software to calculate stability /improving behavior, trawler conversion

[expedition]

We bought a 40 m. fishing trawler to convert into a private expedition yacht.

I wonder how easy it is to calculate the new stability after removing heavy winches (22 tons), main fishing gear (13 tons) and the most cost effective solutions could be to add new weight to the main deck to avoid the ships becomes too stable (and a corresponding fast righting moment).

I was thinking about concrete which would also offer great sound isolation. Any other suggestions.

Regards,

Thorwald Westmaas

[Guillermo]

You are involved in a 40 m trawler conversion and you have not performed a previous stability calculation yet? I'm in fact surprised!
I strongly recommend you to put the thing in the hands of an NA. Much more important than having fancy renderings.
Cheers.

[marshmat]

Thorwald,
From reading through your website I gather you're already in regular contact with the ship's original designers. Have they done any stability checks on the new configuration? They would know this boat and have its original stability data available.
Concrete is a cheap, easy way to add weight, but you need to know where to put that weight, how much, and how to make it stay there. That's the tricky part and is why Guillermo is so insistent you get a qualified NA to evaluate the stability issue.

[mvrembrandt]

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[expedition]

Dear Matt, hola Guillermo,

Yes, I'm certainly in touch with a n/a. In fact, as Matt already noticed from my website posts, I'm in touch with the original designer of the vessel and they are in fact doing the design and now started on construction drawings.

We have the previous stability results but they want to do another test in the near future.

I simply bumped into this great website and wanted to get some feedback from others about my concrete idea. Rest assured I'm not just going to pour concrete and 'see how it goes' :-)

I totally agree that's more important than 'fancy' renderings but, those fancy rendering do visualize the project for people not used to construction drawings that might at some point help me to carry out this project.

Thanks for your comments.

Thorwald

[Guillermo]

Hi Thorwald,
More relieved now....
We've just did the design and calculations for the transforming of a 35 m fishing vessel into a merchant one (And coincidentally it has been registered in Panama!). We were not looking to keep GM in acceptable low figures from the point of view of comfort, the reasons were other, but we added a tank on deck, which certainly contributes to keep GM down.
That pool over the bridge will contribute, too (when full), as well as increased items/areas in overdeck accomodation. Also adding cranes, or stronger masts and poles, as well as deck cargo carrying.
A typical solution to fight short rolling periods and/or large amplitudes is to add active and/or passive stabilizing systems. There are several of those, as you already probably know, being a marine engineer.
Probably unnecessary to say this, but unless you have a 100% trustable lines plan, drafts marks, general layout drawings and tanks drawings and/or capacities, your NA should carefully check what you have against the real thing when performing stability tests and calculations. I've had my share of surprises when dealing with old fishing vessels.
Good luck with your project.
Cheers.

[marshmat]

Active stabilizers would do the trick, but I've heard they can be a real pain in the wallet to install and repair....
Concrete's kind of permanent. Might I suggest a number of small (well, around 2 cubic metres each) water tanks, that can be filled to increase roll period and drained when you'd rather have the high righting moment? One big tank gives you two options, full/empty, with no way of utilizing anything in between without getting free-surface problems. A few smaller tanks would be a more flexible solution.

[expedition]

Hi Guillermo,

I'm glad you're more 'tranquilo' now :-) We have all the drawings and the line plan is accurate so we should be able to do a pretty accurate calculation. I'll attach some other data in my next post.

Matt, yes, the stabilizers would do the job and yes, they're expensive. The folks from Quantum who make the MAG lift stabs quoted something over $ 400 K. USing traditional active fins would give me even more draft and well, not save that much more.

We'll be looking in stabilizing tanks although of course the issue is getting them high enough to really sort effect. Maybe it makes sense to add concrete and to increase the roll and then have a smaller set of Mag lift stabilizers and accept they reduce roll only 60 % instead of 85%.

I'll get in touch with them.

[expedition]

I don't understand all the numbers and since some is in Dutch, you may not either but then again, I guess you recognize some data.

Thorwald

[Guillermo]

Originally Posted by expedition

...We'll be looking in stabilizing tanks...

If you have room for them you may consider flume or wing tanks.
Have a look at:
http://www.utgjiu.ro/conf/8th/S5/03.pdf

From Kasten Marine pages (http://www.kastenmarine.com/roll_attenuation.htm):
"Passive Anti-Roll Tanks: According to published research in Marine Technology, in some sea conditions, with optimized tank / vessel design, roll reductions in both amplitude and acceleration on the order of 50% to 60% have been documented. In other sea conditions, the percentage of roll reduction appears to vary considerably. Vessel speed does not appear to be important to roll damping benefit. There does not seem to be any negative effect on vessel speed or efficiency, except of course for the added displacement required to carry the extra deadweight of the tank contents. Anti-roll tanks seem to vary in size from around 1.5% to around 2.5% of a vessel's displacement. If located higher, the overall weight may be able to be less, since the tank will have a greater effect due to being farther from the vessel's center of gravity. Similarly, if the tank is able to be the full width of the vessel, its effect appears to be greater and there may be the potential for a reduction in tank weight. Space requirements are very difficult for small pleasure vessels (say below 60 feet). Possible undesirable effects on stability, depending on the vessel (large free surface effect). Very unlikely as a retro-fit. Possibly noisy. Relatively complex to design correctly (therefore expensive to design). Relatively inexpensive to build. Relatively simple in use."

[RyanR]

Excluding all the discussion of enlisting the services of a qualified Naval Architect, I can comment as follows as far as the need to look at getting ballast in the ship to correct a ship's motion problem.

Presumably, this is a large GM problem as a result of pulling off a lot of heavy fishing equipment and deck gear reasonably high up on the ship.

As noted in the last post, anti-roll tanks are certainly a good way of reducing roll if you have the space and aren't worried about slowing you down, and don't have any other stability issues. You can also dump the tanks if you are in calm water, or are not looking for reduced ship's motions, and want to get more speed. You can also set these up to be fuel tanks as well, though if you are at the end of an endurance run you may have to sacrifice motions.

On the other hand, if you're looking for a cheap way as you mentioned and just put some fixed ballast in, it is possible to use concrete. However, it does have problems with breaking down over time and chemical reactions that occur when it contacts seawater, oil, and other compounds that are likely to wind up in your bilges and tanks. This can be dangerous if the hull is steel and it sets up corrosion cells between the concrete and the hull.

There is a product out there that is a semi-liquid permanent ballast that can be pumped into and out of tanks, and doesn't slosh around a lot (close to zero free surface moment). I can't remember the name, but the vendors have visited me on more than one occaision and should be pretty easy to find in the commercial marine marketplace.

Lastly, you can do what all yacht designers like to do, assuming you have the money, (and what all Naval Architects hate), put weight up high! Add a wetbar on the flybridge with a 75 mm thick solid marble top! Add a lot of thermal and sound insulation in all the cabins. Perhaps a couple of 40 ft sportfishers on the main deck. You're only limited by your imagination, your checkbook, and when your Naval Architect tells you to stop.

Ryan

[RyanR]

This isn't the exact company I was trying to remember, but below is a link of one who offers ballast solutions for the marine industry:

http://www.euroballast.com/index.html