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Old 24-10-2016, 15:14   #1
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help me understand tonnage better?? (for licenses, etc)

I will freely admit that i'm quite a newb when it comes to anything beyond recreational boating where I never needed to worry about licenses and such at all at any level accessible to me. When I look through some of the licensing rules for the USCG and elsewhere though I get totally confused at what kind of boat is even in what category. I read about Gross Register Tonnage, simplified calculations for smaller vessels, some kind of net tonnage calculation on larger vessels (or luxury yachts where they try to make "exempt space"), and ITC tonnage.

Even looking at official forms I have to be honest that I don't understand it, because following the rules the answers I get don't seem to make sense. Is there a For Dummies version or explanation showing for instance a specific ship, and all the ways it can be calculated? (exclusively for legal/licensing purposes, not interested in historical or volume or weight, just what will answer whether time spent on X boat counts me towards a 25 GRT, or 50 GRT, or 100 GRT, or 200 GRT licensing class for instance. I mean on a theoretical level/if I don't have the information provided by the owner, i'm trying to understand how the calculations vary and why by seeing examples of them applied to different vessels.)
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Old 24-10-2016, 15:43   #2
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Re: help me understand tonnage better?? (for licenses, etc)

USCG Gross tonnage for a Document is a measurement of pretty much a big cubic rectangle. I think it was developed for freight carrying ships. It has little to do with your actual boat.
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Old 24-10-2016, 16:39   #3
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Re: help me understand tonnage better?? (for licenses, etc)

It is a somewhat antiquated system to approximate cargo carrying capacity of ships.

I think most get confused by tonnage measurements because they try and relate it to the displacement or actual weight of the vessel. It has little to do with either, it is really a measure of approximate cargo carrying capacity. Recreational vessels can use a simplified method.

Working thru a measurement may help. Forms here:

http://www.uscg.mil/hq/msc/interactive_tonnage.asp
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Old 24-10-2016, 19:57   #4
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Re: help me understand tonnage better?? (for licenses, etc)

Quote:
Originally Posted by Cheechako View Post
USCG Gross tonnage for a Document is a measurement of pretty much a big cubic rectangle. I think it was developed for freight carrying ships. It has little to do with your actual boat.
Right, I understand that just like I can read the wikipedia entries on what each thing is, and look at the forms. I'm just wanting to see some examples of the math of different ways of calculating applied to the SAME vessel so I can see how it applies in the real world.

For instance I wanted to design a series of half recreational/possible half work (passenger charter for side income or mild cargo use) boats (this is a long term plan/what I plan to build and go to sea in retirement being different than what first goes on the water, etc) and i'm trying to figure out just how large it can be at GRT 25 50 100 200. Yet my math has to be wrong because I hear of the 96m Limitless showing at 300 GRT yet my sketch of a two deck wide catamaran exceeds that at 24m length.

EDIT: Yes that's the form I used (though the interactivity didn't work for me but I was trying to follow the instructions I thought) which confused me to begin. I enclose 10,000 cubic feet and exceed 100 GRT with a 55ft long 26ft wide single deck bridgehouse with 7 foot ceilings. I enclose 2500 cubic feet and exceed 25 GRT on something I could probably haul behind my Dodge Ram Cummins.
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Old 24-10-2016, 20:40   #5
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Re: help me understand tonnage better?? (for licenses, etc)

I had a similar experience trying to figure out the tonnage for an old trimaran I owned.
You just have to get comfortable with the concept that the tonnage calculated for any given vessel doesn't mean that said vessel could physically accept such tonnage without breaking into tiny pieces.
It's just a way to measure size, not a dose of reality.
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Old 25-10-2016, 01:15   #6
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Re: help me understand tonnage better?? (for licenses, etc)

Quote:
Originally Posted by black_sails View Post
Right, I understand that just like I can read the wikipedia entries on what each thing is, and look at the forms. I'm just wanting to see some examples of the math of different ways of calculating applied to the SAME vessel so I can see how it applies in the real world.

For instance I wanted to design a series of half recreational/possible half work (passenger charter for side income or mild cargo use) boats (this is a long term plan/what I plan to build and go to sea in retirement being different than what first goes on the water, etc) and i'm trying to figure out just how large it can be at GRT 25 50 100 200. Yet my math has to be wrong because I hear of the 96m Limitless showing at 300 GRT yet my sketch of a two deck wide catamaran exceeds that at 24m length.

EDIT: Yes that's the form I used (though the interactivity didn't work for me but I was trying to follow the instructions I thought) which confused me to begin. I enclose 10,000 cubic feet and exceed 100 GRT with a 55ft long 26ft wide single deck bridgehouse with 7 foot ceilings. I enclose 2500 cubic feet and exceed 25 GRT on something I could probably haul behind my Dodge Ram Cummins.
The USCG measurement procedures dont apply well to catamarans. My 35' cat has a build spec of ~11,000 lbs and comes in at 11 GRT.

My Hobie 33 monohull, which has a dry weight of about 3,500 lbs, just barely squeaks in over the minimum 5 GRT limit to document.

The measurement system, at least the simplified one, does not take hull curves into consideration...basically they just define a rectangle.

Maybe try calculating tonnage of some known production boats to get your head around it. Or, submit questions to the NVDC, helpful folks.
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Old 25-10-2016, 05:08   #7
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Re: help me understand tonnage better?? (for licenses, etc)

Quote:
Originally Posted by black_sails View Post
...not interested in historical or volume or weight, just what will answer whether time spent on X boat counts me towards a 25 GRT, or 50 GRT, or 100 GRT, or 200 GRT licensing class for instance. I mean on a theoretical level/if I don't have the information provided by the owner, i'm trying to understand how the calculations vary and why by seeing examples of them applied to different vessels.)

If you've been on X, Y, Z, etc. boats but don't know their tonnage, sometimes you can look those up by model, on-line or elsewhere, i.e., without needing info from the owner or master. So then you'd know better how many days you have on 25 GRT, how many on 50, etc.


Quote:
Originally Posted by black_sails View Post
For instance I wanted to design a series of half recreational/possible half work (passenger charter for side income or mild cargo use) boats (this is a long term plan/what I plan to build and go to sea in retirement being different than what first goes on the water, etc) and i'm trying to figure out just how large it can be at GRT 25 50 100 200. Yet my math has to be wrong because I hear of the 96m Limitless showing at 300 GRT yet my sketch of a two deck wide catamaran exceeds that at 24m length.
And then once you've looked up specific boats you've been on, to determine their gross tonnage, you can maybe interpolate/extrapolate to other boat lengths -- with conceptually similar beams, shapes, etc. -- and make some semi-decent guesses, better comparisons to your calculations...

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Old 25-10-2016, 05:44   #8
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Re: help me understand tonnage better?? (for licenses, etc)

I'm just going to kind of reiterate what others have said. If you are tying to figure out the logic behind all this, don't. There really isn't any. Especially when it comes to multi-hulls.

Just work through the calculations on form 5397, following the instructions on the USCG website, and there is your answer. It may not make any sense, but if you follow the instructions, and do the math correctly, then it is what it is--whether it makes sense or not.

Good luck.
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Old 25-10-2016, 09:56   #9
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Re: help me understand tonnage better?? (for licenses, etc)

A lot of it will not make sense to anybody because of the way it is manipulated.

There is a HUGE difference in the laws and equipment required once you exceed 300 tons. I used to drive tugs that normally would measure around up to 1000 tons but had special “tonnage doors” (removable bulkheads)and baffles in the tanks that measured it down to 300 tons. I have been aboard a few Mega yachts that would measure 600 or 700 tons but had an Act Of Congress (read a political contribution to a Congressman) that officially made it 300 tons.

The same with licenses, a USCG license up to 300 tons is pretty easy to get, but once you get to 500 tons they start requiring a LOT more experience, seatime, and training.

And... it changes with the country. If you get a US license for 1600 tons, it is actually a 3000 ton ITU license (international) but 1600 tons in the US.

I have been in the business of driving ships and yachts for many years and the ONLY way to understand it is to have an architect or tonnage lawyer give you a very rough idea.

I hope this is more helpful than confusing.

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Old 25-10-2016, 09:57   #10
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Re: help me understand tonnage better?? (for licenses, etc)

Quote:
Originally Posted by belizesailor View Post
The measurement system, at least the simplified one, does not take hull curves into consideration...basically they just define a rectangle.

Maybe try calculating tonnage of some known production boats to get your head around it. Or, submit questions to the NVDC, helpful folks.
A rectangle inside the curves minimizing tonnage or outside the curves maximizing tonnage?

I can't seem to figure out how to calculate it at all though. :) First thing I did was go here, https://www.uscg.mil/hq/msc/docs/CG-...6-04_v1_0b.pdf though for some reason I can't get the interactivity to work. No matter, I go through manually, and the problem is self obvious when I get to #7 'additional dimension for large deck structures' because all my designs have more enclosed cabin than cat hull volume. Unless some part of an enclosed bridgedeck is exempt the cubic volume goes up very quickly. Unless there is some secret reduction factor.

Alternately if I just have basically an open deck tonnage is very little. So it seems skewed against houseboats and enclosed cabin cruisers.


Quote:
Originally Posted by ranger42c View Post
If you've been on X, Y, Z, etc. boats but don't know their tonnage, sometimes you can look those up by model,

And then once you've looked up specific boats you've been on, to determine their gross tonnage, you can maybe interpolate/extrapolate to other boat lengths
I've only been on monohulls and it sounds even more difficult to know for cats.


Quote:
Originally Posted by denverd0n View Post
I'm just going to kind of reiterate what others have said. If you are tying to figure out the logic behind all this, don't. There really isn't any. Especially when it comes to multi-hulls.

Just work through the calculations on form 5397, following the instructions on the USCG website, and there is your answer. It may not make any sense, but if you follow the instructions, and do the math correctly, then it is what it is--whether it makes sense or not.
Not interested in the logic... just the application. I would like to see for instance a For Dummies walkthrough showing me someone writing in the numbers, then doing a calculation, and showing the result so I know i'm not doing it wrong. Then showing this for more than one tonnage calculation (such as GRT for US use, and ITC for international travel) across more than one boat (mostly enclosed, mostly open deck, cat vs mono), so I see how it's not 1:1 and understand the math each way.

My goal is to sketch up my own boats for the future - it's just wondering how much I can fit within different legal captaining classes of for instance 25 50 100 200 GRT. Since it does no good to sketch something slightly larger than I might legally handle with a 6 pack license later on. I'm aware i'm pretty early in the process but that's not the point, my early brainstorming is dreaming up what kind of work I might do with each license, and my ideal plan designing and building a recreational boat that I later do some side work with once i'm retired and have the license to do it. If I can't even sketch ballpark sizes for cargo holds or passenger cabin space options I can't know if my brainstorming is totally off base or not about what I can do with a given license class.

Hence my desire to come in just under each tonnage limit with my catamaran sketches and then figure out what I can do with a boat like that for different jobs. :)
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Old 25-10-2016, 11:07   #11
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Re: help me understand tonnage better?? (for licenses, etc)

Quote:
Originally Posted by black_sails View Post
I will freely admit that i'm quite a newb when it comes to anything beyond recreational boating where I never needed to worry about licenses and such at all at any level accessible to me. When I look through some of the licensing rules for the USCG and elsewhere though I get totally confused at what kind of boat is even in what category. I read about Gross Register Tonnage, simplified calculations for smaller vessels, some kind of net tonnage calculation on larger vessels (or luxury yachts where they try to make "exempt space"), and ITC tonnage.

Even looking at official forms I have to be honest that I don't understand it, because following the rules the answers I get don't seem to make sense. Is there a For Dummies version or explanation showing for instance a specific ship, and all the ways it can be calculated? (exclusively for legal/licensing purposes, not interested in historical or volume or weight, just what will answer whether time spent on X boat counts me towards a 25 GRT, or 50 GRT, or 100 GRT, or 200 GRT licensing class for instance. I mean on a theoretical level/if I don't have the information provided by the owner, i'm trying to understand how the calculations vary and why by seeing examples of them applied to different vessels.)
Don't worry about it as a recreational boater. If you want a license, with tonnage, showing commercial would be advantageous.
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Old 25-10-2016, 12:14   #12
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Re: help me understand tonnage better?? (for licenses, etc)

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Don't worry about it as a recreational boater. If you want a license, with tonnage, showing commercial would be advantageous.
Well the longer term plan is I would switch from recreational to commercial, wanting to do it for retirement income on the side. Whether its yacht charter or whatever. Before I had a license i'd have to hire someone else to skipper it because I wouldn't be allowed. But I still want the boat designed around certain things.

So anyway, finally got the interactive form to work on a friends laptop, and am coming to the same issues I thought I was figuring out manually - 'large deck structures' ruin it.

Playing with different figures on the form make it clear that once deck structures exceed volume that's an issue, but the whole point was wanting a catamaran design with enclosed bridgedeck volume so that I have space space space while keeping it still relatively light. My sketch jumps from like 20 GRT to 120 GRT once I add the enclosed deck. So it's wondering if there are loopholes or very precise definitions of how to get around 'principle deckhouse or cabin' (two separate 'cabins' separately enclosed?) since I was planning the cat hulls to just have machinery/fuel/heavy stuff and all living space on the bridgedeck.

Another thing that's annoying is the trihull calculation - it says 'applies if no buoyant volume in the structure connecting the hulls' but I assume this means if I have anything except a perfectly flat underside to the bridgedeck, if I use shapes of any sort to try and handle waves better, it's penalized or is the shape exempt as long as it's above the waterline?

It also says to measure by the Convention Measurement System if the boat will be used outside of the USA, so i'm wondering if there's a similar interactive form or directions for figuring that.
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Old 25-10-2016, 12:45   #13
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Re: help me understand tonnage better?? (for licenses, etc)

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Originally Posted by black_sails View Post
I've only been on monohulls and it sounds even more difficult to know for cats.


Fair enough, but you can look up tonnage for existing cats, too. Have a go at yachtworld, find a cat you like and that might be documented, then look it up someplace like here to find the tonnage: Coast Guard Vessel Search

Get enough of those the might be representative of what you're trying to design/calculate/build, and that could give you some empirical data for comparison.

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Old 25-10-2016, 13:48   #14
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Re: help me understand tonnage better?? (for licenses, etc)

I believe just getting a 100T ticket would be all you would want. I am not sure what the knowledge of determining tonnage is of use to you, unless it is on a test. A head boat Capt. or delivery Capt. generally only have that. If it is for a couple of retirement bucks get a job with a tow boat co. Under 24ft. doesn't even require a tow boat license, last I checked. Run a commercial fishing boat no license required.
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Old 25-10-2016, 14:21   #15
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Re: help me understand tonnage better?? (for licenses, etc)

Quote:
Originally Posted by black_sails View Post
Another thing that's annoying is the trihull calculation - it says 'applies if no buoyant volume in the structure connecting the hulls' but I assume this means if I have anything except a perfectly flat underside to the bridgedeck, if I use shapes of any sort to try and handle waves better, it's penalized or is the shape exempt as long as it's above the waterline?
I take "no buoyant volume..." to mean nothing below the waterline.
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