Attaching the inflatable dinghy (tender, Zodiac, or whatever you want to call it) to the boat was always a goal of mine. However, which one was the best for my boat was the question? There are many different styles available, with different mounting configurations. Since I have a massive swim plaftorm on the boat, I came to the conclusion that I wanted a swim platform mount. Some of the more popular swim platform davits are:
Cost varies from hundreds to thousands of dollars for the various systems. And since the systems may not come with any hardware to mount the davits to your boat, there is typically an additional cost in stainless hardware, as well as any additional bracing that might be required on your boat. This cost can be significant, and run into hundreds of dollars more.
The design of my boat is such that I have an integrated swim platform. I did not want to lose the ability to walk on the swim platform while the dinghy is aboard as there is a fuel tank inlet on both the Port and Starboard side of the boat - only accessable via the swim platform. As well, I wanted to retain as much of the access to the boarding ladder as I could. Finally, I wanted a davit system that would be easy to launch and retrieve a dinghy when anchored out in Lake Michigan, with the possibility of some wave action.
The davits I decided would fit my needs were St. Croix Model 400 Lifting Arms, and Model 401 Swim Platform Mounts. These are massive stainless steel arms, with a lifting capacity of 350lbs (Model 488 can be had with a capacity of 488lbs). My estimate of my dinghy is about 85lbs, and 50lbs for the outboard, so I was well within the weight limit of my dinghy. As well, many hard-shell dinghys with motor will be within this weight limit should I decide to upsize the dinghy.
The davits would also retain access to the swim platform when the dinghy was mounted as the dingy would be out and over the water rather than sitting on the platform itself. Finally, the swim ladder's access is not blocked, even when the dinghy is attached.
As it turns out, the davits provide a rather nice railing of sorts - something to hang on to when on the swim platform on a rolling boat. And we have developed a technique where we can grab the lift lines when boarding the dinghy, and simply step on board.
As shown in the drawing, to ensure a solid installation, I plan on constructing mounting brackets to go under the swim platform.
To save on shipping, I fortunately found a dealer in the area that had the davits in stock, so on one beautiful winter Saturday, we drove to the dealer and purchased them. After bringing them home, I built a mock-up in my basement with a few 2 x 4's. This allowed me to layout the desired spacing and attach the necessary fittings on the dinghy, and simply, to see how this would all work. I figured it would be easier to attach the dinghy mounts in the basement rather than in the slip.
The davit arms themselves rotate about 180 Deg, to both allow for different sized dinghys, as well as winter storagge. I came up with a minimum and maximum width dimension that would allow the swing arms to be perpendicular to the transom for both my current dinghy, and the expected size of a new dinghy, should I find one under the Christmas Tree some day. This gave me some leeway in fitting the dinghy mounts to the swim platform.
For the the mounting brackets, I ordered 4 pieces of 4" x .247 American Standard Channel, 30" long, from Discount Steel (www.discountsteel.com). They cut each piece to size, which at $100 for the 4 pieces was cheaper than purchasing at a local mill as they had a minimum order.
I also had a Mill holder set with a Morse taper that I bought from Grizzly.com, and a X-Y table that converted my drill press into a rudementary milling machine. This worked OK for a small project, but it sure doesn't replace a bona-fide milling machine.
Well it may not look too pretty, but I had to mill out the side walls of the channel a bit for clearance. I could have went with wider channel - however, I was afraid that I could not fit the channel into the swim platform as it was.
You will notice 6 holes in each bracket, where there are only 3 holes in each leg of the mount. However, the three mounting holes are a mirror image on each side of the platform mount, so this will allow me to use any bracket on any leg of the mount.
Not shown in the photo is the other end, where I milled elongated slots to make it easier to locate the platform's holes, as there was a slight differnce between each leg of the platform. The milling job was pretty bad, but its out of sight, and it met the needs, so I guess that is the measure of success.
The boat is stored 100 miles from my home, so a little planning is in order. I literally created a machine-shop in a truck, and brought along a Chop Saw should I have to cut the 4" channels.
Installing the davits is a lot easier if the boat is out of the water. You may have to work at a 4" height, but working from a step ladder at the back of the boat is a lot easier.
One factor in delaying my purchase of the davits for a year or so was how to mount them. Or more accuratly, how to access the underside of the swim platform. The swim platform is integrated to the boat, and consequently forms a shell of sorts, and there is limited access to the void between the upper and lower surfaces.
I was faced with two options - either rip out the entire aft cabin, which probably would cause damage, or cut out 8" dia holes in the swim platform so I could get my hands under the mounts and tighten the nuts. I came to the realization that the cuts in the swim platform was necesary, but I delayed that task for a year. Although I am adept at drilling holes in the boat, it doesn't mean I always like it.
The first step then (after deciding to cut the holes) was to layout the mounts using the dimension I mocked up, and as close to symmetry as possible. Turns out I nailed it perfectly and could not have asked for a better setup. There is a slight trip hazard on the platform when traversing it, but it is manageable.
Check your dimensions, check your dimensions, check your dimensions!!!
Check your dimensions, check your dimensions, check your dimensions; you are about to cut some big holes. Be particularly mindful that there is sufficient clearance under the swim platform to access all of the holes you will be drilling.
When cutting out big chunks of swim platform, nothing works as well as a good tool like this Bosch heavy duty jigsaw. Don't use a wimpy Wal-Mart jigsaw; borrow one, or go out and buy one if you have to. You simply cannot do a good job with cheap tools.
After making these cuts, I realized that the swim platform had a Balsa wood core. I was a bit surprised, and figured if anything, it would have been plywood, as the manufacturer specifically states the swim platform is intended for mounting water toys.
This also means that the balsa must be properly sealed, or I'll damage the boat. If you are unfamiliar with this process, read my position about properly drilling (and sealing) holes:
Properly drilling holes in fiberglass
Since I am on the messy side, when applying epoxy to holes, I always like to mask off the area with Blue Tape as any epoxy drippings will look yellow on fiberglass. I typically use West System epoxy, and liberally wet each hole and raw exposed wood edges with several coats of epoxy. A foam brush works well for the larger areas, and a Q-tip works fine for smaller holes. Use gloves, etc. or you will make a real mess of everything.
I sometimes also like to take an old used drill bit - perhaps a dull or bent one that remains in the tool box - the same size as the holes, and manually rotate it in reverse. This helps smooth the epoxy out on the hole's sidewalls.
Fortunately, I was able to fit each channel into the swim platform (but just barely) from an access hatch in the aft cabin. I tied a pull string on each one to help position it into the needed area. As it turned out, their 30" length was just enough, and I didn't have to cut any of them.
The manufacturer recommends grounding the mounts, and while not shown, I drilled and tapped a hole in the side of two channels so that I could attach a ground wire. This ground went to the boat's ground.
One of my thoughts to grounding was:
So if anything gets wet, I could get corrosion around the bolts. Grounding the aluminum channels effectively couples them to the boat's zinc anode, which will dissipate any galvanic current through the zinc.
A liberal amount of 3M 5200 sealant-adhesive was used on the platform mounts. Turns out I had just enough room from the 8" dia holes to get my arm and elbow into the swim platform to secure the nuts, as all of the platform hardware was thru-bolted. One danger area here is if I fell of the ladder, I would surely break my arm. So take it slow and safe.
It is very helpful in this step to have a helper; one that can fetch sockets, hardware, hold the ladder steady, or whatever tasks are necessary to get the job done.
After the mounts were secured, waterproof inspection plates were used to cover the 8" holes. I initally had some worry about leaking, but after two seasons, they have not leaked at all.
The entire system was assembled and the dinghy dry-fitted to the boat. Again, this is easily accomplished when the boat is out of the water. You may notice a slight angle on the boat. This is the normal "storage" position as it allows rain water to run out of the dinghy's bilge. However, when the mother ship is underway, I bring the lower end of the dinghy up to the level as the dinghy can be better secured when level.