understanding GRT (and similar legal tonnage issues)

Discussion in 'Boat Design' started by big_dreamin, Oct 19, 2016.

  1. big_dreamin
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    big_dreamin Junior Member

    I mentioned this in my other thread, but it's potentially it's own topic of interest to others plus likely to have a fairly involved discussion by itself so I opted to set it aside.

    Quite simply i'm trying to understand "tonnage" insofar as it affects license requirements. Whether USCG licensing or international/foreign flag options which don't necessarily measure the same, i'm trying to catch up my understanding and i'm not getting it too well so far.

    I've looked at the USCG chart (specifically https://www.uscg.mil/hq/msc/tonnage/docs/Brochure_Documentation_and_Tonnage.pdf ) which as near as I can tell, if I designed up a single enclosed deck catamaran of 55 feet long, 25 feet wide, with 8 foot interior ceilings I already have an enclosed tonnage in excess of 100 GRT because it's in excess of 10,000 cubic feet of volume. (the above specs are 11,000 cubic feet afterall)

    Am I correct in this? Verifying this before going much further since if I misunderstand the core calculation method everything else will be wrong too. :)

    Relevant in any case is the idea that 'permanently enclosed space' could change, could I design a boat to deliberately enclose less volume at first (like just leaving the top off of a cargo hold since soft tarping doesnt count as permanent) with the idea of closing it it later having the boat re-registered at the greater volume later?

    I also see on the form that "commercial vessels under 79ft can choose one of two older measurement systems" (in addition to the GRT method/thats the third I guess?) dealing with formal/net tonnage. Recreational vessels and sail over 79ft in length can also choose the simplified system but may require "dual measurement". (and that dual measurement i'd like to understand more about, is this ITC tonnage?)

    On the higher end of the scale I hear of private recreational megayacht owners who engage in "tonnage reduction techniques" in the design level which apparently works up to about the 180 foot level to keep GRT under 300. (source http://www.yachtsinternational.com/money/money-flagging/stars-strife/ ) I'm sure this is more a field for higher end naval engineers but i'm just curious how all that works approximately.
  2. Ad Hoc
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    Ad Hoc Naval Architect


    On the one hand yes, you are in the right direction. However, the US calculates its tonnage very very differently from the rest of the world. All US vessels try their hardest to get to below 100GT for crewing reasons - under 100 less crew (nowhere else in the world does this occur). The vessels that end up less than 100GT (sub-chapter K if I recall from memory??) but international standards of tonnage measurement are way way over.

    We designed a series of high speed passenger catamarans. Built in the UK and Asia. Then we had the very same vessel built in the US. In the US, the tonnage is more dictated by the on-site surveyor!! If the surveyor sees longitudinals passing through frames, this affects his calculations for the GT. What???!!...yup....so we had to redesign the whole structure, against our advise and DNVs too, to have all the long.t as intercoastals. Since this significantly affects the GT calculation.

    The end result is that the vessel which was built elsewhere in the world, has a GT of 605, in the US it was just under 100. Go figure!!

    If you can understand the illogical logic of the US system of calculating GTs...good luck!
  3. big_dreamin
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    big_dreamin Junior Member

    Which part am I in the right direction over? :) I asked a bunch of things. It sounds like 'simple tonnage' applies for recreational boats and commercial under 79ft, i'm trying to get some "for dummies" level understanding of at least a few of the ways in which a larger vessel can even start to be designed. Like I was reading about Subchapter T allowing up to 149 day passengers or 49 overnight and trying to figure out how they could fit into an under 100 GRT vessel when my proposed catamaran already seems over!

    Related and out of curiosity i'm also trying to understand upper limits for wooden vessels. One reason being that I prefer wood (and no I wont even START a topic on that because it's been done to death for decades!) and whatever i'm dreaming up whether it's totally private use, or light commercial or whatever, i'd want to build it largely out of wood. If I had a superyacht - it would be wood. Just coz. But i'm wondering if there are legal length or tonnage limits for it to be in wood - whether recreational or commercial use. (if 100 GRT that seems insanely small "for how I understand")

    PS whats the crewing difference for under/over 100 GRT?

  4. TANSL
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    TANSL Senior Member

    How is it possible that there are such big differences ?. Perhaps you are comparing the "Formal" tonnage calculation and the "Simplified" calculation, which can not be applied on all vessels. Keep in mind that "For undocumented vessels, Simplified tonnage is Calculated on an "as-needed" basis by interested parties, and is not certified on Coast Guard documents." (Paragraph extracted form "SIMPLIFIED MEASUREMENT TONNAGE GUIDE 1", by USCG)
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