Looking at Barge Bottom Rocker

Is you a Gazelle?

Or is you a Elephant?

Looking at Barge Bottom-Rocker

One of the distinguishing features of a box barge or scow is a large-ish mid-section deadflat.

By deadflat, we mean flat both athwartships and longitudinally. Another way to say this is that the mid-section has zero rocker. Rocker is bottom curvature as seen in profile, like looking at the bottom of a rocking chair from the side.

When bottom rocker is discussed, it's often asserted that a rockered bottom is faster (more easily driven) than a flat one. In respect to barges, I once read (somewhere) that even a little rocker improves speed.

In Triloboats, I specify deadflats without rocker. 

The primary reason for this has been ease of construction. 

Over the deadflat, everything is four square. The bottom edges of furnishings or structural components don't need to be shaped to fit a curve. Standard house cabinetry or furnishings could be simply fixed in place. As a bonus, during construction, the deadflat is like a very large workbench for building such bits and pieces as bulkheads, transoms, decks, etc.. These considerations, however, fade in your wake, once the boat is built.

A secondary(?) reason is that cutting away from sides of given height and draft also cuts away from interior volume and displacement. In optimized sheet construction, cuts always reduce these values, relative to what's made possible by the materials. To regain volume and displacement, one would have to increase side height (presumably to the next efficient sheet fraction) and cut away from that, and/or increase draft to make up the amount of displacement lost to rocker.

Adding a little rocker - breathing 'life' into the deadflat - is entirely possible, but takes time and a little skill (not much). But the benefits, if any, would last the life of the boat.

But are there benefits?

Well, I tried to hunt down some evidence for the assertion that rockered bottoms are faster, especially with reference to barge hulls. What I found surprised me.  

Here's a sample, typical of what appear to be informed opinions, this one from the world of surf-boards (highly developed 'hulls' of similar footprint, lots of feedback, with many designers being also expert boarders... I omit the caveats about the complexity of the subject):

Some basic 'rules' concerning rocker are as follows: The more rocker or bottom curve that a board has, the looser (but slower) it will be. Water flow has to follow the excess curve, ends up pushing water, and drag is the result. Flatter rocker brings more speed but brings a decrease in maneuverability. Generally, boards with more rocker work better in larger, hollow waves where the added curve and drag can contribute to more control for the rider. Flatter bottoms are normally used on small-wave boards designed for slower, mushier surf, where the speed (and added leverage) help keep the board planing.

As with every other aspect of surfboard design, the best option lies in the happy medium of compromise. The better-designed surfboard steers clear of extremes and finds that an even, neutral rocker serves best: flat enough to be fast and efficient, but with enough curve to let the surfboard fit into the curved face of the wave and allow for the tight turns that are the mainstay of performance surfing.

 -- Dave Parmenter

Key points that emerge are that the question is complex; that more rocker equals more maneuverability; that less rocker is faster... less draggy! 

NOTE: For our purposes, fitting to curved wave faces and tight turns are negligable. I include them to contrast flat and fast vs. rockered and draggy/maneuverable. Among all the practicing sources I found, these associations appear standard (I specifically exclude forum discussions).

CAUTION: Astute reader Glenn (see discussion in comments) correctly points out that surfboards - being planing hulls - aren't relevant to displacement hull design (Barges, among others). 

He further writes: " In a displacement boat the smoother you make the transition between midships and the aft waterline, the better. The more abrupt the run, and the more immersed the transom, the more drag on a displacement hull. Period." Rocker, whether it be full length or partial (between more abrupt end curves), will ease this transition, and, if this be true (likely), reduce drag. 

In other words, some of my conclusions are at best suspect. Even more so than usual, that is! 8)

RE maneuverablilty -- This agrees with our experience on LUNA (fully rockered, Advanced Sharpie), who could turn on a dime. SLACKTIDE (deadflat, box barge) has a wider turning radius, but not bad. Turns on a nickle? Both have high aspect ratio lateral resistance devices (leeboardy off-centerboards) and big rudders, which I'm guessing play more of a role than mid-bottom profile in shoal boats.

RE speed -- In practice, I doubt one could tell a difference in real world situations. Both LUNA and SLACKTIDE get to hull speed, and it doesn't take a fair gale to do it. Meanwhile a lot of other factors weigh in... size of boat,  end curve placement and heights (for another post), ballast and loading, sail area and shape, sea state (bucket o' worms, right there!), angle of heel and course to waves and so on.

RE grounding -- Generally, a rockered bottom is stiffer, all else being equal, than a deadflat. Even if the deadflat has supporting structures (e.g., vertical faces of furnishings), these break the large deadflat into smaller ones, each of which will flex a little more than if they were rockered.

Grounding on rockered 'belly' concentrates the weight of the boat on a relatively narrow, thwartships band running across the point of deepest draft. In LUNA (aggressively rockered) this was an area between one and two feet by eight feet (depending on how yielding the ground). Grounding on SLACKTIDE's deadflat, the boat's weight is spread out, much like an elephant's foot.

Question is, is you a gazelle or is you a elephant?

Actually, I don't think this matters much, either. I've seen hulls of both types and similar scantlings (structural dimensions) do fine, grounding on reasonable bottom. 

Nevertheless, I personally incline to the deadflat here, too.

If one settles onto a sharp rock, PSI (Pounds per Square Inch) skyrockets at its tip. Focused PSI like this is what allows a thin, pointy nail to be driven into hardwood. Fortunately, considerable hull weight is supported by whatever other part of the bottom settles down, relieving some load on the rock. But still.

This is usually happens in a triangular pattern... the rock forms the apex of the triangle, while the baseline, opposite, relieves  load. The baseline could either be an opposite chine, or a thwartships area toward one end, depending on where the rock contacts the bottom (picture the rock like a jack lifting the hull from the ground... around what line will the hull rotate as the jack lifts?).

In a rockered bottom, worst case scenario is a very short baseline (the low point of one chine or the other). A deadflat puts down a long baseline (the chine along one side of the deadflat). It's 'worst' case (settling onto thwartships band) is the rockered bottom's best case.

However, in both LUNA and SLACKTIDE, what slight bottom damage we've suffered had nothing to do with shape, but rather, concentrated PSI's locally exceeding the crush strength of our (copper plated) plywood bottom. Makes a divot.

*****

So. My state-of-the-art preference (where barges are concerned) is for the deadflat... not just as a worthwhile compromise, but as all-round winner. See CAUTION, above.



PS... When rolling LUNA, one annoying habit emerged. It's uphill and down over rollers, and her belly tends to bottom out very shortly between rollers. SLACKTIDE's deadflat makes rollering a piece o' cake!