N215PB, I've been making foam core floats for 5 years and have calculated the displacement of most of the sizes I make. My 34" floats will support a maximum weight of 19 lbs with the tops of the floats at the water's surface (completely submerged) and my 42" floats will support 38 lbs. For the floats to perform properly, I like to keep the flying weight down to around 1/2 the maximum. In your case the 38" floats should support around 28 lbs. maximum or 14 lbs. flying weight. Hope this helps...........Seaplane