David Thatcher wrote: I would have thought that strength would not play a large part, as long as the legs were reasonably substantial, and the same for the mounting point. There are lots of factors to consider including the nature of the substrate the legs are sitting on, and the natural inclination of the particular vessel to sit upright. It seems to me that the legs would need to be splayed slightly outboard at their lower end to help keep the vessel in an upright position.

BUT real life is different and sometime the leg is working in flexion and, as the weight of a boat is expressed in tones, the efforts on a leg, for example, by a boat healing 45° may be important. In the transition periods, from floating to resting, the efforts are for above the crew capacities, the time is short and you are just spectators, expecting sametime that the situation will not degrade more....

BUT Murthies law is not a dream! The leg may brake and ponction the hull... (my brother experienced that scenario, in a gale, in a not well protected harbour).

Thanks Eric for some good practical pointers. And I realise any of these materials would do, the reason I questioned Rob on materials and size that he used in his case, was only because I was trying to get some idea of how strong they need to be. (which sort of depends on how much of a lean one should tolerate, I suppose, like sitting on a stationary heavy weight motorbike and holding it upright.) And thanks David for that link to a very good article by Paul Fay on the subject, which goes some way towards answering that question of how strong they need to be.

But it raises others, ie why the need for special hull fittings, special bolts with the right sheer strength, pads against the hull, deck plates, factory-made extendable legs etc etc.? Not to mention the question of champagne or whiskey corks. 

From the outset and not being a fan of the "yachting industry" I couldn’t see the need for anything more complicated than two legs holding the boat upright by its chainplates, like Rob showed and pretty much what Eric is saying. Which, in the case of a junk conversion, chainplates would seem to be a redundant fitting tailor-made for the job.  Its just a matter of the legs being the right strength, I would have thought.

I hope I haven’t missed anything.

Actually, I had been thinking of the tops of the legs as “sky hooks”, each carrying a short chain down to the chain plate where it is shackled through the appropriate link. Or an adjustable tackle. Maybe Rob’s thru-bolt and choice of holes is a better idea, though I am not sure about that, as that might depend a bit on where the chainplate is located in relation to the belting, and other little details pertaining to the particular boat.

I get that it might be difficult during that transitional period while the boat is settling, to get everything to sit right. And I am thinking of, rather than a fin keel boat, a long keel which, once its on the bottom, can't really pitch, wobble or screw around (if the latter may be taken as a technical term).

Thanks again folks.

A simple inexpensive solution from Paul Fay

Thanks Rob, that looks simple enough.

In my case, draft 1m and round bilge, they might need to be a just a bit longer - and I dare say just a little thicker. What were the legs made of, and do you happen to remember the cross section measurement?

Thanks David, for taking the trouble to follow up on that. Pretty much as shown in the picture, I guess there is no great mystery to it. You never seem to see them used here, though we have something like 3m tides. I suspect that antifouling on a sand beach would not be permitted these days, but a scrub down might be OK and there are still other reasons for needing to get at the hull sometimes. My (long keel) boat will lie down and stand up OK (as I found out at the last junket at Stillwater) - but not very dignified and I don't like to do it on purpose.

I was wondering if, instead of bolting the legs to the hull, it might be possible to deploy them alongside and coming up a little above the existing (bermudan rig) chain plates, so each leg is in compression and the attachment to the chainplate is in tension. If you see what I mean. With judicious use of a line fore-and-aft from each of the feet.

It works OK in a "mind experiment" but I have never seen it done.