On Thu, 03 Jul 2008 22:27:51 -0400, Matt Colie
wrote:

Every boat has the center of gravity above the center of buoyancy.

Why? Ever hear of heavy ballast keels? If the superstructure is heavy
that's one thing, but it doesn't have to be.

Casady

On Thu, 03 Jul 2008 14:34:39 -0100, David dh@. 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?


You probably want to go over to the rec.boats.cruising group and give
Roger Long a shout. I doubt that he will be prepared to tell you
whether your boat is a camper but he probably would get you a
reference to some rough and ready "righting moment" calculations which
you could use to figure out the answer to your question.


Bruce-in-Bangkok
(correct Address is bpaige125atgmaildotcom)

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

On 2008-07-03 11:34:39 -0400, David dh@. said:

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?

Judging from friends' boats of about that configuration, I'd suggest
something quite a bit lighter: a light hard roof and tarps, screen and
plastic for removable "walls", a pop-top camper of sorts.

That said, a friend was for a time a "slum landlord" of a number of
"floating structures" in Key West that were about what I hear you
saying. They (mostly) stayed afloat for some years and weathered some
nasty weather.

Have to say that the inhabitants were not thought well of, though, and
the "boats" uniformly disliked by all. Ugly doesn't begin to say it.

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

"David" dh@. wrote in message
...
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?

Pontoon boats make pretty good house boats. I checked out a 22' that the
owner had built up about 2/3s the deck space with house and a patio roof
over the open front. He used that sandwiched foam/aluminum patio roofing and
it came out real well. Kinda looked like a floating trailer, but it served
his purposes.