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When the line blurs between hull and appendage (and how to model it for seakeeping analysis)
The octopus has a lot of hardware to control, but it manages without sweat. With eight arms flailing around in the water, how does it keep coordinated when hunting or defending itself? There’s a hint in how its nervous system is structured. In the entire body, 66% of their neurons are in their arms. The result is that their arms can work semi-independently, almost like they have a mind of their own. It may seem like there’s a lot of overlap and redundancy, but there are significant benefits. Each arm can automatically hunt for food. They can also react even faster for protection and defence if a threat is nearby. It’s not all chaos, though, and their central brain can directly control each arm when needed, and coordinate things to do more complex tasks.
It all works together: automatic food collection and defence, but when you need to, you can take control. But if more of the neurons are in their arms than in their central brain, it raises the question – is there a single brain or not? In the case of the octopus, the line is blurred.
Sometimes, when solving problems, we find blurry lines and a grey area rather than black and white solutions. This can often be the case in the world of ship motion prediction. It’s particularly the case when zeroing in on the hydrodynamics of hull features and appendages. You may need to look at modelling hull features in multiple ways. There may even be overlap and redundancy in the approach you use. It may even feel like chaos when you build up a lot of detail, yet understanding what details are essential and when is the key. In this article, we will focus on when the line blurs about whether hull features are appendages or not.
What do we mean by hull appendages?
Hull appendages are just that – an add-on to the hull form. They might be bilge keels, skegs, fins, fin stabilizers, or other accessories. Of course, they aren’t added only for looks. There’s usually a specific purpose in mind. It’s very common to add appendages to help with roll reduction or general motion stabilization. But there can be other reasons for appendages, too, like skegs, which can help maneuver. Sometimes, there are also appendages to help shape the flow of water around the propulsion system to increase thruster efficiency, or even to reduce resistive drag on the hull.
How are hull appendages different than the hull?
Most of the time, it’s easy to tell the difference between an appendage and the hull itself. After all, the appendage sticks out like a sore thumb: it is usually visually apparent how they are attached to the hull. Another more subtle difference is that the main hull must resist the hydrostatic and dynamic pressure from the ocean. It better be watertight so it tends ot be thick and solid! Whereas appendages generally are meant to do other things, like help with motion performance or efficiency in some way, and not necessarily meant to resist the pressure of the ocean, so they might be formed from thin plates jutting out into the water.
But sometimes the line blurs between the hull form itself and an appendage. This can happen in some ship designs in the aft portion of the hull. Sometimes referred to as a gondola, a portion of the hull can act more like an appendage to help guide flow for more efficient propulsion. But at a certain point, the hull can start to look like a skeg. For example, when you look more closely at the aft of the Generic Yacht series, the hull contours into a ridge. So what are the implications when using a seakeeping and maneuvering analysis tool?
Seakeeping programs model hydrodynamics in more than one way
The hydrodynamics of the main form of the hull are modelled to capture bulk wave interaction effects, like ocean wave excitation and wave radiation forces. However, the physics of viscous effects like lift and drag are not automatically included in many seakeeping programs. These viscous effects are crucial to capture the effects and forces of appendages on the hull. These forces are then accounted for in seakeeping programs by using specific equations for different appendages. These forces are then superimposed on the equations of motion of the ship. Yet, if there’s more than one way to model hydrodynamic effects, it can lead to a grey area when setting up the ship motion prediction model. Now, what does that mean for an analyst configuring a vessel model in a ship motion prediction program?
It helps to consider the goal of the ship motion analysis
Hydrodynamic effects have different impacts depending on what problem you are trying to solve. For example, a resistance and propulsion efficiency study is one goal. Another goal might be to determine ship motion in waves in a seakeeping analysis. Yet another goal would be to determine how the vessel handles through a maneuvering analysis.
Some hull features have an outsized influence on maneuvering analysis. Appendages and structures like skegs or gondolas are more likely to affect maneuvering, like zig-zag or turning circle tests than ship motion in waves. This has to do with how much appendage area there is and how these structures are placed on the aft and midline of the vessel, where there isn’t always optimized flow or leverage to affect ship motion like roll in waves. On the other hand, bilge keels tend to have a large aggregate appendage area and are spread along the length of the ship to increase the chances of creating forces to resist roll motion. However, how bilge keels are oriented means they aren’t intended to affect resistance or maneuvering effects very much. The purpose of the appendage or structure is one way to look at things. But you might get another perspective by considering what stage of the design process you’re at.
Comprehensive detail in a hydrodynamic model is not always possible in the early stage of design
In the early stages of design, there isn’t much information or developed details. You may not know the rudder size to use or what anti-roll technology will work best for your application. Yet it’s still possible to get a lot of insight into ship motion performance to inform design decisions. Comparisons in ship motion performance that consider the hull form, bow shape, beam size, displacement, and vertical CG may all give naval architects insight into how to improve the design.
Nevertheless, it helps to have some understanding of the goal of the hull appendages to guide how best to model the hydrodynamics. Now let’s look at a few more specific examples.
One example is the Generic Frigate
The Generic Frigate has many appendages configured: skeg and bilge keels, and also the rudder, propeller, and even propeller shaft stays are included. All these details contribute to refining the roll damping of the vessel to some degree. Focusing on the skeg, it has about 7m2 of total appendage area and is concentrated at the aft of the ship. While it can help a little with reducing roll motions, the primary goal is to stabilize directionality during maneuvering. In contrast, the bilge keels are spread along the length of the vessel with almost 20m2 of appendage area. The single purpose of bilge keels is to increase roll damping and reduce the impact of motions in a seaway. They have a higher chance of lowering roll motion by covering an extended portion of the hull as the ship can move in complex ways in a seaway. Regardless, the skeg and bilge keels are clearly hull appendages and can be configured directly in a tool like ProteusDS. So this example is clear-cut. What about when there’s more of a grey area?
Another example is the Generic Yacht series
Things are less clear because of how the hull contours to a ridge in the aft section. It’s not precisely an appendage, but it looks like a skeg at a certain point.
The intent for this kind of hull shape is a little to help propulsion efficiency and a little to help maneuvering. Generally, many larger yachts benefit from a fin stabilizer system. So any contribution in roll damping and reduction in roll motion from this hull structure is a bonus. But how do you proceed to configure a hydrodynamic model of a structure like this? In the ProteusDS ShipMo3D toolset, there is more than one way to do this.
In the early stages of design, there is a lot of value in making quick relative comparisons to consider options. The simplest way to proceed is to incorporate all the hull geometry and ignore the appendage model to represent the aft region. This is currently how the Generic Yacht series sample projects are configured. So what are the hydrodynamic implications for a decision like this? By including all hull geometry, including the ridge, the impact on all wave excitation and radiation calculations is incorporated. The wave radiation calculations will include a damping effect that will help reduce roll. Yet for monohulls, roll radiation ramping tends to be very weak. And modelling the hull with this approach won’t include any viscous effects, like drag. So, how can you tell if the missing viscous effects are important? One way to manage this is through an alternative model setup that applies a skeg hull appendage that overlaps with the fin region, and make comparisons in ship motion prediction.
An alternative configuration shows a skeg appendage model added in the ProteusDS ShipMo3D toolset. The skeg model is based on a thin plate, so there is still some grey area about how much of the fin area configure to represent the hull ridge. We used a total fin length of 12.5m, with height tapering up from 0 to a maximum of 1.5m, which covered the fin region well enough. We avoid double-counting added mass from wave radiation effects by removing added mass effects from the viscous skeg model. This then includes viscous drag forces from the skeg, mimicking the effect of the ridge in the hull, in the ship motion prediction model.
We checked the impact of adding the skeg by looking at the roll motion RAO in beam conditions and then the motion-affected comfort rating to see what difference it makes. When considering the motion RAO in beam sea conditions, adding the skeg model reduces peak roll by 20%. Other motions like sway and heave are almost unchanged. Still, at first glance, this seems to have an important impact. But is it? The beam case at roll resonance is definitely a conservative worst-case scenario. The roll RAO for yachts tends to have a very narrow peak, implying that ship motion in a wave spectrum might show a different picture, too. So what might a more comprehensive analysis like the motion-affected comfort rating show?
When using ProteusDS to evaluate large yacht motion-affected comfort rating in 1.5m significant wave height, the difference the skeg model makes is changing results by only 3%. There are a few reasons for this smaller difference. The comfort rating factors in motion calculations across a wide range of sea states, both zero and forward speed, and in a bow quartering condition. These give a broad view in which all vessel motions, including but not exclusively roll, drive the differences. These differences the viscous effects from the skeg model even still are likely exaggerated because we haven’t included more appendage details like a rudder, prop, or even passive fin stabilizers that each contribute to a reduction in roll from additional viscous damping.
This more comprehensive analysis from the motion-affected comfort rating indicates that the viscous effects from the aft ridge for this particular configuration of ship might not be that significant. But it doesn’t mean these hull structures are useless – just that their intended shape is more useful for other reasons like propulsion efficiency and directional stability, which is more accurately assessed in other analysis approaches.
Summary time
We covered a few details on differentiating the hydrodynamics of hull appendages, and now it’s time to summarize. There are many reasons for incorporating appendages on a hull – controlling resistance or propulsion efficiency, controlling ship motion in waves, or improving stability during maneuvering. Hull appendages are often clearly differentiated from the rest of the hull because of how they are attached. But sometimes there can be a grey area, and you can’t tell whether a hull feature is an appendage or not. This can pose a problem when constructing a hydrodynamic model in some applications because there is more than one way to do things. One way to approach this problem is to have an understanding of what the goal of the ship motion evaluation is for – seakeeping? maneuvering? resistance calculations? – and let this guide the decisions you make in how you set up the software to examine it. Another consideration is what stage you are at in the design spiral. For early-stage designs, there is a lot of value in relative comparisons from significant changes in the system, like VCG, roll inertia, beam, and hull form, where it doesn’t necessarily help the design process to spend time on fine tuning hydrodynamic details and coming up with definitive answers. Much like trying to find a definitive answer of whether octopus arms have a mind of their own – maybe yes, maybe no!
Next step
The motion-affected comfort rating was used in the examples in this article to give some crucial insights into ship motion performance. Learn more about what this motion-affected comfort rating is and how you can easily compute it in ProteusDS in this overview video. Click on the image to watch the video on our YouTube channel:
