"DSK" wrote in message
...
... If conventional boats with heavy, weighted keels, particularly those
of heavy construction, had enough positive flotation to keep the boat
afloat, there would be little room left in the cabin.


Bull****.


Maxprop wrote:
You are correct, but I'd be interested to see the volume of flotation
material needed to compensate for that displacement of water. *The
volume of flotation material required to offset a given volume of water
is not necessarily same.*

I'm not sure what you mean, here.

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.


... Lear Siegler, the builder of O'Day boats, published a lengthy report
some years ago about why larger boats don't have positive flotation. It
was written from an engineering point of view and made sense to me at the
time, albeit I'm no engineer. Their point was essentially what Jim C was
claiming--loss of interior volume in a marketplace demanding more and
more interior space.

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

From an engineering standpoint, there is less than no reason at all why
*any* boat shouldn't have positive flotation. Just fill it all up with
foam.

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.


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.

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.

... They even explored the concept of flotation that could be inflated in
crisis, but cited cost and space requirements for even this more compact
system.

There have been two such systems on the market, both went out of business.
People won't pay enough for such a system... from a viewpoint of market
analysis, a failure. From a viewpoint of somebody who wants as much safety
as practical, and cares less about costs, it's a total success.

I suspect it has more to do with one's desire to save his boat from sinking.
I'd think a life raft would suffice if safety were the only consideration.
I recall such aftermarket flotation systems, incidentally. Older Snipes
occasionally used air bags.


People buy cheap stuff. Why do think Wal-Mart does so well?

Probably the same reason MacGregor sells lots of boats.

... So I'm not quite sure Jim is wrong.

From a marketing standpoint, sure. MacGregor can only afford to offer
positive floation because it's partially installed anyway by their
building method... and their foam is the cheap stuff.



... Can you provide some documentation to the contrary, beyond just your
opinion?


Umm, show me a boat that doesn't float to start with, and I'll show you
one that probably can't have positive flotation.





Positive flotation probably wouldn't be offered by manufacturers
voluntarily.

It already is, by several. Sadler & Etap spring to mind.

I wasn't aware of that. Do their boats all have pos. flotation? I don't
know much about either mfr.--are they higher-end boats?

... It would most likely be the result of a government requirement
(there's that nanny state again, Doug), and it would have to be applied
to all boats, regardless of design.

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.

So according to your last paragraph, such a ruling might eliminate a
whole class of boats. Small class racers like the Mumm 30 come to mind.


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.

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.


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. Unless you can bail the boat out and sail it
subsequently there is little value in keeping at afloat. Near shore may be
another matter entirely, but along the "rocky coast" I'm not sure there
would be any value either. A liferaft makes more sense to me--it will move
along relatively well with the prevailing winds and currents. It will also
be able to stay afloat in rough seas, where the flooded, low floating boat
would simply take monstrous waves over the deck until it breaks up. Here in
the Great Lakes, or on Pamlico Sound, a floating boat could be salvaged.

There are tremendous advantages in a boat that just plain will not ever
sink. It's possible that I would make it a high enough priority to put in
myself. Do I expect anybody else to? Not really, especially the people who
rave about the advantages of Wal-Mart type boats.

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.

Max