buoyancy.

[trebor]

I am re-furbishing a Minisprint, the buoyancy in front of boat looks a bit ropey, is their a way to work out what the newtons are for the polystyrene ?
I would prefer to replace with plastic bottles, they will not absorb water and are easily/cheaply replaced if their is a problem, same question, a 2 litre pop bottle, how much buoyancy ?
The volume of Polystyrene is approx 2 square feet (24" long x 6" x 6")

[Ed]

I have worked out how to do this in the past.....but exactly how I did it seems to escape me at the moment....

I wouldn't worry about it though and would just make sure there is more air than was in before... in other words just stuff as many bags/bottles in as you can.

I use those plastic bags that they use for packing, which work well enough.

This does presume that the polystyrene doesn't also have any structural use of course.

eib

[SoggyBadger]

Convert the measurements to centimetres so that's rough 60 * 15 * 15 which gives yoy 13500 ml or 13.5 litres.

[trebor]

Hi Ed, I am glad you mentioned structural, I had not looked at that.
Hi Paul, Thats surprising, 13.5 litres.

[Max McCarthy]

Hi Rob,

The density (of most, from a quick internet search) is 0.1g/cm cubed of polystyrene. And the density of water (roughly speaking) is effectively 1g/cm cubed.

So, considering your polystyrene must displace more than its weight to float, and it would weigh around (comparing SI to imperial measurements, and the size of your polystyrene) will, therefore weigh around 1.4kg.

So, the volume of your polystyrene, is 14158cm cubed, so when fully submerged, would displace this amount of water, and as water is 1g/cm cubed, it would have a force of around 14kg when fully submerged, of course you take away the weight of the polystyrene which (according to the internets density of polystyrene) is 1.4kg, so 12.6kg of buoyancy would come from the polystyrene, and in Newtons that is 123.6N (when no water is taken in, and it is fully submerged, although, it seems unlikely to be fully submerged).

Of a 2 litre bottle (with the density of air being around 0.000001g/cm) will be, with 2000cm^3 being the same as 2 litres, it would be 0.002g, so it would need to displace that to float, and of course the weight of the bottle, lets says 20g? So 20.002g of water will need to be displaced, being the same as effectively 20 cm^3 of water, it would, supply 2000-20 = 1980g or 19.4N, with that being 1 bottle, with the bottle (when full) being approximately 2 litres, and of the same area as the polystyrene, it would have 14N of extra buoyancy compared to that of the polystyrene, of course assuming all the bottles could fit together perfectly.

And considering you can't have circular bottles perfectly adjacent to each other, with no gaps, I would have thought (until the polystyrene soaks up water) that you would be better off with polystyrene.

But this really depends on how many bottles you can fit into a space of 1/2 a cubic foot.

Please note, I am assuming, in my working that the bottle fit perfectly together, and in reality we know they don't, so I think the space you have available is better off with polystyrene, but of course this depends on how much space there is in the bow, and how much space is taken up by the bottles trying to fit together....

Hope this helps...

Cheers,

Max

[davidh]

I'm with Max on this one! Bottles, loose around in a tank, can create 'hard spots' when you get excess buoyancy in one small part that can easily lift decks. You also want to consider the flip side of your equations - how much the buoyancy compartment would weigh if it were filled with water. Working on the basis that one metre cubed weighs one metric tonne, you want to keep as much of the wet stuff out - Polystyrene, wrapped in plastic and sealed works best at this!

D

[JimC]

I'm with Max on this one! Bottles, loose around in a tank, can create 'hard spots' when you get excess buoyancy in one small part that can easily lift decks.

Surely only if the tank actually fills up with enough water for the bottles to be jammed up against the deck?

[Paul Rogers]

My minisprint uses the polystyrene to add rigidity to the moulding. The polystyrene blocks are attached to the underside of the foredeck with a layer of more rigid expanded foam that was obviously poured in before the deck and hull mouldings where put together. Even removing enough to fit an inspection hatch in the deck has caused the deck to become more flexible. I have used expanding foam to reseal around the hatch and will be using more to add extra boyancy to the bow area. I had considered bottles but having taken pictures of the interior I decided that they could well move around and get jambed in restricted areas and cause hard spots as previously stated.

Paul.