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Does anyone know how much their boat weighs?

Neil2

By Neil2
in General Boating

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Neil2 Veteran

Neil2

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I've been trying to do a displacement calculation as an estimate of how much our boat weighs.

 

I'd like to do a comparative check to see if I'm miles out. Some members may have had their boats craned out or had the weight calculated by a more accurate means - I just used graph paper to estimate the displaced volume.

 

It's a 45 foot trad and by my reckoning she's about 10 metric tons. How does that compare?

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Guest

Guest

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My 57ft semi was 14t when craned in as a sailaway but with all the timber aboard and ballasted. I estimate 15 tons finished with tanks full.
Wouldn't think you are far out at 10.

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churchward Veteran II

churchward

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Posted (edited)

I've been trying to do a displacement calculation as an estimate of how much our boat weighs.

 

I'd like to do a comparative check to see if I'm miles out. Some members may have had their boats craned out or had the weight calculated by a more accurate means - I just used graph paper to estimate the displaced volume.

 

It's a 45 foot trad and by my reckoning she's about 10 metric tons. How does that compare?

A rough general estimate for a steel narrowboat is 1 ton per metre of length.

 

Of course with generalisations a specific boat will be more or less depending on how it is built but I am told it does bear up to measurement by crane give or take a bit.

Edited by churchward
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MoominPapa Veteran II

MoominPapa

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I once did an estimate based on displacement and got a figure significantly north of 20 tons. (60' NB, 2'10" draft). That feels about right when you're on the end of a centre line trying to stop the thing, or (as I was today) standing on the back as the Bill Fen tractor struggles to control the weight as the trailer is backed down the slip.........

 

MP.

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Chewbacka Veteran

Chewbacka

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If the boat was square, so no pointy end and no swim, and is 45ft long by 6' 10" and sits 18" deep it would weigh 13 tons.

So if you reduce the volume by say 30% around the bow and swim, then depending upon the actual dimensions would reduce the 'weight' by a ton or two. So about 10 tons seems not unreasonable to me, given my guesses.

Edited by Chewbacka
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NickF Rising Star

NickF

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I once did an estimate based on displacement and got a figure significantly north of 20 tons. (60' NB, 2'10" draft). That feels about right when you're on the end of a centre line trying to stop the thing, or (as I was today) standing on the back as the Bill Fen tractor struggles to control the weight as the trailer is backed down the slip.........

 

MP.

Surely that assumes 2'10" draught for the whole length of the boat, is it not less at the bow so you have to use an average?

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Southern Star Experienced

Southern Star

Posted
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I've been trying to do a displacement calculation as an estimate of how much our boat weighs.

 

I'd like to do a comparative check to see if I'm miles out. Some members may have had their boats craned out or had the weight calculated by a more accurate means - I just used graph paper to estimate the displaced volume.

 

It's a 45 foot trad and by my reckoning she's about 10 metric tons. How does that compare?

All I'm basing this on is a working lifetime of needing to assess the weight of an object without actually being able to weigh it, but I reckon you are fairly close there. My own boat is a 41' trad and I would bet money on it weighing between 9 and 10 tonnes, I would estimate that a 45' boat probably weighs about a tonne more than that.

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MoominPapa Veteran II

MoominPapa

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Surely that assumes 2'10" draught for the whole length of the boat, is it not less at the bow so you have to use an average?

 

I can't remember the details, but I do remember using the average draft and taking into account the bow and the swims when I made the estimate.

 

If you ignore the pointy ends and assume bow draft is half stern draft, you get a displacement of about 26 cubic metres, so that looks sane.

 

2'10" = 90cm

60' = 18.3m

6'10" = 2.1m

 

2.1 * 18.3 * (0.9 + 0.45)/2 = 25.9

 

MP.

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Laurence Hogg Veteran

Laurence Hogg

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Our large Woolwich 71ft 6" motor, 4ft 9" sides weighs in a 19 tons without any ballast. We carry approx. 5-6 tons of ballast giving us a working draft under way of approx. 3ft. The hold ballast can be topped up with oil drums full of water adding around another 7.

Edited by Laurence Hogg
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Neil2 Veteran

Neil2

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When we had our boat craned out for blacking recently we were told that it weighed 11 tons.

It's only 42ft long, but 30" draft at stern and about 26" at bow.

That's interesting, our boat is about 26" fore - aft (she almost trims level), which seems quite low for a 45' ten ton boat, one reason why I posed the question. But our boat is very "fine" underwater with a long swim, is yours similar?

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Tony Dunkley Influencer

Tony Dunkley

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The displacement ( weight ) of a flat bottomed and slab sided vessel [ ie. most 'narrowboats'] can be calculated accurately enough for most purposes by the following :-

Waterplane area ( in square feet) X mean draught ( in feet)

_______________________________________________ = Freshwater Displacement (in tons)

 

36

 

The small additional displacement from the shallow immersion of the counter can be conveniently reckoned to roughly offset the small reduction in displacement resulting from the flare along the sides immediately above the chine. The waterplane length ( for the purpose of calculating waterplane area) is the distance between the sternpost (where the swim side plates meet) and the stem at the normal load waterline, and therefore somewhat less than the overall length. Mean draught is the average of the draught at the stem and the sternpost.

From previous Posts there seems to be a tendency to use the overall length and the maximum draught as well as not taking into account the reduction in w/p breadth down to nothing at each end. This will result in an overestimation of the true 'weight'.

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Bettie Boo Experienced

Bettie Boo

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By the looks of it, we're a little overweight. Mind you we don't sit very low in the water

 

When we were craned out/in for the road haul down we were 29 ton, I recon we are about 30+ now with all our loose furniture, fixtures, coal & full tanks. 57ft trad x 10.10

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