Davit – Part 2 (Support Brackets Below Swim Platform)

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A Davit is a small crane onboard a ship, especially one of a pair that are used for suspending or lowering a lifeboat. My friend and customer Ray has designed a Davit system to lift up his dinghy onto the swim platform of his boat. This post describes the brackets that will attach to the bottom of the inserts, which were previously glued into the swim platform. The purpose of the bracket assembly is to connect the bottom of the inserts to the Aluminum fabrication that supports and lifts the swim platform to take some of the load off the glue on the inserts.The first picture shows a Davit from another boat, to give you an idea of the typical design of the Davit. The support brackets described in this post would be located below the swim platform.1. Davit

The following picture shows the support bracket assembly for one side of the Davit. The two inserts on the top are 2” in diameter have a M10 x 1.5 thread and were previously glued into 2” holes that were drilled in the swim platform of the boat. (See Davit – Part 1 for information regarding gluing of the inserts).2. Davit support bracket asm

The pair of support brackets that I machined from angle Aluminum are shown below.3. Support brackets

I wanted to clamp the angle aluminum to a piece of plywood resting on the table of the milling machine. The plywood allowed me to drill and mill the angle Aluminum without cutting into the table of my milling machine.  This configuration forced me to use an extra long center drill to reach past the leg of the angle Aluminum.4. Extra-long center drill

5. Plywood under angle

Smith Oxyacetylene TorchMiller Dynasty 200DXLincoln SP-250 MIG WelderClausing/Coldchester 15" Lathe16" DuAll Saw15" Drill Press7" x 9" Swivel Head Horizontal Band Saw20 Ton Arbor Press BridgeportLincoln LE  31 MP & Lincoln 210 MP
Reply:The plywood supported the Aluminum angle to prevent chattering when milling the slot in the angle.6. Milling slot

Below is a picture of all of the parts that I machined for the support bracket assembly except the compound angle spacer which will be discussed later.7. Machined details

I like to stack up details to save machining time.  In this case the pieces were cut from two different pieces of stock so they were different widths and therefore wouldn’t clamp in the vise.  The solution was to “C” clamp the pieces together as shown.8. Clamps to hold pieces together

Sometimes a piece is much larger than the vise.  One solution is to support the cantilevered end with a shaper gage.  The shaper gage has a shallow locking angle, so that it doesn’t typically slip for this application.  Another solution would be to use a second vise that was machined to match the position of the first vise.  Unfortunately, I don’t have a second vise.9. Shaper gage to support cantilevered piece

The two angle Aluminum pieces were cut from the same piece of stock so that the legs were the same thickness.  This allowed me to clamp the two angle Aluminum pieces in the vise at the same time using two 1-2-3 blocks. This was done to save setup time.10. Clamping with 1-2-3 block

Smith Oxyacetylene TorchMiller Dynasty 200DXLincoln SP-250 MIG WelderClausing/Coldchester 15" Lathe16" DuAll Saw15" Drill Press7" x 9" Swivel Head Horizontal Band Saw20 Ton Arbor Press BridgeportLincoln LE  31 MP & Lincoln 210 MP
Reply:One side of the swim platform fabrication was slightly bent in the area that the support brackets attached.  Ray’s solution was to design a compound angle shim to fill the space between the support bracket and the swim platform fabrication.  Ray’s philosophy was to fix what is wrong rather than to modify good parts, to work with bad parts.  Also, in the final assembly the compound shim would be almost invisible. Below is a picture of the compound angle shim.  The second picture shows the dimensions of the compound angle shim.11. Compound angle shim

12. Compound angle shim dimensions

To machine the shim, I used SolidWorks to calculate the angles to set on the compound angle vise. I used a sine bar to set the angles, because the graduations on vise were too course.13. Using sine bar to set angle

This vise can actually set three angles.  I set the first angle to zero.14. Compound angle vise

This is what all of the pieces look like when bolted together.15. Parts bolted together

Smith Oxyacetylene TorchMiller Dynasty 200DXLincoln SP-250 MIG WelderClausing/Coldchester 15" Lathe16" DuAll Saw15" Drill Press7" x 9" Swivel Head Horizontal Band Saw20 Ton Arbor Press BridgeportLincoln LE  31 MP & Lincoln 210 MP
Reply:Here are two pictures of the compound angle spacer in place16. Compound spacer in place

17. Compound spacer close up

Here is a picture of one side of the support bracket assembly temporarily installed.18. One of the installed support bracket assemblies

Ray is planning to remove the brackets and sent them out for powder coating. Before the final installation. The next phase in this project will be to design and fabricate the arms of the Davit, which will mount above the swim platform.-DonSmith Oxyacetylene TorchMiller Dynasty 200DXLincoln SP-250 MIG WelderClausing/Coldchester 15" Lathe16" DuAll Saw15" Drill Press7" x 9" Swivel Head Horizontal Band Saw20 Ton Arbor Press BridgeportLincoln LE  31 MP & Lincoln 210 MP
Reply:Judging by the visible corrosion this is a saltwater boat. I forgot the alloy you’re using but you will need to isolate those bolts with some aluminum NeverSeize and it definitely will need a zinc sacrificial anode somewhere. In fact, despite that excellent machining, I would have welded it all together and then powder-coated it or even better yet, leave it raw. The correct alloy will not corrode past the “gray” stage. Any “stacked” aluminum will corrode like crazy; welding to seal it and prevent any water intrusion, which otherwise will inevitably occur as the boat “works,” will result in corrosion. You can see the white powder on the engine structure in that photo, once the coating has penetrated the finish, the aluminum dissolves like crazy and the only prevention is aggressively grinding off the corrosion to bare metal then another coating.Otherwise, your solution will work but it will attract unnecessary maintenance.Last edited by Xsbank; 04-04-2022 at 02:54 PM.Do not believe everything that you think.
Reply:

Originally Posted by Xsbank

Judging by the visible corrosion this is a saltwater boat.