Help designing simple jib crane

So I want to put a crane on my boat.

I envision a steel tube with a small section 6½' 2m long I beam welded on at the top. Probably a reinforcement triangle welded in. Bottom to drop into cup. Upper bearing to rotate in, mounted about 45", or 1.1m higher. Maximum expected load on the fnd of the jib would be 800#, 350kg.

Aluminum would be much lighter, but I can't weld aluminum. I'm sure I could sketch up something that would do, but I don't have the engineering math to properly proportion the components. What outer diameter tube? Wall thickness, reinforcement detail. Like that.

John

ps- I envision a future where I listen to my spell checker.

 

John we had a discussion about a similar crane here which might be helpful: Material for a small lifting boom. https://www.boatdesign.net/threads/material-for-a-small-lifting-boom.64217/#post-882060

 

Thanks Tiny Turnip! I'll check it out.

 

Autodesk "Inventor" CAD Student Edition FREE software will do "real" stress, bending, etc simulation. You can see which parts are under what stress and run loads to bending/breaking points.
You just gotta find someone officially enrolled in legit 2 or 4 yr college (any major, or non-major) to activate.
The kind of crane you describe would be easy to sketch. Or you could pay some starving CAD student a couple bucks to run some sims at local Junior College.

 

I do not think it is necessary to resort to any type of CAD / CAM. The basic formulas for the strength of materials and calculation of beams would suffice.

 

Unfortunately I don't know these formulas TANSL.

 

Can you post a sketch arrangement of what you would like and the max lift you wish to have...

 

Absolutely. Within the hour.

 

This location.

These proportions.
So the collar at the 1m mark would be a bearing to take side load from the couple. Cup at the foot takes the other half of the couple, plus all vertical load.

 

Port side be okay instead?
Less intrusive to the drivers view (especially to strd, the right-of-way side and all).

I was thinking more along the lines of a schedule 40 Al pipe inside another, stationary pipe so the whole assembly could be lifted out and be removed.
A locking pin could keep it in place in a seaway.

 

I'm thinking mounts both sides and both sides of the back deck too, and 1 post and jib that can be lifted out and moved around for maximum convenience.

The Al tube is preferable to steel, but I can't weld Al.

 

If you do up a good design (or find a better one) have someone weld it up in Al.

Did you talk to your Al contact in Alaska (the Sea Sled guy)?

 

Haven't talked to Marcus in months. I expect AdHoc or TANSL, or other forum members could improve on my sketch. Realistic scantling certainly. Anything I drew would be ridiculously heavy, because I'd do my usual "when in doubt, overbuild" thing.

 

Just a few random thoughts.
If the top section of your crane / davit is an 'I' beam, would it be worthwhile having the lifting point able to slide along on a trolley that has it's wheels on the lower flange? Or is the range of travel not worth such complexity, as the arm is only 1.85 m long anyway?

Would you have a chain block hoist, or just a simple block and tackle system?
I am thinking that if you want to be able to hoist 350 kg, a chain block would probably be easier and safer than a block and tackle (although it would take longer to lift up and put down the load with a chain block).
There is a mind boggling array of chain blocks for sale on Amazon, at pretty cheap prices. Here is a typical example.
https://www.amazon.com/SPECSTAR-Manual-Chain-Block-Capacity/dp/B07TVGZSTN/ref=sr_1_9

As an alternative to having a horizontal cantilevered jib, would it be feasible to have a derrick instead?
One advantage of a derrick is that the end can be 'luffed' up and down. The base of the derrick could be pin-jointed, attached to the mast just above the collar at the 1 m. mark on John's sketch.
And if the derrick is designed so that the maximum load of 350 kg can be lifted at 1.85 m. outreach, then you still have the possibility of lifting a lighter load at a further outreach, or a heavier load with the derrick 'luffed up' a bit more, so that the outreach is less.

 

Most excellent questions! The I-beam is standard, because a traveller that locks onto the lower flange is an off the shelf item.

A derrick is an appealing idea. I need clearance under the horizontal component for a quad, or a large refrigerator, but the joint isn't necessarily fixed. The engineering for a jib crane would also apply to a derrick. No doubt there are many improvements available. I'm trying to minimize complexity, because I need to grasp it all, without dropping any balls.