Forces on a yacht cradle

[Avocet]

Hi All, A mate of mine is designing a cradle which he hopes will suit a variety of yachts at his club. I was wondering if anyone had any rough figures for the kinds of loads that a yacht might exert on its cradle legs in a gale? As a worst case, obviously, something with a long keel and a lot of windage - presumably with the rig up.

 

[BAtoo]

I suspect a fin keeler with rig up will be worse than long keeler; friend had a 29' FK lifted by a 90mph squall coming up the river and dumped onot one leg - needless to say the leg went right thro the hull....

 

[Avocet]

Sorry, poor choice of words on my part! Meant "deep" keel!

 

[savageseadog]

I've seen a few cradle disasters. The design situation is not a piece of mathematical theory. The loads are not great under normal conditions, it's when things go wrong that the loads take off. Basically the stronger and sturdier the cradle the better and the bigger the ground base the better. The other very important factor is in preventing any, even slight movement of the boat.
I would suggest buying an approved manufactured cradle to anyone who wants one. If something does happen at least you couldn't be held liable and your insurance company will pay up.

 

[Guest]

It all depends on the cradle style. Do the legs push up more than they push horizontally. I now have quite a collection of photos of collapsed cradles and domino boats.

It is the main reason I can not get my brother to boat yards as a mechanical safety engineer he wants to reporting every yard.

The main issue is the keel stepping sideways. The wind blows on the rig and if the props are pushing up to much then the weight is transferred to the prop and the keel friction reduces. I saw a 55' boat step its keel about 6 inches sideways it left it standing on 2 props only. The keel had moved different distances foreword and aft.

However, if you use horizontal only props then either the cradle needs to be wide or you must use the Australian method of heavy chains and turnbuckles. The best cradles I have seen in the UK are at MDL marinas and are made in Holland I think. They use fixed turnbuckles and extendable props that can push higher than the water line. The best cradles in the world for boat yard use are in Australia and consist of vertical posts that slide inwards and clamp the boat some where near the toerail. The inward clamping force is applied by chain and turnbuckles to variable points up the vertical post. These fit any boat from fishing vessels to keel yachts as long as fore and aft props are used on some yachts to stop the yacht pitching if it has a tiny base to the keel.

Take care, if you use the example of the yacht earlier the vertical force was the total weight of the boat on 2 props. The horizontal forces on the 2 props were about 10 times the equivalent force on the mast top in 50knot winds.