"Dave Doe" wrote in message
.
Indeed the water ballasting *only* has positive righting
moment
if the
boat is pushed over to where the centerline is above the
waterline (a
fair way!).


Bzzzzzzt! You are wrong Dave.

Nice evidence.

This is ASA, no evidence necassary.




PS: if yer don't believe me, fill a milk carton with water
and
play with
it in the bath


A scientific study if ever there was one.

It sure *is*.


Take a 5 gallon jerry jug, put 5 gallons of water in it. Will it
float? Take that same 5 g. jug and put the same weight ( approx.
40 lbs.) in of lead. Will it float? Which floats higher?
Conclude which is heavier? Make sure you wash behind your ears.

Scotty

In article .com,
says...

Dave Doe wrote:
In article ,
says...
says...
I have been getting interested in building this boat:
http://www.duckworksbbs.com/plans/jim/cormorant/

hmm, not the prettiest boat I've seen.

Of course, I have to finish the Tolman Skiff first, but a trailerable
water ballast sailboat that I can build to my needs is attractive to me
but I have some concerns

Trailerable can be a great way to go. The trade-offs are
that you can cruise much wider territory (VMG to windward
65mph) but have to deal with trailer maintenance & highway
traffic. Frankly I would not want to trailer a 32' boat,
IMHO the 'bigger is better' attitudes leads to some
expensive hassles when you try to take it on the road. My
wife and I cruised a 19-footer (which was water ballasted)
for some years. It was cozy but very practical & we spent
much less time on rigging & other petty hassles than friends
with bigger trailerables.


1. Do water ballasted boats tend to be exceptionally "tippy" before
they are filled?

No more so than the hull form dictates.

2. Could this be alleviated by including a few hundred lbs of lead
sheeting along the bottom?

Possibly but not definitely. Is the added weight no
trailering penalty for your tow vehicle? All the lead is
going to do for initial stability is to sink the hull lower
in the water, it will increase stability if you put the turn
of a hard bilge right to the waterline. Otherwise, it's not
likely to make a great improvement in stability.


Any other useful thoughts? Yes, I know you do not save money by
building boats but its become an obsession.


In that case, build the best one for your purpose.

How about this one?
http://www.dixdesign.com/26didi.htm

I've sailed one of these and they're fast & fun.

Dave Doe wrote:
I have a couple a questions...
1. How much water ballast (approx litres) does it take to equal say
200lbs of lead hanging 6' down (on say a swing keel)?

Umm, about 200# maybe?

Can't be, the 'moment' is entirely different for starters, as: the water
ballast is on the centerline, but the lead is 6' down. What the hell do
you think canting keels are all about!



2. How much righting moment does a water ballasted boat provide given
the ballast is below the waterline?

The same RM as an equal weight of any other material... say,
feathers, for example... providing the same center of gravity.


And a cheeky Q3? - How much does water weigh - in water?


Not cheeky at all, just makes you sound ignorant of physics.
How much does water weigh when it's inside the boat?

If it's below the waterline? - nothing! Sounds like *you're* ignorant
of physics - think! - why did the last round the world yacht race boats
pump water from side to side? A: because they could pump it to an area
well above the waterline *and* to one side of the boat, therefore
providing righting moment.

Water weighs the same above or below the waterline, and the same in or
out of water assuming the same gravitational potential.

No it doesn't!

The *mass* is the same.

Take a plastic bag of water and push if off a bench.

Take the same and do the same off a wee bench in a tub of water.

Sheesh!

--
Duncan
[Farr 7500 'Hi-Time' http://hitime.no-ip.info]

"Dave Doe" wrote in message .

Water weighs the same above or below the waterline, and the
same in or
out of water assuming the same gravitational potential.

No it doesn't!

The *mass* is the same.

Take a plastic bag of water and push if off a bench.

Take the same and do the same off a wee bench in a tub of
water.



Dave, the water in a ballast tank is KNOT IN the water.

Sheesh!!!!

A chain saw.

Thanks Bart:
Knowing a thang er two bout that fiziks an njineerin stuff (MS Physics,
MSEE), I think that water ballast does nothing until it is raised above
the waterline. I'll forego discussion about the diff tween mass and
weight.

wrote in message
oups.com...
Thanks Bart:
Knowing a thang er two bout that fiziks an njineerin stuff (MS
Physics,
MSEE), I think that water ballast does nothing until it is
raised above
the waterline.

Well, you're wrong!

Scotty

wrote:

Knowing a thang er two bout that fiziks an njineerin stuff (MS
Physics, MSEE), I think that water ballast does nothing until it is
raised above the waterline.

If the centre of rotation as the boat heels is in the centre of gravity of
the mass of the water ballast, then that might be the case. If the centre of
the boat's heeling rotation is above the COG of the ballast, then it has to
pull that enclosed mass around - ergo, the water ballast can have an effect
when it's below the waterline. I think this is the same thing as what I was
saying about bouyancy, but from the perspective of te ballast rather than
the air above it.


--
Capt Scumbalino

Scumbalino. agreed

If you want to build a boat, have you considered
the Dudley Dix kits built in South Africa? You
could also buy just the plans.

Their Mini design could be built with water ballast
albeit I don't think the kit includes that feature.

http://www.dixdesign.com/didimini.htm

I've thought about building one. I like the mini's but
the wooden designs are not very competitive, albeit
I'm sure they are a blast to sail.

wrote

Thanks Bart:
Knowing a thang er two bout that fiziks an njineerin stuff (MS Physics,
MSEE), I think that water ballast does nothing until it is raised above
the waterline. I'll forego discussion about the diff tween mass and
weight.

Capt. Scumbalino wrote:

wrote:



Knowing a thang er two bout that fiziks an njineerin stuff (MS
Physics, MSEE), I think that water ballast does nothing until it is
raised above the waterline.



If the centre of rotation as the boat heels is in the centre of gravity of
the mass of the water ballast, then that might be the case. If the centre of
the boat's heeling rotation is above the COG of the ballast, then it has to
pull that enclosed mass around - ergo, the water ballast can have an effect
when it's below the waterline. I think this is the same thing as what I was
saying about bouyancy, but from the perspective of te ballast rather than
the air above it.




I think this is the answer. - The downward forces on the water ballast
tank tend to pull the boat down to the lowermost floating position, at
point which the boat is in an upright position, absent other forces.
For example, ocean liners utilize water ballast below the waterline.
From personal experience, the Mac 26M is initially somewhat tender but
becomes increasingly stiffer as it heels.

Jim