external water ballst

[sand groper]

Possibly dumbo question here but I'm used to it ....

An internal water ballast tank works.
I assume that an external tank, ie. on the bottom of a boxy hull, would also
work in that when full the mass and therefore the 'weight' of it would be
dragged around along with the rest of the hull and still be subject to gravity.

Now if this is true, then would such a tank still work if it were to be filled
and drained by small holes at launch and retrieval of a trailer-boat ?

The assumption here is that an enclosed mass of water would still be
largely 'captive' even though some water would move in and out of the tank
because of the pressure of the water flowing along the hull.

[alan white]

Sure. It's not necessary to actively fill and drain. Water has weight even if it's in motion.
The only requirement would be that water stays in long enough to return the boat to an upright position should the boat capsize. Therefore, short of using a valve, the entry/exit hole or holes should be small enough to slow the process down.

[sand groper]

Thanks for that Alan.
I've not had much to do with power boats but have a vague idea that
some have 'wet wells' for live fish, and some aluminium dinghies have
hollow keels that fill and drain.

[gonzo]

The chinese had bow tanks in the junks to damp pitching. That is thousands of years ago.

[masrapido]

As a deck officer to me every water compartment is internal if it is on board. Extenal would apply only if you place it outside of the boat, say hanging over the board.

In sand groper's example, the tank is internal to me, although it is in a removable canister (did I understand that properly?). It is still inside the boat, at the bottom. And it works in the appropriate manner. The weight is working within the boats structure and it is essentially a part of it and I would treat it as such.

The only problem with such a wight would be if the canister/tank is not secured and can fly around the space. Then you would have a few problems with it.

[sand groper]

Hi Masrapido. What I had in mind is an 'extra skin' on the underside of a hull that fills and drains slowly via suitable holes, not a tank enclosed within the hull itself. Imagine a double-bottomed vessel such as a tanker with water between the shells.

[masrapido]

Ok,thanks. Now I understand the picture., And Alan's response makes perfect sense. I wonder if the drag would not be detrimental though.

[sand groper]

The 'drag' would come from moving the mass of the water ballast system plus the drag of the additional skin or blister or tank or whatever, plus any drag associated with flow through the system (unless it was valved), and water movement within the ballast would have some small effect too perhaps.
Obviously the hazards are corrosion, bio-growth, and so on.
I just want the impossible - a trailer-able, roomy, wide, shallow hull that can stand up to being powered up to windward (even if only to 60º or thereabouts) without falling over. Maybe that's why motors are so popular.

[gonzo]

You are describing a damping tank more than a ballast tank.

[jehardiman]

Gonzo is correct here.

While the mass of the water in the "tank" (actually a free flood) acts as a damper, it cannot be consider "ballast" unless a) it is totally contained AND b) is above the waterline. Otherwise all that happens is you have reduced the "ever buoyant" hull to some strange shape and maybe lowered the CG slightly due to the weight of the shell plating "exterior" and below the ever buoyant hull. But it is not "ballasting" the vessel in any way. If you want some worked examples I can show you some.

[souljour2000]

interesting idea Sandgroper...but with a double-walled " hull-tank" ..how would you deal with fouling in marine conditions...?

[sand groper]

I'm thinking of a trailer-boat where fouling isn't so much of a problem, though corrosion might be.
Not sure about jehardiman's idea(s) however. My cerebral cortex isn't sufficiently developed enough to comprehend a naval architect's series of equations in worked examples, so are there any other views on his argument ?

[souljour2000]

sorry.. obviously missed your earlier post on that subject..I guess if you could design it so you could spray it down really well at an angle with say..a pressure washer...it maybe could work "some"how..

[sand groper]

umm ... http://www.barcrusher.com.au/technology.htm
I found this after a tip from a work colleague -

"STABILITY AT REST
QUICKFLOW tm WATER BALLAST TECHNOLOGY"

"Boat design is about trying to incorporate all the very best design features to ensure the hull gives great performance for its intended application. As boat manufacturers and designers we have listened carefully to our customers and ensured that our design technology is applied to provide the smoothest possible ride whilst achieving optimum stability at rest.

In the past, boat manufacturers have had to compromise by building flatter hulls to achieve the required stability at rest. As we all know, a flatter hull shape means a hard, pounding ride. (You may have ridden in another aluminium boat on a windy day and found the ride to be almost unbearable).

The Bar Crusher design uses a deep-V hull for superior ride and an innovative water ballast system for stability at rest. Enter QuickFlowTM water ballast technology...

Running along the full length of the keel, there is a cavity in the bottom of the hull open at the transom. When the boat stops, this fills, becomes ballast, lowers the chines into the water and provides tremendous stability. The moment the boat moves forward, the water ballast is jettisoned from the hull allowing the boat to fly up onto the plane. Interestingly, there is no lag when accelerating, just a smooth and fast transition onto the plane.

The reality is that boat manufacturers use very different design technology and accordingly perform very differently on the water."

[masrapido]

Gonzo is correct here.

While the mass of the water in the "tank" (actually a free flood) acts as a damper, it cannot be consider "ballast" unless a) it is totally contained AND b) is above the waterline.

Did you mean below the waterline? Ballast can be above, but that is quite unusual for a leisure boat. I do not know one with the ballast above the waterline.

[masrapido]

I'm thinking of a trailer-boat where fouling isn't so much of a problem, though corrosion might be.
Not sure about jehardiman's idea(s) however. My cerebral cortex isn't sufficiently developed enough to comprehend a naval architect's series of equations in worked examples, so are there any other views on his argument ?

Consider copper nickel alloy. Zero fouling, no painting needed.

[jehardiman]

Not sure about jehardiman's idea(s) however. My cerebral cortex isn't sufficiently developed enough to comprehend a naval architect's series of equations in worked examples, so are there any other views on his argument ?

Did you mean below the waterline? Ballast can be above, but that is quite unusual for a leisure boat. I do not know one with the ballast above the waterline.

No, this is not an idea or theory, it IS the physics of the stability equations. Water "ballast" is fraught with perils and quirks, each of which needs to be addressed in the overall whole. It is also important to remember that static and dynamic stability as well as initial and final stability are all seperate subjects that each need to be addressed

Water "ballast" weight does not increase static stability unless it is, or is lifted by heel, above the waterline (see why below). This is why active ballast systems pump the weight to the high sides and passive "ballast" systems for sailboats work best on flat, wide hulls. The contained mass does effect dynamic stability though, both good (retards motion) and bad (continues motion once moving)

Adding water "ballast" volume below the waterline does not increase dynamic stability unless it causes the waterplane inertia to increase in greater proportion than the increase in volume (i.e. the geometry leads to an increase in BM == Iyy/Volume). Again, tanks high and outboard on wide shallow hulls work best.

Adding water "ballast" weight and volume below the waterline to an existing hull decreases both static and dynamic stability by dulting the effect of existing ballast (i.e. it lowers KB in faster proportion than KG is lowered) and it lowers BM by increasing volume on the same waterplane inertia.

Free surface, from any tank system, decreases both static and dynamic stability. This is why most "ballast" systems are operated fully pressed up.

If you want examples, or an analysis of a given case, I can do so. Until you go through all the math, some things are not obvious.

[masrapido]

jehardiman, I'd be interested to see some examples. Iit is true that sometimes one needs to go through the numbers, particularly to understand the physics of moments and their interaction.

It is common to place ballast tanks on commercial ships high above the waterline, for example, but I never thought that the same principle would apply to leisure boats given their small size.

This maybe outside the thread, but I think that other readers may find it interesting too if you show us some examples here.

[sand groper]

This business reminds me of the school question - "what is heavier - a tonne of feathers or a tonne of lead ?".
Does a cubic meter of water weigh a tonne on the deck of a ship and a tonne when submerged ?
A steel box containing a cubic meter of water and welded inside a hull weighs a tonne, and does it also weighs a tonne if welded to the underside of a hull ?
If this water box is welded to the outside of the hull, will the hull displace an extra tonne ? (ignoring the weight of the box itself of course)

[Typhoon]

I know of a small (13-14ft) fibreglass centre console that had a very deep V and a flooding chamber for stability at rest. Was called a Dingo if anyone every comes across one. It needed the ballast to sink the chines into the water at rest, else the boat would flop around up on the deep hull.
It was simply a 6" diameter tube (from memory) running inside the keel of the boat, open to the transom. Coming off plane it filled quickly and getting on the plane emptied equally as fast. It worked very well.

Regards, Andrew.

[gonzo]

A ton weighs a ton, but a ton of steel creates a lower metacentric center than a ton of water. That means that the stability is different.

[sand groper]

Would a thin 'skin' of alleged water ballast give a similar metacentric height to a not-much-thinner steel 'skin' on the underside of a hull ?
If not, might the stability 'improvement' over an un-ballasted hull still be useful anyway ?
To get my floating Winnebago to windward would require enough power to overcome the hull's limitation, ie. being a fat boat. No amount of power will make it close-winded or fast, but it doesn't need to be.
It just needs to stay upright.

[gonzo]

No, the center of gravity of a material of less density will always be higher.

[sand groper]

My contemplated camper boat hull designs are wide and shallow.
I know they'd fall over if given sufficent rig to have enough power to get to windward.
Being a tradesman (and not a naval architect and therefore unable to apply theory) I tend to think of practical problem-solving strategies. In this case that would be to shovel sand into the hull until it stopped trying to flip.
If that worked, then the next step would be to work out what mass of ballast is needed and then the next logical step is to replace that sand with a lump of steel for reduced ballast volume.

If that would do the job, then this steel slab could go on the underside of the hull presumably. If this is the case, then rather than have this slab of 1/2" (or so) steel built-on and therefore having to drag this weight up and down hills when trailing, would it not be possible to attach fore-and-aft aluminium strakes along the underside of the hull and then cover them with a sheet of stainless to make the proposed water chamber ?
Well it could be done of course, but would it work as intended ?

I'm not convinced it would work but have a gut feeling that it should.
Aussie blokes are born knowing how to do everything, and never need to look at the instructions. Well, not really, but you get the idea ....

[gonzo]

That is not a practical solution to the problem. The sand will have a much higher center of gravity. If you put enough sand it will make the boat top heavy and capsize it. Naval architects think of practical problem solving techniques too. The stability calculations are not so extremely difficult. Another solution, is to make a model and try adding steel to the bottom until the boat is stable enough. Then you can scale up the weight.

[sand groper]

How can a layer of sand in the bottom of a hull make it top-heavy ?

[troy2000]

How can a layer of sand in the bottom of a hull make it top-heavy ?


It might not be literally top-heavy. But the boat would be noticeably less stable because the weight would be higher--inside the hull, instead of below it. And since sand is much lighter than steel, an equivalent weight of sand would have a higher center of gravity than the same weight of steel laying inside would.

Of course, on a set of sharpie plans Chapelle drew, he specified bags of sand in the bottom of the flat-bottomed hull for ballast. But sharpies are different animals completely...

[sand groper]

What do you mean by "the weight would be higher " ?

Okay, the whole lot now weighs more, but would the CoG now also be higher ?
Sand is not as dense as steel for sure, but a layer of sand in a hull bottom would have a centre of mass only slightly higher than the center of mass of a slab of steel would it not ?
A little bit higher would not raise the whole boats CoG that much (as compared to a hull with a slab of steel ballast inside) and the extra weight would add stability IMHO.
But I've been wrong plenty of times in the past and no doubt will be in the future.
How are sharpies different as far as ballasting goes ?
(Your quote on idiots reminds me of the Chinese saying "when you argue with a fool, then two fools argue" - and I'm not implying idiocy in anyone here.)

[sand groper]

My thinking on this subject is along these lines -

Here is a barge. To get it to windward it must have a lot of power and therefore a lot of rig, and therefore a high sail plan CoE which will try to make it capsize.
So to stop this it must be ballasted to the point that the rig will blow over the side in a sufficently high wind, rather than have the barge flipping over.
Practically, this point need never be reached with a bit of judicious boat-handling, so ballast can be reduced from this hypothesised required weight.
So the result is a vaguely weatherly, ugly but comfortable and roomy horror.
Think Thames barge, bark 'Endeavour', botter, and so on.
But I want to tow a 6m version of this beast at minimum weight so a water ballast system seems the way to go.
But rather than mess around with pumps and so on, why not have external, self-filling and -draining water ballast.
But maybe there's no free lunch after all.
Oh for a battery as energy-dense as a bucket of petrol and then I wouldn't mind towing that weight.