CCG’s Hero-class vessels are too stiff to be out in a 3 metre sea.
A ship is never too stiff, but the crew might be too soft
The article isn’t really clear on the issue, except that it’s about crew comfort. For the most part it sounds like the ships are too tender:
More than a year later, in May 2018, Wyse relayed an unidentified at-sea testimonial: “I’m rolling 15 degrees port and starboard (30 degrees total) out here today and the winds are less than 10 knots and seas are less than one metre. We need to make this platform more workable.”
“All I know is something needs to be done,” he said. “They want us to patrol through the nights whenever possible. The crews are getting fatigued trying to achieve this in North Atlantic conditions in the winter. Most 45-foot [13.7-metre] fishing boats we work around are wider than the [mid-shore patrol vessels].”
But then at the end:
“The quicker a ship rolls and comes right up, the better stability it has, it’s less comfortable. That’s the difference,” he said.
Which is a gross simplification (confusing initial and ultimate stability), but makes it sound like they are indeed too stiff. Oh, how my heart bleeds for those poor men, having to put up with a 30 degree roll.
EDIT: I say this with the tongue-in-cheekiness of someone who has been tossed around for a living, sometimes dramatically. Of course crew comfort is an important factor in overall platform effectiveness.
Reminds me of the fisheries survey ships NOAA built. Roll like crazy in the smallest sea unless you drop the centerboard. Antiroll tank makes a difference too, but the centerboard is by far the bigger stabilizer that I found.
Try a bulk carrier in ballast. There is nothing much you can do except grin and bear it.
30 degrees of roll can be fairly comfortable, if its slow enough. You got to tie things down and sleeping can be difficult if your bunk is oriented fore/aft, which is why having a couch that goes the other way is really nice. But quick rolling is dangerous for the people and hard on the structure. Deputy Commissioner Pelletier might be trying to explain the difference between tender and stiff and its relationship to stability to a non-expert audience but the quote makes him sound like: Tender bad. Stiff good! which is obviously silly. Just because stabilizers might help doesn’t mean that the ships have a stability problem, but just because there may not be a stability problem doesn’t mean that there isn’t a problem. These ships are not big, so a quick roll could be totally miserable! Totally miserable is, in fact, a dangerous condition. Add to that that its also hard on the vessel itself. I’d like to know if they have bilge keels and if they’ve considered reducing the metacentric height a little bit.
Semi-submersible heavy lift vessel have high deck tanks, which can be filled in order to reduce the GM, thus slower roll frequency.
This reduce the forces on the seafastenings and, especially when carrying jackup rigs with tall legs, reduce stresses at the upper leg guides and the leg structure from fast acceleration.
That it is also more comfortable for the crew and riders is a bonus.
Doesn’t most bulk carriers have high wing tank to be used when on ballast voyages?:
I have experience with a vessel with the exact same problem. Built in 2015. Operates in the Gulf of Alaska and Bering Sea, year round. Carries cargo. 240’ x 50’. The retrofit of bilge keels eliminated the problem. The vessel rolls normally now.
The vessel has plenty of initial stability. She is a stiff ship. Knock her down with a big wave and she snaps right back up. The trouble: it felt like this quick righting effect carried on through due to momentum, causing the the vessel to roll exaggeratedly the other way. Some weird harmonic thing began, and misery ensued.
Once the bilge keels were added (they were in the original plan but left out of construction due to a desire to maximize speed)) the problem disappeared.
CCG, a suggestion: bite the bullet and add the bilge keels.
I need to learn how to do these kind of maths again. Its something to do with eigenfunctions, but I don’t think my understanding was ever clear enough to explain it to someone and now I can’t even convince myself that I ever did understand it. School is a trick: its just time consuming enough to keep you from learning the important things.
If the ship rolling is seen as a mass/spring system… than the bilge keels add dampening. Same as the shock absorbers on a car. The springs absorb the shock in a way, the absorbers dissipate the energy of the compressed spring.
Beyond that it gets complicated and beyond me…an eagle flying in the sky, a snake moving on a rock, a ship moving across the sea etc.
Your picture is accurate and the wing tanks do reduce the GM but it is still measured in metres.
One of the problems in adding bilge keels is when the vessel has an ice class. I’m not sure if these coastguard vessels are or not.
The Damen Stan 4207 patrol vessel can carry up to 12 crew and four passengers, and accommodate 18 standard bunks. It has a length of 42.8m, beam of 7.1m, draught of 2.52m, and a depth of 3.7m at sides.
The vessel has storage tanks for holding 23m3 of fuel, 6.5m3 of fresh water and 8.4m3 of waste water. The auxiliary equipment includes a facet bilge water separator with a capacity of 0.5m3/h, Veth Motoren bow thrusters with a capacity of 75kW, Ajax fire fighting system with water / foam monitor, Westfalia fuel separator, Quantum stabilisers, sewage system, and water maker.
From the CBC article:
An analysis of options prepared for the coast guard by a Dartmouth, N.S., naval architecture firm said stabilizers were intended as original equipment.
“Due to many changes to the original design the vessel had become too heavy and it was decided to drop the stabilizers to save weight and reduce hull resistance,” said an assessment by Lengkeek Vessel Engineering prepared for the coast guard.
I thought that the language used in the CBC article included other possibilities besides “we took an off the shelf design and modified it in a way that made the boat unusable”.
This is funny:
Should be “we decided”
The article does leave a few interesting questions unanswered: On what basis was it decided that the original roll period requirements were extraneous? The dynamics of the hull without fins must have been well documented and understood, but were the end users ever questioned about the suitability of the platform in its current state? If so, why did they think that the ship would still be able to fulfill its mission?
Quantum doesn’t publish the key specs of their systems, and I can’t be bothered to write them and ask, but I’m curious by just how much the mass of the fins would have reduced fuel range. Keeping the displacement down strikes me as a somewhat thin explanation of why the fins were dropped.
All in all, this provides some insight into why public acquisition projects get so expensive, with or without due diligence.
Mass or drag? I would assume that the drag of fins is more of an impact than the weight of them.
A long time ago I helped with a steel boat redesign and one of the first things we did was get the new roll period calculated so we weren’t launching passengers off the boat.
I assume they mean weight when they say so. Stabilizer impact on fuel burn at a given displacement and speed can swing both ways, depending on what you’re doing.
The drawings I could find don’t make the stabilizer arrangement very clear, but I assume that robustness requirements would dictate retractable fins. Again the specs aren’t public, but I can see system weight happily eating up 10% of the fuel load, which is not insignificant.
How this impact on the boat’s considerable fuel range and loitering ability measures up against reduced effectiveness due to crew fatigue and restrictive mission planning, is a subject that goes way beyond my understanding. It looks suspiciously like it went beyond theirs, as well…
It sounds like Commander Lamoutee might also think that they are meant to be retractable, if they are going to be subtracting from the volume of fuel that can be carried. Or maybe not, maybe its the hydraulics and what-have-you that make the tanks smaller. Anyway, as Mike Crottey points out in the article, without the fins the Skipper goes around the weather when he can and so they burn more fuel anyway.
OK - it wasn’t weight of the fins vs. fuel, it was space that apparently became part of a fuel tank. That makes more sense!