Spec'ing out new arch - need help

[zboss]

Hi,

We are looking to replace an existing stainless steel davit system on our Cabo Rico 38 that has been on the boat for many years with a new aluminum arch with integrated davits, solar panel supports, and an engine lift.

We learned long ago to move the dinghy onto the deck when we anticipate rougher weather or when doing an extended coastal or offshore passage, so the davits would only have to deal with unexpectedly sporty conditions for a short period, until such time we could anchor.

The existing davit system has "forward" legs that sit flat on the deck and are through bolted. It then has two "stern" legs that are bolted onto the stern, which is sloped inwards from the cap rail to the water.

Not being a mechanical engineer I am trying to understand the dynamic forces involved and how to best design the arch to support a dinghy in moderate sea conditions, with my main concern being the sheer forces on the stern mounting.

Would schedule 40 tube 2.5" in diameter be sufficient for this design? Would it be best to plan to have the tube project into the stern of the boat by an inch or so and then welded on the outside by a plate that is through bolted? Or is it best to weld the pipe to the plate and only have the plate bolted through?

What angle to the arch should I avoid?

I'm not trying to reinvent the wheel, just design something sturdy with an emphasis on making sure it is not weak at the transom mount.

[CAELESTIS]

Here are photos of an aluminum arch, about 3.5 in Sch40 tube.

[zboss]

Wow... 3.5 inch. I have no idea - how strong is that? I see your dinghy lift swings down... how is it held there or do you crank it back up when the dinghy is on it?

I like your single tube look a lot but I wonder if I could get a just as strong a design using the double arch with smaller diameter tubing?

[CAELESTIS]

Actually, it is not my boat. It is a design I admire and was about to replicate on my own boat.

If you are running fore and aft tubes, then 2.5in should be more than enough.