JimC wrote:


Jeff wrote:
JimC wrote:

And the other issue is that the water ballast extends all the way
from stem to stern. This can't be helping the pitching moment at
all.


Nope. Cross-sections #4 and #5 clearly show that the ballast tank has
narrowed toward the stern such that there is an insignificant volume
(and mass) toward the stern. Instead, by far the greatest volume (and
mass) is in the area near the mast. Thus, the water ballast tank, with
greatest mass located near the center of the boat, would help rather
than augment any pitching moment.

Yes, there is less ballast aft, but it is certainly not
"insignificant." You have a way twisting the truth around. You're
saying, "Yes, I lied about the ballast but it doesn't really matter."

And, as I said, with that large engine hanging off the stern there's a
huge amount of weight back there. In fact, with a 50Hp engine running
well over 200 pounds, or perhaps 8% of the displacement, that's like a
30 foot cruising boat carrying a 800 pound dinghy in davits. Now
try telling us that has no affect on the pitch moment of inertia.






Wrong again. it extends about 2/3rds, and the front and rear
portions of the tank taper to sharp end portions and are therefore
of little mass and no real consequence re the distribution of mass.

Not according to the published diagram:
http://www.macgregor26.com/drawings/drawings.htm

Its pretty clear from this that the ballast extends all the way
forward, and that in fact a substantial amount is forward of the
mast. You should really spend some time learning about your boat, Jim.


I never questioned whether there was a substantial amount forward of the
mast. Nevertheless, since the hull, and the tank, narrows toward the
bow,

Actually, it doesn't look like it narrows that quickly. In fact there
looks like there's a lot more water ballast at station 1 than anywhere
else. Since you keep denying the truth, we'll just keep posting the
diagram so that everyone can appreciate your sense of reality:

http://www.macgregor26.com/drawings/drawings.htm

and since the lever arm extending from the mast to the bow is
relatively short,

Gee, isn't the lever arm forward of the center of mass roughly half
the length? Actually, we want to use the center of buoyancy, which is
well aft, thus increasing the lever arm.

water ballast at the bow would have little effect on
the distribution of mass.

This is total nonsense. Most designers make an effort to keep the
fuel and water tanks as close as possible to the center of the boat.
My large water tank, for example is athartships at the center. Look
at any large Hatteras and many other powerboats and you'll find the
fuel tank is at the center.

You've been making a big deal about the water ballast, but now you're
claiming its not in the stern, its not in the middle (because that's
where the fixed ballast is), and the amount forward would have "little
effect." What are you Jim, some sort of lawyer?



Jeff, did you happen to take courses in geometry and logic in high
school or junior high? The reason I ask is that you obviously know
nothing about either subject.


Actually, I majored in Naval Architecture for two years before
switching to Physics. Then I worked for NASA for 6 years. Any more
questions?


Yes, several. - If you majored in physics and Naval Architecture, why do
you seem to know so little about them.

Right Jim. You sound like a total idiot saying one thing when the
drawing of your boat clearly shows you're lying.

And, what did you do for NASA. -

I was the senior programmer for the data analysis for the Einstein
X-ray Observatory. Everything from telemetry to navigation to image
processing.

Surely you weren't designing any boats for them,

No, but I did work for a year for a successful America's Cup defender.
However, my work was almost entirely in sail research. Does that
count?

I hope. (Incidentally,
I worked for NASA for 11 years, and was recently asked to do more work
for them.)

So how much engineering did you do?



- The fact that the water ballast tank in the Mac extends toward the
bow, forward of the mast, is not determinative of whether it extends
about 2/3rd the length of the boat.


It starts at the bow, and it ends at the stern. The diagram clearly
shows the water ballast running the entire length of the boat. If
anything, it looks that the tanks is deepest in the forward area. The
cross-section at the forward station under the hatch appears to be by
far the largest, indicting that a large portion of the water ballast
is forward.

Here's the diagram again:
http://www.macgregor26.com/drawings/drawings.htm
please tell us if there's any other way to interpret this?


The issue, of course, is whether the mass is substantially centered
or whether substantial mass exists at the bow and stern, which might
affect the distribution of mass throughout the length of the boat. Since
that's the substantive issue, why don't we concentrate on that one.

OK. The boat weighs 3700 lbs with 1150 lbs, or 31%, of that water
ballast. Now you just claimed that at sections 4 and 5 the water
ballast is "insignificant." And the diagram clearly shows that much
of the ballast tank at section 2 and 3 is actually occupied by the
fixed ballast. So in fact, a large portion of that 1150 pounds of
water is well forward. And while the boat obviously narrows at the
bow, since the ballast tank doesn't extend the entire width, there is
actually very little narrowing of the tank until you get to the last
two feet. And, since there isn't much of a bow overhang, its pretty
clear the there is a lot of mass well forward when the tank is full.




And with a 250 pound engine hanging of the stern, that's a lot of mass
in the extremities.


Nope. The hull is built to handle the weight of the motor. As well as
the weight of several adults in the cockpit.

Again, you sound like an idiot here, Jim. You should quit trying to
sound like an engineer. There was no claim that the stern was going
to fall off. The point is that boats are usually designed to minimize
mass in the extremities, and thus reduce the pitch moment of inertia.

By comparison, on my boat, which weighs triple yours, I use the
lightest possible ground tackle to save a hundred pounds or so from
the bows. There is no excess mass at all in the forward ten feet of
the boat.



As stated, the drawings clearly show that the ballast tank has narrowed
toward the stern such that there is an insignificant volume of ballast
water (and mass) toward the stern. Instead, by far the greatest volume
(and mass) is in the area near the mast.

No, the drawing clearly shows that there is even more water forward of
the mast. Have you even looked at the drawing? By claiming there's
little water ballast aft you're claiming there's even more forward of
the mast!

Thus, the water ballast tank,
with greatest mass located near the center of the boat, would help
rather than augment any pitching moment.

Sorry Jim, anyone can clearly see that you're lying. Let's have
another look at that drawing:

http://www.macgregor26.com/drawings/drawings.htm

Yup, it still shows the largest cross section of water ballast at
section 1, well forward of the the mast.

Sorry, Jim. The Republican approach of repeating the lie over and
over until someone thinks it must be true has been discredited.

Perhaps you can get someone else here to explain it to you.