Join Date: Jan 2012
Location: British Columbia
Thanked 47 Times in 45 Posts
Rep Power: 0
Re: Pros and cons of steel sailboats
When comparing metal with other materials, one should bear in mind that only metal has the same tensile strength in all directions. Douglas fir has a tensile strength along the grain only. Across the grain it is very weak. With planked boats ,that means the planking has strength longitudinally only. The frames give the transverse strength, usually much weaker. That is what transverse frames were invented for, a material which has strength in one direction only. It has very little diagonal strength, which is why Herreschoff used diagonal bronze strapping, to prevent diagonal movement, called hogging and sagging.
At best, a tripe laminated cold molded hull only has about a third of its planking in any one direction. Whichever direction you load it, only a third of the wood in it has any tensile strength in that direction, the rest of the load is across the grain of the other laminations. So for a tensile strength of 1500 PSI for fir, a cold molded fir hull will have a third of that strength in any one direction.
Roving has half its fibres in one direction and half in the other. So the best you can hope for is roughly half the maximum tensile strength of a fibreglass hull, in any one direction.
Diagonal strength? Matt?
In a random jumble of logs, there is very little wood and mostly air space. Similarly, in the random jumble of fibres which is fibreglass matt the thickness of the matt is only a tiny percentage glass fibres ( strength )and a huge percentage resin, which has very little strength. As roving is very rarely run diagonally, there is very little diagonal strength in most fibreglass hulls, which is why it is so easy to sag them with a hydraulic backstay adjuster. You will never get the ultimate tensile strength of fibreglass in any hull layup, for these reasons.
You can see this in old fibreglass SSB antenas. Unidirectional layup makes them strong, longitudinally, but weak transversely ,where you can twist them apart by hand, if you cut the metal end off one.
Steel however, has 60,000 psi tensile strength in all directions, period.
Last edited by Brent Swain; 10-09-2013 at 07:05 PM.