We wanted davits for Phoenix that would be strong enough to lift and carry a 5-person RIB with its 15 hp 4-stroke engine. Total weight including RIB, motor, 5 gal fuel tank and misc gear is about 350 lb.
I looked at a variety of commercially available davits but was not satisfied that the conventional tubular steel davits would be strong enough to carry the load. The Kato “Voyager” davits, with welded aluminum box construction, were strong enough (rated at 420 lb safe working load for each arm) but were pretty expensive. The Kato davits with the custom mounting brackets cost about $4400.
I decided to adapt the basic Kato design and fabricate our own davits, customized to fit the stern of our Whitby 42. Rather than mount the davits on deck or on a transom bracket, I decided to extend the davits so that I could through-bolt them through the transom for added strength.
The davits are fabricated from 6061 Aluminum. I increased the thickness slightly from the Kato design. Kato davits are 1/8” thick side walls and 3/16” thick top and bottom. I used 3/16” for the side walls and ¼” for the top and bottom. We were able to cut the 4 side wall pieces from one 4’ x 8’ aluminum sheet. I recommend that you mark all 4 side pieces on the 4 x 8 sheet before cutting though. It’s a tight fit. We used 3” wide aluminum bar stock (1/4” thick) for the tops and bottoms. The fabricator tack welded the tops and bottoms to one of the sides, bending the tops and bottoms as he went. He added a few baffles inside the aluminum box to help bend the tops and bottoms to the side plate curve.
We used 6”wide bar stock (1/4” thick) for the base of the davit to provide a mounting flange to allow it to be bolted through the transom. To fit the curve of the transom, the side wall on the inboard side of each davit must be cut back about ½” to ¾” before the base/mounting flange is welded on. That way, the davit will point fore and aft when it’s bolted to the curved transom. We didn’t get that angle quite right so I built up an epoxy “shim” under the mounting flange so we’d have solid support under the davits.
After my fabricator was done with the welding, the welds along the 4 seams were still pretty rough. I used West epoxy with the fairing powder to fill all the seams. I then ran a router along each seam to round off the fairing and any aluminum that protruded into the corners.
I painted the davits with 2-part polyurethane (Interthane Plus) after priming with Vinyl-Lux Primewash and Epoxy Primekote.
The davits are through bolted through the transom with 3/8” bolts. We used the 6” wide bar stock (1/4” thick) to fabricate backing plates which we placed inside the lazarrete lockers.
We mounted a cross-brace between the two davits for strength. The cross-brace is a 6” wide aluminum “U” channel. We bolted a piece of salt-treated wood inside the channel for added stiffness and to cover the wire going to the stern light. We mounted the stern light and the mizzen sheet on the cross-brace.
We mounted triple blocks in the aft ends of the davits. The lifting lines were run internally to the forward end of the davit where they run through a cam cleat to a turning block. The 6 to 1 advantage of the double triple blocks makes it possible to lift the fully loaded RIB. It’s still a bit of a strain to lift the heavier aft end of the dingy even at 6 to 1 but, when cruising, we routinely lift the dingy every night and whenever we get underway.
What we’d do differently: If we were building another pair of davits, I’d place the transom-mounting bolts a little differently. The top two bolts should be as near to the toe rail as possible to keep a small crack from opening up between the davit and the hull at the top of the mounting flange.
If you have any questions, please contact me;