Net Tonnage to tonnes conversion

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npf1

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Does anyone know how to convert gross or net tonnage, as taken from ship's registry certificate, into metric tonnes? I'm trying to work out if Sealift 2, which has a capacity of 25 metric tonnes, can lift a boat with a gross and net tonnage of 27.4. Thanks
 
I just asked Hubby and he said No ! They are totally different units and not even relevant to each other.

Nett and Gross are based on volume of various spaces on board and what you are looking for is Displacement - the physical weight of the boat.
He also said that the old trick of adding Nett to Gross is horse poo ! He also winced when I said Metric Tonnes, he replied that there are Metric Tons or Tonnes - that they are same but not names together.
 
1 metric tonne = 1,000 kilograms = 2,205 US pounds = 1 US ton = 1 short ton
( in literature equation is like this ..... and it is not =)

1 US ton equals 0.91 metric tonnes.

However, I recommend the following rule to follow when not clear.

A tonne is a metric tonne.

A ton is an American ton.

Without complicating things, the easiest way to relate the two is to think of the American ton as being about 10% smaller than the metric tonne or the latter being 10% larger than the former.
 
I realise they are different, hence the question is how to convert from one to the other.

I thought that the tonnage of a ship is a measure of displacement so not an imperial ton, long ton, short ton or metric tonne. Wikipedia discusses the volume of water displaced as shown by the tonnage, then multiplies by 2.8 tonnes for the mass of water to occupy that space. But this calculation gives a mass in kg which is more than triple that I have seen mentioned for my make/model of boat.

To rephrase my question ... Sealift's capacity is 25 metric tonnes, so I need to estimate my boat in the same units but I only have the displacement tonnage to start from. How do I calculate it?

I'm confused! And probably not the only one!
 
Something is incorrect in that calculation.
Displacement D ( kg) = Volume of water ( m3) * Spec. gravity of water ( kg/m3) ( for sea water !,033)
Result is in Kg - that means Mass in metric tonns!
Displacement is a Force opposite to weight so simply - if you can calculate Displacement - you have boat weight!
IMHO for your question my answer should be positive .......But you are very close to Sealift high capacity value so it would be better to find another one ( even it has spare more than 30% cap.)
 
It is used in Imperial calcs for transformation from Lbs to kg. It is abandoned if the calculation is go in metric system.
 
The gross tonnage is basically the internal capacity of a vessel. Its determined by measuring the volume of all the space below the "tonnage deck" and adding the volumes of all spaces above used for crew, stores or passengers etc That volume in cubic feet divided by 100 is the gross tonnage.

The net tonnage is the above less any non earning space eg the space occupied by boilers, engines, fuel, stores, crew, navigating space etc.

<span style="color:red"> You cannot determine the actual weight of a vessel, which is the same as its displacement, from the gross or net registered tonnages.</span>

(very abbreviated explanation of the gross and net tonnages ...... standby for precise definitions from Brendan!)
 
Please forget about the "Tonnage", actually a bastardisation of "Tunnage" referring to capacity in barrels. Because ships registry is based on commercial shipping requirements, the volume measure (which has its own form of calculation) still persits. As Vic S explains it is a measure of usable volume and nothing to do with weight so impossible to have any conversion factor.

Two types of "Ton" as weight in use. Imperial Ton which is 2240 pounds. The nearest equivalent Metric is 1000 kg or one Tonne. As 2.2 pounds = 1kg, the Tonne is 40 pounds or 18.18 kg or 1.78% lighter. Largely irrelevant with regard to weights of yachts.

Back to your original question, you need to get the displacement calculation (in Tons/Tonnes), add 20-25% for all the kit you have added over the years and full tanks. As a rough guess, unless it is a super heavy displacement it is unlikely to be more than 25 Tons/Tonnes if it is less than 50 foot. For example a Bavaria 50, which would be classed as a medium displacement design has a design unladen weight of 12.6 Tonnes. Add 25% brings it up to 15.75 Tonnes - well within the limit of your lift.

You don't say what your boat is, but if it is relatively modern it will have full design calculations available as these are needed for RDC. If the design data is not available, then ask the lift operator for a view - after all he will have a lot of experience. Some lifts have a weight indicator and an overloead alarm, so you may find the actual weight in due course.

Hope this helps
 
I'm confused! And probably not the only one!
******************************************

You most certainly are.......

You are assuming these are measurment of the same things and so can be converted one to the other. This is NOT so.

Tonnage and Tonnes displacement are not measurments of the same thing and so there is no conversion.

You seem to have picked up the registered tonnage of your boat which is a volumetric measurment of the inside of the boat that you use.Nothing to do with tons weight or tonnes weight. You need to find the Displacement of your boat. How much does it weigh and this is the figure for using to evaluate a sealift.

FAQ

The light weight tonnes displacement figure should be available some where in your papers.
 
The registered tonnage (Thames) of my boat is 9.13 tons. Its displacement is somewhere between 5.5 and 6 tons.

The Thames tonnage was determined with a tape measure, although because the measurer thought the top of the boot-topping was the waterline, he got the draught, and probably the tonnage, wrong. It's a largely meaningless term.
 
You bio says you have a Skookum ketch. I know that the 53' is super-heavy, at about 44 tons displacement, so would be well over the capabilities of the lift.
 
[ QUOTE ]
The registered tonnage (Thames) of my boat is 9.13 tons.

[/ QUOTE ]

Careful. The Thames tonnage is a different thing again, and not related to registered tonnage. It's a yacht handicapping formula developed by the Royal Thames Yacht Club in the 1850s, is based on dimensions rather than weight, but not in the same way as registered tonnage. No relationship to weight; for this job forget it.
 
Been digging through some paperwork and I think I've found the answer. Looking at the stabilty study of a sister vessel, the Arrival Load 10% Consumables is just a shade under 26 metric tonnes. After allowing for me having full fuel but empty water, plus a few other adjustments from the list of equipment shown in the study, I end up with just under 26 metric tonnes.

Doing a similar calc for the 100% consumables gets me to a figure almost equal to gross/net registered tonnage per the registration certificate.

Thanks for the responses.
 
[ QUOTE ]
I realise they are different, hence the question is how to convert from one to the other.

I thought that the tonnage of a ship is a measure of displacement so not an imperial ton, long ton, short ton or metric tonne. Wikipedia discusses the volume of water displaced as shown by the tonnage, then multiplies by 2.8 tonnes for the mass of water to occupy that space. But this calculation gives a mass in kg which is more than triple that I have seen mentioned for my make/model of boat.

To rephrase my question ... Sealift's capacity is 25 metric tonnes, so I need to estimate my boat in the same units but I only have the displacement tonnage to start from. How do I calculate it?

I'm confused! And probably not the only one!

[/ QUOTE ]

First of, here Hubbies passing on again, as he's in Shipping and all this Tonnage lark, it's his job.

Nett and Gross Tonnages are Volumetric and previous posts about conversions of different tons, tonnages are fine - but irrelevant. Second that post also makes fundamental error of Metric and Tonnes in same term. You can forget US Tonnage in this as well as it has no bearing whatsoever.
But now you say all you have is Displacement tonnage. IF you have Displacement of the boat - that is the weight you need to go with. Displacement tonnage is the actual weight of the vessel displaced in water. The true weight.
Looking back at first post - it did seem strange to hubby that you said Nett and Gross both at 27.4 tons. That's not possible unless strange circumstances or improper measurement.

So taking his words, if you have Displacement already stated, then that is the weight you need.
 
[ QUOTE ]
1 metric tonne = 1,000 kilograms = 2,205 US pounds = 1 US ton = 1 short ton
( in literature equation is like this ..... and it is not =)

1 US ton equals 0.91 metric tonnes.

However, I recommend the following rule to follow when not clear.

A tonne is a metric tonne.

A ton is an American ton.

Without complicating things, the easiest way to relate the two is to think of the American ton as being about 10% smaller than the metric tonne or the latter being 10% larger than the former.

[/ QUOTE ]

Irrelevant.
 
Moody 1 - the Part 1 cert, for which it was measured in 2003 by Lloyds, has both the gross tonnage and net tonnage as the same at 27.4. I assume they are borh the same because it is not a ship with cargo area etc.

Feel like I've learnt something today.
 
[ QUOTE ]
That's an imperial ton, still in common use in UK.

[/ QUOTE ]

According to my Hubby, who is in the shipping game. The Long Ton which is referred to here by your UK reference is common in many places. Long Tons and Metric Tons are basic weight units used by cargoes.
 
[ QUOTE ]
Moody 1 - the Part 1 cert, for which it was measured in 2003 by Lloyds, has both the gross tonnage and net tonnage as the same at 27.4.

[/ QUOTE ]

Ok, not going to argue with your Pt 1. Hubby just passed on that on his experience, and he is a lot older than me, he has never seen Nett and Gross as same in Shipping world. If you look at a previous posters reply where he generalises the basis of the Nett and Gross Registered Tonnages, you can see why. Hubby is often involved in determination and checking of such tonnage marks etc.
He has suggested that you may have had the numbers inserted based on information supplied in Pt 1 application originally instead of actual measurement. It has been known.
 
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