On Sun, 30 Dec 2007 19:29:48 GMT, "Dennis Pogson"
wrote:

maybe there IS something in this water ballasting
after all! Perhaps it stops Mari Chai 1V from taking off and becoming a
flying machine!

Water ballasting and water roll stabilization are two different
things.

Our boat is a Grand Banks 49 trawler displacing in the neighborhood of
60,000 lbs dry, 70,000 loaded. It has a substantial flybridge deck
located about 12 feet above the waterline, and about 12 feet wide.
Since we have frequently had a dozen people up there with no obvious
ill effects, I'm assuming we could also get away with several thousand
pounds of water, especially at anchor. The boat already has dynamic
roll stabilizers which are quite effective underway but are expensive
to maintain.

I'm aware of at least one other boat in this size range that is
successfully using water stabilization. They had their system
designed by a Naval Architect which is no doubt the right way to go
for guaranteed results. I was more interested in cobbling up
something cheap and dirty for some informal testing of the concept on
our boat. Key variables are tank size, tunnel size/shape, and
gallons/pounds of water.

Any suggestions for a starting point?

On Sun, 30 Dec 2007 18:01:09 -0500, "Roger Long"
wrote:

Any suggestions for a starting point?

Yes, start by deciding whether you are willing to invest a lot in
construction and additional weight with it's attendant increase in fuel
consumption to make things just a little better. If it's worth it just to
reduce motion by 10 - 15 percent, you can probably do something.

Thanks, some interesting points there. Weight could largely turn out
to be a non-issue if used only at anchor or as a backup to the dynamic
stabilizer system (Naiads). It would be easy enough to pump some salt
water up into the tanks when I wanted to use them, and drain it back
out afterwards.

I was thinking of buying a couple of fiberglass dock boxes from West
Marine and cross connecting them with PVC pipes. The system could be
tuned by restricting flow or adding additional pipes. I have no idea
how to calculate optimal starting pipe size(s) however.

Just for the sake of argument, lets assume 400 total gallons and a
natural roll period of about 3 seconds. Given a 5 degree roll, most
of that water would have to flow to the downhill tank in about 1.5
seconds if I'm conceptualizing this correctly.

"Roger Long" wrote in
:

"Wayne.B" wrote

Any suggestions for a starting point?

Yes, start by deciding whether you are willing to invest a lot in
construction and additional weight with it's attendant increase in
fuel consumption to make things just a little better. If it's worth
it just to reduce motion by 10 - 15 percent, you can probably do
something. Most people who are bothered by motion will still think
there is nearly as much rolling as there was before. If you want to
do something significant, it will need to be huge.

The tanks in the project I was involved with were a dodge around the
MacMullen patents. They were simply rectangular boxes and worked on
the principle that the shallow wave of water rolling from one end to
the other would break, retarding it's speed. Speed was adjusted to
tune to the vessel's natural rolling period by adjusting water depth.

In the MacMullen tanks, the speed of water is regulated by either an
hourglass shape or sometimes by two pipes installed vertically at the
middle to restrict the flow. Tuning is fixed at the construction
phase requiring careful calculations.

You could build the largest tank you think the boat can handle with a
removable panel in the middle of the top. Install two pipes of 2 - 3
inches diameter between the bottom and the removable panel. Then make
larger spools of different diameters. Fill the tank half full and
then experiment by inserting different spool sizes until it seems to
work. If the MacMullen patents are still in force, they might have an
issue with this if you start selling them.

You could also just build a big rectangular tank and try different
lesser amounts of water as was done on the long liners.

Whatever you do, as you are experimenting, bear in mind that getting
the tuning such that is it in phase with the vessel's natural roll
period will increase the roll by as much as the tank is capable of
reducing it. Could be exciting. Even with properly tuned tanks, there
will occasionally be sea that creates a roll just in tune with the
tank so that the vessel will take a larger roll than it would have
without the tank.

--
Roger Long

Most "Flume" Tanks I remember were more on the lines of a "U" shape layed
on it's side (tankers being the exception). At one point they were
looking at a system for yachts which would be installed on the
"flybridge" built into the shape around the upper steering station....
this had the advantage of reducing the size due to the fact that the
higher the tank, the greater the righting moment.
I don't know if any of these were ever installed, but you would have
expected a reduction of @50% of ROLLING (note: rolling, not roll angles).
As Roger is saying, trying to design one of these systems without knowing
the basic formula's they used could lead to some catastrophic results.

One final point: If the system could have easily been designed and built
to work effectively with a minimum loss of space, ease and safety of use,
and maintenance, we'd probably see a bunch of them out there...... we
don't.

The smallest boats I remember these being put on were some buoy tenders
and Tuna Clippers (built into the ramp for the seine boat on the stern)

On Sun, 30 Dec 2007 20:28:13 -0500, Wayne.B
wrote:


I was thinking of buying a couple of fiberglass dock boxes from West
Marine and cross connecting them with PVC pipes. The system could be
tuned by restricting flow or adding additional pipes. I have no idea
how to calculate optimal starting pipe size(s) however.

Just for the sake of argument, lets assume 400 total gallons and a
natural roll period of about 3 seconds. Given a 5 degree roll, most
of that water would have to flow to the downhill tank in about 1.5
seconds if I'm conceptualizing this correctly.

My first shot, off the top: two 30 in diam smooth pipes

Brian W

On Sun, 30 Dec 2007 20:53:17 -0600, Brian Whatcott
wrote:

Just for the sake of argument, lets assume 400 total gallons and a
natural roll period of about 3 seconds. Given a 5 degree roll, most
of that water would have to flow to the downhill tank in about 1.5
seconds if I'm conceptualizing this correctly.



My first shot, off the top: two 30 in diam smooth pipes


That's much too big of course. Can you take us through your
calculations? It would take a large rectangular tunnel to achieve
that much cross sectional flow area without creating an unworkable
obstruction on the flybridge.

On Mon, 31 Dec 2007 07:58:13 -0500, "Roger Long"
wrote:

The tanks I've seen have the restriction at the middle, be it two vertical
pipes or a necking down of the width, about half of the cross section area
of the tanks themselves so you would be talking about pretty big pipes. The
time it takes for water to get all the way from one side to the other is the
issue so, if you are using pipes which are longer and restrict flow more,
they would be even larger.

Your roll period sounds improbably short. If your boat has a typical
trawler hull form, you would have sold her and moved to Kansas after just a
couple of trips if the roll period is that short. I'm curious what it is
and it is fairly easy to check.

On a very calm morning, slack the lines and push the boat off the dock.
Step on and off the boat or push up and down on something. You will see her
move just a little bit. Get into the rhythm and get her rolling as hard as
you can. You can set up an impressive roll on even a 100 foot boat this
way. Let go and time as many rolls as you can with a stopwatch before the
rolling stops. Sight a landmark such as the top of a rail stanchion against
the background to identify the end of the roll. A complete roll is from one
side, over to the other, and all the way back. I suspect your roll period
is actually 6 seconds.


It is definitely not as high as 6 seconds. I did this excercise a
while ago and recall that the actual number was between 3 and 4,
possibly closer to 4 but I don't remember the exact value.

4 seconds is a good roll period for a highly damped hull form such as the
Kingsbury with a flat beamy hull. When checking the roll period, the boat
will roll just once and pretty much flop back upright. There is no
carryover of motion.

That closely describes our boat, perhaps as a result of the full keel
and hard chines. There is some carry over of motion in flat water but
the amplitude is small and damps down quickly.

The roll period matches the period of waves found in
lighter winds. Resonant rolling is thus small in magnitude. When the waves
get big, the boat is no longer in tune and there is little rolling. The
number of times a hull rolls after you let it go during the roll test is a
measure of the amount of damping.


The biggest "rolling at anchor" isue we have ever had was in Bar
Harbor, Maine. At high tide the ocean swells surge over the
breakwater and really get things going. You also have the "Big Cat"
fast ferry coming in at night from Nova Scotia leaving a powerful
wake, not to mention the 40 ft lobster boats barreling in and out at
all hours.

6 - 8 seconds causes the boats natural period to match that of waves
typically experienced on the days when you decide to head for port or more
protected water. If the hull form isn't well damped, rolling can be heavy.
The stability the produces roll periods in this range will usually not have
a lot of inherent damping due to narrower beam and more rounded sections.
This is why your trawler needs those dynamic roll stabilizers.

An 8 - 10 second roll period means your boat could well be dangerous and
should have a full stability analysis, especially if freeboard is low.

It sounds like your issues are primarily at anchor. Before tanks, I would
try this.

Cut a sheet of 3/4 inch plywood into two of the largest equilateral
triangles you can. Drill holes in the corners and make a rope bridle so
they can be suspended flat by a length of rope about 50% longer than your
draft and freeboard combined. Weight the sheets so that they sink.

Yes, the old flopper stopper trick. I built a set for our old boat
and they were quite effective. On the Grand Banks they appear to be
undersized however, with 3x the displacement. Equilateral stoppers,
more than 4 ft on a side, would be quite bulky to stow and deploy.
I'll have to think about it. One trick that I did get some success
with was to leave the inflatable RIB dinghy over the side, half
suspended from the cargo boom. It allowed the boat to roll easily
towards the dinghy but provided 300 lbs of resistance going back the
other way. You could hear the dinghy splashing around at night
however so it was not a big hit with the admiral, and probably tough
on the mast rigging over the long term.

The
weights can be removable to facilitate storage. Concentrate the weight
towards one corner so that, when the sheets sink, the side away from the
weight is nearly floating. Hang these over the side made fast to your
midsheet cleats. When the boat rolls towards one of the paravanes, the
weight will pull it down edgewise and it will sink quickly. The other will
be pulled up flat with great water resistance. These will probably do a
better and quieter job at anchor than a tank.

On our old boat they provided about 50% damping which was a noticable
improvement from its deep-V "snap roll" characteristic.

On Sun, 30 Dec 2007 23:53:18 -0500, Wayne.B
wrote:

On Sun, 30 Dec 2007 20:53:17 -0600, Brian Whatcott
wrote:

Just for the sake of argument, lets assume 400 total gallons and a
natural roll period of about 3 seconds. Given a 5 degree roll, most
of that water would have to flow to the downhill tank in about 1.5
seconds if I'm conceptualizing this correctly.



My first shot, off the top: two 30 in diam smooth pipes


That's much too big of course. Can you take us through your
calculations? It would take a large rectangular tunnel to achieve
that much cross sectional flow area without creating an unworkable
obstruction on the flybridge.

I looked up a table of flow rates for smooth pipes of various
diameters on Google and scaled to a flow rate of 12000 gal/min

Brian W

On Mon, 31 Dec 2007 10:36:35 -0500, Wayne.B
wrote:


Yes, the old flopper stopper trick. I built a set for our old boat
and they were quite effective. On the Grand Banks they appear to be
undersized however, with 3x the displacement. Equilateral stoppers,
more than 4 ft on a side, would be quite bulky to stow and deploy.
I'll have to think about it. One trick that I did get some success
with was to leave the inflatable RIB dinghy over the side, half
suspended from the cargo boom. It allowed the boat to roll easily
towards the dinghy but provided 300 lbs of resistance going back the
other way. You could hear the dinghy splashing around at night
however so it was not a big hit with the admiral, and probably tough
on the mast rigging over the long term.

Since I began reading about "roll dampeners" I've wondered about the
stress on attachment points. Read a few accounts of flopper stoppers
breaking their cables, and it seems the forces are significant.
How do you judge what's too much stress for these setups?
Since cats are resistant to rolling I've pondered whether extendable
amas on strong poles - don't know what material - might be a solution.
Storage, cosmetics, and ease of deployment might be the big issues
with this, but less so on a bigger boat like yours than a smaller
boat. There are some interesting engineering questions about
flexibility of the poles, attachment points, ama buoyancy, etc, which
might be fun to work out. Sometimes I wish I were an engineer,
but I don't even do algebra. Still fun to Rube Goldberg it.

--Vic