Tuesday, July 13, 2010

What is Stability?

By Eric Blumhagen, P.E.

“I’d rather have my gums scraped than go to a stability class.” This is a pretty common sentiment among non-naval architects, and who can blame them? Many old-school stability classes are filled with abstract ideas like righting arm and metacentric height, not to mention pages and pages of calculations. This article isn’t about the numbers – it’s about the basic information you need to operate your boat safely.

Stability is the boat’s ability to ride out wind and seas without capsizing. If a boat comes back safely from a trip, then it had enough stability. The tricky part is knowing whether a boat has enough stability before the trip. To answer that question, the Coast Guard has issued a series of stability criteria for fishing boats. If a boat passes these criteria, then it is virtually certain to come back safely.

How do you know if your boat passes the stability criteria? In general, following the guidelines in the stability booklet will make sure that the boat passes the stability criteria. However, the Coast Guard only requires stability booklets on boats of more than 79 feet registered length (usually 85-90 feet overall). Smaller boats often do not have any stability booklet, and so they have to rely more on guesswork.

In general, there are two types of stability: initial and reserve stability. Initial stability is what you feel most of the time when you’re running your boat. The more initial stability a boat has, the faster and sharper the roll will be. The width of the boat and the height of its center of gravity are the biggest factors in determining initial stability. Reserve stability is what brings the boat back from a deep roll. The biggest factor determining a boat’s reserve stability is reserve buoyancy, or watertight volumes above the waterline. Reserve buoyancy comes from freeboard, poop decks, watertight foc’sles, etc.

All bets are off if the boat starts flooding. Water can come in through leaky hatches, cracks in the hull, or open doors. No matter how the water gets into the boat, it is extremely dangerous. Most capsizing incidents start with flooding. The water sloshing around the bilges destabilizes the boat, making it roll over. The first symptom of water in the bilge is a slower roll. While this can feel more comfortable, it is a big warning sign. If the boat’s roll suddenly slows down unexpectedly, check for flooding.

Installing bilge alarms in all dry spaces can give you early notice, but only if they work. Float switches are notorious for failing at just the wrong moment, so test them often. It’s also important to keep any water that leaks into the boat contained. Close up holes in watertight hatches and make sure you keep doors closed.

The Stability Booklet
There’s nothing I hate more than a lousy stability booklet. You know the type. It’s about 500 pages long, filled with pages and pages of numbers and no guidance to the skipper in sight. Yes, this booklet meets the bare regulatory requirements, but it’s unusable on board. You should look for a few features in your stability booklet. First of all, you should trust the naval architect. The booklet should give you clear and straightforward instructions on how to operate your boat, with a list of fishing gear weights.

The stability booklet reflects the condition of the boat at the time the naval architect produced it. If you add a new deckhouse to your boat, then the guidance in the stability booklet is no longer valid. More importantly, boats gain weight over time. This is just because we always add more stuff each year than we take off. Each individual item is insignificant, but the total can really add up after a few years. On most boats, this weight growth is about one half of one percent of its empty weight each year. Normally, the Coast Guard requires owners to have the stability booklet updated after the boats weight has changed by two to three percent. Because of this, we recommend that owners update their stability booklet every 5-10 years.

If you convert your boat to a different fishery, you should absolutely have the stability booklet updated. Better yet, ask a naval architect if the conversion will make sense before you start the work. You can save a lot of money by finding out that there’s a problem while everything is still on paper and not committed to steel.

You’re trusting your life to the instructions, so you should make sure they match up to your boat and your operation. The instructions should typically reference a loading table. These tables will give you an allowed deck load based on fuel and hold loadings. Unless your boat has exceptional stability or very light deck loads, you should see some differences in the allowed deck loads if the fuel tanks and holds are full or empty.

Because the stability of the boat depends so much on how you load it, the stability instructions may tell you to burn fuel or load holds in a particular order. If that order doesn’t work well for you, then you should have the naval architect change the instructions. This might reduce your allowed deck loads, but this is better than having a stability booklet that you don’t use.

The Bottom Line
If your boat meets the stability criteria, you’re in good shape, but you still need to be careful. Keeping an eye out for flooding or bad weather will help bring you home at the end of the trip. If you want to learn more, we’ve heard good reviews from the stability classes offered by AMSEA and NPFVOA. No gum scraping required.

Eric Blumhagen is Chief Naval Architect, Jensen Maritime Consultants, Inc.

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