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Recipe for a stable yacht

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Ready to launch after your refit? Not so fast. Before you sail, an incline test may be required to determine the yacht’s stability. Be prepared, this procedure takes a lot of time and money.

“Other than outfitting the yacht and paying crew, this can be one of the more costly parts,” said Senior Surveyor Will Nock of United Maritime Survey in Ft. Lauderdale.

 

 

“It can take a month or two to complete the entire procedure, just finishing the test results can take a week,” he said. “This causes frustration for owners and captains.”

 

 

One way to minimize frustration is for captains to understand the entire stability testing process and have a clear schedule and budget.

 

 

With no less than 14 hand-drawn diagrams, Capt Brian Mitchell of M/Y Sealyon explained how the data gathered during the testing is input into complex formulas to determine stability. Mitchell said yachts maintain a stability book that outlines percentages of load and how they affect performance. These loads include all items onboard, fuel and water tanks and more. One of the calculations produces the stability curve that captains use to operate safely.

 

 

“It is in and under this profile line, after calculation, that we must remain,” Mitchell said, pointing to the familiar diagram with curves of righting levers and heel angles.

 

 

“It’s all based on the center of gravity and the righting moment, the G-Z lever. As the vessel heels past its maximum moment, it then begins to lose its righting ability and the center of buoyancy rises above the center of gravity,” he continued. To illustrate the importance of a stable vessel, he offered the occasional news of asylum seekers that capsize boats by congregating on top decks.

 

 

“When that happens, the vessel will loll,” Mitchell said. “If it indeed goes over, it’s ‘good night Irene’ and over she goes.”

 

 

We’ll accept that captains take courses in stability and understand the complex equations and formulas, so we’ll instead explore what a captain can expect as a vessel’s stability is assessed during an incline test.

 

 

Incline testing can be required for many reasons, including commercial certification, flag change, launch of a new build, substantial weight change, a refit or poor vessel handling. The procedure starts with a naval architect and an incline procedure manual that goes to the surveyor who checks it for accuracy, Nock said.

 

 

“This goes back and forth to finalize, and our engineer tells us how to follow the completed procedure manual for the onboard experiment,” he said.

Meanwhile, the captain prepares for a lightship assessment.

 

 

“First thing, we go onboard, walk around and see what is not where it is supposed to be,” Nock said. “The tender has to be in the correct place, everything has to be secured. You can’t do the incline test if up to 2 percent is not in place. On a 100,000-pound boat, that means up to 2,000 pounds.”

 

 

Eng. Andrew Brennan explained the next steps for a captain. Brennan has taught stability as an MCA-authorized instructor for deck officer modules from OOW to master.

 

 

He explained the ingredients for an onboard test: One large crane, large increment certified weights, deck padding to prevent damage, the captain, a class surveyor, a naval architect, and deck or yard crew to move weights.

 

 

Every ingredient in the recipe must be precisely weighed, each person, lifts and dollies that move the weights, deck padding and all equipment brought onboard for the procedure. Each ingredient will be subtracted during stability calculations.

 

 

Captains, concerned about damage to the yacht, often request to use water instead of 500- and 1000-pound weights, Nock said. Although plastic U-tubes can be used when there is no space onboard, it is more difficult because of the difficulty of precisely measuring water.

 

 

“This is harder to do because certification must confirm how much water is there and this is dependant on specific gravity, temperature, many conditions.”

Next is the actual testing.

 

 

“As far as the time required, allow half a day to set up, usually the previous afternoon,” Brennan said. “Then the test is conducted at first light the next morning before the wind picks up.”

 

 

Brennan said the the naval architect and surveyor check the instructed set-up and make marks on deck with blue tape while the vessel is upright. This is done the previous day or in the morning if the test is to be done in the afternoon, Brennan said. Also, he said tanks should be pressed (full) or emptied, according to procedures. And he added that mandatory class engineering spares should be onboard and all crew equipped for radio contact.

 

 

The conditions are included in results and testing is optimized in a safe harbor with calm water, no wind, slack mooring lines and the gangway removed. It may be required to test during a slack water period if tide is a factor.

 

 

The test uses a pendulum suspended from a line tied on a metal washer and hung from a nail or similar metal object from a tripod or trestle. The pendulum is submerged in an oil or detergent bath to give viscosity to slow the sway. The idea here is to reduce friction so accurate readings are obtained, Brennan said.

 

 

“The trestles must not be moved under any circumstance after the first mark has been made,” Brennan said. “After the first weight is craned onboard, then there are eight weight movements.”

During the test, crew report by radio that they are on their mark when required, they mark spots as directed, and they return to their standing marks when instructed.

 

 

Between movements, the vessel is allowed to roll and come to rest. The weights are moved in patterns back and forth in three locations on the yacht.

 

 

The results from the experiments include the establishment  of zero degrees for the baseline and that number is used to determine stability of the yacht, Nock said. Next the surveyor reviews the data, does preliminary calculations and sends them to the authority governing the yacht.

 

 

“This takes about 40 man-hours,” Nock said. “The engineer does theoreticals on how the yacht will handle.”

 

 

Once all the results are interpreted, weight is added or removed from the yacht.

“I’ve done stabilization twice before,” said Capt. Paul Stengel, who oversaw incline testing on M/Y Plan A at Roscioli Yachting Center in Ft. Lauderdale in February. “Ninety percent of the time you have to add weight. My guess is we will add about 12 tons. We hope to add usable weight like anchor chain. Say we add 350 feet of chain. We’ll add it to each side to balance.”

 

 

Captains often want to add useful weight such as fuel or water for emergency, but solutions can include dead ballast.

“Once we filled the keel with little steel balls,” Stengel said.

 

“Some pour concrete, but that is not good because it absorbs moisture. That 5 tons can later become 6 tons.

“You can add equipment, like we’re adding zero-speed stabilizers and furniture,” he said. “Remember, a pound of lead and a pound of feathers both weigh a pound, but take up different space.”

The road to yacht stability is complex but Nock offered captains a few tips.

 

 

“Often, the owner pushes changes like adding marble tile or a helicopter,” Nock said. “But watch out for an incremental change up. Adding a 1 percent weight change in one year is fine, but another 1 percent the next year and adding the same next year can change the yacht’s stability.”

 

 

Results of stability tests are used to define the number of passengers, the draft line, maximum loads and loading conditions for a yacht. But one part of the results Nock said owners and captains often overlook is the area of operation. Stability results can define where a yacht can safely travel and may impose limitations like ‘short range, up to 60 miles from a safe haven’.

 

 

“If the insurance company finds a yacht has traveled outside that defined area, there could be a problem. I haven’t heard of it, but it is a real possibility,” Nock said.

 

 

Nock pointed out that although incline tests are always the same, outcome can vary.

“They are interpreted differently by authorities and can vary by flag state,” he said.

And finally, Nock said he has seen one issue that can be avoided.

 

 

“If you send the actual stability book instead of a correct digital file, that will double the review time,” Nock said. “To save time and money, captains need to be well versed in the entire procedure.”

 

Dorie Cox is associate editor of The Triton. Comments on this story are welcome at dorie@the-triton.com.

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