[By safety I mean: can take a knock] IMHO there is only one material that will cope reliably with an impact at 6 knots and that is steel. I have seen a couple of steel boats that have spent a day pounding on a reef and were still watertight.
I guess there are two ways to see the world: go sailing, or plonk yourself down on a small island and wait for continental drift.
Steel is popular in Arctic regions where you push through ice. Apart from that, alternatives are not so bad. As someone else said in a thread, I’d rather be upside down on a multihull that will still float. The Hummer approach ignores your greatest safety factor: the man behind the wheel.
[and be beached, without too much worry] ANY boat fit for sea can be beached BUT beach anything other than steel on sand with a pointy rock in it and you are looking at a hole. ** Ask the guy who beached the Prout in Fishguard.
Any? I suppose with a 6ft keel you could beach it, but don't have soup for dinner at the angle you'll sit there
In any case, steel is not “the only” – aluminium gets you there, and it has some qualities that make it more resilient in a crash. You mention 6 knots, and that is part of an interesting equation: Speed times Mass. The steel boat hits an object with more or less twice the force of a lighter fibreglass or aluminium body. Also, what hits first? If it is the keel and/or rudder as in most cases, those are the constructions I’d look at, and in that case a comparison of long keelers, fin keels and swing keels might be more to the point?
Construction is yet another factor. There are designs with watertight compartments, and some with inbuilt flotation.
Finally, there’s workmanship. Many steel and alu boats are home built, and you won’t know how good they are without solid research into the welding, surface treatment and dimensioning, not to mention the grade of steel or alu used.
I’d look at the whole package rather than fixating on a hull material. Does it have a reliable and well mounted engine? Are the couplings and gears sound? How protected is the propeller? Failure in any of these are likely causes of washing aground in the first place. Sometimes a relevant question with steel boats: is this a sailboat or a motor sailor? Does it have the rig and sails to haul you off a lee shore in an emergency?
Questions, questions, and so many answers