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Old     (luchog)      Join Date: Jun 2002       09-16-2007, 9:47 PM Reply   
I'm checking fat sacks, but have serious doubts about their REAL weight capacity.

For example a 20''X 20'' X 50'' sack is said to weight 725lbs, the same manufacturer offers a
15'' X 15'' X 50 and that is only 440lbs.

I might not be the math king, but something seems odd to me there.

Anyway I want a sack for the rear of my 80SN, something that weights around 350-450lbs REAL.
Old     (technarider)      Join Date: Jun 2006       09-16-2007, 10:34 PM Reply   
20x20x50 is 20000 cubic inches

1 cubic in =.004 gallons

20000 = 80gallons

1 gallon is 8.33 pounds

80 gallons = 666.4 lbs...

mmm... my sacs are not squares when filled tho...
Old     (byrd)      Join Date: Dec 2005       09-17-2007, 6:17 AM Reply   
Get bigger and dont fill them all the way
Old     (dgunthert)      Join Date: Mar 2007       09-17-2007, 6:54 AM Reply   
Leo rounded off quite a bit in the calculation above when converting 1 cubic inch to gallons that makes the 725# claim seem more exaggerated than it really is. 1 cubic inch = 0.00432900431. So 20,000 cubic inches = 86.58 gallons = 722.5 lbs.

However, the OP point is totally valid. I've got a couch that simply can not weigh the 1200 pounds the manufacturer claims. I realized that when I started thinking about building in a couch behind the engine cover. In order to get the 1200 pounds, I'd have to basically build a box that was as tall as the engine cover and filled all the space behind it. The couch doesn't come close to that.

(Message edited by dgunthert on September 17, 2007)
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 7:04 AM Reply   
I've always wondered why I can pick up my 150's so easily they're probably only 80-90 lbs
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 7:41 AM Reply   
my locker sac is 12x12x80 and is rated at 350lbs and I'm pretty sure it every bit of 345lbs
Old     (showtime)      Join Date: Nov 2005       09-17-2007, 8:08 AM Reply   
DG is correct

J - mont. 12x12x80 = 400lbs
Old     (acurtis_ttu)      Join Date: May 2004       09-17-2007, 8:15 AM Reply   
Those sacs dimensions are more of an approximate dimension...they will expand when filled. I know mine look liek they are going to burst.
Old     (luchog)      Join Date: Jun 2002       09-17-2007, 8:17 AM Reply   
Hi everyone, thanks for the responses, perhaps we should measure the sacks and not guide ourselves for the product specs?

Maybe some weight specs are real, and some other are not?
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 8:23 AM Reply   
I don't usually fill mine up that much my luck they'd bust and flood my boat
Old     (peter_c)      Join Date: Sep 2001       09-17-2007, 8:29 AM Reply   
First rule of not ever believe advertising!!

One of my friends doubted the original Fat Sac so he measured the capacity by filling the sac, then pumping it out into a bucket. Doing the math, it came out way wrong.

Another example was in a magazine I read where one brands sack held "X" amount of gallons and then they posted a weight. Funny thing is when you took their number of gallons times 8.34 for the weight of water, they were over 40 lbs off????
Old     (wakeprodigy)      Join Date: Oct 2002       09-17-2007, 9:17 AM Reply   
water weight changes with temperature, so that could account for the error in the calculation.
Old     (dgunthert)      Join Date: Mar 2007       09-17-2007, 9:46 AM Reply   
Tom, please tell me you're joking. The density of water varies less than 0.01% between 40 and 100 degrees Fahrenheit. That's a difference of fractions of a pound. Even across its entire liquid state, from freezing to boiling, the density of water varies less than 0.03%. That would account for less than 2 pounds.

And Luciano, you're probably right. Some manufacturers are probably more accurate/honest (ascribe motives as you will) than others.
Old     (1boarder_kevin)      Join Date: Mar 2007       09-17-2007, 9:55 AM Reply   
Water only changes 0.6% in weight from 50 F (sg of 0.998) to 100F (sg of 0.9931), so for summer temps, that is negligible.
Old     (luchog)      Join Date: Jun 2002       09-17-2007, 10:00 AM Reply   
I think the 725lbs sack is too much for my boat, I think I'll go with a 440lbs sack for the rear and 2 150lbs blocks on the front, what do you think? that plus 2 or 3 people in the boat.
Old     (showtime)      Join Date: Nov 2005       09-17-2007, 10:17 AM Reply   
I would recommend going w/ the larger sac and test it filling it to different levels..... However if you are talking about hte boat in your profile, go w/ 2 400's or so -- one for each side of the engine.....
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 10:22 AM Reply   
I have a 91 sunsport and I'm currently running a 350 in the rear and 2 150s in the front with 4 people in the boat and the 2 150s up front aren't nearly enough but the sunsport is wider and heavier than your Nautique I'd go w/ what Tickle recommended
Old     (luchog)      Join Date: Jun 2002       09-17-2007, 10:48 AM Reply   
yes I want sacks for the boat on my profile it's a 1980 Ski Nautique, the thing with the side sacks is that my boat has a Ski locker on one side and I dont think the sack is going to fit in that side. Dont even think about taking the ski locker out since it's the only storage area on that little fun machine.

We rode with a 350 on the back and we liked the wake, so I dont know if I want much more than a 440 on the back, problem is storage again, I dont know where to place weight in the front, that why I thought 2 150 plus driver/observer was the way to go.
Old     (showtime)      Join Date: Nov 2005       09-17-2007, 10:57 AM Reply   
get a few lbs of lead.... if nothing else a few dunbells will work
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 11:50 AM Reply   
you should have some kind of a bow access and you can fit at least 2 150s up there
Old     (nauty)      Join Date: Feb 2004       09-17-2007, 12:09 PM Reply   
I've always used this calculation when figuring the weight for a rectangle sac:

Ex. - 20x20x50x.036 = 720 lbs.

This calculation seems to be pretty close. Granted, it is only if the sac were filled exactly to the seams and it remained a perfect rectangle. However, like Adam said, my sacs also look more round then rectangular when they are filled all the way up. They stretch a little past the seams, so they will hold a little more than the calculated weight.
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 12:16 PM Reply   
I've never been able to fill mine all the way up because of the air pockets that get in there but I think that also keeps me from busting the bags
Old     (showtime)      Join Date: Nov 2005       09-17-2007, 1:24 PM Reply   
j -- let the air out or add a vent line --
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 1:55 PM Reply   
hmm wow I never even thought of a vent line damn I need to pull my head out of my ass more often
Old     (luchog)      Join Date: Jun 2002       09-17-2007, 2:36 PM Reply   
would that vent line plug on the other filling port?
Old     (lovin_the_wake)      Join Date: Jul 2007       09-17-2007, 2:41 PM Reply   
That's what I'm gonna try


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