Stability Letters Explained

For small vessels, the major problem with stability letters is explaining it to the captains. Today I will review a stability letter and give plain English interpretation of all the boring mumbo-jumbo.

How to Read the Right Information

1.0 Introduction

READ THE MANUAL!  That is how I wish I could start every stability letter.  In the maritime world, naval architects and engineers, admittedly, create a lot of boring reports.  The stability letter looks like another boring report, but it really acts as a safety manual.  Ignore it at your peril.  Much like venturing into the jungle, preparation is key when going to sea.  And a stability letter is the manual that explains a major part of the preparation.

This was inspired by another article I read in Marine Technology, by Darren Monzingo. [1]  He explained that for small vessels, the major problem with stability letters is explaining it to the captains:

“Backed into a corner and left to their own devices, our mariners will disregard their stability guidance and do what they know has worked before:  they walk blind-folded across the street. . . . nobody has walked them through the contents of their stability letters one sentence at a time to make sure they understand it.”

Darren Monzingo [1]

Today I will review a stability letter and give plain English interpretation of all the boring mumbo-jumbo.

2.0 Stability Letter vs Stability Booklet

First, a bit of context:  what is a stability letter / stability booklet?  First came stability books, which we still use for big ships.  These act as instructions for planning a ship voyage.  They give the data and equations to determine your own limits for ship loads.  More flexible, but also more complicated.  Stability letters are a simplified version, usually used on smaller vessels.  The stability letter doesn’t allow flexibility.  It gives one set of operating limits for the ship, but no math is required for the master of the ship.

3.0 Introduction of Letter

This stability letter is for a small passenger vessel, regulated by US Coast Guard (USCG).  (Figure 3‑1)  I hid all the information that would identify the vessel, to protect my Client.  The first thing we notice is that the stability letter came from USCG, not the naval architect.  Ship owner’s hire a naval architect, who performs an analysis with recommended limits on the vessel.  But for passenger vessels, USCG has the final say.  In this case, only USCG has the authority to provide a stability letter. 

The first line of the stability letter is easy to forget, “You are responsible for maintaining this vessel in a satisfactory . . .”  What that really means is that no science in the world can guarantee your safety.  This stability letter is guidance, but not a guarantee.  Master of the ship, do whatever you need to so you can ensure the safety of your vessel.

The end of that first paragraph also provides a hint when you should check the stability letter.  “After loading and prior to departure.”  The most important limit for a stability letter is the limit on draft (more on this later), which you check right before departure.  Don’t depart too heavy, or with too much trim.

The last item I want to highlight for now is the deadweight survey.  A deadweight survey is one technique we use to determine the weight of the boat.  Just focus on the date.  If that date is 5-10 years old, this entire stability letter may be questionable.  Ships change a lot in 5-10 years.  You may need to recalculate the vessel stability (which requires a naval architect.)

4.0 Operating Restrictions

Next we come to the important part, operating restrictions. (Figure 4‑1)  The first one is the route.  This vessel was limited to Protected Waters.  Those are key words, part of a stupid naming convention used by USCG.  They divide the ocean into three classes of operation, mainly depending on the weather. (Table 4‑1)  As the operator, you need to keep the boat within the limited region at all times. 

For some regions, like the intracoastal waterway on the South Coast of the USA, this classification is not obvious.  If in doubt, call up the local USCG sector, and they will know.  Each USCG command acts as its own fiefdom, aware of local conditions for their region.

I once knew a boat that was limited to Protected Waters, but the captain thought that only applied when passengers were onboard.  They needed to transport the boat and took it across the open ocean in the Gulf of Mexico.  That captain violated the stability letter.  For safety purposes, you have to assume that all the limits of the stability letter apply at all times.

We also see standard language referring to the Certificate of Inspection (COI).  This is because the stability letter restricts the route only based on stability.  The COI also considers life saving equipment.  USCG will not coordinate between the stability letter and the COI.  Sorry.  You have to sit down with both documents and compare between them to find the more restrictive limits.

4.1 Loading Restrictions

Next comes the big restriction.  40 persons or less, 38 of which are passengers.  This is very specific wording.  Persons is anyone with a heartbeat on board the ship.  Passengers are all the people NOT crew.  So in simpler language:  Don’t go over 38 passengers onboard.  You can have more than 2 crew, but no more than 40 heartbeats onboard.  (Babies and children each count as one person.)

Also notice the weight per passenger of 185 lb.  You don’t need to weigh each person stepping onboard.  This comes from a law to deal with American’s getting fatter.  It used to be that the average passenger weight was 165 lb.  Now it’s 185 lb. (in 2022), and USCG is allowed to adjust that every year.  Just know that if you have the old weight of 165 lb. per person, expect trouble on your next inspection.

Freeboard and draft comes next.  The ship has a weight limit.  Do not load down to the deck edge.  We need some reserve stability.  Freeboard is the distance from the deck edge down to the ship waterline.  If you want to make this easy, next time you have the ship out of water, measure this freeboard point on the hull, and paint a short line there.  If you see the paint line go underwater, there is too much weight.

4.2 Modifications

The next two paragraphs are standard lines.  First, don’t modify the ship without engineering review.  As a commercial ship, you are obligated to inform USCG about changes to the boat and prove those changes are safe.  And of course, don’t leave hatches open that will allow a giant wave to swamp your ship.  This also applies to small openings, like tank vents, and doorways.  Of course, some of these openings can’t be closed.  We need them to operate the ship.  So really this should translate to:  “Walk the deck and be aware of your hull openings.  And don’t flood the ship.”  The last parts seems a little obvious.

The next restriction often gets overlooked, weight changes.  Everyone loves to modify their boat.  Whether it is a fresh coat of paint or some new machinery, changes will happen.  But all those changes also alter the ship’s weight.  When USCG wrote the stability letter, they measured the boat with a specific weight and center of gravity.  Stability depends on those numbers.  Too many changes, and the stability letter is no longer valid.  You need to recheck the stability (hire a naval architect.)

The side task to this:  you must track your weight changes to see when they will trigger a recheck of the stability.   I created a separate article about tracking weight changes (Stability Decay).  The biggest thing to remember is to track those changes.  Write them down in the engineering logbook.  Note when new equipment was added.  Take a guess at the equipment weight and add it to a running total.  If you are pro-active on this, USCG is a lot more flexible, allowing you to update the stability on your schedule.

We also see a mention that the vessel has no fixed ballast.  This is a trick we do to fix older vessels that have problems with stability.  We add permanent weight low in the vessel.  the stability letter states that you have fixed ballast, don’t move it.  Don’t remove it.  Just like the counterweight on a crane, fixed ballast is doing a job:  keeping you from capsizing.

5.0 Standard Statements

Next, we have several instructions that add up to:  “Manage the ship well.”  First are the tanks and bilges.  These both focus on limiting the free surface moment (FSM) of your tanks.  Your ship doesn’t have infinite tolerance for FSM.  Too much and the ship capsizes.  The calculations behind this letter allowed for a very limited amount of free surface moment (FSM).  It’s your job to keep FSM to a minimum.  Don’t open the cross connects on tanks.  We don’t want the two tanks sharing liquids.  This includes manifolds for fuel transfer.

Next look at the bilge in the engine room.  The bilge of your ship also acts like another tank.  One that we need to keep at minimal levels.  Water and oil both collect in the bilge of the ship.  Especially in the engine room.  We want to keep the bilge dry.  But how dry?  This is the real world, where we can’t pump out the bilge every second.  If you can see a visible pool of liquid rolling back and forth with the ship, that is too much bilge water.

Figure 5‑1:  Stability Letter, Section 4

Next, we have a warning about list.  A healthy ship does not normally list.  This line is a reminder that a listing ship can warn of a serious problem.  If your ship is listing, run through a mental checklist to see if there are any other potential problems, like tank cross-connects or water in the bilge.

And finally, we discuss freeing ports.  Funny fact, the ship that matches this stability letter doesn’t even have freeing ports.  Some parts of the letter are just standard text and don’t even apply to you.  (Sorry, USCG writes them, not me.)  But many working boats do have a large working deck in back, protected by high bulwarks.  Basically, the jumbo version of the cargo bed in a pickup truck.  Problem is, if a large wave washes over that deck, it will fill the entire working deck and turn it into a swimming pool.  Congratulations, you have a new tank!  One far too large and far too wide for your ship. 

We need to drain that water, fast!  As a general rule of thumb, all those thousands of gallons should drain out in less than a minute.  This is the job of freeing ports.  These are large openings in the bulwarks, covered by hinged steel flaps.  If the deck fills with water, the water pressure opens all these freeing ports, creating lots of openings to quickly drain the water.  Unless the freeing ports don’t work.  Like everything else on a ship, freeing ports can get rusted and stop moving.  So add this to the maintenance list.  Grease the bearings on the freeing port.  Before going into a storm, kick each of them to check that they all work.

6.0 Conclusion

I know that stability letters are boring, and full of useless standard text.  I do this for a living, and even I skim over them, looking for the important parts.  So let me finish with some simple advice that will prevent 90% of problems with stability letters.

1. Skim over your letter and highlight any numbers.  These are the most important parts.
2. Plan ahead when modifying the boat.  Every change affects your ship stability.
3. Free surface moment is your enemy.  Do everything you can to minimize it.

These three items are the best advice I can give.  That’s the major intent behind stability letters:  advice.  As the captain, you have the ultimate authority on how to manage your vessel stability.  It’s your ship.  Your crew.  Your decisions.  The stability letter gives advice for making good decisions.

7.0 References