View Full Version : Help me understand fat sac weights

03-05-2012, 08:15 PM
I have been tempted by some fat sacs for sale on the list, as well as on ebay. My current fat sacs, which came with a 2003 moomba, are listed as 30x16x16. I am trying to find out how much the hold to determine if I should upgrade. These would hold 7680 cubic inches. I know they have an inner bladder so that may be less.

If I go here:


I see a 42x16x16 listed as 400lbs. That is 10752 cubic inches.

If I go here:


I see a 35x20x10 sac. Also listed at 400 lbs. This is 7000 cubic inches.

Something is off here, or I do not undertand how these things work. Based on how I compare to these my current bags either hold 285 lbs or 438.

03-05-2012, 08:42 PM
I like the mathametical way to go.

A cu inch of water weights .036

.036x 7680= 276lbs
.036x 10752 = 387
.036x 7000 = 252

I your concern with descrepency here but the 42x16x16 is the factory size bag and measurments and its 400, none are exact but always close, also bags can be slightly over filled and ballon out a bit.

You best bet is call WakeMakers direct and ask them as they will know the sizes and weights.

03-07-2012, 08:26 AM
BK, dont over think it. Most peolple want to put the most wieght they can without going over boat limit. If you surf you are going to want more on one side than other anyway. I measured the compartment to see what would really fit. I put a 500 sac on Surf side. Peolple put more. My 02 can hold like a 42 inch bag without getting to close to dual batteries and if I push it, the sac can go under the seat if filled all the way. Alot of surfers add an inside bag as well to get a bigger surf.
Go by size and what your after for Boarding or surfing. Wakemakers can help you too.

03-07-2012, 08:52 AM
I agree with Rick - I experimented a bit with different sacks and with my older hull, (and I think both of your guy's is very similar), it seemed the more weight the better. That's not the case with every boat though.

A good rule of thumb is, if you're going to spend the $$, might as well spend just a little bit more and get the biggest bags that will fit. As Dusty's Mom always says, bigger is always better. You don't have to fill them all the way if you don't like it. But having that option is a lot better/cleaner than having a bunch of bags laying around on your seats or floor in search of a better wave.

03-07-2012, 08:58 AM
Berg, Hope Dusty or his MOM dont hear you. Maybe he doesn't like this post.....I got to meet his MOM! LOL................

03-07-2012, 09:31 AM
those sacs are a moot point. You want to get the 750 or the 1100. Personally I would get the 1100 unless you know for certain the 750 will totally fill your compartment

03-08-2012, 03:13 PM
Well, it looks like there must be a typo in that eBay ad, or that is the weight of it "expanded."

Here are some numbers I have used in doing ballast math:

1 gallon of water = 8.35#
1 cubic foot of water = 62.4# (7.48 gallons)
and obviously 1 cubic foot = 1728 cubic inches

So, the meassurements of a bag, 16x16x42 for example is, as you said, 10,752 cubic inches = 6.22 cubic feet = about 388#, but as you realize, the bag is not square and it bulges a bit, so a multiplier can be used which appears to be about 3-4% (388 x 1.03 = 400).

Now, obviously the bigger the sac, the more it can bulge. For example, Barefoot International (the folks who make the Fat Sac brand bags) say that a 20x20x50 is 750# (actual weight of that size squared off is 722#), but expands to 950#. That is about 30% more than what the "rectangle" of water would weigh. If anyone has ever seen one of these sacs totally filled, you can see why it can weigh so much more. For kicks, fill up your sacs with air and see how big they can get. It is pretty amazing. Keep in mind that the sacs are basically trying to become spheres.

The other thing that can affect bulge is the porportions of the sac itself. That 35x20x10 sac is pretty long and flat, so if it was totally filled, it would approach becoming an 19" diameter sac, that is 35 inches long or so, which is about 9800 cubic inches of volume, which is in the 350# range. If you look at the cross sectional area of the 20x10, that is 200 square inches, and a perimeter of 60". A circle with that much perimeter (circumference) has about 280 square inches of area.

So I would have to say the the 30x16x16 sacs weigh about 285 pounds.

Barefoot International will tell you that they will expand to fit whatever are they are in, if they are big enough, which is true, but there are a few things everyone should keep in mind. With the weight of that water, those sacs will put pressure on everything around it. Seats, lockers, engine partitions, amps, etc. I know that quite a few guys have reinforced their engine partitions because the sacs were pushing them too much. Corners of amps can be very sharp. Also, this can put too much stress on latches and such.

So in my mind, getting the largest sac that will fit might not be the best idea, especially if an automatic pump system is used. If a compartment is, let's say 18x18x46, a 20x20x50 sac will fit, but will not fill up all the way and will put extra pressure on the surrounding items. However, a 16x16x42 will bulge slightly larger than its listed size, but will put less pressure on the stuff around it, so it may be a safer bet. The one caveat to that is that it will become more round, as mentioned above, which may lead to it "rolling" back and forth a bit, which may or may not be an issue.

03-10-2012, 01:00 PM
Nice write up. That info should help some people out.