Re: Floatation calculations, or what will it take to float your boat ?
This is a great thread, One thing to consider is that from what i can tell everyone is calculating the amount of foam needed based on the air weight of their boats. However if you calculate the negative buoyancy of the major component you will even further refine the amount of foam necessary.
You can do the calculations either way. The reason everyone is starting with their boat weight in air is that it gives a simple way to do the calculations and come up with a reasonably accurate answer.
You can argue that calculating buoyancy is more accurate, but it's still not perfect, and if you get a usable answer the other way the increased accuracy is extra work for little advantage. Calculating you can put in 27 cubic feet of foam instead of 27.6 really isn't a huge difference.
As to "not perfect".... you could make your calculations more accurate by measuring the specific gravity of your drive train as a whole.. the density (and therefore buoyancy) of your engine will be less than a block of iron the same size because the engine is hollow for the most part. Even if you've accounted for the negative buoyant influence of the iron, you're not counting the positive buoyancy of any trapped air, exhaust gases, or the oil (or for that matter the fuel in the gas tank).
But in the end, the answer you get from more detailed calculations is only slightly less than the estimate you get using the dry weight of the boat plus Archimedes' principle.
One comment on your safety margin:
Fresh water buoyancy is 1696.4
with Safety factor
1696.4 * 1.33% = 22.6
tbw =Total buoyancy weight = 1719 lbs
A 1.33 percent safety factor isn't safe. Think about it... if you happened to have a couple extra downrigger weights, or one more dive tank, you'll sink to the bottom.
A more appropriate safety factor would be one third, or multiply times 1.33, to have a third extra flotation.
Remember, since your increased accuracy in calculating floatation volume reduced the estimate of foam you needed to put in to float the boat, you've got to be equally accurate loading it so you don't exceed the floatation limit.
So how much foam? It depends on the Foam.
What you need to have is F which is the Flotation value of foam in lbs. per cubic foot
then you can take the tbw value and divide it by F to get you necessary cubic feet of foam to keep your boat from going to the bottom.
1719 / f = amount of foam.
The max floatation value a cubic foot of foam can give is about 62.8 lbs.. that being the weight of the cubic foot of water that's displaced by the foam. A cube of foam with a low density, say standard 2 lb floatation foam, would provide about 60.8 lbs of buoyancy when fully submerged (62.8 lbs/cf water - 2 lbs/cf foam). If you used something other than foam like trapped air you could get a little more buoyancy provided the container the air was in weighed very little.
By your estimate, you need about 27.3 cubic feet of foam for positive buoyancy (barely positive, since your safety factor is so small).
1800 lbs is a pretty big aluminum boat. Hopefully you have the space for that much foam
Erik
