Float bulkhead areas and calculation of resin to cloth weight ratios

I've finished a little experiment to estimate the resin to cloth weight ratio that I am achieving with the flat panel vacuum bagging technique. I understand that a good target is 40% resin content by weight to the fiberglass cloth.

I was wondering how best to estimate this ratio because you end up cutting off the extraneous cloth around the perimeter of the part. This makes the estimate of the amount of cloth on the final part difficult to calculate.

A better approach is to use Ian Farrier's dxf files which I purchased to CNC the main hull form frames. With the dxf files the areas of the floats can be computer calculated. These areas are shown in the picture above.

With these areas the specifications for the foam and cloth can be used to calculate weights.

For example consider the forward beam bulkhead

Calculation of the fiberglass cloth weight

• 6781 style s-glass from BGF has a finished weight of 8.9 oz/yd^2
• Both sides of the bulkhead are laminated with the top 18 in of each side getting extra reinforcement.
  
• Full area of bulkhead = 265358 mm^2 = 411 in^2 = .317 yd^2 times two = .634yds^2
  
• Extra reinforcement area = 192756 mm^2 = 298 in^2 = .23 yd^2 times two = .46 yds^2
• Total cloth weight without resin is = (.46 + .634) yd^2 * 8.9 oz/yd^2 = 9.74 oz

Calculation of the foam weight

• The Divinycell H80 foam has a weight of 5.0 lbs/ft3.
• The bulkhead has an area of 2.85 ft^2
• and has a volume of 2.85 ft^2 * .03125ft = .089 ft^3
• and a weight of .44 lb * 16 oz/lb = 7.125 oz
• This compares to an actual weight measurement of the foam at 6.5 oz taken before laminating.
• I will take the average between the two at 6.81 oz +- 9%.
• The final part after laminating was weighed at 25.3 oz.
• Subtracting out the foam and the cloth gives 25.3 - 9.74 - 6.81 = 8.75 oz for the weight of the resin in the vacuum formed panel.
• This gives a resin to glass ratio of 8.75 oz/ 9.53 oz = .92
  

Now consider the centre bulkhead.

• Full area of bulkhead = 230009 mm^2 = 356 in^2 = .27 yd^2 times two = .54 yds^2
• Total cloth weight without resin is = .54 yd^2 * 8.9 oz/yd^2 = 4.8 oz
• Volume of foam 2.4 ft^2 * .03125 ft = .075 Ft^3
  
• foam weight = 5 lb/ft^3 * .075Ft^3 = .375 lbs = 6 oz.
• The final part after laminating was weighed at 14.05 oz.
  Subtracting out the foam and the cloth gives 14.05 - 6 - 4.8 = 3.25oz for the weight of the resin in the vacuum formed panel.
• This gives a resin to glass ratio of 3.25 oz/ 4.7 oz = .69
  

So with ratios in the range from 70-90%, it does not look like I have achieved the theoretical optimum. However, I do suspect that the ratios appear high due to epoxy resin filling the perforation holes that are required in the foam. I do not want to remove any more resin.