Tank baffle spacing

[GILow]

Hi all,

I am soon to build my new fresh water tanks. They will be in the order of 400 litres each (100 gallons), around 1.6 meters fore and aft, average depth and width around 50 cm by 50 cm. (that's 62 by 20 by 20 inches).

Can anyone point me a formula or guide to calculate the number and spacing of the baffles? I'm not concerned with free area effect as the main dimension is fore and aft, however I am worried about them slapping and damaging themselves.

Matt

[Paul Elliott]

Sorry, I can't answer this, but for purely theoretical reasons I would also be interested in any info. For example. I would think that baffles with perforations would be better in damping the slapping or force caused by the pitch/roll amplitude and frequency. But what percentage of perforation to baffle area is optimum?

[GILow]

Well, so far I have googled some specs for fire vehicles, which state no more than 48 inches between baffles. I suppose that's a start and I would imagine the g forces in a fire truck would be more than an old cruising boat.

Hadn't considered the matter of perforations... intuitively it does feel like they would help. But the extra work would be a bit of a pain as I intend to make the baffles out of plywood, which has to be adequately sealed.

[Singularity]

FWIW the following from a Practical Sailor article on building fuel tanks.

Tanks usually require at least one baffle or “slosh plate” between the ends and/or sides to prevent the contents from surging, to improve strength, and to minimize noise. Baffles should be fitted where any distance exceeds approximately 30inches so that tank intervals won’t exceed 20 inches maximum, or so that a maximum of three cubic feet (about 20 gallons) exists between baffles.

[GILow]

Originally Posted by Singularity

FWIW the following from a Practical Sailor article on building fuel tanks.

Tanks usually require at least one baffle or “slosh plate” between the ends and/or sides to prevent the contents from surging, to improve strength, and to minimize noise. Baffles should be fitted where any distance exceeds approximately 30inches so that tank intervals won’t exceed 20 inches maximum, or so that a maximum of three cubic feet (about 20 gallons) exists between baffles.

Thank you, that's an EXCELLENT article with lots of additional info I wish I'd had at my fingertips previously.

It tends to suggest I will need four transverse baffles at least, and maybe one running fore and aft, which would at least support the transverse baffles.

Plenty of other good tips on construction that I will apply to this job too.

Edit: I googled the author of this article and he appears to have written a number of boat building books from 1978 to 2003, so it is not unreasonable to assume that his advice is credible.

[GILow]

Originally Posted by Paul Elliott

Sorry, I can't answer this, but for purely theoretical reasons I would also be interested in any info. For example. I would think that baffles with perforations would be better in damping the slapping or force caused by the pitch/roll amplitude and frequency. But what percentage of perforation to baffle area is optimum?

In case you haven't seen it already, Singularity's linked article states no more than 30% opening on each baffle.

[wsmurdoch]

You might be interested in the US Code of Federal Regulations for fuel tanks... https://www.law.cornell.edu/cfr/text/46/182.440

It says, among other things...

(8) A metal tank exceeding 760 millimeters (30 inches) in any horizontal dimension must:

(i) Be fitted with vertical baffle plates, which meet subparagraph (a)(9) of this section, at intervals not exceeding 760 millimeters (30 inches) to provide strength and to control the excessive surge of fuel; or

(ii) The owner must submit calculations to the cognizant OCMI demonstrating the structural adequacy of the tank in a fully loaded static condition and in a worst case dynamic (sloshing) condition.

Bill (Who bought a replacement fuel tank this year and asked.)

[GILow]

Originally Posted by wsmurdoch

You might be interested in the US Code of Federal Regulations for fuel tanks... https://www.law.cornell.edu/cfr/text/46/182.440

It says, among other things...

(8) A metal tank exceeding 760 millimeters (30 inches) in any horizontal dimension must:

(i) Be fitted with vertical baffle plates, which meet subparagraph (a)(9) of this section, at intervals not exceeding 760 millimeters (30 inches) to provide strength and to control the excessive surge of fuel; or

(ii) The owner must submit calculations to the cognizant OCMI demonstrating the structural adequacy of the tank in a fully loaded static condition and in a worst case dynamic (sloshing) condition.

Bill (Who bought a replacement fuel tank this year and asked.)

Interesting... a bit closer to the fire-truck tank spacing spec and 50% greater spacing than Ken Hankinson's recommendation.

[wsmurdoch]

Originally Posted by Paul Elliott

Sorry, I can't answer this, but for purely theoretical reasons I would also be interested in any info. For example. I would think that baffles with perforations would be better in damping the slapping or force caused by the pitch/roll amplitude and frequency. But what percentage of perforation to baffle area is optimum?

ABYC H-33 does not say what the optimum is, but it does set a maximum baffle openness for diesel fuel tanks. https://law.resource.org/pub/us/cfr/....H-33.1989.pdf

It says...

(4) If baffles are provided, the total open area provided in the baffles shall be a maximum of 30 percent of the tank cross section in the plane of the baffle. Baffle openings shall be designed so that they do not prevent the fuel flow across the bottom or trap vapor across the top of the tank.

Bill

[GILow]

Originally Posted by wsmurdoch

ABYC H-33 does not say what the optimum is, but it does set a maximum baffle openness for diesel fuel tanks. https://law.resource.org/pub/us/cfr/....H-33.1989.pdf

It says...

(4) If baffles are provided, the total open area provided in the baffles shall be a maximum of 30 percent of the tank cross section in the plane of the baffle. ...

Consistent with Ken's article. That is interesting.

[Singularity]

The logic I understand is that a baffle needs to be just as structurally stout as is a tank end; where this is true then the side-to-side baffle should not need some form of internal support from an additional fore/aft structure (just as the ends only connect to the top/bottom/sides).

I think it's very problematic to compare baffle needs of a road vehicle, particularly that of a firetruck, with that of a boat. In a boat CG/handling concerns are really inconsequential on such a small movement area vs such a large boat. Rather, on a boat tank ~5' or less long I think of baffles as most importantly there to help maintain the integrity of the tank, particularly if the tank is not extremely well externally supported circumferentially.

I previously read a bunch online on this subject with respect to boats. To summarize much, people using large rotomoloded tanks (including over 100 gallons) have no problems going without baffles, withstanding XX regulations requiring baffling and YZ opinions about sloshing et al concerns. Accordingly, if I was building a very stout 100g water tank with good external support, I'd put a maximum of 2 side-to-side baffles in and wouldn't consider 1 unreasonable.


PS: Do 50 gallon black water tanks have a baffle installed??

[GILow]

This is very interesting thinking.

I was concerned mainly with preventing "slapping" within the tank, because the nature of my tank construction is such that they will be extremely strong. (I am using the boat hull to form much of the tanks themselves, and my hull is extremely thick at those points, approaching two inches.)

Certainly it would make fabrication easier to minimise the number of internal baffles, AND I am concerned that the baffles are the hardest part to make completely waterproof, so much so that I had considered making them from stainless steel or frp panels.

Hmmm... a bit to consider.

[Singularity]

FWIW my previous research was for DIY diesel tanks. Your searching may have come across the following thread on a DIY diesel tank, but if not you might glean something from it: https://www.thehulltruth.com/boating...n-project.html

Frankly the thought of using a core (wood or foam) in tank construction doesn't excite me...this is one case where I'd waste money and accept extra weight in making an all glass laminate. At the same time, the layup schedules we see are way strong for what they need to do. More important from my eye is the supporting structure allowing for zero movement/chafe potential...same with that about a bladder tank.

If the side/bottom of your water tank is the actual hull...I'm thinking that you have to be particularly concerned that your supporting structure has zero possible movement, lest you point load the heck out of the joint where the ~rest of your tank structure meets the hull.

[Karanga]

I have 2 aluminium tanks that hold approximately 400 litres of water each, 1 tank in each hull. There are hull stringers, but no baffles. When they are about 85 to 100 % full, I get significant slapping.
I am considering cutting lengths of 90 to 100mm PVC guttering downpipe and gluing them together as a raft and fixing to the inside top of the tank.
Baffle balls are available commercially, but seem incredibly expensive for simply moulded plastic...

[Pete7]

The baffles on our 30 Imp gallon water tank are only attached to the lid which makes the tank easier to clean when you take the lid off.

Baffles in road vehicles, not just fire engines, are to stop the surge moving the vehicle forward after it has come to a stop as any liquid carries on moving.