Hi Codger, here is an extract from a link I posted earlier, refering specificaly to titanium and yachts.
With very high strength, extreme nobility on the galvanic scale, virtual immunity to corrosion in sea water and in the atmosphere, and about half the weight of steel, there is no reason Titanium should not be considered the "perfect" hull material.
Among the higher strength Titanium alloys there is little spread between the yield point and the failure point. This reveals a limited plastic range. However, elongation before failure is fairly high compared to, say steel.
Another characteristic is "stiffness" expressed through the modulus of elasticity. For steel, it is 29 million psi. For aluminum, it is 10 million psi. For Titanium, it is 15 million psi. This indicates behavior that is somewhat closer to aluminum in terms of material rigidity. In other words, before Ti is made to yield (the point at which a material is deformed to the point where it will not return to its original shape), it will flex about twice as much as steel, but about 50% less than aluminum. Interestingly, Ti has about the same stiffness as Silicon Bronze, but Ti has less stiffness than copper nickel, which has an elastic modulus of 22 million pounds.
Yet another consideration is the ease of welding, which has to do with several properties.
Titanium melting point is above that of steel (3,042 deg F, vs 2,500 deg F) and about a third that of aluminum (1,135 deg F). Titanium forms a very tough oxide on exposure to the air, so welding must be done only after thorough cleaning of the weld zone, and must assure a complete inert gas shroud of the weld zone as the weld is being made, both on the side being welded and on the opposite side. The weld must be shielded until the metal cools below 800 degrees. These characteristics are surmountable by thorough attention to detail, good technique, and aggressive measures to assure post weld shielding. These characteristics would however dramatically increase fabrication costs over those associated with, say, aluminum.
Among the other material properties that contribute to ease of fabrication of any metal are the heat conductivity, and the thermal expansion rate of a material. Aluminum expands twice as much as steel, per degree of temperature change, and is three times as conductive thermally. The thermal conductivity of aluminum is a big help, but the expansion makes trouble. As a benefit however, an equivalent aluminum structure will have greater thickness and locally greater yield strength, so the score is more or less even between steel and aluminum, with aluminum having a slightly greater tendency toward distortion while welding.
With Titanium, this latter consideration will be the overriding factor in determining the minimum practical thickness for plating. Thermal conductivity is given as 4.5 BTU / Sq Ft / Hr/ Deg F / Ft for Titanium. For steel, it is 31, for aluminum it is 90. Thermal expansion is given as .0000039 in / in / deg F for Titanium, about 50% the expansion of steel and about 30% that of aluminum. These figures seem to indicate that the material would be fairly stable while welding, but that welds would take a much longer time to cool. In other words, the heat would remain concentrated.
Based on these factors, as a very rough guess, a thickness of around 3/32" may possibly be the smallest practical titanium hull thickness for a welded structure. As a comparison, the least thickness for other materials (mainly due to welding ease and distortion issues) is 10 gauge mild steel (.1345") and 3/16" aluminum.
An interesting Titanium alloy is the experimental alloy 5111 (5% Al; 1% SN; 1% Zr; 1% V; 0.8% MO), described as "a near alpha alloy having excellent weldability, seawater stress corrosion cracking resistance and high dynamic toughness." It has a high elongation before failure, a "medium" overall strength of about twice that of mild steel, and has a slightly greater spread between its yield point and failure point than the "high" strength Titanium alloys.
Although I believe Titanium would be an outstanding hull material, it would require extreme care during construction. Titanium is possibly the "ultimate" in terms of heirloom boat hull materials...! "
The site also discusses other metal types and can be found here.... http://www.kastenmarine.com/metalboa...#Which%20Metal