New holding tank - design review questions

[deryky]

Hi Everyone:

I am currently working on adding a holding tank on my primary head, which currently discharges straight overboard. I have spent a lot of time perusing old posts here, and have read Peggy Hall's book. I still have a few remaining questions about the design, and I would like to solicit the collective expertise here.

I plan on using a stock tank, 760mm x 510mm x 310mm deep (it will be oriented flat on its side), and with its longest dimension oriented across the boat. This is probably the worst orientation for having the contents slosh around, but it is pretty much dictated by the space I have for it. My initial thought was to have it sloping slightly downwards towards the centerline. However, my research has convinced me I should be putting the vent outlet towards the centerline (to minimize sloshing into the hose), and consequently is seems prudent to mount the tank horizontally, so the contents of a nearly full tank don't flow into the event hose when the boat is flat. Is my logic sound here?

One aspect of the design I have been unable to find comprehensive advice on is the location of the tank connections, specifically on the outlet side. I am planning on having separate discharges for the deck pump-out and the sea discharge pump. What are the pros/cons of having outlets in the bottom of the tank, vs dip-tubes from the top? Intuitively the bottom outlets are simpler, but carry a greater risk of leakage, and odor permeation of the hoses due to standing effluent. The dip tubes seem to solve these issues, but is there a down-side other that a bit more expensive? I understand that they should touch the bottom, and be cut at a 25 deg angle. This should allow them to drain the tank as empty as the bottom hose, more or less.

My next question relates to the vent hose. I plan on having the vent outlet at the top of the tank, on the inboard end of the tank to minimize waste sloshing into the hose as mentioned previously (note this placement adds a couple of feet to the hose length). I have read Peggy Hall’s advice that the vent hose should be horizontal, or close to it. I don’t understand the rationale behind this (unless the tank gasses are heavier than air?). In fact, it seems like a vertical section near the tank would have the benefit of having and splashes easily draining back into the tank. I could have a fairly straight run of 4' or so, at something flatter that 45 degrees, but for a few reasons it would be better to have a slightly longer run with an couple of extra bends in the hose. My question is assuming a 5' to 6' run with a couple of modest bends in it, would I benefit significantly from increasing the size from 1" to 1.5"?

My last question regards the use of a y-valve between the head and the holding tank. Since I won’t have huge tank capacity (100 litres/26 US gal, plus half that again for the second head), my thoughts are to cheat a bit when capacity will be tight, by sending liquid-only flushes directly out. I appreciate this isn’t strictly legal in many cases, but it seems like a reasonable ethical compromise in situations where the tank is likely to fill before reaching a pump-out station. I would want this to share a single through-hull with the tank discharge, so 2 y-valves would be needed (I would only do this arrangement on 1 of the heads). Is this a common approach, or is the trend solidly towards running everything into the tank?

I welcome your thoughts and advice.

Deryk

[Three Sisters]

partial quote

Originally Posted by deryky

Hi Everyone:

My last question regards the use of a y-valve between the head and the holding tank. Since I won’t have huge tank capacity (100 litres/26 US gal, plus half that again for the second head), my thoughts are to cheat a bit when capacity will be tight, by sending liquid-only flushes directly out. I appreciate this isn’t strictly legal in many cases, but it seems like a reasonable ethical compromise in situations where the tank is likely to fill before reaching a pump-out station. I would want this to share a single through-hull with the tank discharge, so 2 y-valves would be needed (I would only do this arrangement on 1 of the heads). Is this a common approach, or is the trend solidly towards running everything into the tank?

I welcome your thoughts and advice.

Deryk

Lol. You're kidding right.

Apparently the laws instruct that overboard discharge is allowed when holding tanks are full...I think not. However you may find someone to sanction such activity and just play the dimwit while in front of the judge. That's kinda thin though, so hopefully you will receive a "reasonable ethical comprimise" of a fine only without jail time.

[peghall]

Have you checked the Ronco Plastics catalog for a tank? Ronco Plastics Marine Catalog They make TOP quality thick-walled water and waste tanks for a very reasonable price and has more than 400 shapes and sizes, over 100 of which are non-rectangular--which could open up locations for a tank that a "stock" won't fit--and they install fittings in the sizes and locations specified by the customer when they make the tank.

I have read Peggy Hall’s advice that the vent hose should be horizontal, or close to it. I don’t understand the rationale behind this (unless the tank gasses are heavier than air?).

That is the rationale behind it...and, the reason why the vent line should be as straight as possible. The optimal vent line is no longer than about 5', doesn't rise any more sharply than 45 degrees and is as straight as an arrow. However, that's not always possible, so keep it as short as possible and limit the number of bends to one gradual sweep. As for the vent line diameter, it should 1" even if it's only a foot long...bigger if the boat is large enough to make a larger thru-hull cosmetically acceptable.

What are the pros/cons of having outlets in the bottom of the tank, vs dip-tubes from the top?
All fittings on the top with diptubes are the best way to go IF you have the clearance above the tank for the fittings--at least 5" for a 1.5", at least 3" for a 1" vent.

As for plumbing the toilet discharge line to provide a choice of flushing into the tank or directly overboard...nothing wrong with way you want to do it.

If you want to brainstorm all this in a bit more detail than is practical in a forum, you're welcome to give me a shout via email...I'll be glad to help you sort it all out.

Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein
http://www.amazon.com/New-Get-Rid-Bo...dp/1892399784/