Developing hard chine hulls

I wonder sometimes as to how boat designers develop hard chine hulls. Do they first develop round bottom hulls and then convert them into hard chine shapes or do they start with hard chines right from the start? Also, many designers offer round bottom and hard chine versions of the same boat. Which brings me to the main point of my post - how do designers break up a round hull into hard chines? I've noticed that some boats look beautiful no matter what design principle is applied and some hard chine boats look butt ugly. Is there an optimal way to develop a hard chine out of a round bottom or is it more or less just a sense of aesthetics (or absence thereof ) that guides a designer? Also, is there currently any software that allows for the optimal conversion of a round bottom to a hard chine shape?

 

I assume you mean non-planing boats, chines are driven by the sectional area curve, arrangements, and plate development.

 

If you google "radius chine hulls", you will see an intermediate form between the two. Function is the main determinant of the shape of that part of a boat that spends a lot of time interacting with the water, there are boats that would suffer in functionality from any attempt to convert them from round bilge to hard chine, but so too there are hard chine boats that would lose functionality by attempted morphing into round bilge. It isn't all aesthetics, but dare one venture that that which functions well, also looks good, more often than not.

 

Very few "round-bottom" hulls have actual round hull sections. Most have compound curve ones, which have segments that are often very close to being straight.

These are the easiest ones to turn into hard chine. If you are willing to go to the trouble of rounding off the corners where the panels intersect, you end up with a hull that looks very much like a round bottom one. This rounding off is accomplished by use of pipe sections, for metal boats, and often strip planking, for wooden ones.

If one is not willing to go to this trouble, one can merely snip the corners and add smaller panels there, then end up with a multi-chine hull, with each added panel adding another chine.

The name of the game here is get as close to the shape of the original rounded section as one can, with as few panels as possible, while maintaining the exact area of the original rounded section.

This can often be accomplished with just two chines per side (meaning three panels, if the bottom of the section ends with a "V" ). Sometimes a third chine is needed. And even rarer, a fourth one. The rounder the original round section is, the more panels and thus more chines are needed to imitate it.

It is best to keep the panels curved in in one plane, with as little twist as possible.

But often some twist is needed to get the proper hull shape. When this happens, life gets a lot more interesting. To get this twisted panel to follow a curve, it has to be taken from an enormous cone--hence the term "conic development". Sometimes it will be necessary for a single panel to be taken from more than one cone, and the it gets called "multiconic".

Using these two types of panel development, without having to draw the gigantic cones is possible, using several drafting techniques--which I have yet to master.

Right now, I am playing with a faceted imitation of a scaled-down Spray hull by using less than 20 facets (absolutely flat panels), not including the full-length keel.

What I am ending up with, will never be mistaken as a small Spray hull. But it will be clear that it is an attempt to imitate one.

 

It turns out that the actual shape of the wetted section has little effect on the far field wave drag (it's almost all about getting the submerged area correct). The near field effects relate to surface drag and aligning the flow so it doesn't have to bend sharply around a chine. At the bow, most chined boats have the chines that converge towards the cutwater, and you try to put some curve into the panels there (viewed in section) to weaken the chines. This can be done with a developed surface by having the ruling lines with a lot of slant when viewed in profile.

The chined hull will have a bit more surface area than a round one with the same displacement and righting moment curve. But it isn't much of a difference, often about 2%. The chined hull may need to have a heavier shell or more framing to handle the flat surfaces. This is potentially a bigger problem, and it depends on the material and the size and type of boat. Smallish steel boats often have a practical minimum thickness for welding and corrosion allowance that is more than adequate for strength, and the issue becomes designing the panels for minimum waste. Plywood has to be thin enough to bend to the needed curvature and thick enough not to flat-spot during the build.

In my experience, the better the round bilged hull, the easier it is to convert to a chined surface. And the aesthetics are normally fine. But that doesn't guarantee an efficient-to-build surface. So you end up pushing the chines around to make better use of materials, and this can completely ruin any hope of preserving the aesthetics.

So if I'm converting an old wooden design to plywood, I'll start with the midsection and one in front and behind it drawn from the offsets. I'll turn them into polygons and match displacement and the righting moment curve, with an eye towards the panel widths that I intend to use. The only real take-away from this is the location of the first chine below the waterline at midship and how it is flowing fore and aft.

Starting all over, I draw this first wet chine full length and and then construct the hull towards the keel from this chine. The centerline is actually the last thing that gets settled because that's where you can tweak the displacement in each section. This is what drives most people nuts. You don't know what the profile will look like until the very end. You can't draw your favorite bow profile and work towards it. The bow profile ends up being whatever happens when you trim the panels to the centerline. (Wooden ships had much the same problem. They just had a great, fat stem timber that they could adz to the final attractive shape a few hours before launch. So it was also their last last line to be finalized)

Working the topsides up from the chine gives you a bit more freedom since the exact offsets near the bow and stern don't affect stability very much. The trouble here is that the focus points of the ruling lines tend to be a long way from the boat. I used to draw on old salvage doors set on sawhorses. I could draw a 20" hull on one end of a 7' door. Doors were cheaper than drafting paper and I could work outside. When I wanted a drawing on paper, I screwed the drafting board to the door and I had all my reference lines and thumbtacks still on the door. Then I'd build a model from the drawing.

So there isn't too much that's different from designing any boat. Midship sections drive the entire boat, regardless of construction. Centerlines are the result of tweaking the displacement to hit the ideal displacement curve. Fairness in the flow direction is very important. If aesthetics are important, there's going to be more waste.

Chines on sailboats want to be fair over a range of heel angles. If you have a sharp chine, it should be near midship where the flow and the chine are fore-aft, and designs that abide by the Metacentric Shelf method of balance will do better as far as keeping the chines fair to the flow near the ends as the boat heels. And you may have to align a topside chine to the heeled flow as well.

 

The choice between round and hard chine is often decided by the materials and simplicity of build. Designs intended for amateur home builders are often hard chines because the labor is simpler and faster. There are preference trade-offs that are considered, as well. Do you want a planing hull, is initial stability more important, or would you like a boat that doesn't roll or is self-righting? Is it a displacement hull that needs to easily make hull speed? These things help decide what choices are made.

If designing from scratch, design directly in the style you want in the end.

Many designers do both versions to allow for home builders to choose which materials and building techniques work best for them.

My impression about that is that hard chine boats are often designed for the practicality of simple build with sheet materials. Because of this, aesthetics takes a backseat or the designer has a different sense of design than one who designs on curve.

-Will (Dragonfly)

 

Some good answers there, and hopefully the OP wanders back in to acknowledge, at some stage.

 

What about the trend of putting hard chines on fiberglass sailboats lately? Is that for performance or looks?

 

My guess is to benefit from the new planing designs. In the late seventies, the world speed record for a sailboat was held by a sailboard. I thought, back then, sailboats would start to look more and more like surfboards. It took longer than I thought it would, to get there, but here we are.


-Will (Dragonfly)

 

Thanks to all who responded and my apologies for this belated response. Will Gilmore responded that many designers do both versions of the same boat. Surely, they don't start the second project from scratch, but alter the existing design and I have a strong feeling that they start with round hull and then convert it to a hard chine and not the other way around. It just makes more sense this way. The process of this conversion is what's so fascinating to me. While all responses were very informative, nobody mentioned any existing software tools that would be helpful in the process. Or maybe there simply isn't one out there. Is conversion strictly the art and the skill of the individual designer? Also, ugly vs pretty hard chine lines - this can't be controlled simply by "practicality" of the subsequent build process for, surely, "ugly" and "pretty" hard chine hulls don't differ much in terms of difficulty of construction.

 

From a round bilge hull shape, it is quite easy for a software to introduce a vertical hard chine variant :
** in the longitudinal view, just generate a hard chine line, in the top view the hard chine line and the sheer line are identical
** reshaping the sections under the hard chine line in order to have the same displacement, or more exactly the same equilibrium mass / buoyancy at same mass unit per m2 for the hull and the deck surfaces, which gives a slightly different displacement.

I give an example here attached, done with "Gene-Hull Sailboat 3.1" : here just two parameters are necessary to input, one to trigger the hard chine line introduction, one to reshape the sections. And I add a comparison at 20° of heel : the variant with hard chine leads to +13,9% of righting moment and - 3,2 % of wetted surface, so potentially a better performance.

Here attached also the Gene-Hull Sailboat 3.1 file with the example in place and the two parameters to change are in red, in cells B39 (to change 0 to 1 to trigger the hard chine) and B64 (to change 2 to 1 to reshape the sections) , you can do it yourself : change in B39, look at the drawings, then change in B64, look at the final drawings (it is an .ods file, you need Open Office or Libre office).

 

P.S. BTW, I thought I would bring up examples to illustrate what I consider "pretty" hard chines. Caroff's "Chatam" series are exceptionally good looking as are Ovni type boats. Some of Van de Stadt hard chine designs are also very appealing. I'm not going to mention "ugly" designs as not to offend anybody.

 

Clark Mills design, inspired by the sharpie schooner.

-Will (Dragonfly)