Gross Tonnage and Net Tonnage for a Grampian

akin_alan · #1 (**permalink**) 12-04-2010

I have looked on Spec sheets for G30's but these aren't listed. I know that there are ways to calculate it but it seems that the ones that come from the manufacturer would be more accurate if there are any.

Vasco · #2 (**permalink**) 12-04-2010

Manufacturers do not provide this information. You need a tonnage surveyor. This is usually required for documenting a vessel (registering in Canada). The tonnage has nothing to do with weight. 100 cubic feet is one ton for measurement purposes.

sailingdog · #3 (**permalink**) 12-04-2010

You can also use a form from the USCG website to calculate the rough net displacement for the boat. If you're looking to USCG document the boat, you'd need to fill that form out anyway.

boatpoker · #4 (**permalink**) 12-05-2010

Queen Anne assigned responsibility for all British ports to the navy and being a government department the first thing they had to do was devise a method of charging port dues. The most comon import to the UK in those days was port wine which came in casks of 252 gallons and such a cask was called a "tun". A tun took up a little over 40 cubic feet of space. The navy decided to charge port dtuies based on how many "tuns" a ship could carry. "Gross Tonnage is the interior volume of a vessel in increments of 40 cubic feet. From that measurement space is deducted for "contra" ie. space where it is not possible to carry cargo such and engine spaces, galley spaces etc. The resulting measurement is the "Net Tonnage".

If you have a background in engineering or mathematics you should also have a grasp of Simpsons Rule No. 3 which is the main equation used to calculate volume of odd shaped spaces. The mathematical rules are the same for all jurisdictions but there are differences in what is considered "contra" in some countries.

I have never known a manufacturer to publish this information as it is not relevant to anything they do but I am curious as to why you wish to know.
If you are Canadian , there is a simplified method on-line at Transport Canada. You don't actually need a Tonnage Surveyor in Canada unless your boat is over 15 meters.

PS. I am a Transport Canada Appointed Tonnage Measurer.