bow flare

It depends on how the developable surfaces are designed, and how the unrolled/flattened surface is obtained. Working in 2-D, whether on paper, a loft floor, or a computer can be limiting and/or very tedious and time consuming compared to working with good 3-D software provided the user has a solid understanding of developable surfaces.

 

For the fun of it I did a quick scan thru my liberary of designs for the Home Builder, "The study catalogue of Glen-L marine caught my eye in that some of their larger power craft seem to have flare in their plywood builds. One feature i did notice, those with what looks like flare also have some clipper curve built into the bow. Possibly this is how they encourage flare development into the ply flat panel in that area.--

 

Doesn't work that way.....try bending ply or aluminum like that....you'll find that either those hulls are cold-molded or they introduce an additional (knuckle) chine in the topsides to create more shape. The ply edges at the stem are still a series of straight lines, but microballoons can do wonders........

Mertans does the same thing, longitudinal slits (chines) in the ply to introduce shape.......pretty much defeats the purpose because you have a fairing nightmare to get it to look reasonable. May as well cold-mold.......

 

Tad, are you saying that the stem of a plywood boat needs to be a straight edge and not curved for the plywood to be a single, developable surface? Or am I misunderstanding you?

 

I'm saying that the stem (of a single panel in sheet material) can be straight or a series of straight lines, or it can be convex.....but not (IME) concave.

See the paper models below, whenever you bend sheet material in one direction it will (depending on the specific stiffness) want to "cup" across the other dimension of the sheet. That's how all the "origami" shapes below are developed. The problem is that it takes very sophisticated software to predict this "cup". If the first bend is moderate and even (developable) cup can be minimized (again depending on stiffness of the material) and the best you can do is a straight section or stem. Concave (flairing sections or stem) doesn't happen.

 

On small, fast craft, hollow flare is of little use, IMO, unless proceeding slowly, when water is turned down reasonably effectively rather than splashing on deck. Up and running it creates unwanted aerodynamic bow lift climbing waves upwind, and can make broaching more likely when the fuller waterlines above the chines get immersed and apply the brakes to the front of the boat.

 

I determined this evening that a concave stem with a developable suface is possible if and only if the waterlines immediately aft of the hollow portion of the stem are concave.

An example of a boat side with a concave stem which is a single developable surface with continuous curvature is attached. Curves in the illustration are sections, waterlines and buttocks. Also attached is the unrolled surface. (Note: this is an example only and not any particular design.) It was created using Rhino. I created upper and lower defining curves which extended beyond the stem and beyond the transom, and lofted a developable surface between the defining curves. I made sure the defining curves were "parallel" at the inflection points. This is key to obtaining the single surface with continuous curvature. The shape at the stem depends on the shape of the defining curves. The resulting surface was trimmed to the centerplane for the stem, at the desired transom location, and at the desired sheer and chine profiles.

 

I'd like to see a 3D rendering where a concave stem arises from a developed single surface. Never seen it done.

 

See my previous note. That is what it is.

 

Eureka ! A hollow chine !

 

On the Rhino attachment ? I can't see a drawing showing the lines ? Am I too thick to see it ?

 

This is a simple rendering with the lines on the surface. The lines are not on the .3dm attachment.

 

Being possibly the least technical here, in post #17 I stated those Glen-L Designs that seem to have flare also seem to have some clipper curvature built into their bow. In proper terms I meant the actual Stem Frame itself was not straight but had inward curvature (clipper style). Dave am I assuming correctly this is the " Concave Stem" characteristic thats showing up in your design investigation. ?

 

Yes, but that requires the waterlines to be hollow immediately behind the stem if the surface is developable. Note that plywood can be forced into slightly non-developable surfaces. I don't know if the Glen-L designs have true developable surfaces.

 

Bow stem profile