Hatteras

[Greenheart]

I always went with...net tonnage (being) the actual weight, and displacement being the weight of the water that is displaced...I was surprised by the earlier statement that the actual weight of the boat and the weight of the water it displaces are the same...

Be surprised if you like, but don't bother to question it. It's pleasingly logical if you think about it...because how could any floating object displace more or fewer litres of water than its own weight in kilos, ashore?

With an aussie girlfriend, I visited Adelaide's port, where an Australian Navy vessel was docked. Mademoiselle said "I can't believe how anything that heavy, can float."

The beautifully simple factor (which I didn't learn at school, and which most kids & adults don't seem to know) is that one litre of water weighs one kilo, and measures 10cmx10cmx10cm - so one cubic metre of water is a thousand litres, and weighs exactly a tonne. If you adopt this thinking when estimating volume beneath the waterline, you simultaneously estimate displacement.

The fact that any floating vessel has a waterline, shows that the water supports it at exactly the point where its weight in kilos is equivalent to the number of litres volume it occupies. Add a specific number of kilos payload, and the boat will displace exactly the same additional number of litres (or cubic decimeters of volume)- which, spread across the floor of the vessel, may only mean a tiny increase in draft - but it still always exactly reflects the litres displaced/kilos weight of the whole.

Registered Tonnage is a measurement which only belongs on the ancient brass tat which adorns the walls of quayside pubs.

 

[rustybarge]

Be surprised if you like, but don't bother to question it. It's pleasingly logical if you think about it...because how could any floating object displace more or fewer litres of water than its own weight in kilos, ashore?

With an aussie girlfriend, I visited Adelaide's port, where an Australian Navy vessel was docked. Mademoiselle said "I can't believe how anything that heavy, can float."

The beautifully simple factor (which I didn't learn at school, and which most kids & adults don't seem to know) is that one litre of water weighs one kilo, and measures 10cmx10cmx10cm - so one cubic metre of water is a thousand litres, and weighs exactly a tonne. If you adopt this thinking when estimating volume beneath the waterline, you simultaneously estimate displacement.

The fact that any floating vessel has a waterline, shows that the water supports it at exactly the point where its weight in kilos is equivalent to the number of litres volume it occupies. Add a specific number of kilos payload, and the boat will displace exactly the same additional number of litres (or cubic decimeters of volume)- which, spread across the floor of the vessel, may only mean a tiny increase in draft - but it still always exactly reflects the litres displaced/kilos weight of the whole.

Registered Tonnage is a measurement which only belongs on the ancient brass tat which adorns the walls of quayside pubs.

i think our old friend mr Plimsole drew a line on the side of ships to stop overloading of cargo ships, which by the late Victorian era caused a epidemic of sinking vessels.

is the registered tonnage the max cargo a vessel can carry when the waterline is at the plimsole line?

 

[Greenheart]

I shouldn't claim any understanding I only barely have, or much knowledge either. But I'm acutely aware that many privately-owned sailing yachts are terribly overloaded, rarely dangerously so, but almost invariably to the detriment of performance.

It's a critical factor with some of my favourite sailing boats, which really suffer when the owner unthinkingly stows heavy tool kits, countless unnecessary spares, oversized engines and tanks, plus numerous alleged creature-comforts on board, messing up the trim and deepening the underwater form, making her much less willing to move...

...in fact I think the unenviable reputation of certain sailing marques can be attributed to most of their owners' habitual overloading. The designer intended her for use with an ideal maximum displacement...it's not the boat's fault if the owner keeps her 10, 20, 30% heavier than that. I don't know whether motor-yacht owners are so cavalier...

...but if fuel economy is an issue, I'd think it's worth checking your vessel's actual displacement, relative to what she was anticipated to displace by her designer.

 

[Mariner69]

If it helps:-

The old system was as below, now (since about 1966) decimalised and in cubic metres.

The Gross Tonnage (GT) is the total enclosed volume of the ship, measured to set points defined in the rules for measuring and called tons, each ton being 100 cubic feet.

The Net Tonnage (NT) is the measure of the space available to load cargo and is measured and defined in the same way as the Gross but the tonnage for machinery and crew spaces are deducted. It is this figure that the port and light dues were charged upon.

The Register Tonnage (RT) is the same as the NT but for non-working vessels.

The Displacement is the actual weight of the vessel and is found by reading the draughts and using the hydrostatic tables for the vessel to find the volume, now days in cubic metres, which is them multiplied by 1.025 to give the tonnes when floating in sea water.

Figures are carved on the main frame for the Class One Registry not SSR.

 

[MapisM]

its hard to imagine that an extra inch of draft would equate to the enormous weight (downward force) neede to lower the boat in the water by said inch........ perhaps some boating physicist knows the real answer to this......

While I would rather call myself a boating amateur than a physicist , I can confirm you that on my 53' tub it takes 5 tons to raise the w/l by just about four of those inches - and this is by empirical experience, which is something I usually trust more than any theory.

Nonetheless, it did make sense also according to the theory (which dancrane already explained precisely), when I estimated the footprint of my boat.
Which is in fact about 50 square meters, hence by loading 50 tons on her she should go down one meter. Or 10cm for 5 tons, i.e. about 4".
Q.E.D.

 

[Latestarter1]

i think our old friend mr Plimsole drew a line on the side of ships to stop overloading of cargo ships, which by the late Victorian era caused a epidemic of sinking vessels.

is the registered tonnage the max cargo a vessel can carry when the waterline is at the plimsole line?

Thought this had been done to death, however one must simply draw a line under making any connection between vessels weight(displacement), load lines (Plimsoll not Plimsole) and registered tonnage.

Tonnage is used for many purposes in shipping - for assessment of port and harbour dues, pilotage charges, canal tolls such as Panama and Suez, insurance premiums, manning levels, maritime statistics, limitations of liability, and as a criteria for application of regulations made under International Conventions, in particular, SOLAS 74/78.

The word does not imply the ship’s weight or the weight of the cargo but concerns the internal volume of the ship and is therefore an indication of the ship’s cargo carrying capacity. Ships are measured internally and issued with a certificate which states their 'gross' and “net” tonnage. Crudely, 'gross tonnage' is a measure of the volume of a vessel, and 'net' tonnage represents the volume actually available for cargo.

For example many vessels I served on had a 'Tonnage hatch' which was simply a wheeze to reduce a vessels 'net' tonnage. The little tonnage hatch generally used for storing junk and gear such as ships 'insurance' cable. Because the 'tonnage' hatch gave access to the tween decks it reduced vessels 'net' tonnage.

Since typing this I note that mariner69 has also posted same explination.

 

[Mariner69]

Since typing this I note that mariner69 has also posted same explination.

I also sailed on vessels with Tonnage Hatches; between the No.5 Hatch and the access to the crew quarters.

We are of an age!!

 

[SpottyDog5]

Please can someone explain what is meant by 44/100

 

[jimmy_the_builder]

Please can someone explain what is meant by 44/100

I thought it was a decimal fraction ie your RT is 43.44 T.

 

[Latestarter1]

I also sailed on vessels with Tonnage Hatches; between the No.5 Hatch and the access to the crew quarters.

We are of an age!!

R829042

 

[Portofino]

I thought it was a decimal fraction ie your RT is 43.44 T.

I think it is
For Part 1 the registered number (usually 6 digits) should be carved into a main beam of the boat. In modern fibreglass yachts a board with the numbers carved on is usual, displayed in the main saloon/cabin space. (Hence the term "Carving Number"). Also Registered Tonnage will probably be on the board - As in "RT 43 -44/100 " meaning 43 tons and 44 hundreds

The O in ON I would say means " official "

 

[westernman]

The beautifully simple factor (which I didn't learn at school, and which most kids & adults don't seem to know) is that one litre of water weighs one kilo, and measures 10cmx10cmx10cm - so one cubic metre of water is a thousand litres, and weighs exactly a tonne.

Not exactly. But near enough.

One thousand liters of water weighs 1.000028 tonnes.

 

[SLC]

Not exactly. But near enough.

One thousand liters of water weighs 1.000028 tonnes.

The accuracy of this depends upon density, surely, or is 1.000028t per 1000lt the industry standard used for calculation?

 

[westernman]

It seems to be a lot more complicated than at first glance.

It depends on temperature, pressure and also whether other things than H20 is in it (which is the case for anything we drink or sail on).

 

[Bajansailor]

Getting back to Hatterae (plural of Hatteras?)........

A gentleman here bought a Hatteras 36 Convertible new in the early 70's - I think she originally had a pair of GM Detroit diesels in her, and she was very similar to the vessel in this Yachtworld link.
http://www.yachtworld.com/boats/1973/Hatteras-Convertible-2670915/PALM-COAST/FL/United-States

She had a succession of Owners; the previous owner decided to do a complete re-fit on her, but he went about it in a rather peculiar fashion, by first re-painting the hull at huge cost, then gutting the interior and re-building it, and then (finally) turning his attention to the engines - only to find that by then they were apparently well knackered.
And he couldn't afford by then to buy a replacement pair of engines, or get his re-built, so he sold her for a song.
The current owner is a fibreglass repairman and fisherman, and he is now slowly converting her into a commercial fishing boat, as shown in the photos below.

The hull has been 'stretched' at the stern, to make room in the cockpit for a large icebox, the sheerline has been raised, a new superstructure has been built, and a keel has been added to allow for a single engine installation.

I had a look around inside some time ago, and (like with other boats built in the 70's) I noticed that in those days it seemed to be customary to finish the hull laminates with a woven roving, and not bothering to gelcoat the inside of the hull. In contrast to these days when they are usually finished smooth, and gelcoated.

 

[Mariner69]

R829042

Can't find my Discharge Book but it was R906 something. Had it changed at Hull when the new format came out in the mid-seventies. Three of us got sent to the office for the new books and the agent arranged the transport. We arrived at the federation office in a huge black limousine not the best transport in the middle of one of the seaman strikes!!