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Ballast in order to be effective in keeping a boat upright must be kept below the waterline, the lower, the better. However, it seems that many of the plans for shoal draft boats like 'Fancy Free" and others do have ballast and some seem to keep their ballast inside the hull.
Buehler's Backyard Boatbuilding mentions non permanent ballast in the form of sandbags laid on the floor of the hull that can be discarded when not needed and relates the case of a boat grounded in shallow water. All the crew had to do was remove the ballast to free themselves.
Once free, they were able to fill the hull with rocks in order to sail back to England.
Now what I would like to do with my Weekender would be to add about 300 lbs of sandbags to the floor for only those extra heavy gusty days when the wind constantly changes directions.
Correct me if i'm wrong, but I think that the ballast would slow down the boats reactions to sudden heavy gusts giving me more time to sheet out, turn or lower sails. I know that ballast on the floor won't keep the boat upright but I hope it will slow down the reaction to excessive heeling.
the question is: How much flotation is required for 300 lbs of ballast in order for the boat to maintain positive flotation?

Your best bet is to keep the main sheet in your hand and let it pay out as soon as you feel a gust that heels you over until you get uncomfortable.  The whole process should take less than two seconds.  If you let the boat round up while paying out the main sheet, you are doubly protected, but you can over do it like I tend to and have to do a big correction after it is all over.  Letting the main spill wind is one of my favourite things to do in gusty conditions. 

I have been thinking about ballast for my weekender for a while now and if I ever do it, I will mount it on the bottom of the keel for a better righting moment.  When I knocked my weekender down to test it I found that it didn't take much of my ponderous weight to stand it up again.  About 50 lbs seems right to give me just enough righting moment to keep it from going turtle and that is all I want out of ballast. 

Al

This is an ongoing debate for some Weekender owners. Ballast can help some of the issues associated with the boat, but it has draw backs as well.

First of all, to support 300 pounds of ballast, you'll need 4.8 cubic feet of buoyancy. Yep, think about this for a second and picture a nearly 5' long 12" wide and 12" tall box, who's sole purpose is to offer floatation, for the 300 pound ballast. Now, think about where you'll fit this hunk of dead space in a Weekender. Of course, it can be divided up into smaller hunks, but the volume still needs to be there. If you use foam, you have to subtract the weight of it, from the amount of buoyancy it provides, which commonly is about 2 pounds per cubic foot.

Back to ballast, it's difficult to make comparisons between boats, in terms of how ballast is handled. Simply put, ballasted boats are designed to carry it, which usually means there's a little more belly rocker and/or beam, and/or other design considerations, so the result doesn't unnecessarily "burden" the boat. The term burden is an accurate one, as an unballasted boat that gets ballast will usually suffer, in regard to performance, rigging loads, load carrying capacity, being able to sail on her designed "lines", etc.

Weekender is best if kept light. This is often a problem for the home built boat, where lots of "additions" and substitutions have been made. At her designed 550 pounds, she's a little heavier then she needs to be, in terms of her water plane and internal volume. Most build her heavier, which just sinks her down lower than desirable. Ballast does this too and 300 pounds (a 54% increase in hull weight) is a huge amount of weight to burden a Weekender with. It will make the boat more comfortable when it gets rough, but it will also slow her down, increase rigging loads, decrease capacity, drop the transom even further into the water, make her a little sluggish in maneuvering, etc.

A 100 pound weight, usually placed right near the mast box, will offer some steadying of this seemingly "quirky" hull, without over burdening this little lass. You must remember this is 14' LWL boat, so capacity is limited at best and boats of this scale are normally quirky, to the novice skipper.

The location of ballast in this boat will be best as a hunk of lead on the bottom of the keel. Even in this location, it's benefit is dubious, mostly because it's not far enough from the CG, to be particularly effective as a righting arm. It will slow the roll moment a tad, but not much and only once you have significant angles of heel, which isn't the way you should sail a flat bottom boat of this shape.

Lastly, Weekender is a fair weather craft, so when things get rough, you should head for shore, possibly getting wet and taking a bashing in the building wind strengths. You're not going to improve her abilities in this regard, ballast or not, as it's a function of her shape and typical trim level when underway. Part of this is also the skipper being forced to sit in the very aft end of the cockpit, which lifts her bow to slapping waves and sinks her transom. A tiller would permit the skipper to sit farther forward, so he can adjust trim, keeping the transom clear and the bow "engaged".

In my case I have decided to add another boat to my fleet that can handle a blow.  The wife is not happy.  There is a pretty nice CP 16 for sale in town that I am looking at.  I lost a lot of days last summer because it was blowing like stink off Lake Superior.  I took my weekender out on one of those days and learned a valuable thing.  I'm not good enough to go out in steady 20mph yet.  I think the boat can handle it at least in the harbor, but I've got a ways to go.  I think this is going to work out for me real well.  I have my sailing canoe for going fast, my weekender for fun cruising, and a heavy boat for days the others might not handle. 

Al

I agree with Paul.  I put 260# around the compression post on the Vac Saralee to bring her nose down to her lines.  Oon your boat shifting the ballast cooler should be enough.  My ballest fixed the water line issue and some others bur the burdon was noticable with her fully loaded for a 5 day cruse.  I would not waste my time with it on a Wknder.  JUst my 3 cents worth.

To update Craig's ballast, it's nearly 8' long, 4.5" wide (mostly) and 8" tall. It has 6 rows of keel bolts, with the 4 center rows being doubled, for a total of 10 3/8" bolts. This seems like kind of small, but the breaking strength of a 3/8" SS 316L bolt is over twice the displacement of the boat, so each bolt is capable of holding up the twice the boat's weight.

If this ballast casting was divided into eight 1" thick horizontal plates of lead, the bolts will hold will sandwich the assembly of plates. Each plate will be about 225 pounds. This is still a bit of weight to manhandle around, so the plates could be done in halves about 4' long and 112 pounds each, which is easy enough to manipulate around, under the boat. They'd be stacked and the bolts inserted and cinched up. This might be the way to go, maybe casting the 8' long pieces, but cutting them in half as they come out of the mold, so they can be handled easier. Not the alternating bevels to help hold the plates in relative position.

This is the keel assembly and general profile of the structure, greatly reduced and slightly simplified. This ain't your momma's back yard build . . . The keel, deadwood, stem, hog and stern post are laminated assemblies, of mostly vertically oriented  1x6's.

Wow! That is a beautiful boat. I'd love to see a picture of it in the water.

I think that there are different use of ballast. One is to avoid capsizing and to balance sails pressure. Another is to correct the natural yacht trim or the trim involved by variable loads.
Concerning this last, I've found necessary to balance the outboard weight (25 kg) and  I do this with two anchors in the front bay.
I've also used to keep two 5 liters tanks filled with gravel and water in the front bay when I sail alone, while with guests I let them on the pier.
This is in order to avoid beating on chops and to increase upwind performances (this is only my feeling, may be I'm wrong).
Reasons for tanks size are the following:
1. It is easier to lift and place two 5 liters tank  filled with stones and water than one bigger twice.
2. I can place them one above the other far ahead against the stem, in order to increase balance effect with the same weight.
3. 5 liters tanks are free! I recycle those used for cars liquids (windshield cleaner and radiator anti-icing liquid).

Gianluigi

P.S. It is curious how cyclically someone wants to put ballast and increase weight and then someone else asks for floatation increase.

Yes Gianluigi, we call movable ballast "trim ballast" and Craig's boat (above) will likely have in the neighborhood of 500 lbs. (227 kg) of trimming ballast, in the form of ~14 lb. (6 kg) bricks (a few dozen). These will be moved around to trim the boat come launch day and also to offset natural trim changes, such as loading up for a month long cruise or running aground.

Ballast (fixed) affects stability and sail carrying power as you've noted. To improve floatation all you need are sealed chambers or some foam, though both of these methods rob interior volume and are only effective if the boat happens to be full of water. If on the other hand, you want more buoyancy in the stern (a common request on a Weekender for example), you need to alter the hull shape in the aft sections of the boat. This naturally is a much more difficult thing to do, but if you really wanted more, you could glue some foam to the outside of the aft planking, fairing it in smooth and probably 'glassing it in too. This would add buoyancy to the stern and if "V" shaped, would make a better "exit" for water flow too. This additional buoyancy would cause a Weekender to rotate around her CB, placing the pointy end of the boat, in a more desirable bow down position.