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Well....

>Not to be picky, but I did suggest a couple of coats of fibreglass
>(FRP).  I have used this technique in making motorcycle fairings, and it
>is very strong.  The foam just acts as a free-form mold while the FRP
>hardens.  Once the outside shell is formed, the foam is removed.  For a
>fully enclosed shape, removing the foam would be difficult without
>compromising the structure (you would need a hole).

>What I haven't calculated is:
>     1)  How thick would the FRP have to be to withstand
>         X psi at depth?
>     2)  How heavy would it be?
>     3)  Would exotic materials (Kevlar) be required?
>     4)  Could it really satisfy all the requirements WRT
>                -size
>                -drag
>                -flotation
>                -extra weight
>>                -safety
>
>And I am looking through my #%*)@!! Pocket Ref (2nd edition) only to
>find it doesn't list the density of styrofoam.  But, I know it is lower
>that 32lb/cu.ft.  Another material used in boat-making is Balsa wood
>(8lb/cu.ft.).  Too many dependancies in fibre:resin ratios and types,
>but you could ballpark at 120lb/cu.ft.  For 20 lbs lift, whole structure
>would weigh in at 6-10lbs depending on shape and a lot of other things.
>
>More disclaimer:  The above figures were based on a big pile of educated
>assumptions.  Guess-work is no substitute for luck:)

>ttfn
>David Pearson

The strength of a FRP shape in respect to an outside pressure is largely 
limited by the geometry. The foam float in question would have two 
concave sides to fit the tank curvature and two straight or slightly 
convex sides on the edges between the tanks.  This will yield a 
transition between a shell in tension (the concave surfaces) and the 
sides and/or ends in compression. The sharp edge between the concave 
section and the top or end will not easily yield itself to reinforcement. 

Given the geometric complexity of the shape, and the tendency of 
styrofoam to dissolve when anything but epoxy resin is used, a sealed 
rigid foam is likely to be the least expensive per unit. The foam I 
mentioned was used to construct scale models for wave tank testing and 
developing drag co-efficients. I have also used it to provide floatation 
on shallow water ROV umbilicals and sensor cables.  If memory serves cost 
was less than $.05 per in^3 and layers can be bonded together for larger 
sections. Buoyancy works out to just over 48 lbs per Ft^3.

FRP can be made to do about anything not requiring heat resistance. 
Sometimes it just costs too much though.  NO flame intended as you did 
kick off memory of an old solution to a similar problem. I have cast 
lead weights to go between a set of doubles. I just never thought of 
needing flotation there.

BTW most commercial styrofoam is about .5-2 lbs per cubic foot and can be 
neglected in most small buoyancy computations dealing with salt water. 
Styrofoam core removal can be made with several solvents including 
unleaded gasoline.

enjoy, 

FT

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