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The Principle of Archimedes

Posted on September 9, 2012 at 9:45 AM

The surveyor had asked me to find out if possible whether Pentargon had ballast on board. At time of writing late Jan2012 I'd not yet had time to root around under the sole plates.  Pentargon had failed survey because the hull had failed the ultrasound test. 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 v-shaped keel. Of the 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 amount to the addition of 900kg to the all-up weight and both the weight of a sheet and the number of sheets is optimistically erring in our favour.  


900kg would be going on the very bottom and would substantially lower the centre of gravity which is a very good idea. The boat would become more stable, less likely to roll, less prone to wind effect and should be easier to steer and to handle. The extra 0.9 tonne should cause Pentargon to lie lower in the water (increasing her draught by about 2").


The surveyor was concerned that, if dropped low enough by the addition of large amounts of weight, the level of water in the weed-hatch would be too high and the waterline would come perilously close to certain orifices such as the sink outflow and gas overflow vents. He'd asked whether Pentargon had any ballast.  At time of writing I'd yet to root around under the sole plates and after thinking about it I decided i could not be arsed. Instead I gauged the boat by loading it with over one tonne of live-weight.




I could not believe my luck when, peering out of the hatch through the Braunston rain in the mid-morning of Wed. 22nd Feb., I espied a number of gentlemen taking tea under an adjacent tree. Having engaged them in conversation, and established they were a rambler group, I told them they could help me greatly (after tea of course) to put the Archimedes principle to the test in the most blatently obvious way. The Greek is noted for establishing the basics of flotation while soaking in his bath. If you load a ship with extra ballast it floats lower in the water.

My man in Hillmorton would cost me an arm and a leg grinding off considerable amounts of steel, before replacing it by other steel. Convert "considerable" into large wads of time and wonga and I could pay £1000s to swop weight for 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 understood the 'Principle of Archimedes' and/or the original meaning of "Eureka!"

Suffice to say that the ramblers were 12 in number and all admitted to between 80 & 90+ kg in weight. Collectively they 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.


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