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

Sunday, December 29, 2013

A Short Review of Traditional Sailing Box Barges (Scows)



Easy to improve on these lines... or is it?

A Short Review of Traditional Sailing Box Barges

Box Barges (Scows) aren't new kids on the block. After the log, dugout and raft... barge.

For most of that span, they've been rigged for sail. And not just until folks knew better. Right up into the last of the Age of Sail, outlasting the clippers.

Take a look at the picture of the barge-ketch, above. It's likely a Great Lakes boat from near the late 1800s or early 1900s. She likely hauled freight in competition with any number of Curvy Dogs serving the same ports. Not only did she hold her own, but a whole fleet of her sisters were thriving in the same waters.

But look at those lines! Her steep, knuckled entry and exit are close to poor as can be. And this in a day when quality wood and skilled labor were in relative abundance. 

An abrupt entry means plowing water. An abrupt exit means a turbulent (draggy) release.

The economic advantages she gains through simple construction and maximum capacity on given footprint must have been enormous to outweigh making the slightest concession to speed! Hullwise, anyway... that rig looks speedy to me!

Still, if you ask me, those knuckled ends strike me as hard or harder to build than curves. And they wouldn't give up that much displacement. And with that bowsprit, lengthwise port costs couldn't have been a huge factor.

So it's a bit of a mystery that they built them so abrupt. Did they need to make the most of the deadflat for, say, stowing lumber?

ALMA, getting slipperier.

Here's ALMA, a San Fransisco Bay Hay Scow. Similar coasters carried Russian ice cut from Swan Lake, New Archangel (now Sitka, Alaska) to SF... no small feat, today... that's still one rugged coast!

 Lines are getting easier (longer and less abrupt)... more bottom curve and angle at both ends. 

We note that the exit is easier than the entry. Curious. Is this an evolved, hydrodynamically efficient arrangement? Or is it some holdover from the old (discredited) cod's head / mackerel's tail rule-of-thumb?

Let's establish a convention for talking about the proportions of aft curve : deadflat : fwd curve... looking at ALMA, I'm going to guess about 1/4 : 5/8  : 1/8 of LOD (Length On Deck).


Civil War era blockade runner
(replica built by Crystal River Boat Builders)
 
Here's one built to run wartime blockades... in other words, didn't want to be loitering around. I'll assume she wasn't slow, nor does she look it.

But she was also running supplies to ravenous armies, and likely slipping up sloughs and creeks to get out of sight. Of course she's a scow!

Proportions look to be about 1/3 : 1/3 : 1/3 (see also other pics of SPIRIT on related sites).

For the first time, we see an easy entry. Did this contribute to speed?


ANNA, looking sleek!
ANNA is another who's sisters were born in the Great Lakes. No plodder, she, though... This type had a reputation for speed among fast boats.


ANNA looks to be about 1/2 : 1/4 : 1/4... hard to tell... her aft curve is so long and easy that it could be anywhere from 1/3 to 1/2?

Here again, the entry is more abrupt than the exit, though it didn't seem to tarnish her reputation. 


*****

So the question remains... is the blunt-ish bow fast enough to make no never-mind? Is the easy exit and release the real key to increased speed? Or is it an artifact of having aft cockpit and quarters aft, rather than heavy cargo? Or both?

Fortunately, we see 'good enough' anywhere in the range. 

My guess is that a moderate curve forward gives good performance, rises well to waves and pounds less in most conditions, while an easy exit lets the hull pick up and go. Other considerations can push or pull the shape this way or that without undue penalty.


*****


An interesting aspect of traditional boats is that they were seldom designed, per se, but rather evolved.  Any more efficient variant, relative to the job at hand, tended to get copied.

Today, we tend to think in terms of speed or windward ability as the prime criteria for comparisons. But historically, at least among working craft, economy was paramount.

And economy is a gestalt of factors.

Even our first example was economical. We may no longer remember the reasons, but those who built and worked those pug-nosed vessels likely knew to the penny where profit lay and where not.

And that gestalt doesn't begin and end with the vessel itself, nor even its interactions with wind and sea. It perfectly reflects the state of supply and demand of its time; personnel, materials, cargoes, markets, competitors... even laws and the ability to enforce them. Of course speed and windward ability factor in, but they don't always have the only, or even the last say.

Our own lives have their own economic considerations. We want the best return on our investment, but 'best' is a fuzzy, nebulous, personal affair. And we're often led to look where our best interests are not, by those who would sell us a load.

We and the vessel we choose - if the relationship is to be a happy one - must also find economic balance.






Sunday, December 22, 2013

Box Barge Displacement: Archimedes 101

Plimsoll Lines from the Wider World of Displacement
 Box Barge Displacement: Archimedes 101

Story goes that, as Archimedes eased himself into his bath, the water he displaced ran over the edges and onto the floor. 

Nothing but a mess, for most of us, but Archimedes made a sudden connection; he knew, of course, that the volume of water he displaced was equal to his own, immersed volume. But what was not so obvious; The weight of the water he displaced must equal the weight of his entire self... specs, toupe, false teeth and all!

He got so excited by his discovery that he ran down the streets of Syracuse, buck-nekkid, shouting Eureka!!! Must have been somethin'... we're still talking about it 2000+ years later!

The actual Archimedes principle goes on to state that the floaty force acting on a floating object is equal to the weight of the displaced fluid.

So why do we care?

Well, we live aboard floaty objects. We want to know how much they can carry; how deep their hulls reach into the water; how much freeboard will stick out; how much it will settle if we move our anvil collection aboard; how much sail it takes to drive them; how they will float on their sides or upside-down. These and many other questions are informed by calculating displacement.

It involves a wee bit of math (Eeeeek!), but don't be frightened!!  Curvy Dogs require elaborate calculus, but we - box bargers - need only simple 'rithmetic. [Niener!]


The gist is that we want to convert our underwater (immersed) shape into a simple slab, whose volume is easy to work out. Being square sectioned (slab-sided), and rectilinear in plan view, box barges are already half-way there! One cheap trick is all we need.

Here's a walk through: 


Cheap Trick is close enough for Jazz... 
What we're doing, here, is combining the two wedgy slices at the ends into a single slablet, and adding that to the middle slab over the deadflat. We don't really need to flip one, as seen in step three... it's enough to understand that this is, in effect what we're about.

NOTE: Due to plywood standard dimensions, TriloBoat math is a snap in the Imperial System (feet, inches, eighths and pounds). The following can be done in Metric, but sadly, generates funny numbers and mistakes. So we console ourselves with a pint and work in the yoke of vanished Empire.

A, B and C are all linear distances between points along the waterline. Beam, Draft and Total are dimensional distances (at right angles to one another). When we multiply these together, we end up with Volume in cubic feet (ft3).

Once we have Volume, we multiply it by the weight of water in pounds per cubic foot (lbs/ft3). 

Fresh water weighs about 62.4lbs/ft3. Sea(salt)water is heavier at about 64.3lbs/ft3. A designer would choose one or another based on where the boat is expected to be used. It's not a huge difference, but does add up. The upshot is that the boat will float a little higher in saltwater. [Figures like these can be found in the Pocket Ref or searched for on-line.]

Displacement = Volume x Weight.of.Water / ft3

That's the general picture. Let's try an example from my point of view as designer:

Let's say I draw out a T32x8 on graph paper, for use in salt water. I decide that the draft will be 1ft and draw that in. Next I count squares along the water line, and find that:

A = 5.5ft, B = 16ft and C = 5ft

Total = (A+B)/2 + C
         = (5.5ft+5ft)/2 + 16ft
         = (10.5ft)/2 +16ft
         = 5.25ft  + 16ft
         = 21.25ft

Tell ya the truth, if the ends are this similar (which they usually are in TriloBoats), I'll cheat again and just call the wedges equal, and therefore A and C are too. Since they're the same, we don't have to average them; C simply completes A.So the above simplifies even further:

A = 5ft and B = 16ft
Total = A + B
         = 5ft + 16ft
         = 21ft

The difference between them only rounds a skosh downwards; ultimately, a bit of extra displacement is a pleasant surprise.

So:

Volume = Beam x Draft x Total
              = 8ft x 1ft x 21ft
              = 168ft3

And (I reach for my calculator):

Displacement = Volume Weight.of.Water / 1ft3
                         = 172 ft3 x 64.3lbs / 1ft3
                         =  10802.4 lbs                         ... notice that the ft3s cancel out.


Eureka!

This boat displaces about 10800lbs. That is, by Archimedes Principle, how much she'll weigh, fully loaded, including the boat itself, gear, supplies, crew and the dog, when loaded to her Design Water Line (DWL) - the waterline as intended by the designer.

But lo! In the course of time, we tend to accumulate stuff, and each gimcrack and doohickey settles us a little deeper in the water.

For extra credit, let's calculate a displacement related number, Pounds Per Inch of immersion (PPI... somebody, somewhere dropped 'I' for Immersion!). This number is the amount of weight required to lower the vessel one inch deeper in the water. 

Here's the formula for our 32ft box barge:

PPI = DWL x Beam x (1in x 1ft/12in) x Weight.of.Water/ft3

That odd bit in parenthesis is a ratio, which merely converts inch units to foot units (one inch being 1/12th of a linear foot, or about 0.833ft). In our example:

PPI = 26ft x 8ft x 1in x (1ft/12in) x 64.3lbs/1ft3
       = 26ft x 0.833ft x 64.3lbs/1ft3
       = 1392lbs

That's a lot of seashells!





Note: Box barges with high ends load gracefully with weight secured low in the hull. But there are limits, and exceeding them can be deadly. Be aware of them! If in doubt, the Coast Guard is happy to conduct free stability tests for your design or boat.