Developable Surface, Round Chine Hull Design Method

(Also posted on Rhino forum Round Chine Developable Surface Hull https://discourse.mcneel.com/t/round-chine-developable-surface-hull/100081)

Attached is a Rhino file of a sailboat hull shape with developable sides and bottoms, and constant radius “chines”. This hull shape was created as a demonstration of the method used, and is not intended to be used to build a sailboat.


Method used for this shape using Rhino3D V6:

Design initial mid-ships section shape. It as straight segments for the upper and lower panels and an arc fillet connecting the straight segments. The “virtual chine” (intersection of extended straight segments) would be determined using this drawing. Additional sections shapes were also initially created for and aft.

Design longitudinal curves for sheer, virtual chine and keel. I used degree 3 curves. These curves were extended beyond the bow and stern.

Exact ruling lines for developable surfaces were created between adjacent cuves beyond the bow and stern using the method described in Developable surface - exact ruling lines from edge curves. Exact ruling lines were also created at intermediate locations. Longitudinal curves were copied and trimmed using the end ruling lines.

Developable surfaces for the side and bottom were created using -DevLoft (note hyphan version was used) based on the trimmed longitudinal curves. The intermediate ruling lines were used with the AddRuling option.

The side and bottom surfaces were simplified. Rebuild was used to simplify surface. Various numbers of control points in the u direction were tried to find the balance between number of control points and deviation from the original surfaces. Developability was checked using Curvature. The principal curves were visually checked to verify that one curve remained essentially straight as the cursor was moved over the surfaces. The numerical values of curvature was also checked at numerous locations on the surfaces to verify that one principal curvature remained essentially zero (10e-6 or smaller).

FilletSrf was used to create the round chine between the sides and bottom. The surfaces were not trimmed by the fillet.

The side, chine and bottom surfaces were trimmed at centerplane. The were also trimmed by at the stern. Then the side, chine and bottom surfaces were trimmed with each other.

Repeated the process again using modified longitudinal curves which improved the surface shape

Surfaces were mirrored.

Stem was rounded using VariableFilletSrf. Rounded transom surface was created using an arc which was extruded. Deck was created using the method described in Deck Method

 

From what I can see in the screenshot, the hull in the bilge has two curvatures, transversal and longitudinal, therefore it cannot be a developable surface. What do you think?.

 

Correct. The constant radius "chines" are not developable. Only the side and bottom surfaces are developable. This type of hull shape is sometimes used in metal boatbuilding because the sides and bottoms can be quickly plated using large sheets bent the framework. The round chines can be built using constant radius rolled sections. A similar approach is occasionally used with wood-epoxy construction.

Dudley Dix has designed boats using this method of construction. For example Dix 57 https://www.flickr.com/photos/23039635@N03/sets/72157631075265464/

 

I've designed a couple this way for plywood stitch and glue with the round chine closed by strip planking at the very end. The idea was that the developable panels were limited to 4 foot widths, and all the plumbing and wiring could be run from the outside before closing the chine. All hull penetrations went through the strip planking as well. For a sailboat, you have to look hard at the section righting moment characteristics and a constant radius in section view is not the best. But I do think the method has some attractive features for strength and speed of construction.

Full-length keel https://www.boatdesign.net/threads/full-length-keel.27476/page-2#post-275930

(Over the last 15 years, my sketching has gone downhill, my boat smarts have improved a lot, and my spelling remains as crappy as ever I'd tweak the lines a bit if I were to build this.)

 

The constant radius is confined to the bilge/chine area. Righting moment will be very close to a similar design with non-constant radius chine/bilge.

 

The Frégate designed by JJ Herbulot in the late 50's, built in plywood with a hull made of 2 developable surfaces joined by a radius chine, not constant thought.
Résultats Google Recherche d'images correspondant à https://assets.catawiki.nl/assets/2017/11/15/0/0/4/004fa62b-621f-4b8c-a2b2-d44d7425aee3.jpg https://images.app.goo.gl/ruZja5uRqVLEiz6r8

 

I can't see it clearly or perhaps it is that I don't understand the nomenclature but I would like to ask, in which area of the ship, I already know that in the bilge but, longitudinally, in which area is the radius supposed to be constant?

 

Thank you for this valuable contribution.

 

Each half of the hull has a developable upper side surface, a developable lower bottom surface, and a rounded chine/bilge surface between the side and bottom surfaces. The chine/bilge surface is a constant radius fillet along the entire length. Attached is a Rhino V5 file with constant radius curves along the chine/bilge surface.

 

I've designed a few metal sailboats using this method. The hull is essentially a developable hull- (bottom plates & side plates) with radius fillet bilge panel- some call it 'radius chine' or in the case of a large radius, 'radius bilge'. Fabrication is reasonably fast & easy & the appearance is quite acceptable so overall, it's a good option.
Generally, the radius at each section (frame) is the same but we often relax (adjust) the radius at the stern and bow sections to fair in nicely with the adjacent panels. This is to help make the tangent point a fair 'sweep' when viewed from the side. If this 'tangent' **is unfair (like a reverse "S", etc) it may show up by light reflections off the painted surface & detract from the overall appearance of the hull.
** one illustration: some months ago I submitted a 'shell expansion' drawing of a 44' radius bilge steel ketch

 

Example with varying radius chine/bilge. The radius in the middle is smaller than the above example. The radius at the ends is larger.