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Dave and Anke
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Monday, June 10, 2013

An Arc of Arks: Comparing Hull Shapes

An Arc of Arks: Comparing Hull Shapes

Let's suppose, for a moment, that all single, displacement (non-planing) hull shapes are related, and that their relatedness can be ordered by Complexity. For our musings, we won't insist on rigor. Generalities and approximations will suffice. In other words; pull on yer hip waders!

Complexity forms the axis of the cone, above; it is least at the apex and greatest toward the base. Complexity is ordered, here, by geometry, but implies an order of construction effort. Additional hull shape increases lofting, number of parts, shaping and joining; the more complex the hull, the more effort must be expended to achieve it. 

If you prefer, you can see the cone as the space enclosing a Family Tree, tracing from a single ancestor.

Who is that ancestor? We need a reference... some point of origin, from which all others are descended. Simplicity, ground zero. I propose the simple Box; the humble float, dock or swim-platform. If we can build a watertight box, we've already arrived.

Along the left side of the illustration, I've listed features which I feel to represent a leap of of Complexity. Each is matched to the right by select exemplars. The diameter of the cone represents the increasing range of Options. As each new level of Complexity may be combined with all previous ones, our range of possibility widens.


TriloBoats, PDRacers and Thiel Barges are all what I think of as Box Barges; the basic vessel. The problem with our Box, as a vessel, is that it will plow water when it tries to move. So we we slice a wedge from the underside at each end (curved or straigh cuts). Ecco! We mutter with Gallileo, oppure se muove! But it moves!!

Box Barges are highly constrained... there's only so much you can do with Profile Curve, and the range of Options is small (represented by the diameter of the cone ate any, given level). One hull looks very similar to the next.

By the time Plan View Curve and Flare have been added, hulls are starting to differentiate themselves substantially, one from another. By the bottom of the cone, all practical shapes are included (at least given current, hydrodynamic theory), with all the variety of vessels afloat and on the board.

Most, if not all, successful hulls lie somewhere within this cone, regardless of whether they evolved or were designed.

I speculate that hulls beyond this line are beginning to show diminishing returns. Get much (or any) curvier, and construction complexity increases with little benefit. This may be why we don't (often) see hulls with (intentional) bumps, sumps and saddles. A fair hull is, in practice, constrained to a relatively small set of curves.

So, the question arises; does increased Complexity imply increased Performance? I would argue No.

I see Performance as contextual. A context that requires shoal draft will not be helped by the addition of a ballast keel, no matter what its virtues in other contexts. One that calls for amateur fincancing and construction may never materialize if compound, complex curves are involved. The Box itself may perform optimally, out-performing alternatives in the the context of a dock, or stationary houseboat.

Successful vessels appear at every level of Complexity. All are high-Performance, by my lights. Shape matching function and purpose.

Let's take a typical Performance criterion, hull speed, as an example. The upper limit for displacement hull speed is solely a function of waterline length. Among hulls of a given length, hull speed is constant regardless of shape or Complexity. 

Shape influences the amount of energy required to reach that upper limit, and speed made good to windward is an important twist. Still, no displacement hull exceeds its hull speed (for long), and hull speed is simply a function of length.

Given generally low displacement speeds, this is a matter of nuance.

An equivalent Hess CUTTER (inspired by British Pilot Cutters) sailing against a Parker SHARPIE will win the race under some conditions, but not others. The difference of their times will not be large, relative to their common hull speed. Other factors than hull shape tend be decisive. Wind and sea conditions, crew competence, rig and draft vs. lateral resistance all come to mind. But only that. The skipper of a planing hull would consider both to be hopelessly slow.

Is hull speed a sufficient standard for Performance? Only if all else is equal, and it very seldom is. Hull speed is but one factor weighed against  others in the context of use. The very fastest hulls generally make poor cruisers for couples. We constantly trade one virtue for another, according to our priorities.

And the moral is? Hmm. None, really.

I love all the examples I listed, and many I didn't, each for their own reasons. Anke and my context tends toward the Square Boat end. Their performance has met our every need. Oh, sure... if we had a magic wand, our dreamboat (ROMP-like) is high up the scale. If we had eternity stretching before us, we'd sail them all.

But, as a parting thought, if one were to impose a Bang-for-the Buck scale, things look a little different. Replacing Complexity with Bucks (representing time, effort, expense, etc.) doesn't change the conic distribution.  Bucks increase with Complexityl

Bang, however, is more or less a constant. Boats are fun, pure and simple. We don't get more fun with hull Complexity. Ask any Puddle Duck Racer. Who had more fun; the Pardeys on SERRAFYN or the Hills on BADGER? Or even ourselves on SLACKTIDE? Wrong question.

But clearly, the Bang/Buck ratio is relatively high at the Square Boat end and low among Curvy Dogs.

I'm just sayin'.

PS. The astute may have noticed the absence of multi-hulls. I'd contend that each of their constituent hulls would fall within the cone. Complexity-wise, they represent a leap for connective structures and maybe another for multiplicity of hull, in trade for other virtues. But they make my simple cone Complex. No disrespect intended or felt!


  1. Although I do appreciate nice rounded bottoms as much as the next man(!), having been involved in the building of a few, I can say that I am happy to go with the square boats now (you are partly to blame for this conversion, Dave). I am now building our (36ft) (square) river boat and it's way easier/quicker/cheaper (relatively) than anything else I have built before (even my Wharram cat was more complex)
    Mind you, it's nice to have them around to admire and reflect you don't have the problems associated with them.

    1. Hi Joel,

      Square Boats are indeed a fine platform from which to admire them Curvy Dogs! And River Life is ever so loverly!!

      Dave Z