Displacement vs Gross

[goinfishin]

I'm confused by the difference between gross and displacement weights, and which of these applies to towing weights. The Merry Fisher 695 displacement weight is listed as 1.6 tonnes, but seemingly has a gross weight of more than 6.5 tonnes - no way I'd pull that with my Freelander!
Can anyone shed some light - Please!

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[longjohnsilver]

Most definitely won't be 6.5 tons! The 1.6 ton mark seems realistic for a boat that size.

Where did you see the 6.5 tons gross?

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[Birdseye]

The figures you quote are wrong. Archimedes long since established that a boat displaces exactly the same weight of water as it weighs. if it displaced less, then it would sink. it cant displace more except in the situation where you drop it into the water and momentum carries it down a bit. but it then bobs up again until the two are equal.

the situation is sometimes confused when people quote old style tonnages of the sort you see carved into the beams of registered boats. these arent weights - they are a measure of size. bit like the old austin 12hp never developed anything like 12bhp. it was just a number for tax.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[davidbuttriss]

I concur,
Basic physics dictates that an item when placed in a liquid displaces an amount of liquid equal in weight to the weight of the item.
When my Stella was trucked from Maldon to Southampton this question was put by me to a couple of Professors at Soton Uni reference displacement tonnage and weight etc etc.. They both quoted Archimedes and if 2 professors, both naval architects then quote that displacement and actual weight are the same then who are we to argue!

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[Ruthie]

Yup - the 6.5 ton will probably be the gross registered tonnage... which as I understand it is the weight of the boat plus it's cargo capacity.

The smaller figure is what the boat actually weighs, which is the same as the displacement.

The ony subtlety to note is that while the weight displacement stays the same, the volume displacement varries according to the density of the water... so you'll displace a bigger volume for your weight in warm or fresh water than cold or salt water...

Ruthie

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[PaulS]

This is about as good an explanation as I've heard.

http://www.uscg.mil/d13/units/vts/ton.pps

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[goinfishin]

Apparently the 6.5 tonnes is on the bulders certificate, the boat is on ebay - have a look item no. 2486713867.

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[MIKE_MCKIE]

Gross tonnage is a measure of internal volume & is a hangover from earlier days when there were no "pleasure" boats, only working boats. It was used to establish the earning/carrying capacity of a vessel & the consequent ability of harbour authorities & the like to charge vast amounts of dosh to berth/load/discharge whatever. This was actually the Nett Register tonnage & was the gross tonnage, less things like living accomodation, storerooms etc. Basically one gross ton = 100 cubic feet of internal volume.
Many/most boats built in the last 10-20 years have a much higher gross to Displacement ratio than previously. I have a 30+ year old, 26' displacement boat and while there are 4 berths, it is decidedly friendly & weighs about 2.7 tonnes! A friend of mine had a MacGregor 26, which by comparison has vast amounts of internal space & weighs about 1.5 tonnes max.
For towing purposes use the displacement figure, that is how much the boat weighs.
Happy trailing
Mike

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[Mollari]

My Seamaster 34' on a transportation trailer got stopped at a weighbridge, after deducting the weight of the vehicle and empty trailer, the boat, without any liquids on board clocks in at 5846 kilograms that is 5.846 metric tons and yet my displacement in the registry book says 10.13 tonnes.

I am still confused, so what does my boat weigh?
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[snowleopard]

add the crew, normal stores, water and fuel to your 5.8 and you've got it. does your registry certificate give any figure for gross, net or thames tonnage? if not, maybe the 10.13 is actually gross registered tonnage

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[Ruthie]

Hi Mollari,

Your boat weighs 5846 kg - or 5.846 tonnes.

The registered tonnage is the internal volume in cubic feet, divided by 100, expressed as "tons". It's not a weight or displacement.

Interestingly, my boat is just a wee bit lighter than yours - around 5400 Kg, and has a registered tonnage of 9.92 Tons.

Ruthie

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[Guest]

Displacement is actual weight displaced in water ... so equates to real weight.

Gross and incidentally Nett realte to volume measurement and this is then transformed to tonnage ..... it has to do with cargo holds and the fees due for Lights / tonnage tax's etc. on commercial shipping.

You can forget about Gross and Nett Tonnages - they have no relevance to yachts.

Just note that Displacement will be quoted for the boat set for normal use without various additions that owners put in. So add a bit for safety.

As to the boat you quote and a Freelander - no worries there at all.


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Guest]

Basic physics ....

<<Basic physics dictates that an item when placed in a liquid displaces an amount of liquid equal in weight to the weight of the item.>> -

Only when item remains floating and free of all support etc.


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[oldharry]

Tons and Tonnes. 1.6 Tons is the displacement - the actual weight of the boat. Except that it is not! Manufacturers quote the weight of the boat 'ex works' without any of the gear on board, sometimes excluding even the engine!. So you must allow for the weight of all the gear, cooker, batteries, fuel, water, anchors, chain, warps, fenders, food, bedding..... etc, etc , etc!. Maybe as much as a quarter if a ton more! And dont forget to add the weight of the trailer, which for a boat that size will be several cwt. You may in reality be approaching 2 tons - still within the capabilities of your Freelander though.

Tonnage is a totally different measure of the useable volume of the hull originally for cargo carrying purposes, and really only an indicator of the roominess or otherwise of the hull.

So my 26ft Eventide is a 6 tonner, with a designed displacement of around 1.6 tons, and registering just over 2 tons gross weight on the yard crane at lift out with all the gear on board.

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[Guest]

Oldharry ..... would have been better if you had left out the GROSS in your yard crane weight !!!

Oldharry should have said ..... total overall weight ...... 2 tons - which would equate to total displacement .

To give an example from commercial side :

Initial displacement is weight of vessel before cargo operations (load / discharge). Final displacement is weight of vessel after cargo operations (load or discharge) The difference of these is the cargo loaded or discharged (less any changes in stores / fuel during the operation). The displacement is derived by calculation from fore, aft and midships drafts on each side of the vessel. Allowances for hog / sag of the longitudinal form of the vessel and combination of the drafts arrives at a final draft figure to enter displacement tables and so arrive at overall tons of the vessel in ACTUAL weight in salt-water 1.025..... then this corrected for observed density of the dockwater to arrive at true weight at that instant.

Phew ..... difficult to describe as we do this calculation frequently in our line of work........


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[oldharry]

Sorry - SWMBO so often complains that MY weight is 'gross' these days.....

You are quite right of course Nigel - the yard Craney reckons my boat lifts at just over 2 tons 'actual weight', not 'gross weight'

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[Guest]

Another club starters ?

I have an affliction .......called wife who reckons I'm a touch over-stuffed !!! But she loves me.

I wasn't being smart - just wanted to make sure that original postee didn't get confused ..... Gross / Nett / Displ. etc.


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[oldharry]

Re: Another club starters ?

Ditto. And both she and you have rightly pulled me up for making unclear comments late at night - you once, she - well, several times a week.

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[goinfishin]

Many thanks to all who replied, a wonderful response. I have learned a lot!

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