David wrote:

I've got an old boat very much like this one:

http://www.warnersdock.com/images/Fr...fores15yel.JPG

that I'm trying to make into a mini-houseboat,
sort of like a camper. I only expect to have
about 200-300 pounds in the roof and walls,
and the roof will only be about 48" above
the gunwale. The hull will weigh a *lot* more
than what is added, so shouldn't that alone
be enough to keep it from being top heavy
and flipping over? If I add weight, like a
couple hundred pound bags of sand down
in the hull to counterballance, shouldn't
that take care of it? If so, should they
be placed on the sides, or in the center?


Well, boat stability is a complex subject. Here
some loose thoughts (before my first cup of coffee,
so no guarantees that I get all things right. I'm
sure others will jump on the chance to correct my
worst blunders - maybe I can learn something too!)

I would recommend you read a bit more about it. The
"Nature of Boats" by Dave Gerr is a good all-around
introduction, and doesn't get too technical. Many
other books work as well.

When you add weight and windage high up, you reduce
the stability. That increases the risk that your boat
will capsize in high seas (or even in flat water). It
also changes the way the boat rolls in seas, which may
make it more or less comfortable to be inside.

You are right, you can compensate by adding weight (as
low as possible!). This will make the boat sit deeper
in the water. Maybe too deep.

It all depends what you are going to use the boat for.
With a solid house on it, and enough ballast to keep her
upright, she will not want to plane. If you can live with
moving her at hull speed (a few knots), you can make do
with *much* smaller engine, and use the weight difference
as ballast. If you do not plan to move her around much at
all, then you can afford to load he a bit deeper, as the
sailing characteristics won't matter.

You can probably get an idea of the stability by doing a
simple roll period test. Rock the boat sideways, it will
settle on some speed of rolling from side to side. If
this roll time (in seconds) is the same as your overall
beam (in meters), (or 10% more), you have what Gerr
describes a good initial stability. If the boar rolls
much faster, you have more (initial) stability for
good comfort - that would be good news for you. If,
after all the construction, your boat rolls slower than
this, you may want to add some ballast.

In any case, keep the structure as low as you can manage,
and build it light! It looks like a small boat to begin
with, so don't even try to build a large house on her.
Keep it small and simple, and might work well.

Remember that a boat is always a compromise. A heavily
modified boat is a compromise between the original
compromiseand a your new ideas. It is your own boat,
and it is you who will have to live with it. Don't let
us naysayers put you completely off. But watch out, it
is also your own life you are risking, and possibly your
dear ones as well!

Good luck!

-Heikki