No matter which era one was born in, there always seems to be a constant chatter, a muttering, of specialised magical hulls. One would read about such hulls almost defying belief, perhaps even the laws of hydrodynamics, if salesmen are to be believed. There have been claims of hulls that are optimised for their extremely streamlined ability to “cut through waves” that was simply not possible before, and yet, such hulls and their prowess make headlines.
The "before" bit is always intriguing. Before what? Before CFD, before tank testing, before Froude, before water? This is the subversion that appears to occur too much these days, a subtle take on the classic bait-and-switch.
By this, I mean that one is drawn into the narrative that existing hulls are old, slow and inefficient, and no one has come up with a new totally modern superior hull that is efficient beyond any metric known to man, until now! Then, as if by magic, the new world class-leading hull is presented with all its new features and benefits. To the casual observer, such hulls do indeed appear to be impressive and eye-catching. It is the bright shiny thing, the squeaky wheel that gets the oil.
But to any naval architect, and to one with a degree of knowledge and experience in hull form design and hydrodynamics, such claims do not bear the weight of such scrutiny.
What is a hull? In its simplest form, it is merely a three-dimensional shape that moves through the water and that has sufficient volume to support the total weight of the vessel.
With a shape that is a square box, it may perform the supporting the weight function, but the moving through water part? Well, not without a high degree of difficulty, owing to its shape, and being very draggy.
So that’s at one end of the spectrum. At the other end, it is a shape that is like a knife, sharp, very pointed, and performs one function very well, which is that of moving through the water with minimum resistance, with less drag than that of the box but has insufficient volume to be of any use.
Thus, somewhere in between these extremes exists a three-dimensional shape (or shapes) that satisfies the objective.
What is the objective? Isn’t it to move through the water with minimal effort whilst being able to carry the payload that is required for its duty? So, this begins to address the claims by the proponents of such bright-and-shiny things.
To answer such a leading question, one can only do so by knowing the objective, which is outlined in the statement of requirements (SOR). The sole objective is to be “efficient” as noted by the claim of being a magic hull form, but more efficient than what? And a changing payload would mean it cannot be optimum at two different displacements.
We may begin by exploiting the simple rules of hydrodynamics, that being lighter hulls have less resistance than heavier ones on the same length. This is the length-displacement (LD) ratio, which is well-known to any naval architect or hydrodynamicist.
There is a very valid argument for this. Typical high speed fast ferries have a length-displacement ratio of around 7.0 to 9.0, which is a measure of how light and how “slippery” the hull is through the water. All high-speed ferries use displacement hull forms that operate well beyond the prismatic hump on the resistance curve using such hull forms. They can achieve these higher speeds with less resistance owing to the higher LD ratio.
However, when comparing the LD ratio of, say, a typical offshore crewboat, the range is from 4.0 to 5.0. As an example, we can use values of resistance for a typical crewboat hull (i.e., any typical cCatamaran) that has an LD ratio of around 4.5. This difference in LD ratio is the key to understanding all this “magic”, since the amount of power required for the same speed, compared to a hull, of the same length with an LD ratio of 7.0 to 9.0, is significantly less than that of a hull with an LD ratio of around 4.5.
Thus, if one begins to look at the claims of super-efficient hulls/designs, this is, ostensibly, all one is expressing; the hull is lighter for its length than another design of the same length.
However, if the crewboat is kept the same (i.e., no changes to the design nor its layout) but we simply add, say, five metres to either end of the vessel (increasing by a total length by 10 metres) and making a minor weight allowance for the additional structure weight, the LD ratio rises to around 7.0. When plotting the resistance for the original length versus the extended length hull, we find that at 20 knots, 25 knots and up to 30 knots, the difference in required power is, as noted before, significant.
Taking a nominal speed of 25 knots, there is a reduction of 50 per cent power for the same speed for the same design (original length), yet just on longer hull length. Which operator would not like 50 per cent savings in running costs? Is this the holy grail that is claimed?
Some would point out that we have an “optimised” shape that produces less resistance. If we look at a change in shape of a hull with an LD ratio in the 7.0-plus range, the difference is minimal, merely decimal places, as an absolute, but in reality, not noticeable, and certainly not allowing a change in engine size or a saving in fuel consumption that can be bragged about. This has been proven many times by researchers, notably by the series of experiments conducted in the 1990s at Southampton University by Professor Molland, et al, with a systematic series investigation into such effects.
Hull shape plays almost no part in the change resistance. The key parameter that does influence resistance is length, or, more precisely, the LD ratio, the length per unit weight.
So, if these "magic hulls" are noting a more efficient hull or design compared to a similar vessel, it is highly likely that the only "magic" is the reduction in weight for the same given length, since the science (or rather, the hydrodynamics) clearly indicates this and has been known and documented for decades.
There are some minor gains/changes in resistance when changing the shape of a hull in the LD ratio in the 4.0 to 5.0 range. However, like everything in design, it depends, the qualifier being how the vessel sits in the water. But we should also note that low LD ratio hull forms have much higher prismatic hump resistance, too. Putting a massive outboard on a small RIB doesn’t mean it will be going faster if it can’t get over this increase in resistance at the hump, owing to its weight. The systematic data set of planning hulls by Blount et al in 1978 shows this very clearly.
Other similar claims of greater efficiency come from optimising the LCG that improves the resistance. This implies that the design has been able to create a layout that “moves” the LCG to a location that brings massive benefits, which other designs have so far been incapable of accomplishing.
So, in the days that tank testing was the norm, before most offices started to rely on fancy colour plots of CFD output, one of the principal tests done on a hull form was an “LCG chase”. This is classic “old school” naval architecture, long before the days of social media and influencers vying for attention.
In an LCG chase, the hull would be towed for resistance testing in its current level trim condition. Afterwards, a series of runs are conducted with the LCG being moved foreward and aft by increments of 0.5 per cent up to around ± three to five per cent (depending upon the hull form). Obviously, this means the static trim is by the head or by the bow with an LCG that has moved and thus also the running trim will change, and presto, so does the drag.
The result is a shallow-ish lazy curve showing a variation of LCG from foreward to aft versus resistance. Anyone who has done simple algebra at school will know there is a minima on any curve that increases going foreward and also increases going aft. This “sweet spot” is the ideal LCG for minimal resistance. However, it often falls outside the preferred LCG of the design.
This then leaves the naval architect with a choice, to either keep the LCG as it is, or move it to the location that indicates a reduction in resistance. Changes in hull shape with low LD ratios then come into play here, too. Since the change in shape makes the shallow-ish curve a more pronounced "U" shape, this exacerbates any LCG shifts on top of an already increased prismatic hump resistance, simply because the LD ratio is low.
Hence, in the absence of conducting an LCG chase, simply moving the LCG that suddenly yielded a lowering of the resistance would appear to be magic indeed; it is just one spot on the curve.
Thus, there is almost nothing new in hydrodynamics that has not been discovered or known about before. What is new is the subject matter itself to the naval architect or designer that has yet to wade through the multitude of technical papers highlighting such effects.
In other words, the “discovery” is theirs to the subject alone.
