The Disappearing Fillet Radiused Chine

If you watched the last America's Cup races, you may have noticed that the hull shape has basically been turned upside down; flat on the bottom with a radiused chine and veed on the foredeck.  I'm sure that millions of dollars were spent reaching that design conclusion, so the least I can do is figure out a way for the backyard builder to achieve a close result (minus the carbon fiber). 

Radiused chines have previously been constructed with strip planking in between the plywood panels, but fitting the long tapered strips is very time consuming.  What I am proposing is a method that uses disposable fillets in a stitch and tape hull.

Stitch the panels together by any method that you choose.  In this example 
I'm using 6mm (1/4") plywood.

I'm planning an outside radius of 50mm (2") so the spatula tool must be shaped to the correct radius and width.  Install a fillet using the lightest and easiest to shape filler, like microspheres or Q cells.  Even polyester resin could be used because the fillet is only temporary.  Mix it as dry as possible for easy shaping later.

Lay in heavy double bias fiberglass tape.

Remove the stitching and grind or plane off the chine while checking the radius often with a plywood template.  Use a long sanding board to make it fair.  This is the hard part but it will look great.

Finish off with lighter fiberglass sheathing on the outside. 


  1. I think you are right in every respect in your observation of design trends. It makes sense if one is trying to promote planning at the lowest possible speed. Your thoughts of how to construct a radiused chine parallel my own. However I have thought of at least few other ways to construct a radius chine which I think will require less work and materials.

  2. The flat bottoms look like Randy Smyth's second-generation amas on his trimaran Sissor used to good effect in the Everglades Challenge.

  3. Jim Brown had radius chines in his Searunner Series - that's like 50 years ago (better check my math). His technique was rather different as he was not drawing stitch-n-glue plans. Instead of a chine stringer he had two stringers, one each side of the intended chine, which then became a channel into which the interior heavy f/g taping was laid up. The surplus exterior ply edges were then ground/sanded off. Maybe someone has a copy of his construction manual and could scan a picture and post it here.

  4. I built my Searunner back in the 70's and thought about those molded chines when coming up with this method. The two extra stringers would be too much extra weight for what I'm thinking of for a small canoe and also loses the clean interior surface. I have a photo from the Searunner manual here:

  5. A nice idea and good looking of all worlds. I'd recommend keeping a little of the fillet in place so that inner and outer skins remain separated by about the ply thickness.

    As drawn the radiused area looks uncomfortably like a hinge line.

    Cheers, Skip

  6. I've molded quite few canoes with just the 18 oz biax and it's pretty stiff especially when curved in two directions. When you add gunwales, foredeck and crossbeams, it's real solid.

  7. One might also form the glass on a mold of 4 or 6 inch pvc pipe then fasten/form to the hull sides and bottom in green cure stage, avoiding a lot of grinding. Then build thickness (if needed) with phenolic microballoon putty applied by notched trowel, then fair and glass outside. Or even use a tapered cone of sheet metal, plastic or plywood as a mold instead of a pipe.
    Or put sides and bottom together over radiused molds and strip in the chine radius, leaving fairly big gaps to be filled with similar putty - essentially an ugly hybrid strip build to save epoxy and ease shaping.

    But I would rip foam (EPS, XPS or PU surfboard foam) into fat chine logs (e.g. 3x5" or so), then shape the foam outside, glass, shape the inside, glass, done.
    Foam is so easy to shape, by wrapping sandpaper onto any helpful profile. And shaping only chine logs instead of an entire hull or board would make much less dust. You would want the support of glass to shape the inside radius, and could probably gauge the thickness by backlighting the foam.
    One might even perforate the foam at close intervals with a spiked roller to Z-pin the structure before appyling the inner laminate....

  8. Another way I could think of is laminating foam panels to the flat bottom of the hull, then shaping the chine and glassing over.

    This would also add buoyancy if the hull is flooded.

    I'm a complete newbie when it comes to boat building.

    What would be the tradeoffs of this approach?

  9. That is a perfectly good method and I know of at least one of my builders doing that to a flat bottomed hull.


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