isometric view it's apparent what the problem is


plate2

as well as a view from aft


plate3

Clearly this demonstrates that the sheer is a product of many things, not just the profile. What I discovered (and I'm sure you experienced guys are just rolling your eyes) is that the sheer is actually a product of hull volume. This volume also controls whether the sheer is 'planar' or not. Now, the sheer doesn't have to be planar, but on most sailboats it is. Note plate2 and plate3. You can really tell what the results are of a non-planar sheer.

To fix this, adjustments need to be made either by reducing bow volume or increasing stern volume.


plate4

In this case, the starboard sheer (with less volume) is planar. The Port sheer is not quite as planar and would probably require even more volume aft. Not a good approach. Or I suppose I could pull up the port aft corner and put more spring aft, but I really don't want tons of spring.


plate5

Note that even after those adjustments, the spring remains with its low point around sta 7.

Or, I suppose I could just take out the spring altogether but that wouldn't really go with the aesthetic of this boat.

Hopefully others will find this helpful. Maybe obvious to some, but it was a revelation to me. I also have to say this really goes a long way to show the benefits of using 3D design (these models were done in Rhino).

 

I think that computers usually produce ugly boats. The program is limiting what you can do. The easiest way to get a graceful sheer, is to cut the plan view in cardboard, and then hang it from the ends on two supports. Measure the curve and transfer it to your drawing.

 

Dreamer, now you know why I'm building a wooden model instead of fiddling around with a CAD program. I'm not an experienced designer, and the CAD draws what I tell it to, no matter how stupid or impractical it is. Whereas when I'm working in wood, the wood often tells me what to do instead.

 

It is hard to force a piece of wood into an ugly curve.

 

That is a good point , and drawing give me a better feel for the shape.
Programs give quick technical info , but I think a lot is lost in not drawing it out at some point . Drawing also trains the mind in a different way .

 

It's a shame we'll eventually lose the "organic" based lines of analogue design as craft commercially available will cluster around digital design techniques. A certain sense of proportionality and grace will disappear, the loss for the most part unnoticed and unappreciated by generations unaquainted with the art rather than the science of naval architecture. John Henry was a better man than I.

 

While I started designing before cad based hull design programs and enjoy drawing on paper with splines & ship's curves, I would have to say that I can create a sheer line as good or better in cad as I can on paper. Like most things in life it comes down to practice, practice, practice. The more familiar you are with your tools (manual or cad) the better your work will be.

Gonzo, I like your method of hanging the plan view but how do you locate the low spot fore and aft?

Tolly, I think the fact that many beautiful boats with classically prportioned lines have been designed in cad and produced shows that proportionality and grace are not necessarily linked to the design method. As long as the designer studies the old boat designs and tries to emulate them then those attributes won't disapear. Remember, it is the designer and his vision not the program or tools who create the product.



Dreamer, I would say that the sheer is not necessarily a product of hull volume but is definitely affected by it.

 

It is the method that Herreshoff used. The curve forms naturally. You may tweak it a bit later, but it will need very little correcting.

 

Dreamer your problem with the sheer line looked more about the position of the max beam in plan view being a bit far aft than the curve of the sheer in profile.

 

Thanks Joe for injecting some reason here.

I'm still fiddling with this and agree that it's not only a product of the volume. But I have to say I was a bit surprised at how much of an affect it has. Something that's not very intuitive and also difficult to show on paper.

As for 'computers' producing ugly boats, if 'computers' ever get to the point of producing boats, then we're all in trouble. Apparently, you've never seen Terminator. Where the machines take over the world. That **** would suck.

The people who worship the computer and think it is more than just another tool are the PEOPLE producing ugly boats. I would guess their boat would suck even if it was drawn on paper.

For those who think that boats drawn using a computer are ugly, then I guess you pretty much cannot find a pretty boat drawn in the last 20 years or so. That would sure leave out a whole helluva lot of very beautiful vessels with very seductive sheer lines. In contrast, there were sure a lot of very ugly boats drawn by hand and cut into models. Personally, I think the sheer in the model above even looks pretty good. Probably better than I'd ever be able to make drawing by hand.

It's not about computers 'designing' boats, it's about very skilled creative people mastering a tool to produce a viable end product. Anyone looking to make a living at producing beautiful vessels will do so with their hands on a mouse and keyboard.

Now, if you're simply talking about the 'look' of the drawings, that's another thing entirely. I can find enough clip art of wrinkled paper, coffe cup stains and smudge marks to make a CAD drawing look like a paper drawing.

Really though, the point of this thread was not CAD vs. paper. It was simply to illustrate how hull volume can affect the sheer. In addition, I'm finding that beam also plays an important factor as to where the low point in the sheer is. This also affects volume. If nothing else, it shows that you cannot focus on one particular characteristic of a hull without it affecting other pieces. That is where true skill comes in and is something that can only be gained from experience.

 

Gonzo, Thanks for that tip. I'll have to try it.

Joe, hang it from a level, then use a ruler to measure the low point.

Dreamer, I appreciate you sharing your revelation. I'm not sure that I understand what you are describing however. When you say planar, is that the same thing as Developable? Or do you mean the entire curve must lie in a plane?


~ Michael

 

I have read more than one designer discuss how sheer curves must be compatible in both the profile and the plan view for them to look right in 3D. I'm not certain what their definition of compatible is, but I think it means the extrem points need to be at about the same longitudinal position, and the general shape of the curves are the same. For example one can not put a straight line in plan view and an arc in profile in the same portion of a sheer line.

 

I believe that there is no way you can see if curves are fair or not with a computer screen. That forces you to put all your faith in a computer programmer's skills and aestetics. My battens are, in my opinion, much better.

 

That may be true, but the issue then becomes whether or not we want a planar sheer. Certainly not a necessity to have a planar sheer, but for my goal that was what I was trying to achieve. I feel for me personally that a planar sheer is much more elegent.

If the beam is moved forward and you wish to maintain your lowest point in the sheer at around station 7, the sheer will not be planar as it will start 'rising up' to meet the beam (as looking at it from aft).

 

What is a "planar sheer". With my battens I draw any sheer I want.