All of the above factors but the last may be obvious; a low coefficient 
of expansion is critically necessary to keep the total buoyancy system simple. 
lf the buoyancy material changes volume drastically with depth for any reason, 
then the buoyancy will not be constant. If the modulus of compressibility is 
low, the net buoyancy decreases rapidly with depth. Acting in the same direc- 
tion would be a loss in volume with cooling, which can be expected to be an 
important factor in all but polar seas. Whatever the surface temperature of 
the water, in most situations the deep water will be colder, and flotation 
materials will lose further buoyancy on cooling at the bottom. This may 
take considerable time but will eventually occur. Some form of trimming 
control on the bottom might be necessary but would be complicated. An 
alternative which is only slightly less attractive would be to precool the mass 
by soaking In acold bath at the surface. 
A method of improving wood to obtain the desired strength charac- 
teristics for fabricating buoyancy shapes has been proposed (Beck, 1967), 
but to date the necessary development and testing has not been done. Of 
the 13 references in the proposal, the most important for discussion are 
Barnes (1964) and Kukacka and Manowitz (1965). The former gives an 
excellent condensed account of the status of wood as a production material. 
The latter discusses the change in compression strength and hardness which 
may be accomplished by impregnating hard woods with highly penetrating 
monomers and then irradiating them with gamma radiation. Figure 43 
(Barnes, 1964) shows a greater than two-fold increase in strength with heavy 
impregnations. 
Proportional limit (psi x 103) 
0 10 20 30 40 50 60 
Parts polymer/100 parts wood 
Figure 43. Compressive strength of sugar maple filled with plastic monomers and 
polymerized by gamma radiation (© Barnes 1964. Used by permission of 
Machine Design.) 
By) 
