Please Critique the Engineering of My Dinghy Lifting Beam

[Dockhead]

Some of you know that I have been tortured for years by my Simpson S175 davits and problems with lifting and keeping my dinghy up.

I am trying to finally track down the parts to get the original electrified lifting mechanisms working again, but I have decided in any case to have a robust backup system.

The problem with using a normal block and tackle is that there isn't enough vertical space. The original system used slings from four lifting eyes, and the two legs of each sling met at a shackle which would just touch the davit when the dinghy was fully lifted, leaving zero space for any block and tackle. The distance between the lifting eyes is 60cm, and there is just about 35cm of height to the davit when the dingy is fully lifted, so there's no way to shorten the legs.



So I've decided to make a beam to bridge the lifting eyes, in order to gain vertical clearance for two Wichard 35 plain bearing double blocks. I can order rectangular section 316 stainless steel tube, and judging just by eye -- since I lack the skill to do the engineering calculations -- I think what I need is 50mm tall x 25mm wide, with wall thickness of 1.5mm. It would look like this:




I would attach 80mm forged stainless caribiner snap hooks to each end of the beam by through-bolting with M8 stainless bolts. In the middle, I would put in another M8 stanless bolt to which I will shackle the Wichard double block.

At the top, I will hang another Wichard double block. I will use a 9mm double braid line which I will lead through a Spinlock XAS clutch back to an electric sheet winch.

The total vertical height of all of this mechanism is less than 300mm, which should leave at least 50mm between the two blocks when the dinghy is firmly pulled up to the davits.


The dinghy weighs about 175 kilos, with most of that weight at the aft end where the engine is. I tried to imagine that any one of the four lifting eyes and everything attached to it should be able to manage maybe double the entire weight of the dingy, as SWL. I know that's not really "engineering", but it seemed to be a reasonable, conservative way to do it. The snap caribiners have SWL of 600kg each; the blocks have SWL of 480kg with breaking force of 900kg. The Spinlock XAS clutches will hold 575kg.


My main concern is whether the beam itself will be strong enough. I will be lifting it from the middle, but it's bearing weight from either end. The box section would seem to help. What do you guys think?

And is the attachment method ok? The caribiners and the block will all be on one side of the box section, through-bolted with M8 stainless bolts.

As always, I'll be grateful for any tips.

[mike d.]

You can add center strength inside the s.s. tube by adding a smaller diameter aluminum channel or T section. Our boats boom has a 4" square box extrusion about 5' long inside, (round boom) on mid boom sheeting. This original from the factory!

[monte]

Having trouble following the description and drawins, but you can't do something like this?

Nb. Small forward lines stop any sway
Lifting point too low will make dinghy tippy when lifting
The two clips fall out of the way on the floor when not in use
Make sure stern is lower so it drains

[Dockhead]

Originally Posted by monte

Having trouble following the description and drawins, but you can't do something like this?
Attachment 98869
Nb. Small forward lines stop any sway
Lifting point too low will make dinghy tippy when lifting
The two clips fall out of the way on the floor when not in use
Make sure stern is lower so it drains

That angle of those sling legs will create tremendous loads. OK on yours because that will be light weight; not doable on mine because the stern of my dinghy weighs probably 130 kilos if not more.

[monte]

Yes the bow and stern require different approaches. Note the bow is raised to touch the davits. The stern can't be raised quite as high to allow the dinghy to drain. All straps are doubled as well so if one gives way there a backup, but so far no chafe or sign of trouble. The loose strap you see over the top goes underneath as an extra backup for crossing oceans but usually left off in average conditions.
The rear lifts by the lines shown below. The transom is secured in place by 2 additional straps that clip onto the davits as well, this gives 3 support points. The lifting support, as well as the 2 direct to the transom. Lifting takes 30 seconds, connecting the additional clips takes 10 seconds. Note also 2 lines onto the clip, the second purely as backup should the first chafe.

[Stumble]

Dockhead,

What you are designing is called a lifting beam, and is pretty common in the lifting world. Take a look at http://files.engineering.com/downloa...ting_beams.pdf for a detailed analysis of it. My guess is you are massively overbuilding yours, you might want to think about switching to aluminium just to keep the weight down.

[goboatingnow]

Dockhead , before I sold it I was lifting a 180kg aluminum rib and 40 hp yam on those davits. I used a beam just like you.

Dave


Sent from my iPad using Cruisers Sailing Forum

[jimbunyard]

I'd increase the wall thickness to 11 gauge (.12 inch or 3.08mm) so that I could drill a hole at each end to put the carabiners through, to balance the load (otherwise the beam will sit cockeyed, putting a side load on the pulley). I'd also increase the center bolt size to at least 10 mm, and feel more comfortable with 12mm, and mount the block so that it is centered on the 25mm width, again to keep the direction of pull aligned. (either slotting the upper surface of the rectangular tube and through bolting or using a clevis of some type would be my choice)


For a static load, 1.8mm wall seems plenty strong, but bouncing around in a sea, 3mm seems much safer, 5mm would be bulletproof...I overbuild sometimes though

[Dockhead]

Originally Posted by jimbunyard

I'd increase the wall thickness to 11 gauge (.12 inch or 3.08mm) so that I could drill a hole at each end to put the carabiners through, to balance the load (otherwise the beam will sit cockeyed, putting a side load on the pulley). I'd also increase the center bolt size to at least 10 mm, and feel more comfortable with 12mm, and mount the block so that it is centered on the 25mm width, again to keep the direction of pull aligned. (either slotting the upper surface of the rectangular tube and through bolting or using a clevis of some type would be my choice)


For a static load, 1.8mm wall seems plenty strong, but bouncing around in a sea, 3mm seems much safer, 5mm would be bulletproof...I overbuild sometimes though

Thanks for that. I thought about the centering of the load problem, and thought I would just double the beam if needed, and put the hooks and block in the middle.

But is that really necessary? The loads are all lined up; just the beam on the side. Won't that work ok?

Concerning wall thickness -- do we know how to calculate what is adequate, and what is overbuilt? 1.5mm "looks" pretty good to me, but I don't know how to make an actual calculation of the strength.

[Dockhead]

Originally Posted by goboatingnow

Dockhead , before I sold it I was lifting a 180kg aluminum rib and 40 hp yam on those davits. I used a beam just like you.

Dave

"On those davits" -- you mean Simpson ones like mine?

[conachair]

What about using back to back angle and a very shallow dyneema bridle?
Hope this link works... scuse my awful artistic skills ...

https://awwapp.com/s/5f/ec/51.html

M10 or 12 bolt each end with a spacer supporting the carbine hook with the eye of a doubled piece of dyneema sitting each side. 25 x 6 equal angle might do it? Force along the angle is in compression, the dymeena does all the work transferring the centre point load to the 2 cantilevered ends. You would gain back any height lost with the bridle by a smaller section beam and it's easier keeping the load central in plan view.

[jimbunyard]

You could double the beam and it would certainly center the load, but would double the cost and is not exactly what I would call 'elegant'.

One could live with the 'cockeyedness' and it would probably work ok, but again not so 'shipshape' looking (and it will put side loads on the blocks, for which they're not really designed and cause accelerated wear in the lines).

A link to an 'easy calculation' site:

https://www.easycalculation.com/engi...ular-beams.php

shows that at a 100 lb load, a 2 foot section of 1" x 2" rectangular steel tube with 1/16 (.0625") wall deflects .0825", a .12" wall deflects .0477" and a .1875" wall deflects .0346". Engineers, in my experience, are (rightly) leery of specing dimensions for materials because of unknown dynamic conditions, but you may be able to find someone. I have no formal education in engineering but plenty of real world experience. For me the baby bear size is .12".

A couple of descriptive sketches:

[conachair]

Or....

Back to back unequal angle bolted with spacers.



Easy to fabricate.

If you really wanted to save on headroom the end connections could be pins picking up dyneema loops on the dinghy, or maybe dyneema with a toggle to go through the gap and sit ontop.

Box seems a bit over the top as most of the strength is only needed in the bottom face to stop it buckling.

[conachair]

Another thought, if there is a heavy outboard attached and the centre of gravity ends up above the pickup point, which it could well do with a spreader beam low down, then only thing the dinghy wants in this world is to turn upside down...

[jimbunyard]

I should add that, as long as the elements are in the same plane (as I just see you stated), the degree of cockeyedness will be slight at first, but if through bolted, over time, will cause the tube to collapse inward, inducing more out of plane pull, which will cause the tube to collapse more, which will induce more out of plane pull, which will... you get the picture. If you bolt the carabiners and block to one side of the tube, this will be minimized, but you don't get the full use of the strength of the tube, and distortion will still eventually result, probably in the form of twisting.