umm, i guess i'd be one of them..
'conversion' of weight on land vs weight in water is the difference of the objects volume times its density, less water.
for example, consider a cubic foot of each-
doug fir is about 35 lbs per cubic foot, or pcf. in water, it 'weighs' 35-62.4 = (-)27.6 pcf, so it floats. it can float 27.6 lbs of weight as well and still be neutrally buoyant.
ironwood on the other hand is like 70pfc, so in water, it 'weighs' about 8 pcf, it sinks.
a cubic foot of steel, is about 398pcf. in water, a cubic foot 'weighs' 335.6 lbs.
got it? its that easy. the hard part is figuring out what somethings volume and density are.
so, what does the 140 weigh? somewhere around 400 lbs? lets neglect the buoyant weight of the motor and just use gross, ok? so, for your df140, i assume 200 kg = 440 lbs (yes, we are going to do this in metric because counting in units of 10 is easier than ounces and gallons)
seawater~ 3% salinity? 1.03 kg/liter (i am a sweet water guy).
=> 200 kg /(1.03kg/liter) = 195 liters. to be approximately neutrally buoyant, you need to displace 195L of seawater. that is about 100, 2 liter bottles to float the motor. count how many pop bottles you used, times their volume.
to put it in perspective, 9 cubic feet, (cubic yard) is about 250 liters, of 1/4 a cubic meter. 254.5 actually, but close enough for gob't work. DAMHIKT.
your 28, 20 oz bottles are 0.6L each, or 16.8L, 28- 1L bottles, 45L total, for 46kg of seawater or 102 lbs of NET buoyancy.
fenders, large are about 18L each, small about 8L each, total of 53L, or 55kg of seawater or 120 lbs of NET buoyancy.
so about 222 lbs of bouyancy.
your fenders are much more buoyant compared to the pop bottles.
don't forget the other 'heavy' stuff like batteries. lets guess a battery weighs 44 lbs, that is 20 kg, or about 20 liters of displaced volume of seawater. that is 10, 2 liter bottles.
you need more bottles.
where can i send my bill? its due net 30.