Material for a small lifting boom.

I'm in the process of fabricating a small lifting boom. I'll post a drawing (I'm not an artist so please bear with me) that will hopefully clarify some things and a photo of the stern of my boat.



I built an Eastport Nesting Pram recently. I have room on the swim platform to store it. I'll have to rearrange the fuel cans and generator but it fits. The larger piece of the pram only ways around 50 pounds but it's bulky. Rather than try to muscle it onto the platform and fall off the boat or get hurt I though that a small hoist would help.

I did some research on rigging and can use a double sheave swivel and a couple of single pulleys and make up a rig (4;1) that will take a mere 13 lbs. or so of pull to lift the bigger half of the pram.

My question(s) for some of you are this:

I'm thinking of using 2" 6061 square aluminum tubing for the boom. the top of the vertical section would be 77" off the cockpit sole. It will fit under the bimini top. I can securely anchor the bottom of the tubing to a stainless steel foot. I can also stabilize the vertical tube about from 17" to 22" up from the cockpit sole. I'm thinking of using a piece of aluminum plate (1/2 x 5" x 12") anchored vertically to the inside of the cockpit and bolting the tubing to the plate.

The "boom" will extend 4' out and the anticipated load should be no more than 100 lbs, usually less. Am I correct that this places a twisting or torque load of 400 pound/feet at the junction of the tubing?

I've done some homework on this and it appears that I should use gussets at the 90 degree junction between the tubes. This keeps the load and the forces on the tubes centered. I can get heavy cast aluminum clamps like these,


But when these are used the load isn't centered so I'm not sure they would be appropriate, even though they have a working load limit of 1,200 lbs.

So my questions are these.

Will 2" x 2" x 0.25 (wall) tubing be appropriate? I can use the thinner stuff (1/8th wall) but there isn't much difference in weight or price.

How large should the gussets be? I'm thinking of 1/4 inch aluminum plate either 8" x 8" or 12" x 12". I'll probably use bolts to secure the gussets to the tubing. I could have the gussets welded but I'd like to be able to disassemble the rig for storage, at least the horizontal boom section.

Finally, should I consider increasing the angle between the vertical tube and the boom? This would allow me to shorten the vertical tube somewhat and decrease the length that would be above the aft deck and unsupported. I think this would increase the stability of the rig but I'd need to extend the boom to compensate for the increased angle so there would be more unsupported boom. Maybe it's just a trade off?

Any advice or thoughts would be greatly appreciated.

Regards,

MIA

 

I can't see your drawings very well and no idea what the clamp is for, but if you had a round, base tube from the floor to the gunnel permanantly attached, a round tube as the post of the boom part could fit inside the base attached to the gunnel and thus be able to turn/swivel. The boom and post could be removed and stored when not in use and the base tube could then be used as a drink holder or rod holder, or a small bbq with a short post could be mounted there.

 

What SamSam said about swivelling brackets/socket. This sketch arrangement (longitudinal section) below might be easier and more efficient structurally. plus easy to dismount. If you need the mast to fit under the bimini, and need height to clear the outboard, the boom stay could be used, with an additional pulley or two, to raise the boom, also.

 

Hi Sam, good to hear from you.

I could use round pipe and rig it to rotate. I don't need it to though. Maybe I need to explain the goal a little better.

I'll use the boom to lift 1/2 of the pram at a time. It's a "nesting pram". You build it with a double bulkhead in the middle and then cut the boat into two pieces. All I need to do is lift the two pieces, one at a time, up and onto the swim platform. They'll stack vertically right up against the transom of the boat. I'll fashion a little cradle on the swim platform so they won't slide around. I hope that makes sense.

As for the clamps, those are strong 6061 extruded aluminum pieces designed to be used in the motion picture or theatrical industries. You use them with 2" square metal tubing. Typically heavy lights or equipment are attached to long pieces of tubing. They have a working load limit of 1,200 lbs. I may try these in my boom set up but my concern is that with the clamps, the vertical and horizontal tubes are no longer in line so the load is off center. Probably not a problem with a static load but when I'm raising a load it may cause the whole boom to want to rack to one side. My thinking is to keep everything lined up so the forces act in only one direction.

I like the drink holder BBQ idea.

Thanks,

MIA

 

That's got me thinking. If you look at the picture of my boat you'll see a swim ladder in the middle of the transom. With your arrangement I could lift the boom after raising the pram piece with a block and tackle. Then raise the boom and lower the pram piece right down between the ladder and the transom of the boat. As the boom is raised the angle changes and the lifting point moves forward. Just what I need.

Another thought is to build two of these booms and leave the nesting pram in one piece.

Thanks!

MIA

 

The movable boom Tiny Turnip drew is good, it turns it all into more of a crane type thing. If it swiveled around you could lift a big cooler off the dock and put it right on the cockpit floor, lift the motor off the bracket and put it on the dock or the pram transom etc. Maybe load and unload the pram with supplies.

With the movable boom, the works would fold up like an I and store very easily compared to a big piece of welded tube shaped like an L. You could also have interchangeable booms (2x4 maybe) for different purposes. What they would be, I don't know, you might figure something out.

The pram, can you assemble it in the water, like out at anchor?

 

You both had good ideas Sam. I'm going to have to think about this. With Tiny Turnip's crane I'm thinking that with two assemblies mounted in the back I could lift the dinghy in one piece and not even have to take it apart every time I wanted to stow it. Two cranes makes it a more flexible design, the ability to rotate the crane even more so. I could still add a small table back there as you suggested. I have an idea about that too. So thanks again to you both. All I need to do is have one of the guys down at the boat club do a little welding for me for attachment points. Someone must make some ears that I can weld to the tubing and run a shoulder bolt through so that the arm extending over the swim platform can articulate. Is there a name for those types of fittings?

As for the dink. Yes you can take it apart in the water, but not while you're sitting in it. I can take it apart while kneeling on the swim platform and then lift the bow section and then the aft section.

MIA

 

MIA - do you need two cranes? if you have the headroom, might just a 4 point lifting sling do it? (not as long as this one perhaps- but looks like they are just using cam buckle webbing straps) (and $143 seems a bit steep - I can get 5 metre cam buckle straps for about $4)

Adjustable Davit Lifting Sling for Dinghies, 1 Arm Davit, 4 Pt. Lift https://shop.inflatableboatparts.com/product/adjustable-davit-lifting-sling-for-dinghies-1-arm-davit-4-pt-lift/

Cam buckle tie down straps (Choose length and colour) https://slingsandstraps.co.uk/products/cam-buckle-tie-down-straps

 

Well, a single lifting point would be easier but I'm wondering about keeping the dink from swinging around when things get a little rough. 3 foot seas are my limit but if I'm taking them on the beam I can roll pretty good. I'll have to go down with a sketch pad and a tape measure tomorrow and play around with it. I'll attach a sketch similar to what you were nice enough to make Tiny Turnip. It will give you a better idea of what I'm trying to do with your design.
I was thinking of using 2" square tubing and making a fixed boom. I'll have to think about using round tubing so that the boom would rotate as SamSam suggested. I do have a question though. What would one call those attachment points where the boom attaches to the vertical tubing and where the sheaves/pulleys attach to the boom? Someone must make those so that they can be welded to the tubing. I can't find them but I think it's because I don't know what they are called. Maybe I can find a sailor. Also, if anyone can weigh in on whether 1/8" wall tubing or 1/4" wall is needed for this project. I'm leaning toward 1/4" 6061 aluminum but maybe it's overkill.
$143 for that sling is pricey. 4 carabiners, a SS steel ring and some webbing. You'd need to do a little sewing at the carabiners but if you can whip a line it's easy.
That's why I started this thread. Have you looked at what people want for a small hoist? Your idea is more practical and I can build it for a fraction.



Regards,

MIA

 

To be honest, I suspect a fabricator would just knock them up from plate or sheet. The joint is typically called a pin joint or perhaps hinge joint over here - I don't know a specific word for the little plates that are welded to the tubes I think 'ears' is as good as anything.

If you do a google image search for 'pin joint plate' it will bring up lots of examples of large scale joints used in building frames.

I've not worked in aluminium so I can't really help on the sizing, but I can't imagine the loads are that great - there's a bit of bending force in the tubes. If you took the stay to the end of the boom, it would eliminate the bending force in the boom, but ask the cable stay to work harder, as the angle would be shallower. You'd still have some bending force in the mast.

I'm sure a fabricator would be able to size up the sections if you show them your drawing and the maximum weight of the dink.

Not sure that two cranes will eliminate the dink swinging about - if its choppy, maybe some help from the crew, or some deft work with the boat hook?

 

The drawing is maybe simplified and you understand, but I was talking a short piece of pipe, say 2" diameter, the height of the gunnel, permanently attached to the gunnel. Then the vertical post of the crane, say 1 1/2" diameter, would fit in that, going all the way down to the deck, and that post would swivel.

For the boom, which could be round or square/rectangular tubing (or wood), attachment to the vertical post would be a simple bracket somewhat like this gate thing, but with only 1 bolt or pin, so the boom would be able to articulate up and down, the round pipe in the photo being the vertical post of the crane. It's just a short piece of channel welded to the post. The bracket could also be bent to shape from a piece of strap iron, or it could be short piece of rectangular/square tubing with one side cut off.

Maybe you got all that from before, I didn't know.

 

Being a boat, you might want to incorporate one of these so you can lift loads or apparently push rope out to things as theses guys seem to be doing.

 

Hi Sam,

Between the two of you I've a number of good ideas, thanks! You know, I read your post and looked at those guys with the capstan and practically fell off my chair laughing. I'm still laughing. Then I show it to my wife and she says "I don't get it, what's so funny?"

I spent a few hours down in the boat yard playing around with this. I've got to do something with that dink but it's really busy back there by the transom. So I carried the big half of it up to the bow and set it on the ground, tied a rope to it and climbed aboard. I lifted it up and got it over the bow rail and got it settled on the front deck. It actually fits pretty good up there and I can see over it if I'm running the boat from the lower station. It's too heavy to horse around every time I want to use it so I'm still going with a boom similar to what Tiny Turnip and you suggested. If I were to place a boom on the foredeck it would not need to reach out very far, maybe 2 to 3 feet at most. Much less moment-arm to deal with. The trick would be to lift the dink over the bow rail but the bow rail tapers to the deck as you go aft. In the right spot the boom will only need to lift the dink 6 inches or so above the deck, I can then swing it around and lower it onto the foredeck. Since there isn't nearly as much torque on the mast to boom joint, maybe I can have an aluminum fabricator weld me up a sturdy base with something like a 12" round plate that's welded to a 2 1/2 inch base that I can slide a 2" mast into. Think telescoping. One way or another it'll work out.

Cheers!

 

Like the capstan thing, I saw an old oil painting once of these guys in a small boat in a storm at night, water and waves crashing, froth flying by. A guy at the tiller leaning forward, with one hand on the tiller, the other shielding his eyes from the storm as he peered forward looking for the way. Two rough sailors were at the oars, pulling with all their strength and determination. Just a very dynamic and realistic painting of a dire situation, all their faces just a concentrated effort to survive. But then I thought, hey wait a minute...they're all facing forward! Those guys are rowing the wrong way!

 

MIA

Your sketch is missing some dims.. but I did a few back of a fag packet calc's...and some assumptions.
The 50mm RHS, or 2" box section seems about ok.
The 2" (50mm) round tube, base.. i'd go for the 1/4 " wall rather than the 1/8" (which fails).

But of course how often will it be used too.. plays a big part in the FoS.

If you can reduce this distance here too:


Less bending moment on the boom.

EDIT... for ref, the 1/4 tube is "ok" it is on the limit.. but only if unwelded value is used for design. If a welded, since you are welding to it, may be more appropriate in which case a thicker wall or greater diameter would be better. But it depends how you finally arrange it.