S/V FROG'S LEAP.com
DAVITS

There is nothing like dropping your dinghy in the water and making it ready in minutes. Davits are a great think for coastal cruising and keeping the dinghy safe when anchored.

Since I had somewhat achieved proficiency in 316 Stainless steel Tig welding, I initiated the project that I had wanted to achieve for a few years: Frog's Leap would get a set of Davits.

I ended up designing the davits much in the same way as I did the Arch. The picture of the boat and the design drawings for the arch could be augmented with the davits.

Frog's Leap davits were constructed of 1.25" diameter type 316 stainless tubing. The tubing is 0.089" wall thickness. This makes them quite beefy, and they completely supportmy weight without any noticeable deflection when I climb on them. The davits are bolted onto the transom inboard of the Arch aft pads and slightly aft. They are also bolted onto the deck aluminum cross piece at the transom of the boat.

The prep work for the davits was relatively simple, but the davits themselves took a lot of fabrication time. I estimate about 100 hours of work went into these: Tube bending, notching the tubing for the cross pieces, fabricating the davit ends, and finally the pads. Since the tubes were subjected to stresses from the weld material filler rod cooling, there was some straightening out of the davits. Also, I ended up with a splice as a result of having to adjust the height of one of the davits during the fitting.

The final assembly of the davits took a fair amount of welding on the boat. The double cross tubes that act as stiffener for the davits had to be welded on. The davits were secured to the arch piece using a cross tube of 1" diameter

A whole weekend was spent on the engine crane and a weekend and a half on the outboard mount. The engine mount is a standard off the shelf EDSON motor mount, but I fabricated a fairly elaborate framework to fit this engine exactly in the space between the Arch and the davit on the starboard side. The crane is a triangle of two tubes and a vertical shaft and bearings made 100% of 316 stainless, with some high density polyethylene washers to deal with the friction.

The most time consuming and messy part of the job is the polishing. I don't plan on polishing inside the garage any longer. The dust is horrendous. My yard has a dedicated polishing shed outside the fabrication machine and welding shop to deal with the dust problem. If I do this again I would recommend some plastic and a 2x4 frame, with an evacuation method for the dust. Wear a respirator!

Finishing touches: Finally, cleats were added to the davits to secure the lift lines, and in the spring of 2016 the stern light is being moved to clear the arch and davits, allowing for unobstructed view from other vessels.

The Davits being fabricated. There are five tubing bends, these were carefully made to match vertical and horizontal datums. The lower part of the legs are vertical. Only the top piece is horizontal. The inbetween connectors to reinforce the structure are following their natural angle.

The rough bend of the Davit post was brought to the boat at the end of the 2014 season. Notice that a couple more degrees of bend are needed for matching the arch.At this point the davit was a single piece of tubing with a single bend.

The design technique ia the same as the arch: Starting with a picture of the boat in the water, a 2D drawing program is used to visualize the Davits.
From that point, the angles and dimensions can be calculated. The angles were then checked to the davits would match the arch angles. IIt took about three trips to the boat to match the angles, going in one degree increments to tweak the pipe bends to match.

The JD square tubing bender is a very powerful tool that is affordable. Dies can be purchased for just about any bend radius one would encounter on a boat. The pedestal is optional, and I modified it by welding die holders made of 1" tubing. These are stainless and welded directly to the base steel. The base is mounted on a couple of 4x4 pieces of wood fitted inside a table saw rolling base. This gives me portability by being able to load the thing inside my trailer. Finally, because I am not mounting the base of the bender onto a concrete floor as it was indented to be, it is unreasonable to put the forces needed to bend steel of 1 to two inches onto the bender. The solution is available from SWAG. This small business made a plasma or laser cut adapter which mounts right on the bender. The adapter allows you to mount a harbor freight hydraulic cylinder to do the work. You can manually pump the cylinder or also use its air pump. It's not super fast, but does the job. The total cost of the hydraulic conversion is about one fifth of the price of the JD square hydraulic kit.

 

The Davit feet. The one on the left goes against the stern deck plate (about 5/16 thick aluminum) of the boat. The right one fits against the transom and is backed with a 1/2 piece of FR4 or G10 epoxy board on the inside of the transom.

In order to match the angles of the boat, the feet were first cut with a straight hole saw on the drill press. I get about two cuts out of the hole saw despite generous use of tapmatic cutting fluid. After the hole saw cut, I can enlarge the hole manually using a right angle air grinder on which I mount a 1/4" carbide bur. Cylindrical or Christmas tree shape works fine. This allows setting the angle properly.

 

The feet were tacked on while on the boat, with the through transom holes drilled in the right place. This ensured the proper angle match. After tacking on, the davits were removed, taken home, and the welds were completed. Needless to say that all this fitting, checking, and travel to and from home, settting up the welder and taking the welder back home, took an incredible amount of time.

Note the large aluminum bars and clamps. The idea is to keep the steel cool during the welding process. It worked well, as seen on some tig welding videos.

In the end, the welding is the easier work. The fitting and measuring, and the polishing, and of course the cutting of steel without access to a mill, is very time consuming.

The Tubing Notcher is made by BAILEGH and uses a 1/2 milwaukee right angle drill. The hole saws at the end of the shaft are controlled at a preset angle so the tubes match perfectly. This makes for a good weld and clean assembly. I purchase high speed steel saws in quantities of five or 10. Each is good for 5 cuts or so. A milling machine would be alot better but this is a portable contraption which can be moved if needed, an important consideration when your boat is not in your yard.

The tool is mounted on a piece of 1/2 mild steel which can be clamped onto a welding table or mounted on a piece of plywood on sawhorses. The steel and the angle iron are all welded together The weight of the bottom piece allows the tool to stand on its own without being toppled by the weight of the drill. This makes things convenient.

Note the large circular pipe vise and the bronze bearing on the vise screw. This tool is built to withstand large forces when nothing, without deviation to the arbor shaft holding the saw. U recommend Lubrimatic water based cutting fluid.


After I write this I am cleaning and oiling the tool. This is embarassing!.

The Frog is Happy to have the Davits. I still need to tune up the strapping system and add a couple of pad eyes to the transom in order to secure the dinghy in waves.