Davit – Part 1 (Inserts to Attach the Davit to the 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 inserts that I made, as well as the drilling and gluing fixture which were used to glue the inserts to the swim platform. The inserts will eventually support the davit. The first three pictures show a pair of Davits from another boat, to give you an idea of the typical design of the Davits.1. Dinghy on davits 1

2. Davits 2

3. Davits 3

The next picture shows one of the 2” OD Aluminum inserts.  The inserts were tapped for a M10 x 1.5 Helicoil.  Notice the slight chatter on the grooves in the following picture. I machined this insert using a long piece of stock with steady rest, to minimize the stock wastage.4. Insert

I left the chatter because it won’t hurt the function of the insert, but I did eliminate the chatter as shown in the following picture by using a more rigid setup.  5. Cutting groove

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:I used a spiral fluted tap to tap the hole for the M10 x 1.5 Helicoil.  This tap is ideal for blind holes because it pulls the chips out of the hole. If you look closely, you can see the chip coming out of the tap.6. Tapping hole

7. Inserts machined

8. Inserts powder coated

Here is a picture of Ray installing the Helicoils.  Ray specified Helicoils that were 2.5 times the diameter or 25 mm long, which made them difficult to install. 1.5 times the diameter, would have been more than adequate and would have been much easier to install. 9. Ray installing Helicoils

I try to be as efficient as possible, so I clamped the two rails to the table of the Bridgeport, at the same time. I used 1-2-3 blocks to space the support rails above the table to avoid drilling into the table.10. Support rails clamped

 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:In the following picture you can see how I positioned the 1-2-3 blocks so that the drill would miss them.11. Drilling support rails

In a similar fashion I positioned one block directly under the place, for the milled slot for maximum rigidity.12. Milling support rails

And here are the support rails all machined.13. Holes and slots machined

14. Support rails done

The next step was to build a drill fixture that actually had three functions:1. Drill a ¼” pilot hole for the pilot drill for the 2” hole saw, which would drill the holes in the swim platform for the inserts.2.Allow Ray to transfer drill holes in a backup bar, which will mount on the bottom of the inserts on the bottom of the swim platform to help maintain spacing of the inserts.3.Will be used with some custom studs to hold the inserts in position, while waiting for the epoxy to dry. 15. Welding drill fixture

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 some of my welds on the drill fixture.16. Weld 1

17. Weld 2

Here is a picture of the custom stud that I made to allow Ray to use the drill fixture to hold the inserts while waiting for the epoxy glue to set, which glued the inserts to the swim platform.18. Stud with nuts

Below is a cross section of a SolidWorks assembly model showing how the stud will pilot on the drill fixture and hold the insert.19. Stud asm

The first step in manufacturing the custom studs was to make a spacer with a M10 x 1.5 tapped hole in the middle and a tight fit to the hole in the headstock of the lathe.  The purpose of the spacer is to prevent the M10 x 1.5 grade 10.9 threaded rod from vibrating as I machined the other end of the threaded rod.20. Support for threaded rod

 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:My 15” Clausing Colchester lathe has an English lead screw and I most frequently cut Metric threads, so I have lefthand tooling, which allows me to start threading from the undercut of the thread, run the lathe in reverse and thread to the end of the shaft. I added extra material on the end of the part to allow the threading tool to clear the center as the tool exited the thread. I chased the threads using a center in the threaded shaft to improve the true position of the ¼”-28 thread relative to the M10 x 1.5 thread.21. Extension for threading

Here are all of the custom studs completed.22. Studs threaded

I allowed .001 to .003” clearance between the custom stud and the hole in the drilling fixture.  As a result, I checked each stud in all of the holes and reamed the holes in the drilling fixture where necessary.23. Checking stud for fit

Ray used the drill fixture to transfer drill holes in the backup bar, which will support the inserts on the bottom of the swim platform. Here you can see the back up bar and inserts mounted to the drill fixture.24. Backup bar fit

Here is a picture of one of the plugs cut in the swim platform for the inserts.25. Plugs cut with hole saw

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 final step was to use the drill fixture and the backup bar to hold the inserts in position until the epoxy glue cured.  Notice the black spacer, which Ray 3D printed to hold the top of the insert in the correct position.26. Fixture set for gluing

27. Close up of gluing fixture

Last but not least is a picture of the backup bar holding the bottom of the inserts.28. Backup bar in place

-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:Impressive build, just a note: once the dinghy is mounted the sternlight will be obstructed. Bob
Reply:

Originally Posted by rhunt

Impressive build, just a note: once the dinghy is mounted the sternlight will be obstructed. Bob
Reply:Ray got back to me regarding the stern light.Yes, the dingy will obstruct the lower stern light, but my boat has 2 stern lights. The upper one is at the top of the electronics mast.
Reply:What I think, as an owner of 5 boats…Is that 20 horse outboard is going to make for a slow damn boat!Very nice build…and I appreciate your documentation and photos.As far as that light goes…I see several options.   Either move it higher…but that will get banged/bent.  Move it to the back of the cabin/wheelhouse…but that may be a tough install…Or, make a fixture/jig that attaches to the dingy set up that can be easily install when the dingy is up.  I would leave that existing light where it is…and add a plug/outlet wired off that that they could use when the dingy light is used. I am picturing something like a stainless steel curved base that could easily be strapped to the dingy tube…I know…easy to say!
Reply:

Originally Posted by Tarmstrong

What I think, as an owner of 5 boats…Is that 20 horse outboard is going to make for a slow damn boat!Very nice build…and I appreciate your documentation and photos.!
Reply:Looks like a good solution. My only comment is that with a cored swim grid with holes on the horizontal surface is that walking on it or raising the dinghy will eventually crack or cause the sealant to fail and that swim grid will turn to mush. I would have added one more step and drilled out the holes too large, filled them up again with resin, allow it to fully set and then drilled the holes in the resin for your machined plugs.Do not believe everything that you think.
Reply:

Originally Posted by Don52

The 20Hp outboard can be lowered into the water through a hole in the swim platform.It is used strictly for trolling.The steering on the outboard motor is hydraulically coupled to the position of the rudders on the boat.Each of the two propellers on the boat are coupled to a Diesel engine.Below is a picture showing the relationship between the outboard motor and the twin screw propellers.

-Don
Reply:

Originally Posted by Xsbank

Looks like a good solution. My only comment is that with a cored swim grid with holes on the horizontal surface is that walking on it or raising the dinghy will eventually crack or cause the sealant to fail and that swim grid will turn to mush. I would have added one more step and drilled out the holes too large, filled them up again with resin, allow it to fully set and then drilled the holes in the resin for your machined plugs.
Reply:More of Ray’s comments: The penetrating epoxy has a pretty long working life, so I had enough time to move to each location numerous times, allowing the epoxy to penetrate, until each hole was filled to the top. I still may have to top off a little when I get back to the boat.
Reply:

Originally Posted by Tarmstrong

So what is the light solution so far?
Reply:Best to be safe than sorry, you need to come up with a better solution.  Rarely cruise at night leaves a lot of chances, something to think about.  I don't think that excuse would go over big with the Coast Guard.  BobLast edited by rhunt; 03-21-2022 at 11:45 AM.
Reply:

Originally Posted by rhunt

Best to be safe than sorry, you need to come up with a better solution.  Rarely cruise at night leaves a lot of chances, something to think about.  I don't think that excuse would go over big with the Coast Guard.  Bob