Please Critique the Engineering of My Dinghy Lifting Beam

[Terra Nova]

Originally Posted by Dockhead

...I have been tortured for years by my Simpson S175 davits and problems with lifting and keeping my dinghy up...

Attachment 98860

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Originally Posted by Dockhead

...lift from a point alongside the davit...

That both your design approaches ignore the ramifications of side loading makes me ask why you have not yet thought to seek professional help for your davit system.

[Dockhead]

Originally Posted by Terra Nova

That both your design approaches ignore the ramifications of side loading makes me ask why you have not yet thought to seek professional help for your davit system.

Side loading of the davit? You mean off center load? Off-center load is a non-issue. The first design had the load taken exactly in the center of the davit. The second has the load distributed between the center and a point not more than 45mm off center - an obvious non-issue for this massive construction.

If you mean actual side loading, as when the boat rolls or is heeled, these lifting systems don't change anything about that. The davits are designed to take it, and manage fine as long as the bolts are properly torqued.

[Terra Nova]

Yes, off center is what I meant.

A professional is what you seek.

[Dockhead]

Originally Posted by Terra Nova

Yes, off center is what I meant.

A professional is what you seek.

What problem do you see here? Even I can calculate that --

Even 200 kilos (whole weight of dinghy, motor, contents) exerted 22.5mm off center will exert only 44 newton-meters of torque on the davit arm, or 32 foot/pounds. That's entirely trivial for davits designed to carry 200 kilos about a meter and a half off center from their attachment points.

[thinwater]

The PO had built a polyethylene version with a greater Y dimension, which eventually cracked. This re-build from 1" aluminum square tube with 1/8" walls is lighter and still overbuilt. Of course, the front end of this inflatable is only ~ 100 pounds as loaded on davits. Note that I used a Spectra sling in the center rather than cut away metal to fit a biner or large eye.

As for tipping (my low rig is sufficiently stable, but I can lean the dingy easily, which actually helps drain water), while it needs some stability while lifting, any movement underway should be handled by tricing lines. No problem.

[Terra Nova]

Originally Posted by Dockhead

What problem do you see here? Even I can calculate that --

Even 200 kilos (whole weight of dinghy, motor, contents) exerted 22.5mm off center will exert only 44 newton-meters of torque on the davit arm, or 32 foot/pounds. That's entirely trivial for davits designed to carry 200 kilos about a meter and a half off center from their attachment points.

You asked for critique but, when given it, you seem to have all the answers. Carry on.

[Dockhead]

Originally Posted by Terra Nova

You asked for critique but, when given it, you seem to have all the answers. Carry on.

Well, the most helpful critique is when it contains something I don't know

Which is almost everything, but at least not this

But in any case -- sincere thanks for weighing in.

[Dockhead]

Originally Posted by thinwater

The PO had built a polyethylene version with a greater Y dimension, which eventually cracked. This re-build from 1" aluminum square tube with 1/8" walls is lighter and still overbuilt. Of course, the front end of this inflatable is only ~ 100 pounds as loaded on davits. Note that I used a Spectra sling in the center rather than cut away metal to fit a biner or large eye.

As for tipping (my low rig is sufficiently stable, but I can lean the dingy easily, which actually helps drain water), while it needs some stability while lifting, any movement underway should be handled by tricing lines. No problem.

Thanks! That's inspiring. That's a very good design, it seems to me.

I would have avoided alu because of unlike metal, since I would not have had room (unlike you) for the sling in the middle.

But I think I am still convinced that this won't work for me, because I have a heavy (25 horsepower) outboard which make the dink very tippy if I lift it like that.

I think I will probably stick with Plan "B", plus following Poiu's suggestion to try harder to get the internal mechanisms working.

* * * *

OR, Plan "C" --

Simplest of all -- a separate line to each lifting eye, running through a double block, running through two clutches, and then joined before going to the winch . . . .

Solves also the line handling problem -- I can bend on a nice comfy 12mm line to go back to the winch.

The winch is 6 meters from the pushpit. A two meter drop will require 4 meters of pull -- so 2 meters to spare.

I even have the suitable block.

Hmmm.

Would cause a bit of profusion of lines but the lines to the lifting eyes may be only 8mm, which are not too bulky.

[monte]

Or plan c, fit an inner and outer block on the davits instead of centre double block so the lines are more vertical and separated = more stability when raised. Not too wide that they chafe the sides of the dinghy
Nb just tie them on with webbing or dynema loop for simplicity and no drilling the davits

[Dockhead]

Originally Posted by monte

Or plan c, fit an inner and outer block on the davits instead of centre double block so the lines are more vertical and separated = more stability when raised. Not too wide that they chafe the sides of the dinghy
Nb just tie them on with webbing or dynema loop for simplicity and no drilling the davits

Those are all great ideas , but not quite applicable for my particular case, because the central lifting point is as far out as the solid part of the davit goes. Further is just an extension arm to stabilize the dink.

If I use separate lines, then I don't have to drill the davits (how much I hate drilling stainless steel) except for the clutches. The double block can hang directly from the existing eye at the end of the solid part of the davit.

[Terra Nova]

Originally Posted by Dockhead

...The winch is 6 meters from the pushpit...

This seems an unnecessarily great distance to place a winch from where you need to use it.

[Dockhead]

Originally Posted by Terra Nova

This seems an unnecessarily great distance to place a winch from where you need to use it.

I agree that it's a big disadvantage. But it's that far away because it's an existing electric sheet winch -- so that I don't have to install a winch on top of everything else, which would add a lot of cost.

If I didn't mind spending the money, a dedicated winch right on the davit would make a fantastic setup. This is the way davits should be built, in my opinion. Not with custom mechanisms buried inside the davit, but with a standard winch on the outside -- child's play to service, no problem to get parts if necessary, etc. /rant

[Terra Nova]

Originally Posted by Dockhead

...so that I don't have to install a winch on top of everything else, which would add a lot of cost.

If I didn't mind spending the money, a dedicated winch right on the davit would make a fantastic setup...

This is what I was getting at. Many owners trip themselves up by poor-boying it too hard. Rube Goldberging it 6 meters away is disgraceful.