Lifting loads

[Lady Campanula]

Further to my last....

I'm trying to estimate the lateral compression loads for a sling-lift early next week.

The boat's mass is 3 tonnes. The 8m./16.6' slings are well up to the job. The upper triangle in the illustration is equilateral.

Would someone with trig rather less rusty than mine kindly help me gain a 'first order approximation' of the likely compression load?



Slings 8m. / 26'6" new and certificated. ( flinger trubl! )

 

[boomerangben]

Total compression will be approximately 3te x tan 30 deg = 1.73te

Hence the need for long slings or a spreader beam, or indeed a boat lift. I have little experience of lifting boats but used to do a lot of rigging design in a previous career. I would say that you want longer slings or some means of preventing that load being imparted onto the boat. Moreover, you have only show a 2 dimensional problem (or have I missed something). I presume you are going to be using two slings, one fore, one aft, so that load will be spread between two parts of the deck, although the total load would be the same. If you are using a single crane hook, those loads might be a little bigger. Then you want to add some form of safety factor.

I don't know how confident you are with the boat's displacement. We got a bit of a shock lifting what we thought was a 5 te boat. Turned out to be over 6.

 

[VicS]

Yes if the angle between the slings is 60° the total compression is 3 x tan30 divided between the fore and aft slings

The pic below shows how to do it with four brothers and long slings.
With long keelers its often a good idea to put lashings between the fore and aft slings each side.

Your crane operator will know how to do the job. Trust him, don't try to tell him his job!

 

[William_H]

Sling lift

Clearly by comparison with Vic's photo you need a taller crane with longer distance from deck to the hook.
If this can't be done then you need spreader bars on the deck to push the slings apart or take the compression loads of the sling. Of course in asking the question you already know that anyway. I would have thought any good crane operator will have on board spreader bars that would suit. good luck olewill

 

[Conachair]

Total compression will be approximately 3te x tan 30 deg = 1.73te

Are you guys sure about that???

Cut the triangle in half to get a 90 deg angle, vertical load gets halved down each leg as well, horizintal load is 1/2 x 3 (tonnes) x tan 30

Equal and opposite from the other side of the halved triangle.

Allowing for some dynamic and a bit more horizontal from the angle between the front and back bridle legs could be about a tonne, maybe a bit more. Still quite a lot. Spreaders are good.

edit. a tonne ish overall, spread between front and back bridles.

 

[SHUG]

Ahem...errors I think!!
The trigonometry is 3/x =tan60 therefore x= 3/tan60 ie x= 3/1.7321 = 1.732 tons
Answer , the compressive load is 1.732tons if it was a single sling.
The load will be divided between two slings so the compressive load will be halved giving an answer of 0.866tons which looks like your boat will survive lifting...again!!
The longer you make the slings, the less the compressive load will be and if you had a frame to make the slings vertical so that your angle increases to 90degrees the compressive load would come down to zero. If the angle was 89.9degrees (so that my calculator can deal with it) the compressive load would be 0.005tons or 0.0025tons at each sling

 

[Conachair]

Ahem...errors I think!!
The trigonometry is 3/x =tan60 therefore x= 3/tan60 ie x= 3/1.7321 = 1.732 tons
Answer , the compressive load is 1.732tons if it was a single sling.
The load will be divided between two slings so the compressive load will be halved giving an answer of 0.866tons which looks like your boat will survive lifting...again

Eh?? Can you do trig on a triangle without a 90deg angle in it?

I ran it through LD calculator just to be sure. 3000kg on a 2 leg even bridle with 60deg angle between the bridle legs results in a horizontal load of 866kg.

1/2 * 3 * tan30

View attachment 10801

 

[misterg]

Are you guys sure about that???

Cut the triangle in half to get a 90 deg angle, vertical load gets halved down each leg as well, horizintal load is 1/2 x 3 (tonnes) x tan 30

Equal and opposite from the other side of the halved triangle.

Allowing for some dynamic and a bit more horizontal from the angle between the front and back bridle legs could be about a tonne, maybe a bit more. Still quite a lot. Spreaders are good.

edit. a tonne ish overall, spread between front and back bridles.

 

[sarabande]

umm, chaps, those figs are all for point loads. LC's slings will be in contact with the side of the boat for - perhaps - another 500 to 900 mm or more (as per the OP's diagram) - thus spreading the load into the hull away from the hull/deck interface. Also germane to this is the width of the slings and whether any pads are used to ensure conformity with e.g. gunwhales, rubbing strakes, etc.

As the greater part of the boat's mass will be taken at the bottom of the keel by the lifting strops underneath, this reduces the figures at BMax anyway.

 

[Conachair]

umm, chaps, those figs are all for point loads. LC's slings will be in contact with the side of the boat for - perhaps - another 500 to 900 mm or more (as per the OP's diagram) - thus spreading the load into the hull away from the hull/deck interface. Also germane to this is the width of the slings and whether any pads are used to ensure conformity with e.g. gunwhales, rubbing strakes, etc.

Makes no difference, at the point of contact between the slings and the boat you'll have a horizontal load, what goes on further down the slings won't actually alter that. Though the width of the slings will help spread the load.

You can draw it and measure to get the same answer as the formulas. If you draw a triangle with a 90deg angle in it, a 30deg angle in it, one side 1500 somethings long then the other side will be 886 somethings long.

 

[VicS]

I would have thought any good crane operator will have on board spreader bars that would suit.

Not necessarily . Brian, our usual crane operator and the guy you see in my picture, hires them in if/when he needs them.

For several years Clive, the owner of the boat you see in the picture below, insisted that spreader bars were used for his boat. Brian hired the spreaders. He used them for Clive's boat only. Clive paid for the hire of the spreaders. He eventually realised he was paying extra for something he did not need.

 

[SHUG]

Conchair quote:
"Makes no difference, at the point of contact between the slings and the boat you'll have a horizontal load"

Incorrect...If the slings are vertical at the point of contact there will be NO horizontal force at that point.
In practice the slings will angle in after this and you get a force on the hull. In hulls with tumblehome (eg Sadler 25) you should put packing in at the gunwhale to distribute the load. There is also a real problem with long keeled yachts where quite often the slings have to be restrained to stop them sliding forward due to lack of force and friction to the hull.

 

[Conachair]

Conchair quote:
"Makes no difference, at the point of contact between the slings and the boat you'll have a horizontal load"

Incorrect...If the slings are vertical at the point of contact there will be NO horizontal force at that point.
In practice the slings will angle in after this and you get a force on the hull. In hulls with tumblehome (eg Sadler 25) you should put packing in at the gunwhale to distribute the load. There is also a real problem with long keeled yachts where quite often the slings have to be restrained to stop them sliding forward due to lack of force and friction to the hull.

Correct.
The statement was in reference to the original post, where the bridle legs angle inwards. In this case what happens below the point of contact can't lessen the horizontal load at the point of contact.

You can't push a rope.

 

[Main Event]

Eh?? Can you do trig on a triangle without a 90deg angle in it?

Yes - use the sine rule or the cosine rule !

 

[Lady Campanula]

'Curiouser and curiouser....'

Acting on reliable advice(s), I've acquired two 4T x 8m. certified lifting strops. These will be used in a 'basket lift' ( or 'U' ) configuration. I've also acquired two new 3T lifting shackles, and borrowed several new 10mm x 6m. Grade 80 chains c/w appropriate shackles.

I'm working on spreader bars.

FWIW, there won't be a crane as shown in photos above. My local crane hire crew - Sparrows - wanted £1500 + VAT to turn up. OTOH, my local plant contractor will do the necessary lift for £50 - but on my insurance, and if I provide the strops, etc. The sons sail an Elan 340....

The static lift will be about 18", permitting a transfer from road trailer to 'yard trailer'.

I appreciate all the input and promise to wear a hard hat.....

 

[Conachair]

Yes - use the sine rule or the cosine rule !

http://www.ucl.ac.uk/mathematics/geomath/trignb/trig11.html

That's so tidy! Never heard of that one before, thanks.

I wonder if they taught that at scholl and i forgot or if it wasn't taught. Doubt if i'd forget something as neat as that. But......

Can't see how it would work on this one though, you still need the vertical to get 3t in there.

Could it work?

 

[VicS]

Sine rule... Everybody who was awake during their maths lessons knows of that.

 

[SHUG]

I'm working on spreader bars.

Its sounds like you are on the ball!!
Just remember my comment about retaining the forward sling so that it doesn't slide up the keel when the load comes on. Some people take the slings behind a stanchion post if it doesn't distort the line of the sling and others just tie the forward sling to the stern one.
Good luck!!

 

[VicS]

Just remeber my comment about retaining the forward sling so that it doesn't slide up the keel when the load comes on. Some people take the slings behind a stanchion post if it doesn't distort the line of the sling and others just tie the forward sling to the stern one.

Yes I already mentioned in early in the thread but with the short slings proposed its probably not going to be an issue.

Lady C
remember those spreader bars constitute part of the lifting tackle and are subject to the same insurance, certification, and inspections that the rest of the lifting tackle is.
Thats why our regular crane operator only hires them them when needed.

 

[Deleted member 36384]

.... Your crane operator will know how to do the job. Trust him, don't try to tell him his job! ...

My advice would be to understand what the crane driver is about to do by getting an explanation before your boat is lifted.

The crane operators at Tilbury Docks crushed a wooden motor boat that had arrived from America because they used spreader bars that were too short. I stood on the dock with the owner thinking, "that sounds expensive." Tilbury paid in full to have the boat repaired.

The point is you should not be put off questioning so called professionals.