Positive Buoyancy Floatation foam & Ullage Equation

:?:

Hi guys...

Some years ago... a naval architect who designed my boat gave me a equation to calculate how much positive bouyancy foam i needed to include in my hull to achieve positive bouyancy when swamped.

Like usual I wrote it down on the back of an envekope and later lost it!

I can't remember it.

Anyone have it handy?

From memory (which is hazy this many years later) it was the weight of hull plus weight of OB - and batterys and other onboard machinery such as anchors - bailing backets, tackle etc etc etc etc plus ~10% of the weight of passengers and so many % of the weight fuel etc etc...times or divided by the weight of frsh or salt water - the number of cuboc feet (cubic meters) of displaement foam than needed to be placed to achieve positive bouyancy..

Then there was the hulls ullage calcs to coampare to, too see if there was sufficient room under decks etc etc to pace required amount of foam...

Does qanyone have this actual equation please?

I remeber that fuel tank for example (if full) is lighter than water and floats in a swamping situation so only s any % of it's weight counted and so on - it was quite a complex equation - but made a lot of sense if you fiollowed it through logically - I think passengers was allowed at 80 kilos each times the rated passenger carrying capacity plaque etc...

I'd be very appreciative if anyone has this equation - I'd like to keep it to help answer posts at various boating boards for those doing "projects/restorations" on boats or building their own new boat - so they can work out how much foam they need and whether they have the space to actually fit it.

I'm tired of all the old wives tales about using ping pong balls - and PET drink bottles - etc - I'd like to get some "factual information / advice" that I can keep on file and cut paste each time the question comes up.

Many thanks in advance, I know you naval architects like Will A etc will have this info tucked away somewhere.

Cheers!

 

Total weight of boat plus passengers (allow for full fuel tanks always) plus all their/your gear (i.e. weight of boat including a full load) in pounds (for this example). Take that number, and divide it by (64 minus the weight of the foam in lbs/cubic foot). That will give you the volume (in cubic feet) of foam required to float (just) the boat, etc. If you have empty space in the fuel tanks, that just adds more safety, but don't assume it will sink with half-empty tanks - they may be full. Add, say, 25% for the fact that you want it to float with the gunwales above water .
As far as how much volume you have available, that will vry from boat to boat, but should not be too hard to figure.
It is vital that you deduct the weight of the foam from the density of seawater (that "64" figure) or the weight of the foam could sink you.

Steve "rule of thumb, baby..."

 

You can still have a boat that floats, but not upright...! I strongly recommend that, if you make the effort of making your boat "unsinkable", you check stability first. You might otherwise waste your sweat!

Zed