Hehe, fun thread.
There's an episode of Gilligan's island where Gilligan decides to take matters into his own hands and builds himself a set of large bird wings out of feathers found on the beach. The skipper catches him flapping and hovering 15 ft. or so in the air - before declaring "Gilligan!! you can't fly!!"...(Gilligan) "I can't??"...(Skipper) "No!!!".......Not one to defy physics, Gilligan promptly plummets to the ground.
There are thousands of water-ballasted boats sailing every day including my 26M (which sails just fine on Lake Huron). There is a video of an M sailing in 12-15 foot waves in gale-force winds on the MacGregor website. The stability looks just fine.
But can all of this be? What about the "physics"? Dang, I just heard my boat sink.
Something's up - so let's take keel boat and slowly make a Mac....
Start with a keel-ballasted boat. Lead keel, heavy construction, lot's of nice woodwork, D/L of say 200. One myth about this boat is that the VCG (vertical center of gravity) is very, very low...as in below the COB (center of boyancy). That might actually be true in a IOR racer, but not in a typical performance cruising boat. All of the extra weight above the waterline raises the VCG well above the top of the keel. Still, it's reasonably low. Why doesn't it fall over, though? Why doesn't a row-boat fall over, for that matter?
The answer is hull-form. The row-boat's an easy one - the COB shifts so wildly with heeling that boat is said to have "feet". Even our keelboat has "feet" - it's just (usually) an elliptic shape, but the COB still shifts, such that the VCG needs to lift upward as the boat heels. Nature couldn't care less what's causing the VCG to be where it is, whether it be lead, iron, or whatever. That being said, there are a couple of things that do also factor in....1.) boat's that are more massive react less quickly to wind and waves because they have more intertia (that's a good thing), and 2.) if the overall boat is made out of heavy materials, you're gonna have to use lead, or iron - or something good-and-dense - in the bottom, in order to get the VCG respectably low. There's no getting around that.
If we take our keelboat and replace the lead with water, we have - well - a piece of crap. The VCG is probably now about 5 feet above the waterline and our vanishing stability around 60 degrees - (the amount of heel where it tips over). We could do a little better by using a lot more water - putting it in the bottom of the hull instead of our keel - maybe 20 times the volume of water that one could put in a keel. So we scrap the keel altogether and put the water in tanks in the bottom of the hull. FWIW - we'll use a righting-neutral center/daggerboard for sailing off and into the wind.
At this point, forget all of the smoke-and-mirrors about water-floating-in-water, bla bla bla. The water outside the boat does not analyze the contents of a floating container - it reacts to overall density, intertia, gravity-center, and container-form (COB).
Still, so far we're not doing very well with our water-ballasted boat. The VCG is too high for the hull-form. There are only two things left that we can do to make this a decent sailing boat. 1.) Lighten-it-up above the water-line and 2.) Reduce the dead-rise (i.e. flatten the hull-bottom, giving us a bit more "feet").
That's exactly what Roger MacGregor did. He retained a fairly large amount of fiberglass lay-up in the hull-bottom and filled it with ~1500 lbs of water (and some lead). He made the: topsides very thin, the deck of minimal thickness, interior furninshings scantily-clad, and topped it off with a light rig (but one that is still fairly decent for a boat with a 4000 lb displacement). BTW - People often think that the entire hull is as thin as the topsides - it's not. Like I say, the bottom is reasonably thick and heavy. FWIW - If I had to hit a rock, I'd rather it be a lightweight boat with a thick heavy bottom.
So in the end our new boat is lightweight for sure, has no keel, uses water-ballast - but - the VCG is back to a very decent position for sailing - one that gives a righting profile that's not that far out-of-line with other boats of the same displacement (watch Roger's video again). Three things are a bit different though - just so you don't get the idea that I think this boat is "all that".
1.) The Mac's are lightly rigged and really don't belong out in the conditions that you see on Roger's video (that's why it does 20MPH with the engine).
2.) The boat has a lower D/L (about 145), so again, not a bluewater boat - something that all (but a very few) owners get.
3.) The boat is slightly flatter in hull-form (dead-rise), so it's more tender in heavier waves - again, no big deal if you treat the weather with respect.
My M sails just fine - not quite as fast as some boats, faster than others. Either way, when I'm sailing, I'm not in a hurry anyway.
Happy sailing,
~Bob
On edit: I reponded to the thread before reading all of the posts
Looks like I was a little late to make a difference
The arguments over water-ballast have come up so many times - bit it's always interesting
