[Public-List] Holding tank vent

[brooks.glenn at comcast.net]

John, Great question... 


I dont think one vent line can back fill. the ambient air pressure in a closed tank system will prevent sea water intrusion through the vent line. Certainly did (prevent backfill) in my fishing boat in Alaska, which had a fuel tank vent at the bow. 


But with two 3/4" vent lines, the back pressure in the tank is neutralized and it seems like water could then enter, particularly when bashing to windward. One of the vent lines would be in a gravity feed position above the tank, so conceivably could the tank be in a position then to syphon water back into the boat through the commode? A n asty result to say the least... 


Maybe its worth putting a loop or vent or one way valve in each line to prevent back fill? aha, another reason to buy yet more hardware, a back stop valve between tank and commode. 


Is this my wildly over active imagination, or has anybody with two vent lines ever experience this? 


G lenn 
Dolce 318 

----- Original Message -----
From: "John Riley" <jriley at dsbscience.com> 
To: "Alberg 30 Public List -- open to all" <public-list at lists.alberg30.org> 
Sent: Monday, March 4, 2013 7:11:20 PM 
Subject: Re: [Public-List] Holding tank vent 

I asked this same question before drilling holes in my bow...or a 
similar one. What about waves in rough seas. 

I believe it was Mike Lehman that provided the obvious (once I thought 
about it) answer that George gives below...any water intrusion into the 
vent merely would go into the tank itself. 

At worse, it would help clean the tank out. 

I should add that I have a hand operated pump to pump the tank overboard 
offshore where it is legal to do so vice a head that can be pumped 
directly overboard. Taking sea water into the holding tank, which has 
never happened to us by the way, would only mean having to pump the tank 
empty a bit earlier than I planned. 

Regards, 

JR 


On 03/04/2013 04:57 PM, George Dinwiddie wrote: 
> Randy, 
> 
> On 3/4/13 4:00 PM, Randy Katz wrote: 
>> Greetings, Glen, 
>> 
>> I ran my vent tubes (2 of them) from the holding tank (in the same 
>> location 
>> as yours) through the bulkhead and up into the dorade over the 
>> toilet. No 
>> bad smells fliter back to the cockpit. The photo in the A30 
>> collection of 
>> ideas and photos shows vents at the bow, forward-- what about water 
>> coming 
>> down them during heeling? 
> 
> I've never had any trouble with water in the vents. I used standard 
> right-angle fuel vents (one was already there from the Mansfield TDX 
> installation done by the previous owner) and have them angled down and 
> aft. The bow is pretty high, and the cockpit goes under long before 
> the bow does due to heeling. Sometimes the cockpit dips when crashing 
> through waves, but the angle of the vents doesn't seem to pick up any 
> water. 
> 
> Of course, if they did, it would likely just drain into the tank. I 
> worked hard to eliminate any low spots where water could collect. 
> 
> Putting both vents in the same place would seem to me to be 
> ineffective for setting up a flow of fresh air. I've had lots of 
> conversations with Peggie Hall (who has sailed on Calypso, BTW) and 
> she recommended the strategy of venting on each side of the hull. 
> 
> - George 
> 

-- 
John S. Riley 
S/V Gaelic Sea 
1972 Alberg 30 #521 

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