Anti-Rolling Tank

Hello guys,

I have a vessel which currently has a very high roll motion. A suggestion was made to convert the aft and fore ballast tanks into Stabilizing tank. I want to know what all will be the constraints I will be facing while I'm doing this. Also, I'd press on minimum possible structural modification. Please enlighten me on this and dont forget to recommend me a bunch of reading materials.

 

http://www.powerandmotoryacht.com/features/0704stabilizers/index.aspx

 

The Frahm tank is not a bad idea but it is difficult to tune up.
It should be above the center of gravity of the ship, and a system of valves will tune the for and back of the water.
I tryed one on a fishing vessel, and put it on top of the pilot house. The reason why is the following.
When going to the fishing ground the vessel was empty and had a very short period of roll, and when back, with full load, the GM was reduced and the period of roll was very long.
We started with 5% then to 10% of the weight of the hull (meaning the plating, the framing, the keel, the beams, the deck and superstructure) in the tank, and a serie of valves to tune the tank.
It was a complete waste of time.
Then we tryed to put the tank on deck, but the space needed was taken from the work platform, and we raised the amount of weight by 12% of the weight of the hull. The tank was U shaped with air valves.
This time the tank was closer to the G, therefore it was not very efficient.
Again a complete waste of time.
This is my very personal assessment that the Frahm tank is to complicate to tume and position properly, take to much space. and it is by nature when empty, to heavy due to the renforcement needed.
But as I said, this is my own experience, and I hope somebody else has also experience on that and have more success.
Daniel

 

Conceptia, do you have some drawing to show, where the aft and fore tanks are visible? I would just like to point out that fore and aft tanks are not the best choice for roll damping, because there is a possibility of undesired roll and pitch motion coupling during their operation. It could result in steering problems in heavy seas.

You could also consider a fin roll-stabilization system, like this one by Vetus:
http://www.vetusweb.com/manuals/files/Hydraulics_-_Power_Steering/120402 r03 12-04.pdf
Or this one by wesmar:
http://www.wesmar.com/pdf/Stabilizers/stabilizer_brochure_web.pdf
They appears to be suitable for retrofits.

 

There are a bunch of variations on Frahm Anti-Rolling Tanks. The U shape is one, another system is tanks open to the sea and cross connected, a modern variation is the Flume tank. http://pws.prserv.net/flume/flumetankstabilizers.htm

One example of the Flume Tank that apparently works was designed by Professor Don Bass of St. John's Newfoundland. This tank was mounted on a motoryacht called Swan Song, a Roughwater 58' designed by Edwin Monk. The rectangular tank is on top of the pilothouse, and is 12' long (athwartships), 4' fore and aft, and 16" high. Inside are 5 T-shaped baffles !8" from either end. The tank reportedly weighs 250 pounds (foam-cored glass) and has 1550 pounds of water in it, about 6.5" deep when floating level. The owner it totally tickled with it. Zero maintenance, zero drag, but of course real concern about high-angle stability and a simple system to dump the weight if needed.

 

Be very careful using water-ballast to reduce roll. It usually works very well at one frequency, and possibly very badly (ie reduce stability) at all others, unless you have some form of control mechanism. This requires an in-depth understanding of the fluid mechanics and vessel dynamics involved. There are companies who specialise in this.

I think (as I read it anyway) the suggestion was to use non-moving water ballast (ie full tanks) to increase the rolling mass moment of inertia. This will not affect the roll angle particularly, but it will affect the natural frequency at the boat rolls. Again, dependant on your situation, this could be advantageous or dangerous.

You need to contact a naval architect who is good on vessel dynamics. They will be able to help you.

Tim B.

 

Tim the proposed are passive roll tanks.
Daiquiri, Im awaiting the drawings from the client. I will post the data soon. The above was regarding the enquiry. Thanks

 

hey the vessel is a supply vessel with 63(L)X14.6(B)x4(T). Please find the drawing showing the positioning of the tanks. aft tank Fr 11 to 22. fore tank Fr. 63 to 74.
Is providing fins a better option than the U-tank stabilization?

 

The fin effectiveness will depend mainly on their dimensions and on ship speed. More speed translates to more roll damping (what is the estimated vessel's speed?). Also, actively controlled fins will be more effective than the fixed ones. The bad thing about fins is that holes and the local structural reinforcements for torsional loads need to be created in the hull.

On the other side, the bad thing about U tanks longitudinally so distant is that they might create an additional torsion in the hull in case of malfunction of the active control system, so the hull scantlings will have to be checked. And you will still need a way to discharge the water from the tanks - so, holes again. You will have some fun in the weeks to come.

Do you have a section view of U tanks? It would be interesting to see their vertical dimensions, in order to calculate the volumes.

 

daiquiri the speed of the vessel is 12 knots. Also is that torsion or the sloshing which will be a bigger constraint for the U-tanks?

 

As an expert in the application of fin stabilizers and nearly so with roll tanks and pendulum systems...

The latter two are weight and space intensive and the entirely passive flume tanks are only effective when properly designed and tuned for exactly the natural roll period of the vessel. The active roll tanks (Intering, for example, as well as some tested in our own labs over a decade ago) are more effective and over a slightly wider range of roll periods, but there is still the weight and space peanty.. And frankly speaking, I would not touch the idea of trying to use tanks separated fore and aft by large distance for a flume tank solution.

Active fins at 12 knots..would have to be quite large in planform area to be effective. Passive fins or bilge keels..always useless. Only NAs are ever happy with the single digit percentage reduction they might measure on a good day. Operators would rather see noticeable results; Bilge keels - 8 % reduction in roll amplitude; 4 fin active stabilizer system on same boat..89% reeduction in roll amplitude. That is what I mean there..and those are very typical numbers. But like I said..4 large fins and associated control and hydraulic package..big cost.

The active pendulum solutions..all custom protoypes (Koop, for example) and none, to my knowledge, ever developed in to standard production hardware.

Potentially the best off the shelf and really effective solution for yr particulr vessel, IMHO?...the actively gimballed anti-roll gyros sold by Seakeeper Inc, in Pennsylvania (distributed through Yanmar I believe?)...

 

The gyros named by BMcF have been used by Feretti. With success, they claim. Though they will continuously drain electric energy, I've read somewhere a figure of around 4-5 HP during (a long) startup and 2-3 HP when the nominal RPM is reached. And they will still require some beefing up of the hull framing, because large stabilizing forces and moments (which act in a relatively small area occupied by the gyro) need to be transfered to the rest of the structure.

As about sloshing, it is an issue in partially filled tanks with large free surfaces, where water can build up kinetic energy before hitting the wall. Much less an issue in U-tanks with small free surfaces like the ones you have here.

 

Hello Conceptia, any news about this project?

 

"Passive fins or bilge keels..always useless." - Not my experience. "4 fin active stabilizer system on same boat..89% reeduction in roll amplitude." - Four fin is for very few boats and, I contend, the trouble and expense involved would usually have been better spent designing a different boat. 89% must be what they use in the ads.
It seems that there was an article in National Fisherman some years ago about flume tanks, H , U tanks of Nova Scotia (?) some years ago. I can't remember but maybe somebody has access to this...

 

Nothing untill now.. got to check, daiquiri..