... ... ... Seven years in an unassuming narrowboat on the waters of the "longest, friendliest village in England"  


The Principle of Archimedes

Posted on September 9, 2012 at 9:45 AM

At time of original writing, late in Jan2012, I'd not yet had time to root around under the sole plates. (floor!)

My surveyor had asked me to find out if possible whether Pentargon had ballast on board.

Pentargon had failed it's survey because the hull had failed the ultrasound test ... (story elsewhere)

Momentous decision was made. I purchased her as a 'write-off'  (at a write-off price) and decided to pay to get the hull done properly.

This would involve using 6mm sheeting, pre-formed in a press to the profile of the Springer bottom, incorporating chines, tacking the sheets and stitching them together. Serious engineering. Serious money. But it would give Pentargon a 25yr. lease of life and see me off the planet. Also would make Pentargon a realisable future financial asset. In a previous life I had training or experience in, among other things, sheet metal fabrication and design.  

6mm steel weighs about 50kg per sq.m.

Pentargon is 6'10" in the beam and has a [very shallow] v-shaped keel. 36' LOA,  6' is counter and maybe 3' is bow. The "bottom" for this exercise would be about 27' long. Steel sheet comes in 2m x 1metric, although 8'x4' imperial can be found. Metric sheets, (9 of them at 100kg each) would add 900kg to the all-up weight of the finished boat. and realistically, my previous experience working with sheet steel all those years ago had taught me to allo a 'margin of error' of about 10% and always MORE than expected. Call it a metric tonne then ...   

 This is what concerned my surveyor.

Even though the 900kg+  would be going on the very bottom, lowest point of the boat and would lower the centre of gravity which is a very good idea. The boat should become more stable, less likely to roll, less prone to wind effect and be easier to steer and to handle.

The extra tonne should cause Pentargon to lie lower in the water (in theory by increasing her draught about 2").

 My surveyor was concerned though that, if lowered too much enough by the addition of large amounts of weight, the level of water in the weed-hatch might be affected (there has to be a clear 10" in there for the BSS!)  and the waterline outside might come perilously close to certain orifices such as the sink outflow and/or the gas overflow vents.

He'd asked whether Pentargon had any ballast.  At that time, I had yet to root around under the sole plates and anyway, after thinking about it, decided I really could not be arsed. Lifting sole plates would involve moving furniture and once one thing leads to another, leads to yet another ...

I would use my 1960's schooling in Physics and weigh down the boat using a long-forgotten Greek to help me.


Basically, it does not much matter what kind of tonnage you use as long as it gives you a load you want to press the boat down in the water.

I would gauge the boat by loading it with one tonne of live-weight. Cattle or sheep or even PEOPLE, indeed anything with legs that can  be herded on board would do the job. 

From the deep resesses of memory past, I remembered that a crew of 12 sailors weigh about one metric tonne.

trust me on this ...


I could not believe my luck when, peering out of the hatch through the Braunston rain, in the mid-morning of Wed. 22nd Feb.2012, I espied a number of ladies and gentlemen taking tea under an adjacent tree. Having engaged them in conversation and established they were a rambler group and twelve in number, I told them they could help me greatly (... after tea of course ...) to put Archimedes to the test in the most blatently obvious way.

The Greek established the physics of flotation while soaking in his bath. Load a ship and it floats lower in the water. One of the motley crew pipes up and sez ... "We are having a day off from Warwick University. We 'erm' are part of the engineering faculty. In all of England I would be the one to pick 12 engineers, all of whom were more than casually acquainted with yer ancient Greek ...

My man in Hillmorton would cost me an arm and a leg grinding off considerable amounts of unwanted old steel, before replacing it by new steel. Convert "considerable" into large wads of time and wonga ... I could pay £1000s to swop old weight for new weight with no appliance of science whatever and no improvement in performance.

If I could show that NO steel had to be removed, we could just get on with the business of cleaning up the bottom and putting on new steel. The 'problem' with both surveyor and fabricator was that neither appeared to fully understood the 'Principle of Archimedes' and/or the original meaning of "Eureka!"

Suffice to say that the ramblers, 12 in number, weighed in at about 1000kg and there was little need for precise measurements. They gladly went along with my idea of standing around on the boat roughly distributed the same way as the weight of the steel would and in a jiffy I got a couple of reference photos.

The old bargees had a rule of thumb that a ton of cargo would drop a 70' narrow boat one inch. It was a standard way of meas cargo at tolls.

The end result of my little experiment with the Warwick Engineers is that it does not covert into reality on a 36' steel Springer. The boat should have gone down about two inches. It actually made no measurable difference having twelve apostles on board.

WIP 13.41 2018-12-12 at Kilburn Library

WIP 12.02 2019-06-04 at Hertford Library


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