As I get ready to sell my boat, I thought I'd contribute a design concept as a way of paying back for the advice and education I've received from this community. This was one of the most useful improvements I made on my boat. Photos are included below.
Folding Swim Platform Design.
When I purchased my C34 from the estate of the deceased prior owner, there were numerous parts missing that I assume disappeared when his family cleaned out his home. One of the major pieces that was missing was a transom ladder. I didn't feel like paying the high prices for a new ladder, so I started looking at alternatives. There have been some nice swim platform designs I've seen on this and other sites, and I felt that my increasingly stiff joints would benefit from a platform for getting in and out of the dinghy. However, I didn't like the idea of a 'permanent' platform for a couple of reasons: 1.) Marinas will charge you for the extra footage, and 2.) We have otters in our marina, and it gives them easy access to getting onto the boat and doing significant damage. (Geese and ducks also like to roost on the platforms, leaving their associated messes.)
Given the above parameters, I decided to create a fold-up platform. However, this required some custom-made parts for the hinge mechanisms. I'm fortunate that I have a relative who has an extensive machine shop in his garage, and he instructed me on the use of his large mill and lathe machines to make the parts, so I got a good education in the process. We made full CAD drawings of all the components before milling all the parts.
The platform attaches to the transom with only four ¼" bolts (with appropriately robust backing plates inside) so there isn't much drama in mounting it. The hinge mechanism uses a ½" clevis pin, so if I need to remove the platform, I just undo 2 pins. Designing the hinge was tricky, because the transom isn't entirely flat, so there needs to be some swiveling design built into the hinge to be able to have both hinges align correctly. We used a spherical bearing in the hinge to adjust for the slightly different angles.
The support frame beneath the platform simply drops into place when the platform is lowered, held at the appropriate angle by a simple pair of steel cables. The threaded pivoting pads that rest against the transom allow me to adjust the overall level of the platform.
I had a decision to make about the fore and aft dimension of my platform. I felt that 18" was a comfortable measurement for getting in and out of the dinghy, however when the platform is raised, it will rest against the transom step above it, rather than tucking beneath it. That isn't really any big deal to me – I'd rather have the deeper platform. I just use a small rope to raise it and cleat it on a stanchion cleat and the platform rests against the step. The support frame legs just fold back against the platform by gravity.
I thought about buying either a pre-made platform or else making one out of Starboard or teak, but for the past 12 years I've been running a remodeling and handyman business, and I build a lot of decks. I frequently use a synthetic decking material from Lumberock, a Chicago based company. It is made of solid HDPE (High Density Polyethylene – the same stuff milk bottles are made of) and comes in standard lumber sizes (2x4s, 2x2s etc.) I had a bunch of 2x2s left from a previous job, so I thought I'd see if I could make use of those. It's nice stuff to work with, and machines just like wood.
The challenge, as anyone who has tried to glue HDPE can tell you, is how to laminate together a bunch of smaller pieces of HDPE in a way that will support 400 pounds (I wanted it to support 2 people). I found a very special (read expensive!) glue from 3M that is specifically designed for this kind of material. It is their Scotch Weld DP8005 product, and costs over $40 for 1.5oz. To make it worse, you need a unique dispenser gun that costs another $22. The good news is that the glue works. I test glued a couple of pieces, and after they were cured, I put them in a vice and smacked them hard numerous times with a dead blow hammer to see if I could break them apart, and there was no separation. Encouraged by this I proceeded to build the platform, but being a belt and suspenders kind of guy, I also drilled the platform and inserted 3 stainless threaded rods through the width of the platform, which also were drilled through the aluminum angle that makes up part of the hinge mechanism. These add compressive forces that are extra insurance for keeping the entire unit from falling apart.
The platform works great, doesn't interfere with any of the workings of the boat, and is very comfortable to sit or stand on. I mounted a telescoping ladder beneath it, which when deployed hangs deeper into the water than a typical transom ladder, making it easier to climb. I've sailed with the platform in both positions for 4 years now and had no problems either way. It doesn't flop around when the wave action gets aggressive. I suppose a large boarding wave from astern could lift it, but in those conditions, I certainly would have already had it in the raised position. I consider it a practical addition to the boat.
Tom,
Sorry to hear you and your boat will be parting ways but I guess you have your reasons.
Your swim platform turned out really nice it looks well thought out and executed. Mine is similar to yours but fixed. It's mounted a bit lower and runs the full width of the transom, they're worth the weight in gold when it comes to boarding from a dinghy, people, dogs, groceries, ice, etc.
Originally I built something similar to yours but out of teak. After a while I decided to simplify by replacing it with a single piece of 1" marine plywood cut, shaped, glassed, and painted. As they say "more than one way to skin a cat".