Arc-deadrise

[Dennis Marshall]

I have been re-reading Parker's "Sharpie Book" and was especially captured by his discussion of Larry Huntington's adding arc-deadrise to his sharpies which, according to Parker and a Chapelle quote, allows increased beam, sail carrying power, diminishes drag, and increases speed in light air.

What Parker does not comment on, and what I want to know is, what is the proportion of "arc deadrise" that one adds to such craft to benefit from it? Anybody know?

Dennis

[Russell Sova]

Bend a batten of the width of half the bottom and you've got an idea of what your arc deadrise will look like. It's part of a circle. You'll have to determine the center of the circle to get the arc drawn on the frame, this center is probably located on the oposite side of the frame. I'm just completing a radius bilge boat that uses a similar aproach. Get a pencil on a string, attach the string to the arc center with a small nail and draw in the arc. Some arcs can be used more than once particularly in center run of planking but you'll need new arcs near the ends. You'll find it takes a day or two of eyestrain and etc. but it's worth it for sailing properties and resail value. Hope this helps.

[Sam F]

I'll be facing a similar problem if I ever get started on my next boat...
Could the arc radii be determined empirically by running battens longitudinally (maybe 4 or 5) from the center arced ribs to bow and stern?

[Dennis Marshall]

Russel, being geometrically challenged, why can it not be as simple as striking a fair curve from the midship of the frame to the chine in the same way that one adds camber to a deck or transom? Would one get the same results?

Dennis

[Aramas]

Hey, I know! Why not throw in a radius chine and it would be almost boat shaped! smile.gif

[Sam F]

Dennis,
If I understand what you're getting at and if I understand the geometry... Two not very likely circumstances.
If the boat has any amount of rocker and an arc bottom then it won't be "developable". One could strike a fair curve for the amidships radius, but it won't necessarily be the same as one proceeds toward the bow or stern. The only shape I can think of where this might work is to imagine an icecream cone cut in half (lengthwise) with the two wide ends glued together. You’d still have decreasing radii toward the ends, but that would be a developable shape for sheet materials. Not very pretty for a boat though! There are ways to approximate this by “deconstructing” the bottom shape into sections of cones but for a small boat it loses the advantages inherent in plywood.
This can be easily fudged with planking stock but not plywood or other rigid sheet materials as they only bend in one direction at a time.

[Russell Sova]

You probably could use a batten to make a fair curve like has been suggested. You need much more curve at the bow than amidships and possibly more at the stern depending on the design. Ted Brewer invented the radius bilge. They are almost indistinguishable from round bilges. Brewer states that continous radii are not as pleasing as gradually increasing/decreasing radii. You can use double the curve at the bow for a start and calculate the radius in between, so much percentage from the bow, etc.

[ahp]

I have a sort of an arc bottom pram built of plywood, BUT one cannot have a compound curve with plywood. All of the rocker curve is in the midsection where it is straight across from chine to chine, then the arc is in the fore and the aft thirds of the bottom. You can get the idea with a sheet of paper.

Star Boats have a true arc bottom thoughout their length, but they are planked.