Crane lift without a frame

Stellina

Stellina

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Trondheim, Norway
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At my club all the organised lifting uses a mobile crane with long straps but without a box frame to reduce the horizontal compression. The old salts say that having the hook 10-12m over the deck is enough. Should I be worried?
 
I would have thought so but a lot depends on the size and weight of boat. I recently lifted Avocet off a truck with no spreader bars and the hook about 5m off the deck. Avocet only weighs 3 tons and is very narrow (7'6" in the beam)! Also, there are stiff bulkheads at about the same sort of place as the slings were. If you had (say) a 10 ton boat, 10' wide where the hook was about 10m above the deck and (as a worst case) ALL the weight of the boat was on one sling, you'd be looking at about 3/4 ton of inwards load on each side of the sling at gunwhale height. So that's a 1.5 ton compressive load across the deck. Obviously, as the boat gets narrower and / or lighter this is less of a problem but I'm not sure I'd want to risk it on a lightly-built but beamy racer!
 
Fact remains that your boat is never designed to withstand those forces. So, my guess is you have a good chance the boat wil take it, but still a small chance the forces will put excessive load on the hull-deck connection.
It all depends on the size and weight of the boat.
To rephrase the dilemma: if this is were no issue, then why do most cranes use a frame?
There is also the problem that the straps at the front and the back of the boat should also not be allowed to creep to the middle of the boat's lenght axis, just to keep a safe balance.
I would be very reluctant to make use of this crane.

Cheers,

Arno
 
The mobile crane which comes the yard I am based at always uses long slings on "4 brothers" (chains) without spreader bars. We have been using the same "owner operator" for many years. He knows boat lifting well and all of us. The long slings make spreaders totally unnecessary,

If you insist on spreaders, as one of our owners did for a couple of years until he realised they were unnecessary, they may cost you extra. In the case in point they had to be hired in specially, were only used for his boat and he had to pay the full extra cost.
 
Spreader bars are seldom required for monohulls with reasonable structure and weight. In fact, as a rule of thumb most riggers will tell you with the hook lifting from a height above deck of 2 1/2 to 3 times the beam it's no problem. I have had my 6 tonne cat lifted on a hook 15 metres above the deck and she is 5 metres beam. With ring frames and/or bulkheads taking the compression, not to mention the deck itself, you would have to have a pretty flimsy boat to suffer damage.
 
Is this good enough answer .... boat is pitch pine on oak frames - NOT in flush of youth ... each year gets a lift in / out this way ... and she's mid 70's build.

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It all depends on the height of the hook above the deck and the beam of the boat.

If the height is greater than the beam no problem but if the height is less use a spreader beams
 
Have only had to use a spreader frame with very beamy motor boats. I use 15metre twelve ton rated strops, and 10metre chains. This puts the hook at or slightly over the mast on most 35 foot yachts, and gives the strops a nice shallow angle without much compression. If the boat has a particularly proud rubbing strake then we use blocks of 75mm square timber wrapped in carpet placed under the strops just below the strake to relieve the upward pressure as the strops come tight. If the boat has a cutaway forefoot then I tie the two strops together, but under normal circumstances the inward angle will hold them without any slippage. So in other words... no I wouldn't be worried!
 
Are the lifting points marked on a boat (mine is a Gib'Sea 33 by Dufour) put on to indicate a stronger deck or equal length around the hull? Or where to avoid the keel?
 
The sling marks should indicate "Bulkheads, balance, and bits!" as one guy put it to me... i.e. a bulkhead for strength, at the right distance from the ends for good balance and avoiding any bits - prop, shaft, log impellor etc. Do remember that the engine is a concentrated lump of weight in one end, and try and get one strop at least close to the position of that lump. Also beware of blue water cruisers which may be carrying half a mile of chain in the anchor locker, as this can occassionally affect balance! /forums/images/graemlins/shocked.gif
 
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