Any crane experts on here?

[Graham_Wright]

I am currently on shore for scraping, sanding and antifouling.

As in a previous post, the boat was wet and I have had the benefit of inside storage in an attempt to dry her out with infra-red lamps.

Despite 4 kw worth in a test area, the humidity gauge showed no improvement. On the basis that the boat will live longer than me (and probably my heirs) I abandoned the task and epoxied and Coppercoated.

To enter the shed, the mast had to be unstepped. The only crane available at the club had insufficient height.

So I volunteered to make one.

It didn't go well.

After scouring the Hornblower book of problems solved, I dropped the mast using the club hoist and a forestay arrangement. It was less exciting than it could have been but only just.

I now have to restep the mast. W

With half the crane built and proved, the problem remaining concerns the boom. The section I opted for proved to be too light and bent under a test load. It was circular Ø60mm with a 5mm wall. The full length available (7.6m) was used. It was horizontally pivoted to the winch platform which is capable of nearly a full rotation horizontally.

Seeking professional advice, the section choice seems to require the thrust line from the lifting point to rest well within the interior of the section ("middle third"). This recommendation is based on countering the sag in the boom due to its own weight.

The recommended size is 160 X 80 X 6. This seems to me to be a monster.

On sailing boats, masts have a similar problem but it is solved by attaching lines to appropriate points. I find it difficult to understand why a similar approach cannot be made for a crane boom. This view is supported by quick glances at fishing boats which have a derrick to allow the unloading of their catch. Universally, it seems, the boom is supported at the lifting wire end and at a point half wasy between that and the base. The routing of the lifting wire confers the winch pull force to the centre of the boom and twice that to the end of the boom by virtue of the sheave at that end.





Typical safe working loads are plated at 125kg. My mast weighs 170kg and a target safe load is set at 250kg.

I tried a similar arrangement with the light boom mentioned above but, as expected, the force in the centre was too great resulting in an upwards bend of the boom which would be equally effective in crumpling.

My latest thoughts are to fine tune the lift at the centre of the boom to counter the inherent sag by an appropriate routing of the lifting wire.

I wonder if routing the wire from the lifting winch round a sheave at the boom end, round a similar sheave attached to the centre of the boom and back to a point attached to the boom at a height to provide sufficient resolved force to counter the sag.

All expert opinions welcomed.

 

[NormanS]

I did something similar some years ago. I had to rig spreaders on the lifting tube to prevent it from bending. Then it was OK.

 

[coopec]

I am currently on shore for scraping, sanding and antifouling.

As in a previous post, the boat was wet and I have had the benefit of inside storage in an attempt to dry her out with infra-red lamps.

Despite 4 kw worth in a test area, the humidity gauge showed no improvement. On the basis that the boat will live longer than me (and probably my heirs) I abandoned the task and epoxied and Coppercoated.

To enter the shed, the mast had to be unstepped. The only crane available at the club had insufficient height.

So I volunteered to make one.

It didn't go well.

You've got more guts than a Sherman Tank!

 

[Graham_Wright]

I did something similar some years ago. I had to rig spreaders on the lifting tube to prevent it from bending. Then it was OK.

I suggested that to my peers. It was shot down. Seems an obvious way to go.

 

[Daydream believer]

We are able to move our mast removing crane around as it is fixed to our gantry. However, if you can bring the boat to the crane then you do not need a long jib.You could have a taller central mast suitably stayed. That could be by 3 /20 ft scaffold poles with rings welded to the ends. These can quickly be shackled to the upright when erecting. The base can be to anchors on the ground. Then the jib only need to be about 8 ft long. ie enough to reach the middle of the boat. It can be hinged to the upright a few feet from the top & have a block & tackle to hold it out at rt angles. You only need to lift the mast a couple of mm, release the shrouds, then lower the mast onto the deck. Then leave it there on timber frame, or manhandle onto the floor - None of us bother.
Here is a picture of our gantry on our website & you can see the jib folded down at the side 3rd pic down. I did the load test for our club's insurance & set working max at 250KG. I could have set it higher but this covered all our masts so I erred on the side of caution
From memory the sections are only about 105 * 105 but thick steel
Gantry mast crane

 

[Graham_Wright]

Yours is a well thought out procedure.
We have the benefit of a launching trolley so can work on the hard.
This is our hoist around my boat. The crane is being constructed on the cross beam where the scaffolding is. The mast is 14m tall and the deck is around 4m to the ground. The attachment to the mast needs to be at 12m from the ground (second spreader - although there is a way round that).

It is straightforward to lower the mast (using £££££££s!) but, with the main part of the crane finished and working, I need to finish the job.

 

[greeny]

Not wishing to dampen your enthusiasm and hard work but have you thought this through from a legal liability perspective. Using a derrick arrangement on your own boat to raise/lower the mast is one thing. Creating a lifting device on top of a communally used gantry will almost certainly fall under the regulations as regards design build and testing and its use and management will fall under LOLER. (Lifting operations and lifting equipment regulations).
Apologies if I've mis-understood your intent or you've already covered these issues in your plans.

 

[dunedin]

Why not launch the boat with the mast on deck, and motor to the nearest marina or boatyard with a proper crane

 

[Graham_Wright]

Leaning on my natural philanthropic nature, the aim is to provide the club with a useful asset for others. There already exists one crane on the old hoist but it also is old and the reach is insufficient.

There is no real reason not to employ a mobile crane save for the cost.

The club's launching trolley is a wonderful asset. A small leak after first launching in Sharpness, cost me a few hundred in lifting out and back in. At Barry, the cost is peanuts. It is easy to dunk for leak and engine testing. Wonderful peace of mind!

 

[Mudisox]

Gantry mast crane

What an excellent website full of useful information.

 

[Daydream believer]

What an excellent website full of useful information.

Thanks. I try but getting other cruising sailors to contribute is hard work. Some of the cruise info now needs updating. We are already discussing next years itinery -- However, one never knows how the covid & Brexit stuff will work. Hopefully Ok

 

[Kelpie]

Would it help to ballast the lower end of the mast? That would mean you wouldn't need to have your lifting point as high up.

 

[Graham_Wright]

Would it help to ballast the lower end of the mast? That would mean you wouldn't need to have your lifting point as high up.

I still need a crane! Might as well make it as anticipated. There are bigger masts than mine in the club.

 

[NormanS]

I

I still need a crane! Might as well make it as anticipated. There are bigger masts than mine in the club.

I'm with those who say that you can do what you like, as long as it is only and strictly for your own use. Designing and making any lifting apparatus which is going to be used by others, is on a completely different level. Compliance with the regulations becomes paramount. Beware!

 

[Graham_Wright]

I'm with those who say that you can do what you like, as long as it is only and strictly for your own use. Designing and making any lifting apparatus which is going to be used by others, is on a completely different level. Compliance with the regulations becomes paramount. Beware!

Noted and all prepared for.

Finding the relevant regulations is something else.

 

[Kelpie]

Presumably it's LOLER?

 

[earlybird]

Finding the relevant regulations is something else.

I don't think that you'll find much black and white regulation to specifically help. LOLER is fairly general and applies mainly to workplaces. If you wish to use your club insurance or, worse, satisfy an accident investigator, you will probably have to show evidence such as the equipmnt being competently designed with appropriate calculations and load testing. Not easy for a club.
Actual lifting also needs to be properly planned and supervised, even at club level. There's plenty of scope for trouble in this whole area.

 

[AntarcticPilot]

I found this easily enough: Construction - Lifting operations

Right at the top is the following:

The law says that all lifting operations involving lifting equipment must be properly planned by a competent person; appropriately supervised; and carried out in a safe manner.

Cranes and lifting accessories such as slings must be of adequate strength, tested and subject to the required examinations and inspections.

All crane operators, and people involved in slinging loads and directing lifting operations, must be trained and competent.

I've highlighted the bits that might be tricky for a shared facility (as others have said, no problem for a lash up for your own use). The web page has lots of links to other information. The fact you're asking for advice here about structural strength suggests that satisfying the second paragraph might be a problem.

 

[Graham_Wright]

To complete the story, the boom showed an unacceptable sag. The consequence was that the compressive load due to the lift wire lay outside the boom section (at least towards the middle of the length).
A solution was tried which was ill thought out resulting in too much lift being applied at the centre to counteract the self weight. The compressive load then lay below the boom causing it to bend downwards
The fishing boat illustrated has the winch pull force at the centre of the beam and twice that force at the end.
Unless the wall thickness is great, that seems to me to risking the same problem we experienced. However, it obviously works!

My proposed solution is to route the topping lift round a sheave at the end of the boom (as with the fishing boat) but to return it via a sheave at the centre of the length to a point on the boom displaced vertically from the boom at the pivot point on the crane base.
(Such a point exists to prevent the boom falling backwards). The height of that point can be adjusted such that the resolved force at right angles to the boom at the centre point is equal to the inherent sag force.


Worth pursuing?

 

[greeny]

Not at all. How do you intend to stop interaction at the boom end between the two wires? Get a lifting engineer involved to help you. I think you're treading on dangerous ground by self designing and building / testing / using unless you involve professional help. Your choice though.