Vessel Stability

Stability is a topic for vessels of every size. Let's start with some basic theories and physics of vessel stability. Those basics don't have to be complicated just yet. We can get into more complicated formulas later if you're responsible for your vessel's load and trim dynamics. Just remember some of the following general rules for now.

The wider the beam of a vessel in relationship to its length is important. The more beam you have, the more stable the vessel is. Unfortunately, as a general rule the wider the vessel is, the slower or more inefficient it is.

The lower the center of gravity, the more stable a vessel is. Only have one person stand at a time in a small boat. Load cargo as low as possible in large boats, and beat off that ice build up in cold waters. Strictly enforce passenger load limits especially on upper decks. Remember this when lifting cargo with a crane or boom- the center of gravity immediately goes to the top of the crane or boom the second it leaves the deck! 

Don't add on tarps, canopies, sunshades, umbrellas, and other wind catching sail areas without first calculating their affect on vessel stability.

Sea state (wave conditions) and vessel speed and direction in relationship to those waves and winds also affects vessel stability. They say that quartering seas are the worst and slightly off head on seas are the best. Why did I mention winds? Heavy winds blowing on your hull and superstructure from abeam can help tip you over. Think about this before you add on after market canopies, tarps, and umbrellas not already included in your vessel stability calculations. Slower speeds generally aid in vessel stability.

Don't store cargo high and off the center line. Stow it low and on center instead!

Shifting ballast from port to starboard and from midships to fore and aft also has an affect. Shifting weigh from midships to the extreme forward and extreme aft ends of your vessel is generally a good thing. The reverse is generally a bad thing. Opposite of this theory, it is better to keep weigh along the center line of your vessel rather than to the port or starboard. 

Liquid tanks without Baffles are dangerous. The free surface affect just sloshes over and helps to tip your vessel over!

Free surface areas (sloshing water or other fluids) in a vessel has a negative affect on vessel stability. Tanks and liquid bunker locations and heights are important. Amount of tank baffling and tank liquid levels are factors too. It's better to have one tank pressed up full to the top rather than have two sloshing tanks. Keep your bilges dry!

Put Baffles in your liquid tanks to reduce the sloshing and shifting free surface affect.

Sure, those are some basic general rules to help you make your vessel more stable, but what if you want to know what the specific limits of your wet boat are? There are formulas that you need to know. You also need to know some simple algebra in order to work them out. You need to know what the specific design characteristics of your vessel are too. You also need to have some loading tables for that vessel.

Yes, I promised to keep this simple. So, we will learn about one concept at a time. Then we will see how all those simple concepts fit into a more complicated formula. The more complicated formula then won't seem so confusing after all. Diagrams make all of these concepts easier to understand so long as we take it one label at a time.

G is the label for the center of gravity. B is the one for the center of buoyancy. Those are easy enough to understand. So, what is a metacenter? It's a calculated point just like the center of buoyancy and the center of gravity are calculated points. Just tip a vessel over a bit to find the metacenter and see what else happens to the center of gravity and the center of buoyancy.

As the vessel rolls, the center of gravity shits from G to Z in the above stability diagram. The center of buoyancy also shifts from B to B1. Just draw a vertical line from B1 through Z and continue it up until it crosses the old B to G line. That's the metacenter. It's now labeled M. The metacentric height is the distance between G and M. Now that you know that, here is another basic concept that I want you to learn. The longer the distance between G and M, also known as the metacentric height, the better your vessel stability will be. There are a few tricks of the trade to lengthen the metacentric height.

One way to increase the metacentric height is in the hands of the naval architect's design parameters. The other way is in your hands. How? Lower the heavy items in your vessel such as cargo. If you're going to mount equipment, can you place it low? Make sure cargo and equipment is secured so it won't shift and then inadvertently raise the center of gravity. The lower you alter the center of gravity down, the longer the metacentric height becomes.

There's another concept for you to learn, and it's a good one. The good news is that when the center of buoyancy shifts to B1, it means that the center of buoyancy is always trying to push the vessel back to a stable or neutrally stable position. The more it tips over, the stronger buoyancy wants to push it back. That's called the righting arm. You just learned another new concept in vessel stability. Now here's the bad news of this new concept you just learned.

The bad part is that the righting arm has a limit. If you pass the limit of the ability of the righting arm to right the vessel, then the vessel tips all the way over! Every vessel has it's own unique tipping point. You need to know what your vessel's righting arm graph is. You need to know how and why it changes as you change the internal and external arrangements of that vessel, namely the center of gravity, the addition of sail areas, the direction of the wind and waves, and other factors. Remember this; righting arms are significantly reduced when the vessel's center of gravity is off-centerline.

You also need specifically designed loading and trim charts for your specific vessel. This is also where you need to know your stability formulas. You can get those more detailed formulas elsewhere. I'm trying to keep this a general stability knowledge article without a lot of complicated formulas. All of these stability formulas have a safety backup known as a Plimsoll mark.

Plimsoll marks are welded and then painted into the side of a ship. There's one on each side of the vessel. Just like all of the stability and loading graphs and charts are unique to each vessel, so are Plimsoll marks. Why have physical load limits burned into a ship if we have all those graphs, charts, and formulas? Those calculations are subject to the accuracy of the information used within the formula. The more complicated the formula, the more likely it is to make a mathematical error. Even the best math wiz can't be accurate if he or she is unaware of cargo, stores, fluids, or passengers that have shifted or are unreported. How are you to know if a valve leaks fluid from one tank to another?

The Plimsoll mark never lies. It should never be ignored, nor over ruled by a stability calculation. Let's dissect a Plimsoll mark and decode it so that you can understand it better.

The horizontal lines are one inch thick. They represent the maximum water line levels according to the type of cargo, the geographical location of the vessel, and the time of year of the loading. The benchmark for all of the other load lines is the summer line. All other lines are based off of the summer line which is the same line as the one that passes through the circle mark.

You can find out more about Plimsoll marks, but here are a few of the letter codes you might find there.

C1 or P1 = Passenger load lines

T = Tropical waters

F = Fresh water

TF = Tropical Fresh

S = Summer

W = Winter

WNA = Winter North Atlantic

LTF = Lumber Tropical Fresh water

LF = Lumber Fresh

LT = Lumber Tropical

LS = Lumber Summer

LW = Lumber Winter

LWNA = Lumber Winter North Atlantic

The upper most horizontal line may be unmarked, but it represents the main deck line.

There are letters on either side of the summer line that passes through the circle mark. Those letters vary widely, because they represent the various international governmental agencies that certify the Plimsoll marks in their respective nations. For example:

AB = American Bureau of Ships

LR = Lloyds Register

DL = Danish Load Mark

GL = Germanischer Lloyd

K = Indonesia

PC = Russia

RI = Registro Italiano

... and so on and so forth.

You can now understand how important vessel stability is to your vessel's safety. The above information is most important to know. 

 Bilge Keel

Disregard vessel stabilizers and bilge keels with respect to vessel stability. The term "vessel stabilizers" is really a misnomer. They increase your vessel's comfort level by reducing the degree of roll, but they don't affect the time of roll. So, the vessel equipped with stabilizers may roll less far, but it rolls slower. The period of roll remains the same. The righting arm and the metacentric height are still the same.

By Captain Marc Deglinnocenti

OldArmada@Gmail.com