Please visit our home site at www.TRILOBOATS.com.

Anke and I are building our next boat, and writing about it at ABargeInTheMaking.blogspot.com. Access to the net comes and goes, so I'll be writing in fits and spurts.

Please feel free to browse the archives, leave comments where you will and write, and I'll respond as I can.

Fair winds!

Dave and Anke
triloboats swirl gmail daughter com

Monday, November 4, 2013

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!

22 comments:

  1. Interesting, and re-assuring at the same time. While building the flat bottom on my boat, I've wondered (a bit) about possible damage to the large flat surface. Mind you, although always possible, it won't be too likely, simply because it's a river boat and most rivers around these parts are pretty tame!

    ReplyDelete
    Replies
    1. Hi Joel,

      I've heard of such places... the fabled mud banks, bottom and berth! *Sigh* Sounds pretty nice!

      Dave Z

      Delete
  2. Hi Dave,
    With my little experience with scow hull shape with a small rocker, i think the main effect is that for a given displacement, a rocker give you a longer waterline, as so in theory a higher speed limit.

    ReplyDelete
    Replies
    1. Hi Jerome,

      Hmm... hullspeed is calculated as 1.33 times the square root of the length AT the WaterLine (irrespective of underwater profile). This has to do with wave dynamics, however, something certainly affected by hull shape below the WL. A hull that generates more wave ('fuss', as some call it) will have a harder time reaching hullspeed (this is often reflected in a lower value for that 1.33 multiplier).

      Many rule-beating racers increased their WL when heeled, picking up speed. This, I'd think, would tend to confirm WL primacy, though it should be remembered that underwater shape and associated hydrodynamics (even on the same boat) changes as the boat heels.

      In the case of rocker, according to what I've recently read, it won't raise the WL (and therefore not the hullspeed), but does produce drag, which can make hullspeed harder to achieve, or even lower the multiplier (reducing hullspeed).

      This is so pronounced that surfers (apparently) use rocker to ADD drag, slowing the board for better control in certain conditions. I found similar results in rowing shells, kayaks, canoes. Alas, direct barge/scow info was scant.

      Dave Z

      Dave Z

      Delete
    2. Hi Jerome (and others),

      In case you miss it, I stand corrected re Surfboard theory as applied to displacement hulls (eg Barges). See discussion with Glenn, below, where I eat my hat. 8)

      DZ

      Delete
  3. Rocker, more or less mimics the natural shape of a wave, and lets the boat be a little faster. Commercial scows tend not to use it primarily to reduce draft and spread the load when grounding out to handle cargo (Thames Barges especially). Thames barges do have short, but nicely shaped bows and sterns to get better speed without resorting to rocker.

    For my money, I'd say, as with any boat design, you really need to look at how the boat is to be used. Experience in a similar boat helps a lot. I am still refining my ideal Crabber for oar and sail. I've tried heavy gaff sloop, pram dingy, canoe and small Peapod. Next up, lug yawl flattie skiff. Size, weight and rig seem more important than precise shape so far.

    ReplyDelete
    Replies
    1. Hi Glen,

      I've read that assertion many times on forum discussions, but without a theory proposed as to why mimicry should add speed. Got any citatations? I'd be very interested to follow 'em up!

      Among the pro designers I've been able to find, while wave mimicry is occasionally mentioned, it's more in reference to motion and handling benefits (not speed).

      The surfing reference I quoted went so far as to state that rocker is added to INDUCE drag, slowing the board for better handling in certain conditions.

      In general, our deadflat stays well below the surface of waves, even in short, steep chop. I'm wondering if mimicry would even play a role, except right at the surface of the wave? Water is only negligibly compressible, so there aren't even appreciable pressure gradients within the wave body (assuming solid water, that is, rather than aerated).

      As to your second paragraph, I wholly agree. I'm a form-follows-function kind of a guy, myself. Shape is just one of many competing considerations. As a designer, one tries to maximize synergies and minimize opposing features. Makes it fun!

      One thing I should have mentioned (full disclosure) is that TriloBoats are heavily constrained by sheet materials, and the push to optimize their use (savings in money, time and materials). Thus, the bias is toward a deadflat, regardless of any (minor) virtues we may discover to rocker. It's entirely possible this has colored my research!

      Dave Z

      Delete
  4. Dave,

    Phil Bolger says it better and more succintly than I can. I won't argue the point. Read what he has to say about flow. More to the point, one of the primary constraints for large commercial scows was and is draft, so they tend to use more deadflat and less rocker. It also, as you say, gives greater capacity for any given set of dimensions. But to return to empirical design; historically, any time speed was more important than capacity the deadflat was reduced and the entrance became finer and the run longer in proportion to length (note, rocker was not necessarily made deeper, just longer, the smoother the line the better). How far this process went depended on the relative importance of speed v.s. capacity. And this is from people who had to balance tonnage/mile/speed and make a living. In other words, if you can make more faster trips with a smaller load, will it off set the reduced cargo capacity? The bottom line is how many tons can you deliver a given distance in a given time. This changes according to the value of the freight too. The most extreme clippers were the opium carriers...

    Surfboard data is not relevant to any craft that doesn't "break out" and plane. As soon as they start riding a wave, their shape is more relevant to power boat design. I won't go into detail. Read Dynamite Payson's "Build Your Own Boat." If you're planing, horizontal buttock lines and an immersed transom are good, rocker is bad. 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. I've been sailing and rowing displacement hulls for over 50 years. This is experience as well as theory. For hulls that exceed displacement speed under sail, but retain good sub-planing performance (measured as speed v.s. power) see Iain Oughtred's double ended Shetland Yawl series.

    I've read all your blogs, so yes, I knew your design parameters are constrained by attempting to use sheet materials efficiently. Bolger has some amusing remarks about how minor "improvements" can dramatically increase the number of sheets of plywood required. It's one reason I favour traditional wood construction; but I can't enclose the volume or tonnage that you can for the same material or labour cost.

    I'm not saying rocker is better than deadflat, just that it's faster. "Performance" is not speed alone. Capacity, comfort, economy, safety, stability; there are a large variety of factors. As I said, the most important thing is what the boat's actual use is, or will be.

    ReplyDelete
  5. Hi Glenn,

    Excellent p.oints all, and well put!

    It had totally escaped me that surfboards plane (duh). I've run into similar comments in canoe and rowing shell design, but hmm... you're persuasive!

    You write:

    "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."

    I'd have to agree, and might add that it's likely true forward, as well. But maybe less so?

    It's interesting to see that Great Lakes (box) scows have a single, fair curve aft of about midships. Forward, they have a relatively abrupt bow curve faired into a short deadflat or slightly rockered area forward of the beam. ALMA and others are similar, but with longer deadflats.

    What got me thinking about all this was seeing several tow-barges hauled out. None of those hauled out at that time had any rocker... they looked just like humongous TBoats. Since they're servicing a long run from Seattle/Tacoma to SE Alaska (900 miles, each way), I surmised that the flat was more feul efficient. Planned to call a tow-barge designer, at some point.

    But now you've got me re-thinking. They are, after all, also made from sheet materials, with labor being a big, initial cost. Fuel has been cheap for a long while. These factors could well influence design in favor of a deadflat.

    Thanks for sharing your experience and expertise! One of the main purposes of the blog is to get MYSELF an eddication. I truly appreciate correction when it's due!

    Dave Z

    ReplyDelete
  6. Dave,

    Sometimes I forget how obsessively I've researched boats over the years. Chapelle in American Small Sailing Craft gives a description of the the ideal chine profile for flat bottom skiffs. Like I said, speed isn't the only thing, and certainly not everything. In the case of freight barges, capacity certainly comes into it. And they are very large, read "long" even if broad as well. That means they are being towed at something less than their displacement "hull speed". And yes, efficient use of sheet materials, draft, and the ability to fill the hold with boxes come into it as well. A bulk barge that never goes in shoal water might be a different shape, but in an age of mass production the extra design expense and changing the yard's building set up mitigate against possibly reduced towboat fuel costs.

    Design is a complex subject, and the commercial types have to make it pay. I'm not familiar with the Lakes barges, though I am tolerably familiar with ALMA. There was a fair amount of variations in the Scow Schooners depending on which trade they were in. In general they tried to maximize capacity and kept the ends as short as possible and still sail decently. They were faster when light, not just due to a more favourable sail area to displacement ratio, but better lines at the ends, the water just didn't have to move as far to get out of the way.

    The short answer is that water doesn't like to go around sharp corners. You might note that small boats that plane under sail are all light enough that crew weight is the major ballast; windsurfers are the epitome of this. Multi-hulls are interesting; the extreme beam combined with light weight lets them carry a large enough sail area that they have the power to break hull speed, not by planing, but by cutting through their own displacement wave. Also, since each hull is very slender, the water, again, does not have to move very far to get out of the way. Bolger remarks in one of his essays that if the power to weight ratio is high enough, you can't stop a boat from planing regardless of shape.

    But, that's a far cry from a live aboard barge. Yours works for what you want it to do; and that's what's important.

    ReplyDelete
    Replies
    1. Hi Glenn,

      Research definitely pays... I often hear that books don't hold a candle to experience. It's true, but no one gets to experience it all, and the books let us stand on the shoulders of giants!

      I ran an image search on "barge design" then added "bottom profile" and "bottom rocker". I saw no commercial barges with mid-ships rocker [I know of a 'landing craft' with rocker... owner flinches at the term 'barge' 8) ]. Doesn't answer the why, of course, but interesting.

      Great point, BTW, that the big, long haulers tend to run BELOW hull-speed... that certainly ups the latitude for any hull induced fuel inefficiency.

      You also bring up the advantage of loading square containers... That's another perk I'd forgotten to mention, where cargo is loaded in the hull (as opposed to on deck). A cargo deck can be added and faired flat over a rockered bottom, but again, more time, material and expense.

      The question I've been pondering is fairly limited: given a flat-ISH, barge mid-section, would a skosh of rocker help or hinder? (Let's assume we ADD rocker, increasing displacement slightly but keeping the end-curves substantially the same).

      On THIS question (as opposed to the more general question of full-length rocker vs more abrupt curves at the ends), I'm still wondering... is there any reason why water might flow more efficiently around slight rocker than along a deadflat?

      I'm thinking that it creates a longer line for water to travel, pushes/pulls it further out of place as the hull travels by and increases wetted surface (though perhaps not relative to displacement gained).

      The only smattering of theory I've heard concerns 'laminar flow'... that a bit of curve helps water stay 'attached' and not break free into turbulent eddies.

      Any thoughts?

      Dave Z

      Delete
  7. In millpond conditions, laminar flow can exist and rocker can help if all you care about is "efficiency" in moving a sail craft. However, once ripples appear, flow is always turbulent.

    I then look at rocker and side curves as increasing the frontal area of what the barge must contend with for a given displacement. Dead flat bottom with constant beam sides will win that design decision in real world conditions.

    Turbulent flow is not always a bad thing. It can introduce energy back into the boundary layer, keeping the flow attached. Consider the Kamm tail for autos and dimples on golf balls.

    Ultimately, I look at Triloboats as cargo haulers for me and my stuff. Small ones are pickup trucks; large are semis. Not much rocker for those vehicles

    ReplyDelete
    Replies
    1. Hi,

      I know I'm over my head when I have to google two terms in a discussion (Kamm Effect and Boundary Layer), and fail to understand the Wikipedea articles!

      Um. If I understand correctly, you are saying that a bit of rocker in the deadflat would help, but only under ideal conditions... that in real world, benefits would be cancelled out?

      In a side note (related to your golf balls comment), one theory has it that Crab Claw and (fanned) Junk Rigs generate and benefit from turbulence. Maj. Bunny Smith went so far as to increase turbulence generating bits and bumps (leeside battens, lashings and more), and reported significant improvement in windward efficiency.

      Dave Z

      Delete
    2. The WW2 P-51 Mustang was supposed to have a laminar flow airfoil. It did, in theory. A .003-.005 inch bump would destroy that laminar flow. Dirt, paint and bugs in the real world made the flow turbulent in practice. It was "almost" laminar and more so than most others. Except for rigid, precise structures in a uniform flow (water, air, or Bolger's frozen peas) laminar flow just ain't gonna happen.

      Now consider the heeled hull. With dead flat bottom and sides, there are no offset curves to push the hull off course. With rocker, the helm can change from weather to lee and back again as the heel changes. I do not want or need my reflexes to be exercised so. I want a steady, sedate helm. I'm cruising, not racing.

      Look at the bed of a pickup. Worst streamlining ever. But, it can haul stuff more efficiently than a sedan. I can always put some rocker in my dinghy.

      One reason for rocker is to compensate for the weakness of the hull membrane. GRP, carvel, and lapstrake hulls NEED curvature to survive. Steel, aluminum, and especially plywood, not so much.

      Fair winds,
      Dick Pilz

      Delete
    3. Hi Dick,

      Ah turbulence... the ancient Greeks would have pitched a fit!

      Another aspect of the deadflat is that, with parallel sides, we have matching bottom and side curvature in profile and plan view, respectively (both equal zero). This should tend to minimize cross chine flow for the length of the deadflat. Introduction of rocker (without matching side curvature) would also introduce pressure differential between the two, and hence turbulent, cross chine flow. Especially when heeled.

      In theory, this should tend to reduce drag and improve tracking.

      Of course, if the NON-matched curves at the bow initiate turbulence, it will roll aft, mitigating improvements along a deadflat... a full AB (Advanced Barge) would match curvature at the ends, as well.

      RE Bottom Curvature as Stiffener - I think you're quite correct... SLACKTIDE's plywood deadflat has no supporting members for a span of 10ft x 7ft, acting rather like a trampoline. It flexes, but appears to perform comparably to LUNA's strongly rockered, relatively rigid bottom. That is, in terms of resistance to breakage, rather than to movement.

      Dave Z

      Delete
    4. Hi Dick,

      The Kamm Effect tickled a memory loose...

      George Beuhler reported (in Beuhler's Backyard Boatbuilding), something similar about rudder design...

      He reported that a rudder that wasn't tapered aft, overmuch, and was chopped off like a Kammtail, produced a V of 'hard water' (if I remember his words, right) that effectively added to the lateral plane/effectiveness of the rudder.

      Interesting...

      DZ

      Delete
    5. (Posted on behalf of JOHN):

      My intuition about the Kammback is that it works at higher speeds, but maybe not at the slower speeds that sailboats often move at. At those slower sailing speeds I speculate that an abrupt below-water transom would create drag, compared to a tapered hull.

      John

      Delete
    6. Hi John,

      I'd agree... no doubt about that! Just try rowing a boat that drags its transom, sometime, for a visceral difference.

      I left a comment about the rudder effect that Beuhler mentioned. I think it must drag, but it may pay with the extra effect he reports?

      Where it might well make a difference would be above the water.
      Displacement boats only go so fast, but the WIND can whoop along.

      Kammbacks might well be used to advantage in a streamlined
      superstructure, similar to Blondie Hassler's cabins on JESTER and SUMNER. Looks like a key to the concept is to start air flowing along the airfoil before chopping short.

      But this is mere speculation on my part... I'm way out on a limb in this discussion. 8)

      Dave Z

      Delete
  8. Every change of flow creates resistance. A large continuous curve will only create one long and as smooth as possible change in the flow of water around a hull. Any change in that curve will create additional "chock-waves" from forcing the flow of water to do a new faster or slower flow imposed on the old flow.

    Cheers,
    Johannes

    ReplyDelete
    Replies
    1. Hi Johannes,

      I agree.

      But, assuming that we're going to have two changes of curvature in a bargy bottom (at each end of the flattish mid-section), would a smidge of rocker help the hydrodynamics or hinder?

      It is certainly true that any midship rocker whatsoever will reduce the differential between endcurves and midsection.

      Dave Z

      PS. Drop me a line, if you like... I've got an AS idea for your proas.

      Delete
  9. Dave,

    I'll be out of the discussion for a while. I broke my right elbow on Tuesday, and had surgery to re-attach bone and tendons on Thursday. I have plenty of use of my fingers, but it is a tedious position to maintain and left handed typing alone is very slow.

    ReplyDelete
    Replies
    1. Glenn,

      So sorry to hear of your accident, both for your sake and ours! Here's wishing you a speedy, full and comfortable recovery.

      Dave

      Delete