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Ballast Passive Vent & Considerations

A forum for discussing boat or trailer repairs or modifications that you have made or are considering.

Moderators: Paul S, tangentair, beene, Heath_Mod, Hamin' X, kmclemore, Catigale

Ballast Passive Vent & Considerations

Postby Frank C » Sat Sep 22, 2007 9:11 pm

For starters, I'm writing here about the 2000 26X, which is my Mac-of-choice. I'll leave it to other owners to attempt extending this logic to other models. Additionally ... as for all Mods, this is not a recommendation, or endorsement, or suggestion. All owner's change their boats at their own judgments, & risks. This is simply my personal analysis of why I changed my later model X-boat, what happens regarding the forward ballast vent, and why.

I long-ago tapped a threaded bronze hose nipple directly into my forward vent hole, and then routed a 1-inch OD reinforced vinyl hose forward & up to the bow. It is T'd into factory thru-hull at the anchor locker. I leave that hose simply open to the atmosphere.

Of course, the vent hose must always remain securely clamped to the bronze nipple, which remains thoroughly "threaded" into the fiberglass ballast tank. I've used this passive ballast venting for several years now. I just open the aft gate valve to flood the tank, then close the gate valve after 6 or 7 minutes. If I have crew aboard, I like to send some 'live ballast' to the bow pulpit, displacing a last couple of gallons of air from the forward tank, before closing the gate valve.

I elected venting to the anchor locker because it's the "high ground" and should absolutely prevent ballast tank contents from ever dribbling out that hose (later reconsidered, see following post). I also wanted to preclude any mud or rainwater from the anchor locker draining down into the vent hose. So, the vent hose was looped up & over the thru-hull, then T'd into the anchor locker hose. I realized just this week that these pix never got to the MacMods pages, so here they are with full description, and I'll add to MacMods later.

Frank C

Postby Frank C » Sat Sep 22, 2007 9:26 pm

I guess most are aware by now, I contend that it's virtually impossible to empty any significant portion of the ballast tank by leaving my passive vent hose uncapped, unplugged and un-valved. Further, using 20-20 hindsight, it was quite an effort to T-into that bow thru-hull, and I believe that the 'high ground' attempt was totally unnecessary.

It would have been much easier to tap my vent hose into the sink drain line in the X's head. Doing over again, that's where I'd vent the ballast tank, again with a generous upwards loop to prevent taking drain water back toward the ballast tank.

As for ballast escaping through the vent hose, I've contended before that the ballast tank is airlocked, but maybe not why I believe that. So here now, my TWO arguments that a ballast vent at the sink drain won't allow ballast tank drainage.

FIRST argument, just physics:
  • For ballast to escape the tank it must be replaced by air;
  • Given that air cannot enter through the gate valve (secure gaskets);
  • Air & water cannot 'pass each other' in the vent hose;
  • An "air bubble" must naturally rise to the highest portion of a drain-loop;
  • Ergo ... atmospheric pressure effectively traps ballast in the tank.
Frank C

Postby Frank C » Sat Sep 22, 2007 9:29 pm

SECOND argument . . . physics again, though with lots easier reading.

The waterline will actually migrate according to the center of buoyancy as heeling increases. My waterline estimates are simply circumferential ... so I'll grant that much illustrative license already taken. But my waterline SWAGs don't significantly change the placement of air ingress. So IMO, these drawings still illustrate the realities of ballast venting. To wit:


Given that air must displace ballast-lost, how's the air gonna do that??
Counterpoints invited.

I have a nifty photo manager (FastStone Image Viewer) that permits some clever manipulation of photos or drawings. One of its features is rotation to a precise number of degrees. Neato ... and, it's freeware~!!!
Frank C

Postby bastonjock » Sun Sep 23, 2007 1:44 pm

dunno frank but im itching to find out the answers from the more experienced 8)
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Postby ALX357 » Sun Sep 23, 2007 2:23 pm

Frank, I think you have the concept accurately, but I beleive that the actual location of the vent is more centered than your diagram indicates, and the outboard ballast tank extends further to each side than your diagram indicates. The ballast tank in your diagrams is not significantly above the level of the outside water, and would be ineffective in countering heeling until lifted higher.
These two changes in your diagrams would show that the ballast tank is lifted higher than the entire vent hose system as shown, (on the port tack) and would cause ballast loss to the point of equal levels, the ballast contents and the hose system's highest point, at any given angle of heel.

THERE is always SOME air in the ballast tank. Trapped air is not going to prevent ballast loss indefinitely, since that air would allow SOME expansion to let SOME ballast water escape thru the hose to the outside of the boat. Only a little at first, but each time the boat changes tacks and heels over the other way, more air would re-enter the ballast tank THRU THE OPEN VENT HOSE, to occupy the space and partial vacuum from the previous loss, and the increasing air would allow even more water loss the next and each following tack and change of heeling side.
:arrow: After a few hours of sailing, if changing tacks on a regular basis, you would definitely have a PARTIAL BALLAST situation, until the level of water was reached in the system where the heeled ballast level on both tacks was no higher than the highest part of the vent hose.
:arrow: That is why the anchor locker vent is the only safe viable option, because it lifts the hose higher than the highest point of ballast.
Note; you could take the hose up there, then turn it back down and into the liner and then back to a sink thru-hull, but why bother since it would take two hoses passing thru the bow in the cabin, and if you are already there at the anchor locker drain.
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Postby Frank C » Sun Sep 23, 2007 3:03 pm

It's not my drawing. It's from Roger's 2002 website. Agree that the forward ballast tank is probably wider ... the drawing shows only the aft tank. But I'm pretty sure the vent collar is accurately placed. Remember, the cabin sole is offset portwise from the mast post.

Alex, I understand your position. When heeled the weight of ballast might pull a negative pressure on the trapped air. Fact is, even absent an air-trap, the hull bottom itself could compress a tiny bit, oilcan. Even so, I would personally choose to route my vent hose high into the head compartment, then back down to the thruhull.

The problem w/the anchor locker ... it's a b~tch of a mod. You'll find that Roger connected the drain hose betw locker & thruhull with 5200. It prolly took me a couple of hours of reaching & stretching to completely remove hose remnants from both nipples. Finally, there's very little space between those drain nipples, so it's not easy to insert the T and still avoid creating an un-wanted P-trap.

If you're commited to the mod, you can easily test your theory. Connect your hose to the forward vent and zip-tie it up the compression post, leaving it unplugged. See if water enters the hose on port tack. I kinda doubt it, but the pudding IS the proof~!
Frank C

Postby Frank C » Sun Sep 23, 2007 3:23 pm

Regarding heeling angle versus righting moment . . . this from earlier thread:
... It is not until this "ballast" is lifted above the waterline that the weight of the water becomes true ballast. It is at this point that the boat will stiffen up.

It's a fallacy that water ballast only works as it rises above the waterline.

Proof: Imagine a bucket of water as a 'pseudo' ballast container. Submerge it into the lake ... as you lift it from the lake, you feel a gradual increase in its 'weight' as it emerges from hydrosphere to atmosphere - this is the effect that many ascribe to water ballast (erroneously). But, now carry that filled bucket down into your cabin, and gradually lower that bucket to the cabin sole, which IS below the waterline.
Does that bucket weigh less as you lower it closer to the sole .... ???

'Course not! Truth is, the entire ballast tank IS ALWAYS above the effective water level, since it's contained within a vessel - the hull. All mass contained within the boat's hull are suspended fully within the atmosphere, not the hydrosphere. All materials suspended within that hull, including the ballast tank & ballast water ... are acting within the atmospheric locus ... just like your bucket of water.
Frank C

Postby ALX357 » Sun Sep 23, 2007 4:59 pm

The ballast vent on my 2000X is almost on the centerline, just 5-1/2" to the portside.
I agree that a given ballast weight is always the same in the boat, no matter how it heels, which seems your point, but weight by itself will not act as RIGHTING ballast.
:arrow: That is why it is dangerous to have partial ballast. Half ballast will assume a low level in the tank, draining the high side ballast tube, due to gravity, and keep pulling straight down on the center of gravity of the boat, where its ballast weight will not help right the boat.
:arrow: The only way the water ballast can help right the boat is if it is kept, or lifted, beside and then above the center of gravity, in the side tubes, its leverage acting against the bouyancy, pulling the boat back to vertical.

:arrow: Another way to think of ballast .... In a fixed ballast boat, the balllast may be concentrated in the center, but of a deeper V shape hull. The ballast weight cannot shift its center of gravity, and when heeled, the ballast's center of gravity is lifted higher than its resting level, causing it to pull back down and give righting force to the boat.

:arrow: Same thing in a full ballast tank on the Mac. It's center of gravity will not significantly shift when it is full. Since the boat is not deep enough to have 1400# of water under its centerline sole, the side tanks are needed to spread the ballast around in the relatively flat hull. Heeling the boat, by necessity and definition, lifts the high side ballast tube above the water level.
Last edited by ALX357 on Sun Sep 23, 2007 5:54 pm, edited 1 time in total.
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Postby tangentair » Sun Sep 23, 2007 5:51 pm

Looking at this with a critical eye, the only point not addressed is a partially filled ballast tank (for deliberate or inadvertant reasons) and the effects of sloshing that could and does create high internal tank pressures. If there is no way for the ballast to gain momentum, it sounds right but with any momentum, water could be ejected on the pressure surge and air sucked back in on the return which could increase the problem. I sort of had in mind doing what you have done, venting to the anchor locker, but adding a long bicyle cable to a valve handle to open and close everything. I just had not figured out a remote way to tell when the tank was full yet.
on edit reread
I also agree with the ballast being in the hull not in the water but it is much closer to the center of gravity than a keel boats ballast (at least ours) so its effective arm is less.
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