Maxprop wrote:
A cube of lead one inch per side will not necessarily float with a cube of
floatation material of the same size attached. Depending upon the type of
flotation material it might require more or less than a 1" cube to float the
lead cube.


Of course. The flotation has to be of sufficient volume &
density to bring the average specific gravity below 1.0

A point that is occasionally overlooked is that the
flotation also has to be structurally sound. I learned this
lesson in practice, trying to install positive flotation on
the cheap for an old racing class boat.



... "marketplace" and "Engineering" are usually two viewpoints in
conflict.



Engineers, fortunately or not, work for the same companies that also employ
the marketing gurus. While their philosophies may differ radically, the two
disciplines are not mutually exclusive.


Sure. Occasionally you see both talents combined in the same
guy. But just because a product won't sell profitably, that
doesn't mean it isn't possible or even practical.



From a more practical standpoint of a useful cruising boat, then you (as I
believe you were driving at above) all you need is a flotation volume
equal to the difference between the boat's volume of material and the
immersed volume needed to float that weight. I've worked out such figures
for a couple of production boats and the answer is that the volume of the
seat & berth cushions is pretty close to enough.


If I'm interpreting you correctly, that would allow a capsized vessel to
float with virtually nothing above the water level.


At minimum, yes. But that wouldn't serve much purpose other
than to make the recoverable after an accident, at which
point it would be worthless... no value to the crew, who
would still need a survival craft, and no value in the
marketplace. So that is not a good enough answer, which is
why I then said:



Of course, you need a safety margin, and that volume needs to be both
*secure* and also distributed in such a way that the boat floats in it's
proper attitude (ie not bow pointed down, or leaned over 45 degrees) & has
some stability.


. . . and at least some of the boat out of the water and able to support
the maximum allowable crew complement. Boston Whaler is renowned for this.


Definitely agreed.


I'd think a life raft would suffice if safety were the only consideration.

Depends. The whole boat is more desirable than a life raft.
Otherwise why have the boat, why not just cruise in the life
raft in the first place?

It's a bit more of a challenge to build a boat that would be
liveable and operable (even sailable) after severe flooding,
but it's certainly possible. And I think, for some types of
sailing, it's highly desirable.

I can't believe that there are still some centerboard racing
classes that are not self-rescuing; some don't even have
positive flotation. WTF are they thinking?



Show me where I suggested that it be mandated that all boats be required
to have positive flotation.


Easy there, Doug. I didn't say you did. That nanny comment was mine, and
intended as a gentle elbow to the ribs.


Ah so, got it now.


Heck, the Mumm 30 would be real easy to put positive flotation in. Not
much of a premium on cabin space, anyway.


The last Mumm 30 on which I crewed was owned by a sailor who bitterly
complained about the inability of his boat to carry adequate spares due to
the limited interior volume. Then again he believed that nothing short of a
dozen sails was minimal in order to be prepared for any sort of weather.
When I pointed out that those extra sails add lots of weight, he poo-poo'd
the idea. Of course he never finished all that well, either.


Sounds like he didn't have his priorities quite in order.
Well, it's his boat, his priviledge.

I think J-24s should have positive flotation. AFAIK the
Soling class now requires it (sinkings were fairly common
back in the day); not sure if the Etchells does. The 1D-35
and the new Farr 36 both have positive flotation. A Mumm 30?
A bit harder to sink but still possible...




The bottom line is that positive flotation is *definitely* possible... as
I said, all you need is to fill the boat with foam up to the static
waterline, and put your cabin floor over that. Or apply that same volume
of foam to a carefully distributed set of unused voids & crannies.


Or at least part of that flotation foam could be used as hull stiffening,
ala Boston Whaler. My sailboat has an Airex foam core between the hull
laminates. It's not particularly thick, but it does add a substantial
amount of rigidity, and the builder claimed it even provided enough
flotation effect that it wouldn't take a lot of additional flotation
material or air bags to make the boat float in event of capsize. Not that I
exactly care one way or the other.


True.
Some people hate foam core, though.



Is it desirable? Depends. If I were going to do a lot of ocean crossing,
making passages along rough & rocky coasts, etc etc, I would want it.


Why? If making open-water passages, what would you achieve by keeping your
boat afloat. A capsized cruising sailboat a thousand miles from anywhere is
a total loss, floating or not.

Because if I went to the effort, the boat would not only
remain afloat but have a good positive range of stability &
reserve bouyancy... ie be operable and liveable after severe
flooding...


... Near shore may be
another matter entirely, but along the "rocky coast" I'm not sure there
would be any value either.

Not much sense in making sure theboat remains afloat if it's
going to be smashed to pieces, sure. But if all the pieces
still float, the people have a better chance IMHO.


... Here in
the Great Lakes, or on Pamlico Sound, a floating boat could be salvaged.


Shucks, in Pamlico Sound... or many places along the
Chesapeake... you could just wade ashore. The boat wouldn't
sink very far.





I asked Ted Gozzard about positive flotation at Strictly Sail in Chicago a
few years ago. He just laughed at me, as if I were some idiot. I asked him
to elucidate, to which he responded, "See those little cat boats over there?
(18' Marshall) That's what you want if you want positive flotation." I
said I wasn't personally interested in positive flotation, but was asking
the question hypothetically. He just laughed again and turned away. I'm
not quite sure how to interpret that, but it would appear that he regarded
positive flotation as a non-issue. FWIW.


For him, it almost certainly is. I'm not surprised he's a
bit of a reactionary (I mean, look at his boat designs) but
I'd be surprised if he didn't have a pretty good grip on the
practical issues involved. But then, talking to boat
designers at boat shows is often a futile endeavor...
they're there to sell boats.

DSK