Boat length measurements

[WotEver]

5 minutes ago, Mike the Boilerman said:

As I said, measuring the length of a narrowboat is remarkably difficult!

• Drain the pound 
• Drop a plumb bob from each end
• Drive in a stake where each plumb bob reaches the bottom
• Fill the pound and bow-haul at least one boat length down the cut
• Drain the pound
• Measure between the two stakes

simples. 

[MtB]

On 20/09/2017 at 13:58, WotEver said:

• Drain the pound 
• Drop a plumb bob from each end
• Drive in a stake where each plumb bob reaches the bottom
• Fill the pound and bow-haul at least one boat length down the cut
• Drain the pound
• Measure between the two stakes

simples. 

 

Having tried to measure the width of a boat accurately whilst docked using a plumb bob on each side and a plank underneath, I can confirm your method will not work if there is the slightest breath of wind. The PB will swing around by half an inch with even the tiniest bit of air movement.

 

[dmr]

17 minutes ago, Mike the Boilerman said:

 

This bit makes two assumptions not necessarily correct. 

1) The front cabin bulkhead is actually vertical, and 

2) There is no cratch and cover in the way of the tape measure.

Also, I rather doubt the accuracy of your length measurement of 70' 8 3/4" unless you also did a leetle pythagarus to account for the gradient of the gunwale over the length of the boat. I'd suggest the true length of your boat is closer to 70' 8 5/8"

As I said, measuring the length of a narrowboat is remarkably difficult!

 

Edit to add: And I'd like to congratulate you for being the first person to ever come up with a legitimate use for a spirit level on a boat 

and pythagarus suggests that the boat length changes by 9 thou depending upon the fullness of the water tank, I am surprised its so little, think I better check that.

.................Dave

[WotEver]

4 minutes ago, Mike the Boilerman said:

Having tried to measure the width of a boat accurately whilst docked using a plumb bob on each side and a plank underneath, I can confirm your method will not work if there is the slightest breath of wind. The PB will swing around by half an inch with even the tiniest bit of air movement.

Ya need a heavier plumb bob. 

Inertia and all that. 

[Tonka]

mild steel rusts at 7 thou a year untreated in salt water. so your boat is always getting smaller

[Jen-in-Wellies]

Has no one mentioned thermal expansion yet? The temperature at which it is measured must also be specified as well as the length. A 70' steel long boat will expand by 0.01" for each degree C rise.

Aluminium has a higher thermal expansion coefficient than steel and GRP is even higher. This is why most Sea Otters and plastic boats are not full length. Too much risk of them jamming in a lock on a hot day. CaRT should mandate that all boats are made from Invar so the length measurements and licence categories are consistent from summer to winter.

Jen

Edited September 20, 2017 by Jen-in-Wellies

[Murflynn]

6 hours ago, Neil Smith said:

• Murflynn was happy to get a 61 foot boat when ordering a 57 but I would not as I wanted a go anywhere length.

no I wasn't.

I sent it back for a full refund.  ...............  no, I jest ................  they took it into stock and started again.

22 minutes ago, Tonka said:

mild steel rusts at 7 thou a year untreated in salt water. so your boat is always getting smaller

yes but ...................  have you allowed for the paint thickness?

it is always better to thin your paint sufficiently to ensure you don't exceed the stated dimensions.

[MtB]

3 hours ago, WotEver said:

Ya need a heavier plumb bob. 

Inertia and all that. 

 

I still think even the smallest breath of air will move even a whopping great plumb bob too much. I was trying to establish the width to a tolerance of about +/- 1mm.  Closest I could get on a fairly calm day with a bob weighing about an ounce was about +/- 15mm.

[PeterF]

Quite an interesting topic, I moor on Calder & Hebble and have what was quoted as a 55ft boat, using the length quoted by the broker I brought it from, which was as stated in the original builders manual. I like the length because I can open lock gates whilst lying along the lock wall, no need for jiggling. We are now looking for a new boat so I measured the boat and came up with 54ft 6in over steel not including fenders. I measured it from the pontoon and bit by bit through the boat, with measurements within 1/2in of each other. Now I need to look for a 54ft 6in boat.

[ditchcrawler]

24 minutes ago, Jen-in-Wellies said:

Has no one mentioned thermal expansion yet? The temperature at which it is measured must also be specified as well as the length. A 70' steel long boat will expand by 0.01" for each degree C rise.

Aluminium has a higher thermal expansion coefficient than steel and GRP is even higher. This is why most Sea Otters and plastic boats are not full length. Too much risk of them jamming in a lock on a hot day. CaRT should mandate that all boats are made from Invar so the length measurements and licence categories are consistent from summer to winter.

Jen

Is that all, in hot weather its sufficient to make my sliding toilet door stiff to move

 

[dmr]

40 minutes ago, Tonka said:

mild steel rusts at 7 thou a year untreated in salt water. so your boat is always getting smaller

This is clearly wrong, my boat gets longer every year. I often pace out the towpath to see if we can fit into a tight spot. When we got the boat, ten years ago, it was 24 paces long, last year it was 27, today it was almost 30.

................Dave

[David Mack]

45 minutes ago, Tonka said:

mild steel rusts at 7 thou a year untreated in salt water. so your boat is always getting smaller

And rust is about 10 times the volume of the steel from which it came, so your boat is always getting bigger...

[MtB]

21 minutes ago, David Mack said:

And rust is about 10 times the volume of the steel from which it came, so your boat is always getting bigger...

 

This is an urban myth I reckon. Rust seems to me to be far more than 10 times the volume of the donor steel. 

Only my perception though. I'd be interested if there is any research on it.

[Phil Ambrose]

On ‎25‎/‎08‎/‎2017 at 09:24, bizzard said:

LOA length overall.  LWL length waterline.

LOA length overall.  LWL length waterline.

The above is frequently seen on The Broads 

Phil 

[Murflynn]

2 hours ago, Mike the Boilerman said:

 

I still think even the smallest breath of air will move even a whopping great plumb bob too much. I was trying to establish the width to a tolerance of about +/- 1mm.  Closest I could get on a fairly calm day with a bob weighing about an ounce was about +/- 15mm.

in the bad old days before the electronic age we used plumb bobs to set up theodolites over the target - heavy bob, thin wire, accurate to within about 2mm.

measuring a fabricated box, that will suffer from welding deformations, rough/ground corners/edges and normal levels of dimensional control in the workshop, to an accuracy of 1mm is just plain impractical.

[MtB]

2 minutes ago, Murflynn said:

in the bad old days before the electronic age we used plumb bobs to set up theodolites over the target - heavy bob, thin wire, accurate to within about 2mm.

measuring a fabricated box, that will suffer from welding deformations, rough/ground corners/edges and normal levels of dimensional control in the workshop, to an accuracy of 1mm is just plain impractical.

 

I was trying to measure the fattest point by far, where a four figure sum (not beginning with a '1' or a '2') had been spent slimming it.

But I take your point. 

In the end I just took the boat through the lock where it previously got jammed fast for 5 hours. Went through with 2" space at the fattest point so money well spent.

Still got stuck in Hurlesedon a year later though, grrrr......

[Murflynn]

even if you made a frame with internal dimensions of the supposed maximum dimensions of the boat and checked by sliding it along the boat it wouldn't help much because it wouldn't take into account the slight banana shape of many (most?) boats.

probably the most reliable way of checking the 'effective width' of your boat would be to set up string lines along both sides of the boat fixed to cross pieces secured to the bow and stern decks, getting the string really tight and then measuring the separation at bow and stern.

[dor]

On 8/25/2017 at 07:25, Athy said:

Certainly I have never measured Trojan, but then I have never tried to sell her.

I am always impressed by the brokers (I think ABNB is one) who advertise a boat as being for example "45 feet 2 inches". This, to me, is an indication of a careful attitude and thorough approach which will extend to their description of the boat's other features.

In my barman days, when bar staff knew all the prices and added up in their head as they went along, I was told never to ask for an exact number of pounds, as it looks as though you were making it up.  So you always added a shilling or two to the amount and, if feeling generous, said "call it £x" to please the purchaser.

[alan_fincher]

17 hours ago, Jen-in-Wellies said:

Has no one mentioned thermal expansion yet? The temperature at which it is measured must also be specified as well as the length. A 70' steel long boat will expand by 0.01" for each degree C rise.

Provided you measure a steel boat with a steel tape measure, surely the measured length is unaffected by the ambient temperature?

[dmr]

17 minutes ago, alan_fincher said:

Provided you measure a steel boat with a steel tape measure, surely the measured length is unaffected by the ambient temperature?

A greeny for that one.  Strictly you are suggesting a thermally compensated tape measure.

.................Dave

[MtB]

18 hours ago, Jen-in-Wellies said:

Has no one mentioned thermal expansion yet? The temperature at which it is measured must also be specified as well as the length. A 70' steel long boat will expand by 0.01" for each degree C rise.

 

Point of Order M'lud...

I don't think longboats are ever made from steel. I'm pretty sure they are usually wooden or fibreglass.

https://en.wikipedia.org/wiki/Longboat

Longboats being dispatched from a tallship:

 

P.S. Longboats are sometimes confused with Viking longships.

[dmr]

Think it was just a dyslexic moment, he meant "long steel" rather than "steel long", just like I have a long narrowboat but I don't have a narrow longboat .

..............Dave

[Tacet]

For the really fussy, measurement with a Total Station would be a good method; it's a sort-of theodolite that also takes laser measurements.

The advantage would be, providing sufficient points are taken, is that it would identify banana boats and irregularities in the vertical plain.  So you could identify, for example, the minimum distance between two vertical walls required to pass the boat.  Which is one interpretation of the maximum width of a boat

Practical problems would lie in ensuring the boat was very well tied up and not bobbing around whilst the exercise was undertaken.  And measuring all the locks and narrows.

 

 

[ditchcrawler]

11 hours ago, dor said:

In my barman days, when bar staff knew all the prices and added up in their head as they went along, I was told never to ask for an exact number of pounds, as it looks as though you were making it up.  So you always added a shilling or two to the amount and, if feeling generous, said "call it £x" to please the purchaser.

Same when I took deadweight pressures of gas wells. it was never 1040psi

[Murflynn]

1 hour ago, Tacet said:

 

Practical problems would lie in ensuring the boat was very well tied up and not bobbing around whilst the exercise was undertaken.  And measuring all the locks and narrows.

 

 

which is why traditional methods work best.

when I started on site as a setting-out engineer all that was required for each building was four 90degree corner frames, each set at a fixed datum level and with a nail to hold the string line in each of the 2 directions; brickwork was always straight and true, 'cos the brickies understood the system.  Surveying has become a dark electronic art and for simple structures is just a load of bollix.