MacGregorSailors.com

I too performed the glass bottle test, with an empty wine bottle...I have plenty of those around!

I tried both vertical & horizontal, reasoning that weight to surface area applied, horizontal would best represent a boat hull. The results were nearly identical, adding water ballast to the level where the bottle floated when empty, approx. half it's volume, the bottle floated lower - significantly (it sank)! Predictable, considering the weight of the glass bottle.

Comparing the glass bottle results (sinks) to the results of the same size plastic water bottle (floats same) & to the Mac (marginal difference), I reason that a necessary variable to consider is the weight of the hull material to the volume it is displacing.

After all this talk, I forget what it is that we are testing for!!

The ballast tank of your Mac is in the very bottom of the hull. When it is empty it is occupied by air, so its volume is added to the flotation of the hull - but the weight of the boat forces the hull deep enough that its "empty volume" is partially below the waterline. When you fill the ballast tank, its volume is no longer helping to float the boat, so the hull is pushed down deeper...

The effect may be the same, but I disagree with the reasoning, that the empty ballast tank is adding flotation to the hull. It is more that the configuration of the hull, in the water that it is displacing, is allowing it to float.

Consider a simple open steel hull...a hole in the water surrounded by steel... You say that the air in and above the hull is giving it flotation. Now take that same steel hull, flatten it out, and put it in the water. Does the air above it, acting upon the same surface area, aid in it's flotation? No... it sinks!

An interesting discussion indeed!

I am starting to agree with everyone as it becomes clear to me that we are all saying just about the same thing, just in different ways, and the disagreement is in most part theoretical, not in practical realities.

More fuel for the fire, what if we had two (or more) ballast tanks instead of just one? Would it be useful to maybe fill the one in the front and leave the back empty? Vice-versa? Being able to adjust the trim could certainly have an impact on the handling and sailing characteristics of the boat.

As for me, I'm not gonna worry about having to pump water out of the ballast tank to float off a sand bar. I'll just go over the side to lighten the load!

Russell

hoaglandr wrote:More fuel for the fire, what if we had two (or more) ballast tanks instead of just one? Would it be useful to maybe fill the one in the front and leave the back empty? Vice-versa? Being able to adjust the trim could certainly have an impact on the handling and sailing characteristics of the boat.
Russell

Good idea. There are performance sailboats that transfer water ballast between port & starboard tanks. Some members here remark about placing drinking water tanks under the V-berth and it's favorable affect on fore/aft trim. When I place my heavy items about the boat - blue cube water tanks / spare anchor / etc... it has a noticable effect on adjust for listing at anchor.

K9Kampers wrote:Consider a simple open steel hull...a hole in the water surrounded by steel... You say that the air in and above the hull is giving it flotation. Now take that same steel hull, flatten it out, and put it in the water. Does the air above it, acting upon the same surface area, aid in it's flotation? No... it sinks!

The quote above proves the point that the air inside the hull is providing positive flotation!

Consider - An empty hull displaces water equal to its own weight. If the volume of the water displaced is less than the volume of the hull, it sinks. (We're talking density here.) If the volume is greater, it floats. Because the hull is hollow, whatever it contains inside either adds to it's density or detracts from it. If it was filled with water (neutral bouyancy) or flattened out, the steel hull would sink because steel is denser than water. If the hull is filled with air (or partially filled with styrofoam ), it floats because air has positive bouyancy, it is less dense than water.

One thing that is causing some confusions is the issue of whether air is sealed in. The hull of the boat is providing the seal. It holds the water out and lets the air in. That's why we don't like holes below the waterline of the boat. The top is open but the only thing it is letting in (usually) is air, so it doesn't need to be sealed. Air, because it is a light gas, is easily displaced by a heavier liquid. We just have to limit the opportunities for water to displace the air. (Closing the vent of an empty ballast tank would do that but that would only help if the boat was filling up with water!)

Think of it this way, what if the air was solid? Like styrofoam but even lighter. Would you still be concerned about it being sealed? It's value is not that it pulls up on the boat, but that it is better than the alternative, water (plus we can breath it!). Even better would be helium or even a vacuum, but they would have to be sealed in at the top to keep the air out! The only reason you would need to completely seal in the air would be to stop it from being displaced by something else.

Russell

It's pretty obvious that the presence of water ballast in the boat will make the boat float lower in the water than without.

If you brought another 162 gallons of water on board, the boat would sit even lower, no matter where you put it, as long as it was bearing on the hull. Keep on doing that, and the boat will be swamped eventually.

ALX357 wrote:It's pretty obvious that the presence of water ballast in the boat will make the boat float lower in the water than without.

If you brought another 162 gallons of water on board, the boat would sit even lower, no matter where you put it, as long as it was bearing on the hull. Keep on doing that, and the boat will be swamped eventually.

From the above posts, it doesn't seem "pretty obvious" to all of us.

Rather than continue to argue my point (and Archimedes'), I shall treat this as a "Back Room political disputation" and continue to sail my boat my way. If your boat doesn't apear to ride lower in the water with the ballast filled -because you haven't subscribed to my viewpoint, much less even tested it by measurements - that's not my problem, and obviously it's your non-problem (even non-existent datum). I will continue to fill and cap-off and close-off my ballast tank, and sail as if it matters that I handle my boat's buoyancy this way, just as Roger MacGregor and Archimedes have persuaded me that it does matter.

Pre-post edit - I was writing this and when I came back, saw what Rick posted - Rick, the boat DOES sink lower in the water because of the water ballast - or lead ballast - or people on board - or ANY weight you take on board...
Please let me try to explain one more time - this is actually pretty simple, but it get's mixed up by too much intuition about the air vs. water, and above/below the waterline... The experiments with bottles and such are GREAT! Don't abandon them...

This is what I was writing:

I actually think this whole displacement thing is a really really important thing to understand when you're in a boat! Please allow me to point back to the comments about the air/water/displacement issues.

Rick - please try your experiment again, but this time, try this...
1. Fill the bottle as before to the line where it initially sank, the "shake" line - then weigh that water so you know how much it is in WEIGHT (not volume).
2. Instead of putting water in the milk bottle up to the "shake" line, this time use...rocks, lead fishing weights, steel balls, just something more dense...equalling the weight you found in 1 above - the weight of that amount of water (for example, six ounces of lead weights). Don't fill it up to the "shake" line with weights, only put in as many pieces as equal the weight of that water you weighed in #1. Equal weight is what we're talking about, whether it's rocks or water...same amount of weight.
3. You will find that the bottle again sinks to the same place, the "shake" line...even though you didn't have to fill the rocks up to the "shake" line...so it's NOT because you displaced the air that was inside the bottle and the weight of the bottle then mattered, but because the WEIGHT of the bottle plus whatever is inside displaces that WEIGHT of the water on the outside of the bottle.

Forget that the ballast tanks are above or below the waterline - that is confusing the issue and truly doesn't matter... trust me on this one.

(The steel example is a good one I'd like to comment on also, but one at a time.)

Let's do K9's empty 1-liter plastic water bottle test again...I like it, it's a good tool we can use to discuss this! (please adjust the 1-liter in my notes below to 20 ounces or whatever size the bottle actually is...) To start with, let's assume the bottle doesn't weigh anything itself - it's so light we're going to ignore it. I realize it has a slight amount of weight, but for all of this discussion it will help to have it NOT weigh anything...

I'm going to start from the standpoint of displacement, which is volume...the water bottle displaces (takes the place of) one liter of water (assuming it doesn't crush when put fully under water). If you take that bottle and force it all the way under water, it will displace one liter of water on the outside. Assuming it stays rigid and doesn't crush, then it doesn't matter what it is filled with, helium, air, rocks, sand, plutonium...it still displaces one liter (volume!) of water when fully submerged. What gets displaced is volume...the amount of water equal to the VOLUME that you put under water. - the shape of the thing determines the total volume - it's a one-liter bottle, so it displaces one liter!

One liter of water is about 1/4 of a gallon, and water weighs just over 8 pounds per gallon (a one-gallon jug of water weighs just over 8 pounds - that's a constant never changing thing...sea water weighs slightly more). So the one liter of water that is being displaced by the plastic bottle weighs about 2 pounds...about 1/4 of the 8 pounds...let's just call it 2 pounds to keep this simple. That is how much buoyancy you get - equal to the displaced water - you get 2 pounds of "lift" or "buoyancy" or "flotation effect" by displacing a one-liter bottle's volume of water (by pushing the bottle all the way under water). One liter of volume of water weighs about 2 pounds.

So if you put inside that plastic bottle 2.1 pounds of rocks, 2.1 pounds of plutonium, 2.1 pounds of feathers...I didn't say fill the bottle up, just put that much WEIGHT into it (which is more than the weight of the displaced water) and it will sink. It doesn't matter if the bottle has half rocks, half air in it, if it weighs more than the 2 pounds of displaced water, it will sink, regardless of what is inside!!

If you fill it with only ONE pound of rocks, or whatever, then the bottle will weigh one pound, but can displace up to 2 pounds, and thus you will have leftover ONE pound of lift, buoyancy, or flotation. If your crew, gear, and supplies all weigh 1/2 pound, then you can put them on your "bottle boat" and you'll have 1/2 pound left that still allows the boat to float. What actually happens is the bottle only sinks half-way...as you add more weight, but less than 2 pounds, it sinks lower, until the VOLUME of water outside displaced has the same amount of WEIGHT as whatever is in the bottle... 1-1/2 pounds of weight in the bottle will sink the bottle 3/4 of the way (3/4 of 2 pounds is 1-1/2 pounds).

So what happens if you fill the plastic bottle with...WATER? The reason this seems confusing is because it's water and water we're comparing, but don't let that change anything!! You STILL have to fill the bottle with as much WEIGHT in water (on the inside) as the WEIGHT of the displaced volume of water (on the outside)...it just happens to be exactly the same amount, so you have to fill the water bottle up completely with water. BUT (and it's a big but!) you can't get the inside of the bottle to weigh MORE than the displaced water by putting water in it (the way you could by putting rocks in it)!! Because water weighs the same as water, if you fill the bottle with water, it will weigh THE SAME as the water it displaced, but not more, so the filled plastic bottle doesn't really sink. I think this is what is so confusing!

So the VOLUME of the Macgregor boat (like your water bottle laying on its side) is large and has the potential to displace a large amount of water. It displaces a volume of water that weighs as much as the WEIGHT OF THE BOAT! If the boat weighs 3500 pounds, it will displace in volume the equivalent of 3500 pounds of water (about 437 gallons in a rough calculation). The boat will sink down in the water until you have displaced about 437 gallons (437 times 8 pounds per gallon is 3496 pounds). When you add the ballast - STOP! - let's say you add blocks of LEAD instead of opening up the water ballast valve - to achieve 1300 pounds of ballast - you bring aboard 1300 pounds of lead and lay it on the floor...you've just added 1300 pounds of weight, and the boat will sink further until an additional 162 gallons of water has been displaced outside (162 gallons times 8 pounds per gallon is about 1296 pounds). So if you use water instead of lead, and you add 1300 pounds of water to the inside of the boat in the form of water ballast, it has the same effect as 1300 pounds of lead - it weighs the boat down by 1300 pounds and the boat has to sink in the water by an additional 162 gallons of displacement. It wouldn't matter if you had brought 1300 pounds of feathers on board - same effect. It doesn't matter what the weight is made up of, it only matters that it is INSIDE the boat and adding to the weight of the boat.

Where it is in the boat, above or below the waterline, attached to the floor or hanging from the ceiling, doesn't matter, only that it is bearing on the inside of the boat (as ALX said).

I figure not everything has to be electric. I built an automatic vent from a PVC check (flapper) valve. Put a piece of foam inside to close the valve as the water comes up. It sits on a matching flange sealed and screwed on in place of the plug. I'm sure I posted it in the mods, but if you can't find it there and are still interested let me know and I'll get back to you with specs/plans/pix.
Dave

I did see it Dave - good work!
They make them for water piping systems (and big ones for municipal water systems too - I sell those sometimes) called "air-release valves"...they are designed to let the air out of a pipe as the water fills it up, and then they have a float inside that rises when it fills with water and seals off the hole...just like what you made.

Rick Westlake wrote:...I will continue to fill and cap-off and close-off my ballast tank, and sail as if it matters that I handle my boat's buoyancy this way, just as Roger MacGregor and Archimedes have persuaded me that it does matter.

Well put, and 99.99% of the time, this is all we need! ...(to show that white hulls are faster than blue hulls!)

hoaglandr wrote:If the hull is filled with air (or partially filled with styrofoam ), it floats because air has positive bouyancy, it is less dense than water.

Respectfully...No, No, No! Read Vizwhiz's post starting with "Pre-post edit - I was writing this and when I came back..."

On your next outing, remove all of the factory foam from your boat. Does it then float any higher in the water because now there is more air the boat? Except for the difference in weight of the solid mass removed - the foam - NO!...OR...Fill the entire cabin with foam. Does it then float any higher in the water because of the addition of floatation foam? NO!...actually, it would ride lower because of the weight of the mass added - the foam!

Now if we were to apply the air & foam to objects under the water, that is something else...but that is not what we are talking about here.

vizwhiz wrote:This is what I was writing:
I actually think this whole displacement thing is a really really important thing to understand when you're in a boat! Please allow me to point back to the comments about the air/water/displacement issues.

Vizwhiz – thank you for posting a very eloquent and reasonable desription of the issue.
(I'm glad nobody resorted to cut & paste during this discussion.)

vizwhiz wrote: That is how much buoyancy you get - equal to the displaced water - you get 2 pounds of "lift" or "buoyancy" or "flotation effect" by displacing a one-liter bottle's volume of water (by pushing the bottle all the way under water). One liter of volume of water weighs about 2 pounds.

This is what I agree with describing "lift" or "buoyancy" or "flotation effect" in regards to displacement of boats at the water’s surface.

vizwhiz wrote: It doesn't matter what the weight is made up of, it only matters that it is INSIDE the boat and adding to the weight of the boat.

Where it is in the boat, above or below the waterline, attached to the floor or hanging from the ceiling, doesn't matter, only that it is bearing on the inside of the boat (as ALX said).

Agreed

Vizwhiz - Well said - I completely agree with you. I wish I was as eloquent!

K9 - My point was that the shape of the boat allows it to fill with air rather than water, which is a "positive" thing! As long as what is inside (or on the boat) weighs less than the water displaced, it floats! And yes, unless the styrofoam is submerged, it only adds weight to the boat, not flotation.

I've enjoyed the discussion!

Russell

Vizwhiz, thank you for your thorough treatment of displacement. I didn't perform your suggested experiments, though, and I only posted my photos to make my point about displacement and ballast - though I will add that an equivalent weight of lead shot, in the bottom of my milk bottle, would stabilize it in a mouth-up position when I float it in the sink.

This is important to our discussion of ballast, and especially water ballast, because ballast is in place to stabilize the boat, to keep the round sealed part down and the open populated part upright. And it's even more important in a sailboat, to counteract the pressure of the wind on the sails.

Water ballast is actually pretty poor stuff, as ballast goes, but it has this one very useful quality in a trailer-sailor ... it can be recycled, poured back into the sea or the lake, just by opening a valve or two. Thus we don't have to haul it down the road - which makes our boats lighter on the trailer, and less work for the tow vehicle to haul it - and it takes just a few minutes to refill the ballast tank with fresh local water at our next destination.

I have been thinking hard on the idea of installing a vent tube up to the anchor locker, as several folks have done; 99% of the reason I haven't done so is laziness. But there is that other 1%, when I go up forward and dig under the V-berth to plug the vent hole, and I can see and feel that the ballast tank is full. When I put that plug in place, I also know it's going to stay full, as I want it to stay.

Probably, if I were doing more "hurry-mode" motoring, I'd install a vent tube for my own convenience.

It's is a FACT that my ( with ballast vented thru hose that rises to the height of the deck in front of the anchor locker, and then turns back down to "T" into the anchor locker drain hose ) sails and performs the same whether the ballast vent hose ball valve I installed is open or shut. On any possible heel, the tank cannot drain out the open vent hose because the highest point of the hose is higher than the water level of the high-side ballast, even at its max lifted point. The water cannot drain out unless the boat were flat on its side, and then only a bit at at a time, until the level dropped below the centerline.

Open to the atmosphere or sealed shut, no difference while the boat is in any usable point of heeling. IF this is not obvious, keep thinking until the light comes on.

Archimedes findings are being misunderstood if interpreted otherwise.

But it is essential that the hose NOT be simply led up to the bottom of the anchor locker to join the drain there. It has to be led up to the highest point possible under the deck in front of the anchor locker where it is at the highest point possible, then turned back down to the level of the bottom of the locker where it can be joined by a T fitting to the anchor locker drain.

To do the vertical 180-degree turn, instead of trying to bend the hose into a hairpin turn up there under the deck, where it would probably collapse into a pinched bend, I put two elbows together to form an upside-down U, and hose clamped the hoses to that made-up fitting. The fitting is actually pushed all the way up to firmly touch the underside of the deck. The effect is an upside-down P-trap, however you make it up.

That keeps the rainwater from going into the locker, and thru the hose to the ballast tank, because even if the locker drain itself gets plugged and rainwater or seawater splashing fills the anchor locker, it will overflow and run off the deck before it reaches the higher P-trap at the bow. That also keeps the ballast tank from escaping thru the locker drain when the boat heels.