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Old 09-05-2016, 07:58 PM   #1
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GB 36 Holding tank

I have a GB 36 Classic. 1984. Does anyone know the capacity of the holding tank?
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Old 09-05-2016, 08:18 PM   #2
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The only thing I can add is our GB42 has a 48-gallon holding tank.

Our tank is under our Onan genset. Try to measure your holding tank in feet (and decimals of feet) and multiply the length x depth x height. Multiply that product (cubic feet) by 7.48 gallons per cubic foot and subtract about 5% for fittings and tubes inside the tank. Voila!
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Old 09-05-2016, 08:28 PM   #3
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It's possible the tank may not be original. But if you have access to it, determining the capacity is easy: L x Wx H in inches divided by 231 (cu. inches/gal). You'll likely get a number something like 26.743. Because you're measuring outside dimensions, and because you can't fill every single cu. inch in a tank without overflowing it, the usable capacity would be 25 gallons.

If the tank isn't rectangular, you're gonna need some additional math skills to arrive at the number of cubic inches.
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Old 09-06-2016, 02:18 PM   #4
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My 36 has a centerline tank between the engines of 80 gallons
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Old 09-06-2016, 02:46 PM   #5
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It's possible the tank may not be original. But if you have access to it, determining the capacity is easy: L x Wx H in inches divided by 231 (cu. inches/gal). You'll likely get a number something like 26.743. Because you're measuring outside dimensions, and because you can't fill every single cu. inch in a tank without overflowing it, the usable capacity would be 25 gallons.

If the tank isn't rectangular, you're gonna need some additional math skills to arrive at the number of cubic inches.

And then sometimes the fitting placement reduces capacity even more.

Our nominally 40-gallon holding tank will only take about 25 no-kidding "measured" gallons, partly because it would only handle about 37 anyway (from dimensional calculations), and then partly because of inlet and vent hose fitting locations on the side of the tank.

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Old 09-06-2016, 04:04 PM   #6
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What location is it in? Is it for the forward or aft head?
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Old 09-11-2016, 04:11 PM   #7
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Our 1990 - 36 ft CL has the original holding tank - owners manual saids it's 40 gals - I have no reason to think otherwise - it's located on the starboard side in front of the 200 gal fuel tank -
- for what it's worth, I had to change out all of the brass/copper plumbing including the pick up tube and the aft head discharge elbow - they were " gone " - a real s**ty job -
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Old 09-11-2016, 04:51 PM   #8
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Holding tank

Thanks. That sounds like my set up. I think I may have to do the same thing. I have a metal manifold on the top where the discharge and vent hoses attach. What I don't know is whether the pick up tube inside the tank is metal or rubber. I suspect it's rubber and has partially rotted off since I seem to have to pump the tank more often than I should so it's probably not emptying completely. I'm thinking of just replacing the tank and all the plumbing and then providing a fresh water feed to the heads. Then I could glass the two head through hulls over and eliminate the salt water smell and get the added safety benefit of two less through hulls.
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Old 09-11-2016, 05:05 PM   #9
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Metal is far more likely to have corroded through than rubber, but the pickup tube is far more likely to be PVC than rubber if it's not metal. If you had/have any metal fittings in the system, I'd bet it's metal too. Urine is the culprit. It's so corrosive that it' typically turns any metal holding tank into a colander in an average of about 10 years. Metal pickup tubes can develop holes, and even fall completely off in less time than that. And it doesn't matter in either case whether the toilet uses salt or fresh water.
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Old 09-11-2016, 07:28 PM   #10
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Bryant - note Peggy's comments above, she's the " guru " of marine sanatation systems - if you don't have her book - " the whole story of poop ",( or something like that ) - you and every boat owner with a marine sanatation system should have one in your boat library -
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Old 09-11-2016, 07:46 PM   #11
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...her book - " the whole story of poop ",( or something like that )...

Close (and thanks for the plug, Tracie!) "Get Rid of Boat Odors" A comprehensive guide to marine sanitation systems and other sources of aggravation and odor (2nd edition updated, revised and expanded) is available from Amazon in both hard copy and kindle.

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http://www.amazon.com/New-Get-Rid-Bo...dp/1892399784/
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Old 09-11-2016, 08:18 PM   #12
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Thanks. That sounds like my set up. I think I may have to do the same thing. I have a metal manifold on the top where the discharge and vent hoses attach. What I don't know is whether the pick up tube inside the tank is metal or rubber. I suspect it's rubber and has partially rotted off since I seem to have to pump the tank more often than I should so it's probably not emptying completely. I'm thinking of just replacing the tank and all the plumbing and then providing a fresh water feed to the heads. Then I could glass the two head through hulls over and eliminate the salt water smell and get the added safety benefit of two less through hulls.
Years ago I wanted to check if our holding tank was emptying properly. I knew by the external measurements that the tank was between 45 and 50 gallons. The water hose I was using, at the time, filled a five-gallon bucket at a rate of about 12 gallons per minute. It took about four minutes to fill our holding tank. So I was satisfied that our drop tube was intact and free of holes. A couple of years later I couldn't stand it any longer. I followed Peggy's suggestion. I swapped the metal tube for one made of PVC!
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Old 09-12-2016, 09:57 AM   #13
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Yep, if you can't measure it on the outside, empty it and then see how much water it takes to fill it. Disconnect any vent filter before you do this so you don't get it wet and ruin it.


Of course you could fill it and then measure what you pump out but that can be a bit messy.
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