Displacement and ballast

[Polux]

Originally Posted by valhalla360

Displacement is simply the weight of water that the boat will displace when put in water (or you could use mercury but hard to find a pool of mercury big enough). In terms of a boat that is not sinking, that's the same weight as if you hung if from a big scale on land, so yes, it very much has a lot to do with it.
..

I am not sure you are right. Displacement is the water that is displaced by the boat. Imagine a tank full, the displacement is the volume of water that comes out when the boat is put on the tank. That displacement would not be the same in sea water and fresh water. That means that the waterline of the boat would not be the same.

On the RCD the displacement of a boat is measured with a precise determined water density and the weight of that volume of water with that density constitutes the weight of the boat. The word weight is starting to be more and more used in Europe to refer the displacement of the boat, in generic therms.

Regarding definitions, most boats are designed to comply with RCD so their definitions of different weights of a boat tend to become standards. Here some:

empty craft condition: empty boat including fittings and equipment as listed below but excluding all optional equipment and fittings not included in the manufacturer’s basic outfit: a) structure: comprising all the structural parts, including any fixed ballast keel and/or drop keel/centreboard/ daggerboard(s) and rudder(s); b) ballast: any fixed ballast installed; c) internal structure and accommodation: bulkheads and partitions, insulation, lining, built-in furniture, flotation material, windows, hatches and doors, permanently installed mattresses and upholstery materials; d) permanently installed engine(s) and fuel system: comprising inboard engine(s), including all supplies and controls as needed for their operation, permanently installed fuel systems, including tanks; e) fluids in permanently installed systems: residual working fluids as needed for their operation (see examples below), but excluding contents of fluid ballast systems and tanks, and main storage tanks (which are included in maximum load); EXAMPLES: fluids in hot or cold water, fuel, lubricating or hydraulic oil systems f) internal equipment, including: — all items of equipment permanently attached to the craft, e.g. tanks, toilet system(s), water transfer equipment; — bilge pumping system(s), cooking and heating devices, cooling equipment, ventilation system(s); — electrical installation and equipment, including permanently installed batteries mounted in the position intended by the builder;— fixed navigational and electronic equipment; — fixed fire fighting equipment, where fitted; g) external equipment, including: — all permanently attached standard or specified deck fittings, e.g. guardrails, pulpits and pushpits, bowsprits and their attachments, bathing platforms, boarding ladders, steering equipment, winches, sprayhood(s); — awning(s), cockpit tables, gratings, signal mast(s), where fitted; — mast(s), boom(s), spinnaker poles and other pole(s), standing and running rigging, in the stowed position ready for use; all standing and running rigging in place.

light craft condition: empty craft condition plus standard equipment (3.6.12) plus removable ballast (whether solid or liquid) when supplied and/or intended by the manufacturer to be carried when the boat is afloat, with elements positioned as follows: a) where provision is made for propulsion by outboard engine(s) of more than 3 kW, the heaviest engine(s) recommended for the boat by the manufacturer is(are) mounted in the working position(s); b) where batteries are fitted, they are mounted in the position intended by the builder, and if there is no specific stowage provided for batteries, the mass of one battery for each engine over 7 kW is allowed for, and located within 1,0 m of the engine location. c) all upwind sails supplied or recommended by the builder as standard, onboard and rigged ready for use, but not hoisted, e.g. mainsail on boom, roller furling sails furled, hanked foresails on stay stowed on foredeck.

minimum operating condition: boat in the light craft condition with the following additions: a) mass to represent the crew, positioned on the centreline near the main control position of: — 75 kg where LH ≤ 8 m, — 150 kg where 8 m < LH ≤ 16 m, — 225 kg where 16 m < LH ≤ 24 m; b) non-edible stores and equipment normally carried on the boat and not included in the manufacturer’s list of standard equipment; EXAMPLES Loose internal equipment and tools, spare parts, dishes, kitchenware and cutlery, additional anchors or sails, dinghy and outboard if carried aboard.

maximum load: load which the boat is designed to carry in addition to the light craft condition, comprising: — the crew limit at 75 kg each; — the personal effects of the crew; — stores and cargo (if any), dry provisions, consumable liquids; — contents of all permanently installed storage tanks filled to 95 % of their maximum capacity, including fuel, drinking water, black water, grey water, lubricating and hydraulic oil, bait tanks and/or live wells; plus ballast water at 100 % capacity; — consumable liquids in portable tanks (drinking water, fuel) filled to 95 % of the maximum capacity; — dinghy or other small craft intended to be carried aboard, and any outboard motor associated with them; — liferaft(s) if carried in excess of the minimum required in essential safety equipment; — non-edible stores and equipment normally carried on the boat and not included in the manufacturer’s list of standard equipment, e.g. loose internal equipment and tools, spare parts, additional anchors or sails, dinghy and outboard if carried aboard; — an allowance for the maximum mass of optional equipment and fittings not included in the manufacturer’s basic outfit.

maximum load condition: boat in the light craft condition with the maximum load added so as to produce the design trim.

loaded arrival condition: boat in the maximum load condition minus 85 % of the maximum capacity of fixed or portable storage tanks for fuel, oils and drinking water, and minus 90 % of edible stores, but including the worst combination of optional fittings or equipment with respect to stability

It seems a lot of sh*t but the definitions and the different weights are indispensable to calculate the different stability curves that are used to certify the boat.

[Bazz]

Originally Posted by Paul J. Nolan

Paceship 29 was designed by C&C. Most of the boats designed and built under C&C's own name had balsa cored hulls. This is the first thing I'd check.

Paul

Paul,

My P29 has a solid glass hull and a balsa cored deck. Three years ago, I had a localized delamination problem near the bow, a port side stanchion being the source of infiltration, and it was dealt with. Still, to that effect, I'll conduct a real thorough examination of the hull and deck in the Spring.

[Bazz]

Originally Posted by Don C L

Just as a matter of historical interest and to the point of displacement considered in an architect's calculations, I posted the letter from Olin Stephens regarding metacentric height of a Columbia 29 in members images. Click on below my info at left.

This is great data!
We have it directly from the source: range of variation in built weights is in the 10% order of magnitude. At least in boats having sufficient similarities with the Columbia 29: (7769 - 7135)/((7769 + 7135)/2) = 0,085

[valhalla360]

Originally Posted by Polux

I am not sure you are right. Displacement is the water that is displaced by the boat. Imagine a tank full, the displacement is the volume of water that comes out when the boat is put on the tank. That displacement would not be the same in sea water and fresh water. That means that the waterline of the boat would not be the same.

On the RCD the displacement of a boat is measured with a precise determined water density and the weight of that volume of water with that density constitutes the weight of the boat. The word weight is starting to be more and more used in Europe to refer the displacement of the boat, in generic therms.

Regarding definitions, most boats are designed to comply with RCD so their definitions of different weights of a boat tend to become standards. Here some:

empty craft condition: empty boat including fittings and equipment as listed below but excluding all optional equipment and fittings not included in the manufacturer’s basic outfit: a) structure: comprising all the structural parts, including any fixed ballast keel and/or drop keel/centreboard/ daggerboard(s) and rudder(s); b) ballast: any fixed ballast installed; c) internal structure and accommodation: bulkheads and partitions, insulation, lining, built-in furniture, flotation material, windows, hatches and doors, permanently installed mattresses and upholstery materials; d) permanently installed engine(s) and fuel system: comprising inboard engine(s), including all supplies and controls as needed for their operation, permanently installed fuel systems, including tanks; e) fluids in permanently installed systems: residual working fluids as needed for their operation (see examples below), but excluding contents of fluid ballast systems and tanks, and main storage tanks (which are included in maximum load); EXAMPLES: fluids in hot or cold water, fuel, lubricating or hydraulic oil systems f) internal equipment, including: — all items of equipment permanently attached to the craft, e.g. tanks, toilet system(s), water transfer equipment; — bilge pumping system(s), cooking and heating devices, cooling equipment, ventilation system(s); — electrical installation and equipment, including permanently installed batteries mounted in the position intended by the builder;— fixed navigational and electronic equipment; — fixed fire fighting equipment, where fitted; g) external equipment, including: — all permanently attached standard or specified deck fittings, e.g. guardrails, pulpits and pushpits, bowsprits and their attachments, bathing platforms, boarding ladders, steering equipment, winches, sprayhood(s); — awning(s), cockpit tables, gratings, signal mast(s), where fitted; — mast(s), boom(s), spinnaker poles and other pole(s), standing and running rigging, in the stowed position ready for use; all standing and running rigging in place.

light craft condition: empty craft condition plus standard equipment (3.6.12) plus removable ballast (whether solid or liquid) when supplied and/or intended by the manufacturer to be carried when the boat is afloat, with elements positioned as follows: a) where provision is made for propulsion by outboard engine(s) of more than 3 kW, the heaviest engine(s) recommended for the boat by the manufacturer is(are) mounted in the working position(s); b) where batteries are fitted, they are mounted in the position intended by the builder, and if there is no specific stowage provided for batteries, the mass of one battery for each engine over 7 kW is allowed for, and located within 1,0 m of the engine location. c) all upwind sails supplied or recommended by the builder as standard, onboard and rigged ready for use, but not hoisted, e.g. mainsail on boom, roller furling sails furled, hanked foresails on stay stowed on foredeck.

minimum operating condition: boat in the light craft condition with the following additions: a) mass to represent the crew, positioned on the centreline near the main control position of: — 75 kg where LH ≤ 8 m, — 150 kg where 8 m < LH ≤ 16 m, — 225 kg where 16 m < LH ≤ 24 m; b) non-edible stores and equipment normally carried on the boat and not included in the manufacturer’s list of standard equipment; EXAMPLES Loose internal equipment and tools, spare parts, dishes, kitchenware and cutlery, additional anchors or sails, dinghy and outboard if carried aboard.

maximum load: load which the boat is designed to carry in addition to the light craft condition, comprising: — the crew limit at 75 kg each; — the personal effects of the crew; — stores and cargo (if any), dry provisions, consumable liquids; — contents of all permanently installed storage tanks filled to 95 % of their maximum capacity, including fuel, drinking water, black water, grey water, lubricating and hydraulic oil, bait tanks and/or live wells; plus ballast water at 100 % capacity; — consumable liquids in portable tanks (drinking water, fuel) filled to 95 % of the maximum capacity; — dinghy or other small craft intended to be carried aboard, and any outboard motor associated with them; — liferaft(s) if carried in excess of the minimum required in essential safety equipment; — non-edible stores and equipment normally carried on the boat and not included in the manufacturer’s list of standard equipment, e.g. loose internal equipment and tools, spare parts, additional anchors or sails, dinghy and outboard if carried aboard; — an allowance for the maximum mass of optional equipment and fittings not included in the manufacturer’s basic outfit.

maximum load condition: boat in the light craft condition with the maximum load added so as to produce the design trim.

loaded arrival condition: boat in the maximum load condition minus 85 % of the maximum capacity of fixed or portable storage tanks for fuel, oils and drinking water, and minus 90 % of edible stores, but including the worst combination of optional fittings or equipment with respect to stability

It seems a lot of sh*t but the definitions and the different weights are indispensable to calculate the different stability curves that are used to certify the boat.

Below is the nautical definition and it the WEIGHT of the water. The dictionary definition can include the volume but if you look up the specs for boats, they never list X cubic feet displacement. They list a weight. The WEIGHT won't change regardless of fresh vs salt vs mercury. What you are listing in the later part of your post are displacements for various conditions. The same definition for displacement applies to each just how much the boat is loaded changes. This is similar to my example of a sinking boat. As the bilges fill, the displacement increases.

Definition: Displacement or displacement tonnage is the weight of water that a ship displaces when it is floating, which in turn is the weight of a ship (and its contents).


Yes the designer needs to know where she will sit on her lines at various loadings to make sure everything is stable and seaworthy. That is more closely related to bouyancy.

[Polux]

Originally Posted by valhalla360

..
Definition: Displacement or displacement tonnage is the weight of water that a ship displaces when it is floating, which in turn is the weight of a ship (and its contents).

Yes the designer needs to know where she will sit on her lines at various loadings to make sure everything is stable and seaworthy. That is more closely related to bouyancy.

If you are not talking technically you can simplify that way but a displacement is a volume and it is that way that is defined on the RCD:

displacement volume: VD volume of displacement of the boat that corresponds to the appropriate loading condition, taking the density of water as 1 025 kg/m3 NOTE Displacement volume is expressed in cubic meters.

Some NA measure boat displacement not in Kg or pounds but in volume of the displaced water, in m3:

[valhalla360]

Originally Posted by Polux

If you are not talking technically you can simplify that way but a displacement is a volume and it is that way that is defined on the RCD:

displacement volume: VD volume of displacement of the boat that corresponds to the appropriate loading condition, taking the density of water as 1 025 kg/m3 NOTE Displacement volume is expressed in cubic meters.

Some NA measure boat displacement not in Kg or pounds but in volume of the displaced water, in m3:

Two issue:
- You listed the definition for "volume displacement" not "displacement". I know it's all technically and stuff but if you are going to argue the technicalities. You listed a different item from displacement.
- Your example is the first time I have ever seen a boat's displacment listed by volume. It's just not a common usage.

For the designer there are advantages to flipping back and forth between displacement and volume displacement but every buildiers broucher I've ever seen lists displacement as a weight. Likewise, having just submitted paperwork for documentation, when they ask for displacement, it's given in weight not volume (both the deregistered UK paperwork and the new USA paperwork).

Your example of fresh vs salt water is the reason why. If you list the displacement in m^3 maximum load, it actually will change based on the salinity of the water. If you say the boat weighs 10,000lbs maximum load, it will always weight 10,000lbs maximum load regardless of the liquid you put the boat into. Since we are defining a quality of the boat, it should not be dependent on the water it is floating in.

[Don C L]

Of course buoyancy and displacement are two sides of the same coin. A body is lifted by a force, the buoyant force, equal to the weight of the fluid it displaces. (thanks Archimedes) If you displace 204 lbs. of seawater (measure on your bathroom scale) and you weigh 204 lbs. (to choose a purely random weight!) on that same bathroom scale, the force of gravity on you and the buoyant force of the fluid you are immersed in are equal: you neither rise nor fall in the fluid. If you displace 204 lbs. of water and you weigh 205 lbs, you are sinking. It takes very little volume of anything to displace 204 lbs. of mercury, it takes a lot of volume to displace 204 lbs. of air. Ships steaming into the Amazon slowly gain draft as they come into the fresh water from the sea water. A displacing object has its own weight. If the object is hollow then you can add weight and it will lower into the fluid to a point where it displaces an equal weight of fluid. Battleships float very well because the weight of the water they are displacing is FAR greater than the weight of all the steel they are made of. So to make a long story short, displacement AND the fluid density will determine the buoyant force. If a boat weighs 10,000 lbs. it sure as hell better displace 10,000 lbs before the gunwale hits the surface. And to use the term "displacement" and not weight, for a boat, you will need to have a standardized figure for the fluid density if you want to mark the waterline.

[transmitterdan]

Originally Posted by Don C L

Battleships float very well because the weight of the water they are displacing is FAR greater than the weight of all the steel they are made of.

Actually the weight of the water they displace is exactly equal to the weight of all the steel they are made of plus the crew, provisions, fuel, gunpowder, etc.

[Don C L]

Originally Posted by transmitterdan

Actually the weight of the water they displace is exactly equal to the weight of all the steel they are made of plus the crew, provisions, fuel, gunpowder, etc.

OOPS! yes of course, there is plenty of freeboard keeping that displaced ocean out! Thanks for keeping me honest.

[Paul J. Nolan]

Originally Posted by jimbunyard

Interestingly, it the ocean were mercury, since lead floats on it, you'd have to use gold, platinum, tungsten or uranium as ballast.

I use my boat exclusively for racing, so I have aluminum ballast. Balsa-cored aluminum, actually.

Paul

[K_V_B]

Originally Posted by valhalla360

For the designer there are advantages to flipping back and forth between displacement and volume displacement but every buildiers broucher I've ever seen lists displacement as a weight. Likewise, having just submitted paperwork for documentation, when they ask for displacement, it's given in weight not volume (both the deregistered UK paperwork and the new USA paperwork).

In the metric world of course it's easy to flip between weight and volume where water is concerned. For me it has always be natural to think of "a ton of water" to mean "1000 kg of water" and "1 m^3 meter of water" as well as "1000l of water".

[K_V_B]

Originally Posted by jimbunyard

Imagine two boats, identical in every way, one floating in the ocean and one floating in a pool of mercury. The boat on the ocean floats on its' proper waterline, but the one in the mercury pool would float on it side....even though their respective weights on land are the same.

Of course both boats would displace the same. A 10000Kg boat floating in water will displace 10000Kg of water. The same boat floating in mercury will displace 10000Kg of mercury.

[StuM]

Originally Posted by K_V_B

In the metric world of course it's easy to flip between weight and volume where water is concerned. For me it has always be natural to think of "a ton of water" to mean "1000 kg of water" and "1 m^3 meter of water" as well as "1000l of water".

To the nearest 2-3% for most sea water

[StuM]

Originally Posted by K_V_B

Of course both boats would displace the same. A 10000Kg boat floating in water will displace 10000Kg of water. The same boat floating in mercury will displace 10000Kg of mercury.

And of course 10000Kg of mercury is a lot less volume than 1000Kg of water.

Which is why Pollux's VD "Displacement Volume" is rarely used and D "Displacement Weight" is the standard.

As a vessel leaves the sea and heads up a river it's draft changes because although D is the same VD changes due to the difference in density between salt and fresh water.