Doug's Speargun and Specific Gravity considerations.

For those of you (all of us) who are interested in these things, here are what the individual components of Doug's 40" gun weigh in air. I only have a rough estimate of specific gravity in salt water. That comes into effect at the end when the gun is trimmed.

Webster's Dictionary definition:
Specific gravity is the ratio of the mass of a body to that of an equal volume of some standard substance, water is the case of solids and liquids, and air or hydrogen is the case of gases. It is a numerical measure of density where water is considered to be 1 (one).
Examples:
As an example a cubic foot of maximum density pure water is 62.4 lb. A cubic foot of cast aluminum weighs 160 lb. The specific gravity of aluminum is 160 / 62.4 =  2.56.  As another example, lead weighs 710 lb. per cubic foot. The specific gravity of lead would be 710 / 62.4 =  11.38.  When substances lighter than water are considered the specific gravity value is never a negative number. As an example, white pine has a weight of  about 24 lb. per cubic foot. The specific gravity of  white pine would be 24 / 62.4 =  .38. This also tells us that white pine floats upon water. Certainly if you take the specific gravity of water it's self, you would have 62.4 / 62.4 = 1. The specific gravity of water is 1 (one).

 Specific gravity of a few relevant materials:

Mahogany wood (variable natural material)  .51 - .85
Oak wood (variable natural material)   .59 - .90
Fir (variable natural material)  .54 - .56
Teak (variable natural material)  .60 - .70
Polyurethane foam (5 lb. / cu. ft.)  .08
Divinycel Foam (6 lb. / cu. ft.) .10
Pure rubber  .97
Hard rubber compound (tires) 1.99
Speargun rubber probably is close to the same density as water. It does not contribute much to the buoyancy.
Stainless steel  8.0
Aluminum  2.56
Glass 2.40
Polyester resin 1.11
Epoxy resin 1.10
Salt water 1.03 (It's heavier than fresh water. Fresh water floats on salt water)

Doug's Speargun 40"  Weight in Air of Components

Component                                wt. gms      wt. lb.(decimal)     Specific gravity

1. Prodanovich Handle                   505       1.11                       Medium 3.0
2. Aluminum nosepiece                   144       0.32                       Medium 2.56
3. Stainless cocking stock               321       0.71                       High  8.0 slightly less, foam filled
4. Gunbody, stainless tube,     
   foam, track, glass, dowel           1862       4.10                       High+low, overall low (floats) .70 (guess)
5. Rubber gun butt                         112        0.25                       Low to neutral  1.1
6. Fasteners, stainless     
   breakaway fitting                           45        0.10                      High  8.0
7. Five SS wishbones                       57        0.13                      High  8.0
8. Trim leads    
   primary                                       230        0.50                      Very high  11.38
   secondary                                   111.5     0.25                      Very High 11.38
   Bands                                            NA                                  Neutral  .97
   Sum of gunbody                        3387.5     7.46                      Neutral to low  roughly .90 (floats without shaft)

9. Rigged shaft                               901        1.98                      High 8.0 (gun sinks with shaft in)
 
   Total dry weight                        4288.5     9.45

 Conversion factor     
 1lb. = 454 grams

John Warren
12/22/02