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Radiused Chine

This is a type of construction that has come from building with steel and has been around since the 1980s. It effectively simulates a round bilge boat through the use of a large, constant-radius bilge-plate that is typically rolled with a radius of about 25-30% of the full monohull beam. Short pieces of plate are all rolled to this same radius, (wide enough to typically cover an arc of about 60-70 degrees) and these are laid and welded over steel frames (also cut to the same constant radius), side by side over the entire boat length. This 'bilge plate' is then cut to a tapered shape—deepest from midship aft, but tapered towards the bow where it may only be used for about 30 degrees of its width, depending on the specific design.

Flat plates, either cylindrically or conically curved, are then welded to this bilge plate 'insert' to complete the bottom and topsides, and the result will frequently pass for a round bilge boat—though far easier to construct, having less lofting, less forming, less welding and therefore, less distortion.

Although used by some world famous designers in the 80s, the concept was truly put to good use through the ingenious and attractive monohull designs of Dudley Dix and here is an example of how it looked.

The reason I am presenting this here, is that the same design approach could be used by any panel material that can be bent and edge joined (steel, aluminum, etc.) but then, with a little ingenuity, one might even conceive interesting ways to use laminated plywood or even foam composites following the same simple design philosophy. Whether such a system could ever replace or be preferable to those already existing and proven, would depend on the ingenuity and subsequent experience of the experimenter. For now, I just throw this out as 'potentially interesting' and a system that's worthy of being remembered—at the very least for successfully plated boats.

Well, well! That was originally the end of the article, but subsequent research of Dudley's designs has dug up his personal 'Black Cat' design and even more recently, a 55-ft catamaran that clearly reflects just about what I was indicating 'might' be possible!

So here now, with Dudley's kind permission, are some photos and description of a PLYWOOD version of his radius-chine concept and the painted hull shows how great the result was. His monohull Didi 38 (called 'Black Cat') has since sailed about 20,000 miles (incl. 4 trans-Atlantic trips) and despite occasional speeds of 20k+, showed no signs of failure from this method of construction at the time of her sale.

And here are excerpts from Dudley's own description, as they apply to the radiused bilge for his 38' monohull Didi38

Stringers at the tangent join between the radiused bilge and the flat areas, are 12 mm shallower than the others but are set in slots of the same depth. The difference is then made up with 12 mm plywood doublers 100mm wide which are used to give a gluing surface for the intersection between the radiused and sheet sections of the skin. Before fitting the sheets, each must be prepared for the junction with the radiused skin. This involves routing a rebate 25mm wide and half the skin thickness in depth for the overlap of the second layer of the radius. The sheets are glued to all contact surfaces, ie to the sheer clamp, backbone, stringers, tangent stringers, bulkheads and joinery.

Skinning the radius
For the Didi38, the radius is skinned in two layers of plywood working from the stern forward as far as the mast and in three layers from there to the bow. The first layer is glued to the stringers and tangent stringers between the side and bottom plywood, with the follow-up layers overlapping the routed rebates of the side and bottom plywood to form a stepped joint. [Ed: layers are also offset longitudinally, as the small panels are simply butt joined, so one layer supports the other.]
I personally found that 250 to 300 mm wide plywood strips are the easiest to fit when working single-handed and gave faster coverage than wider ones. This is primarily due to the need to hold the panel accurately in place around the curve while also inserting the fasteners. The girth of the radius results in relatively efficient material usage as well because only about 200 mm length is lost to wastage from each strip.
After sanding and filling of fastener holes the hull is ready for epoxy coating and any fairing that you feel necessary. My hull was close enough that it just needed touching up in areas where I had been a bit careless with the belt sander. The final finish with minimal labour is good enough for her to be mistaken for a GRP boat by most who see her.

Economical Construction
I had felt for a long time that steel was the least costly material for boatbuilding, but this project proved otherwise, by a long way. There was also little expense on fairing the hull, as required for a quality metal hull, because plywood sheet is largely self-fairing. The labour input was also extraordinarily low so (for a monohull), this method of construction can hardly be beaten in terms of the most boat (or the best speed) for your buck.

See here for more details of this specific monohull:

As far as the more recent 55' catamaran is concerned, here are some photos of her under construction in Trinidad—and one can see that with the narrower hull, it's even easier to fair the separate bilge panels as the curve over the boat length is very slight. (This boat was a joint design effort with builder Philip Harvey—hence the DH550 model number.)

Mr Dix reports that he has sold over 360 sets of plans for his Radius Ply designs since his first Didi 38 was designed, and will no doubt be adding to the designs that use the concept as time goes along. His Didi Mini and Didi 26 have been particularly popular. (Ed note: and they look great too!)

Advantages: The ability to cover large areas of the bottom and sides in full length plywood sheets, helps to make a fair hull as well as permitting one to proceed quickly with the skin. Filling in the bilge area with a double layer of small panels, is also a job that can fit well for someone with only short periods of available time. With hulls of wood, these designs can be built in places close to a house, as there are only short periods when resin coating would require exterior ventilation. These designs are attractive and the end result can look quite professional.

Disadvantages: There is no doubt that this system involves some hull fairing (as with most), but how much time that will involve will depend on the care taken with earlier woodwork. The base framework is also fairly labor intensive and requires good basic woodworking skills, although the work is typically pleasant and satisfying. Good workmanship will certainly pay off as far as hull strength is concerned, and if not initially comfortable with sharp woodworking tools, one might first start on a framed dinghy or other small boat, to learn some tool control. This system does not permit reverse curves, flare or knuckles, without some extensive additional woodwork, perhaps combining plywood with strip planking. Although this might not directly concern the owner, most wood boats generally have lower resale value than glass ones—whether this is justified or not.

Dudley Dix was and is, still a surfer at heart. Although he studied naval architecture through Westlawn, he clearly has a very practical approach to boat design and all his creations thankfully reflect that. Mr Dix lived and worked most of his life in South Africa but is now residing in the USA and has lived in Virginia since 2004.

You can read more about Dudley Dix at

See also article on Compound Curves.


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