New diagonal hull using Flatfoot method

Just for fun, I made a 2/3 scale of Ian’s Flatfoot design. This is a size that fits on a standard 8.5 x 11" piece of paper. Then I modified the pattern to make an approximate copy of Bill’s diagonal Cobra 3 design, but with a round bilge. It folds up into a reasonable hull shape. But there is a problem: most materials will bend in only one direction. This technique will result in unwanted flats and protruding corners in places where the round cross-section meets the rounding of the rocker. My paper models exhibit this problem. It may be necessary to revert to hard chines.

My first full-scale building attempt will use a polystyrene sheet; two different thicknesses (.010" and .015") are on order. It may be possible to soften the corners by heating this material over a contoured mold (a 100 Watt light bulb has the right shape, and also can provide the heat). This material is more friendly to heating than the bottles that I have been using, and also is available in large enough sheets that I don’t need to patch multiple pieces together.

This could also be done with 1/64 plywood, in which case the hard spots could be back-filled with wood filler, then ground down.

Hi Walt -

we successfully “rolled” our own “A” Class and 18 Sq. Meter catamaran hulls using 1/8 plywood. I don’t have the line/cut drawings any longer, but the typical hulls were as you woud suspect - very narrow an plumb bows with “V” bottom, running back to semi-circular stern with circular underwater sections. This took place in an 18 foot (5.5meter) overall length.

Take a trip to local library and look through kayak building books, and you will see how the hull shape is cut onto flat plywood which can then take on compound bends. Look specifically for non-strip and non-hard chine plans. A kayak called SEVN might be an example. is all in Dutch, but it shows what can be done in turnigg a flat sheet of ply into a round bilge (RG65) hull.

Go to and paste the url that Martin gave, in the box, and choose Dutch to english (or any other language) and hit translate! The webpage, complete with images and links, is translated. Or pretty close to translated.

I think you might find that the ply is unable to bend around compound curves as much on a Footy as it does on kayaks. The bends tend to be proportionally sharper because of the small size of the boat.

I think Walt’s use of a more flexible material makes sense, but every material has its limits. I used cardboard to do it on one Footy, but then you have issues getting it well sealed…cardboard and water don’t get along…don’t ask me how I know :wink:

I’d suggest trying a foam material like EPP…it’s more flexible than Depron, and I think it could be cold moulded to some degree. It might handle the compound curves better as a result.

Keep us posted, Walt…you’re always taking a different, and interesting, path.


I like quick, easy but good too, so enjoyed the RG65 in ply, however, if you look at the beautiful glass one displayed above, that could be done in a flat medium too, with a fully round hull?
I am not sure why the sharp transition from round to flat at the rear?
A suitable material for this would be high impact .75mm styrene sheet (rather than polystyrene?). has to be the definitive depron footie thread.

Thank you all for the many useful comments, especially the references to other builders and their techniques.

My objective is a simple labor-free method. The bottle boats started out that way, although it has taken 2 years of continual improvements to finally become truly water-tight and competitive against our local fleet.

The depron hulls built by Keven illustrate the complexity of building a fair hull with compound curves, however he has discovered some interesting ways of distorting the depron.

As Bill has mentioned, the curves required to imitate a Cobra 3 are much greater than some of the other hulls mentioned. The J65 has very little rocker; Keven’s boat has a dragging stern, and a bow piece; and kayaks also have very little rocker.

I have on hand some strips of polystyrene, which I have been using for battens. A quick test has shown that it softens nicely with heat.

First Attempt

The first full-size diagonal hull has been built, using a single sheet of 0.010" polystyrene, about 7.1" (180 mm) x 14.75" (375 mm). The tapering slots are taped together, not glued. The shape is reasonably fair. It is smooth with a round bottom in front, and a more squared bottom toward the stern.

Fortunately, I have also built a rule box. Unfortunately, the hull doesn’t fit. There was a miscalculation of the pattern at the stern (too fat). So I cut off the stern to fit the box. But now it is only 12.5 inches long.

Obviously, there will be a second attempt, with a re-calculated pattern. The plastic is cheap.

Good sailing to my friends in New Hampshire this weekend, and also to Andy T in his Pirate regatta.

Three Footies of this type now exist.

Footy #2 attempted to correct the pattern problem in Footy #1, using the same material and technique. This came much closer to the desired shape, but still not quite fitting in the box. The stern was shortened to fit, and the prow was re-cut to get a plumb stem. This resulted in 12.8" (325 mm) overall length.

The tapered cuts in the hull were taped up thoroughly to make it reasonably water-tight, and the hull was tested in the bathtub to determine the water line and center of buoyancy. SURPRISE - it sunk!! The thin plastic collapsed inward at the gunwales, folded up, and sunk. Reason - there is no deck to prevent this. Therefore some cross-bracing was added to simulate a deck. This appears to have solved the problem, without reverting to a heavier material.

The pattern was redesigned again, and built as Footy #3, using the same material, but black instead of white. It now fits nicely in the box, with a length of 13" (330 mm). The weight of these hulls (without deck) is about 0.6 oz (17 gm). Pictures of the 3 hulls are attached.

Before investing a lot of effort into completing this hull, I decided to do some drag testing, similar to the stuff that was described on an older thread. Brett had kindly sent me the design for his Bob-About 2, which I found to have the lowest drag of the hulls tested at that time. So today I did a comparison test with my beautiful new black hull.

Alas, my ego is wounded. Brett’s hull has at least 3% less drag at low speeds, and the margin gets larger at higher speeds. His hull is also fatter in the ends, and has slightly better pitch stability.

Prior to this testing, I had cut about 1/4" off the stern of Brett’s hull, to make it fit better in the box, with less of a vertical slant.

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