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How design DNA can misdirect comparison of ship concepts with different hull forms (and what you can do instead)

River Mussels have a sneaky way to travel upstream. It might surprise you to think of mussels traveling around in the marine environment when we see them anchored to a rock for their life. But movement is an important part of their lifecycle, and how they colonize an area. Mussels travel by freely floating in the water column as larva. In the ocean, they will drift around, and eventually latch on to a rock and make a go of it, growing into an adult if there’s enough food around. But there’s a big problem with living in a river. Namely, the constant flow is far too powerful for a larva to overcome to get upstream. So how do River Mussels colonize upstream? This is where their sneaky trick comes in.

They have a way to hitch a ride on fish that also live in the river. Incredibly, river mussels manufacture a lure to attract local fish. The lure has colour, texture, and shape that all resemble a small fish. But it’s completely constructed of juvenile mussel larva. It’s strung from a transparent strand of mucus from the Mussel, giving the lure a fluttering motion in the water. This creates very lifelike behavior, even though it is a complete fabrication. Once a nearby fish gets close, and even tries to take a nibble, poof! The lure disintegrates into a cloud of mussel larva.

With the fish inside a cloud of thousands of larva, many have a chance to temporarily clamp on to the fish. The fish then eventually swims around the river, including upstream. While the clamping doesn’t seem to really bother the fish, it’s certainly disappointing that they missed a meal! After some time, the larva let go, giving them a chance to find a new home everywhere in the river. It’s incredible that an animal without any central nervous system can create such a convincing decoy. Ultimately, the entire process for a Mussel to travel upstream hinges on this fundamental misdirection.

Misdirection in the case of a River Mussel is a survival strategy. But often misdirection is something we want to avoid in our life and work. Getting sent in the wrong direction will waste time because of the resulting confusion. It’s even worse if you miss a good solution to a problem as a result. This applies very much in the world of ship motion prediction. When you are deep in the details of concept design, comparisons of seakeeping performance can give valuable insight to inform decisions. Usually, ship design DNA – length, GM, and displacement – can help us make sure we make fair comparisons in seakeeping performance between variations in ship design. But when comparing designs with different hull forms like monohulls and multihulls, using ship design DNA may lure you into making the wrong conclusions. One design might seem vastly superior – but it’s just a fabrication. In this article, we’re going to talk about why ship design DNA might mislead you in making fair comparisons between dramatically different hull forms – like monohulls, catamarans, trimarans, and SWATHS – and what you might consider instead.

Why can ship design DNA lead you astray?

The three factors of length, metacentric height (GM), and displacement are crucial to ensure you are considering fair comparisons in seakeeping performance evaluation between concept designs. But this applies best when the ship design variations are adjustments on the same hull form, like adjustments to a monohull in isolation or adjustments to a multihull in isolation. But the problem is that concept designs of completely different hull forms have wildly different characteristics. A realistic catamaran with similar displacement to a monohull might have dramatically different lengths. Catamarans also typically have substantially higher GMs than monohulls, and it doesn’t make sense to try to normalize equivalent designs by these parameters. Completely different hull forms have their own inherent advantages, and it makes sense to consider other factors that aren’t directly linked to seakeeping when comparing which concept might be best. This makes it easier to explore design concepts at least – don’t fret about finding equivalent lengths, GM, or displacement – and use seakeeping analysis for a direct comparison. But then this leaves us with which general parameters to consider for comparing concepts. In this article, we’re going to cover three alternative parameters that can guide in making absolute comparisons to concepts derived from different hull forms:

1. Deck area
2. Gross Tonnage
3. Roll natural period

First, we’re going to talk about deck area.

Deck area can be a major advantage of multihulls over monohulls

This is purely about geometry: there’s often more potential space available in a multihull like a catamaran when compared with a monohull of the same length. It’s not necessarily about storage space: sometimes handling equipment or doing work while offshore is just so much easier when there’s a lot of room to move around.

For very bulky but not heavy cargo, the deck area may be a driving factor, like in certain kinds of ferries or research vessels. Cases like this might mean multihull concepts have a natural advantage. That doesn’t mean deck area should always be used for making comparisons. One challenge is that the deck area doesn’t obviously link to seakeeping performance. But nevertheless, it’s an alternative way to consider an equivalent concept design between vastly different hull types.

Deck area is an intuitive and clear parameter to use. Yet deck area doesn’t necessarily translate to cargo capacity. On the contrary, many multihulls don’t have nearly as much displacement to for the same length of monohull to handle dense and heavy cargo. This is when another parameter to use for comparison of designs comes in that better links to cargo capacity: Gross Tonnage (GT)

Gross Tonnage gives an idea of how much you can carry around at sea

It’s a number steeped in history, used to track how much is being transported by a ship. There are often systematic rules for calculating GT for a specific hull form. It’s because the GT to carrying capacity that it becomes useful as a metric for tracking an equivalent kind of ship with a drastically different hull form.

However, the GT doesn’t have anything to do with the displacement of the vessel. It doesn’t directly represent the vessel’s mass or inertia. However, in general, the larger the vessel, the larger the GT will be. It’s one way to help make a systematic comparison between concept designs with dramatically different hull forms.

Now, both deck area and GT correspond to geometric factors. Yet both can be tricky to relate to physical parameters that indicate seakeeping performance. The final parameter to consider is quite different in this way and can give a lot of intuition.

The roll natural period gives a lot of insight into seakeeping performance

The roll natural period is almost like a shorthand for estimating what sea state the ship may be most active in terms of motions at sea. In other words, a ship with a roll natural period around 7 seconds may have the most responsive ship motions in sea states with wave spectrums with dominant periods around 7 seconds as well. But it also gives an expectation about motion at sea because ships tend not to respond much to waves that are below their natural periods. But the flip side is that they follow the water surface and the wave slope for waves with periods above their natural period. So a ship with a 7 second natural roll period would likely not respond strongly to very short period, choppy waves, in coastal areas. However, it would potentially move more in roll in more exposed areas with wave spectrum periods 7-10 seconds and higher. However, a SWATH multihull with a natural roll period of 20 seconds would not respond much in roll in a wide range of commonly occurring seas 3-15 seconds!

But the roll natural period is not the final say on ship motion performance

The point of ship motion prediction is quantify how much a ship will move in different conditions. In other words, the roll natural period gives a qualitative indicator of motion, but it doesn’t tell you how much. A fast crew boat of 40m length and an 80m Frigate might have similar natural roll periods around 7 seconds, but the amount they roll at resonance might be very different from each other for a multitude of reasons that a ship motion prediction program would factor in. In addition, ships operate in a variety of seas of different magnitudes and headings. Roll motion and resonance are important conditions, but they’re only one metric among many others that should be considered systematically in a detailed seakeeping performance. Still, roll motion, typically the most sensitive degree of freedom for ships, makes sense as a guiding parameter for comparison.

Example time

Let’s illustrate some of these concepts with an example. Here are two concepts for crew boat design. One is a monohull, and the other is a multihull catamaran. The natural roll periods were calculated using hull forms generated with Orca3D/Rhino and the ProteusDS ShipMo3D toolset. In this case, a driver for the application is carrying capacity, so the most useful metric for comparison is GT.

The monohull crew boat characteristics are:

• Length: 35m
• Beam: 7.5m
• GM: 1.4m
• Displacement: 140 tonne
• Gross Tonnage: 250 GT
• Natural roll period: 5s

The catamaran crew transfer vessel characteristics are:

• Length: 30m
• Beam 9m
• GM: 6m
• Displacement: 150 tonne
• Gross Tonnage: 250 GT
• Natural roll period: 3s

Ship design DNA of length, displacement, and GT make it a challenge to find equivalent between these designs. Because of the dramatically different hull forms, the GM is completely different. This also affects the natural roll period, which we can see from the hydrodynamics evaluation is quite a bit shorter for the catamaran. Yet for a smaller crew boat, which often operates in coastal areas with relatively mild sea states, seakeeping performance may not be a limiting factor. The similar GT helps show that these two concepts are roughly in the same ballpark.

Deck area, GT, and natural roll period aren’t the only parameters to consider though, too. There may always be other factors that give a concept an edge. For example, catamarans have stable push-on capability for crew transfer via the bow at sea, while monohulls don’t. This along can give them a significant advantage for crew transfer vessels when compared to monohulls.

Summary time

Ship design DNA – length, GM, and displacement, are valuable to guide comparisons of seakeeping performance between concept designs. However, these values are much less sensible to use when the hull form dramatically changes. Using them to normalize seakeeping comparisons between monohulls and multihulls might be almost impossible because of how fundamentally different these floating systems behave, and it could compromise the inherent advantages in using a different hull type. The more different the hull form is, the more important it is to consider other aspects of the project requirements and evaluate seakeeping performance in an absolute sense of the design. For example, there may be requirements on a certain amount of deck area, or cargo capacity in the form of Gross Tonnage that drives the project. Regardless, the natural roll period of a concept design can be useful for making comparisons, as it gives an intuitive sense about what conditions a concept design may be most responsive. Like all tools and techniques, it’s crucial to understand the advantages and limitations of each approach. It’s a nasty pitfall to sink your teeth into a ship design project but find yourself lost in a cloud of confusion – like a river fish who thinks he’s got a delicious meal but fell for a river mussel’s decoy.

Next step

For exploring design variations on the same hull type, ship length, GM, and displacement can be very useful. But going through the steps to do this is not always obvious. There are a few steps involved in generating hull variants using Orca3D and Rhino, and then comparing motion-affected comfort ratings with ProteusDS. Click the image to follow along and use the workflow for your own designs using this video tutorial on the DSA Ocean YouTube channel:

Thanks

A special thank you to Rob Sime from Chartwell Maritime for providing the parameters for an equivalent monohull and multihull used in the example.