C Of G of ships

[Soton sailor]

I'm perhaps coming to the conversation a bit late, but maybe I can help a little (I'm a naval architect specialising in aspects of stabilty).

As mentioned by the other well-informed posters, stability is a double-edged sword. A vessel that has a small amount of stability (which can be quantified as a small metacentric height), will for a given energy input such as a gust of wind, heel over to a large angle. However, with a proportionally small righting moment, she will take a while to roll back upright and will slowly oscillate through larger angles until coming back to rest. A vessel with larger stability (which can be quantified as a larger metacentric height) will, for the same gust of wind, heel to a smaller angle. However, as she rolls back, she will do so with a greater "speed" of roll. The stability for ships is a balance between heeling so far over that gravity assists the deck cargo off the vessel, or rolling so fast that the forces from roll acceleration do similar! As a very crude example, based on a "static" waterline, an example vessel with a metacentric height (aka GM) of 0.15 metres (the legal minimum) and a typical mass distribtion would have a roll period (upright to extreme port to extreme stb to upright again) of around 40 seconds, whilst increasing the GM to 0.50 metres would reduce this to around 23 seconds for the same mass distribution around the centre of gravity. Increasing the GM further to 1 metre would bring the roll period down to around 16 seconds. For the same three examples of metacentric height, the list angle for the same gust of wind would be 7.6 degrees (GM 0.15m), 2.3 degrees (GM 0.50m) and 1.1 degrees (GM 1.00m). This book has some plain-English explanations in more detail.

When combined with the dynamics effects of the sea, and the hydrodynamic reaction of the hull to the forces around it, it becomes rather complex! In extreme cases, excessive stability can be very dangerous - a sad example (where excessive GM was a contributory factor) was the Chicago Express containership accident in 2008 (a quick Google will provide lots of analysis and a detailed accident report).

 

[Buck Turgidson]

I'm perhaps coming to the conversation a bit late, but maybe I can help a little (I'm a naval architect specialising in aspects of stabilty).

As mentioned by the other well-informed posters, stability is a double-edged sword. A vessel that has a small amount of stability (which can be quantified as a small metacentric height), will for a given energy input such as a gust of wind, heel over to a large angle. However, with a proportionally small righting moment, she will take a while to roll back upright and will slowly oscillate through larger angles until coming back to rest. A vessel with larger stability (which can be quantified as a larger metacentric height) will, for the same gust of wind, heel to a smaller angle. However, as she rolls back, she will do so with a greater "speed" of roll. The stability for ships is a balance between heeling so far over that gravity assists the deck cargo off the vessel, or rolling so fast that the forces from roll acceleration do similar! As a very crude example, based on a "static" waterline, an example vessel with a metacentric height (aka GM) of 0.15 metres (the legal minimum) and a typical mass distribtion would have a roll period (upright to extreme port to extreme stb to upright again) of around 40 seconds, whilst increasing the GM to 0.50 metres would reduce this to around 23 seconds for the same mass distribution around the centre of gravity. Increasing the GM further to 1 metre would bring the roll period down to around 16 seconds. For the same three examples of metacentric height, the list angle for the same gust of wind would be 7.6 degrees (GM 0.15m), 2.3 degrees (GM 0.50m) and 1.1 degrees (GM 1.00m). This book has some plain-English explanations in more detail.

When combined with the dynamics effects of the sea, and the hydrodynamic reaction of the hull to the forces around it, it becomes rather complex! In extreme cases, excessive stability can be very dangerous - a sad example (where excessive GM was a contributory factor) was the Chicago Express containership accident in 2008 (a quick Google will provide lots of analysis and a detailed accident report).

Is there a term for a roll oscillation that is coupled to yaw in naval architecture? In aviation we call it dutch roll and many aircraft require external damping to suppress it.

I ask because my boat definitely suffers from it in certain wave/speed conditions but it is self damping after 4 or 5 cycles as she becomes out of synch with the waves.

 

[Laminar Flow]

Is there a term for a roll oscillation that is coupled to yaw in naval architecture? In aviation we call it dutch roll and many aircraft require external damping to suppress it.

I ask because my boat definitely suffers from it in certain wave/speed conditions but it is self damping after 4 or 5 cycles as she becomes out of synch with the waves.

Unless you are traveling in a submerged submarine, I doubt that there is much correlation between aviation and the behavior of surface vessels.

Waves and the wave systems generated by a moving (surface) vessel have a significant and diminishing effect on stability and consequently on the roll attenuation of the hull. Particularly, as a (displacement) vessel approaches hull speed, even in flat water, stability can be quickly reduced by as much as 40% as the wave trough, apparent between bow and stern waves, deepens. Sharp bowed and narrow stern vessels are particularly affected by this (low CP). Most cruising yachts do rarely, and most commercial vessels, particularly large ones, never travel at hull speed.

Yaw angle in a moving boat is more likely caused by the influence of external forces, be they aerodynamic or by waves (wind waves, swell, etc.), than by the wave system generated by the boat itself. As the boat speed varies considerably and constantly, due to effects of waves and the intermittent changes in aerodynamic impact of the rig (assuming we are still talking about sailboats, though wind pressure on superstructure in engine driven vessels can play a role), this will have an effect on roll period. Just as waves can induce and increase rolling, if the roll periods of hull and sea coincide, it can, equally, diminish it when they do not. Speed through the water improves the roll dampening efficiency of underwater appendages. Low aspect appendages, i.e. long and longer keels, are more stall resistant and in this respect perform better in turbulent conditions with rapidly changing aspect angles.

 

[Soton sailor]

Hello,

Laminar Flow is correct in what they post - for ships the wave development around the hull can cause all sorts of issues. When combined with swell this can cause a change in the pressure distribution and hence the forces and centre of forces acting on the hull. There is a coupled motion between heel and yaw, as the vessel heels the resistance characteristics change, moving the vessel out of equilibrium and causing yaw and a heading change (known as the Munk Moment). For most vessels, the heel angle is constantly changing in waves, hence there is a very transient change in the yaw. On larger vessels the response speed of the rudder is significantly slower than an aircraft or smaller vessel, so there is little dynamic input or equivalent of pilot-induced feedback. On smaller vessels, you will get a yaw moment as the vessel rolls, but with a faster helm response or a roll decay that is possibly a bit faster than a large vessel (thanks to more added mass from the drag effects of nice big keels) it should reduce fairly quickly if there are no longer any external forces acting.

There are some dymanic effects that are perhaps closer to a Dutch roll, known as synchronous roll, when the frequency of energy inputs into the hull is similar to the natural period that the hull wants to roll at, with an effect not dissimlar to resonance. There is an issue with similar symptons but a different mechanism called parametric roll - normally on vessels with large bow and stern flare. There is a good video of a simulation of it for a containership here:

 

[boomerangben]

Some really interesting stuff here. In response to the OP, I remember as a student of naval architecture seeing the designs for a passenger ro/ro ferry have ballast tanks high up to reduce the GM to make the motion more comfortable. The greater the GM, the greater the righting moment. Bigger moment = bigger accelerations = ?. Thats if I remember it correctly - it was a long time ago since I tried to make steel float.

 

[Soton sailor]

Some really interesting stuff here. In response to the OP, I remember as a student of naval architecture seeing the designs for a passenger ro/ro ferry have ballast tanks high up to reduce the GM to make the motion more comfortable. The greater the GM, the greater the righting moment. Bigger moment = bigger accelerations = ?. Thats if I remember it correctly - it was a long time ago since I tried to make steel float.

Your memories are right! RoRo ferries need an optimised GM - too much and the accelerations in roll could in theory shift vehicles (and passengers), too little and there is no reserve against Free Surface Effect if water gets on the car deck (and we have a lot of very sad evidence of the outcomes).

 

[Laminar Flow]

Hello,

Laminar Flow is correct in what they post - for ships the wave development around the hull can cause all sorts of issues. When combined with swell this can cause a change in the pressure distribution and hence the forces and centre of forces acting on the hull. There is a coupled motion between heel and yaw, as the vessel heels the resistance characteristics change, moving the vessel out of equilibrium and causing yaw and a heading change (known as the Munk Moment). For most vessels, the heel angle is constantly changing in waves, hence there is a very transient change in the yaw. On larger vessels the response speed of the rudder is significantly slower than an aircraft or smaller vessel, so there is little dynamic input or equivalent of pilot-induced feedback. On smaller vessels, you will get a yaw moment as the vessel rolls, but with a faster helm response or a roll decay that is possibly a bit faster than a large vessel (thanks to more added mass from the drag effects of nice big keels) it should reduce fairly quickly if there are no longer any external forces acting.

There are some dymanic effects that are perhaps closer to a Dutch roll, known as synchronous roll, when the frequency of energy inputs into the hull is similar to the natural period that the hull wants to roll at, with an effect not dissimlar to resonance. There is an issue with similar symptons but a different mechanism called parametric roll - normally on vessels with large bow and stern flare. There is a good video of a simulation of it for a containership here:

Thank you for posting that, Soton Sailor. I appreciate your professional input.

Of particular interest to me was the part concerning parametric roll. I had not heard of this before, but it describes, to a point, the behavior of my own boat which has a strongly flared bow.

 

[dunedin]

There is a big “a but!” about heavier rigs on cruising yachts - which often had heavy furling main mandrels, radar etc etc onto a rig that may not have been designed for it originally.
These can raise the CoG considerably over what the designer intended. And a high CoG and high moment of inertia will slow the rate of roll - but will also slow the cessation of the roll. So if a boat with a heavy rig gets a serious force to start rolling, it will keep on rolling to a much larger angle of heel - which can be very disconcerting for crew in moderate conditions, and potentially leading to a knockdown in serious conditions. Plus it will reduce the Angle of Vanishing Stability, AVS, quite considerably.

So putting a heavy rig on a boat is not necessarily a good thing.

On the sub-theme about applying this to yachts, interesting announcement by Selden that they are working in conjunction with Halberg Rassy to introduce carbon fibre masts with furling mainsails, to reduce the excess weight aloft with a furling mainsail. Seldén introduces carbon furling masts

I guess at least initially this will come into the “if you have to ask you can’t possibly afford it” category, but likely will filter down to more affordable boats in a decade or so.

 

[Laminar Flow]

On the sub-theme about applying this to yachts, interesting announcement by Selden that they are working in conjunction with Halberg Rassy to introduce carbon fibre masts with furling mainsails, to reduce the excess weight aloft with a furling mainsail. Seldén introduces carbon furling masts

I guess at least initially this will come into the “if you have to ask you can’t possibly afford it” category, but likely will filter down to more affordable boats in a decade or so.

Most older yachts ( definitely pre mid to late seventies) were never intended to have furling systems. I have sailed quite a few miles on an old and narrow Robert Clark design that had been ruined by a previous "renovation" when the last owner replaced the old tapered, hollow and keel-stepped mast with a heavier, deck-stepped aluminum one and then added twin head stays with a furler, a retrofitted behind-the -mast main furler, (massive) mast steps and a honking great radar dome.

Weight is the biggest enemy of performance. With a lighter rig, perhaps HR can save a few extra pounds of lead or gain a bit of extra driving power, but when it comes to cruising where comfort may be of concern and with all the clobber that entails, you quickly reach a point of diminishing return.
Lighter boats with a high waterline plane/mass ratio suffer from much higher rates of acceleration due to sea state.
Yer pays yer money, yer picks yer ride ...

 

[Tiderace]

I'm glad "Solent Sailor" came along. I was dreding having to dig out Roawson and Tupper; its been some years! and would struggle to explain it!

 

[Soton sailor]

I'm glad "Solent Sailor" came along. I was dreding having to dig out Roawson and Tupper; its been some years! and would struggle to explain it!

One of my favourites!

 

[Ric]

So does a bilge keeler or shallow draft keel with bare poles drifting give a more comfortable motion than a same design with a deep fin keel in the same situation?
Does a shallow draft motor boat roll less than the yachts of similar size if left to drift in the same sea?

Broadly, yes. This is one of the reasons that aluminium centreboard boats with internal ballast are popular blue water cruisers because generally the motion is more comfortable than a fin-keeler.

 

[Laminar Flow]

Broadly, yes. This is one of the reasons that aluminium centreboard boats with internal ballast are popular blue water cruisers because generally the motion is more comfortable than a fin-keeler.

I should be very interested if you would have some research on this particular matter.

There is a considerable volume of scientific study, including tank testing, which indicates that vessels with little or no salient keel are at greater risk of roll-induced capsize.
These findings would coincide with my personal, as well as a friend's, experience in regards to lifting keel boats in heavy seas.

Thank you, A

 

[boomerangben]

I'm glad "Solent Sailor" came along. I was dreding having to dig out Roawson and Tupper; its been some years! and would struggle to explain it!

Rawson and Tupper. No that’s a blast from the past! Somewhere I’ve got a relatively unused PNA which one day I might more interest in than I did at Uni!