Hard dodger and mainsheet arch

[RMA]

I see a lot of comments and threads discussing some variation of adding/constructing hard dodgers. A common challenge in older boats is that adding a hard dodger often complicates or downright obstructs main sail sheeting. Depending on the existing configuration, the dodger ends up providing little shelter or sacrifices the sheeting ability. That said, everything is a compromise. Given the recurring interest in this topic, I offer my solution for my boat. Take from it what you will.

This is a fully cored composite structure, engineered to take all of the force from the main sail (plus a hefty safety factor of 5x). For my needs and wants, it works great and I think it fits the boat well. The compromise is the loss of a traveller but since removing that, I've never looked back.

There are lots of itterations that one could adapt for their own needs, for example, if bringing the boom to windward in a close haul makes a big difference on your boat, you could easily adapt my system to German sheeting.

For the technically oriented:

The arch and dodger were build seperately.

The arch is 75mm (3") balsa cored, tapering to 125mm (5") at the legs. Its fore and aft width is 150mm (6") and tapers to 200mm (8") at the bases. The beam span is 1500mm (60") and the span at the legs is 2235mm (88"). The laminate schedule is 4 layers 955g/m2 (28oz/yd2) unidirectional e-glass and 3 layers 350g/m2 (9oz/yd2) unidirectional carbon on the top and bottom skins. The entire arch is then wrapped in 3 layers of 440g/m2 (12oz/yd2) biaxial (-45/45) e-glass. It weighs ~32 kg (70lbs) and my rough engineering estimates that it should hold ~44kN (10,000 lbs force) in core shear. The laminate schedule is overkill, as the core would shear long before the skins would yield but I wanted it to be bullet proof in case the boom (or worse, the mast!) ever fell on it. It is attached to the 30mm (1.25") thick cockpit coamings with thickened epoxy, and a substantial amount of uni and biax tabbing, as well as two 15mm (5/8") stainless threaded rods embedded into each leg that bolt through the coaming and are secured with a 10mm (3/8") aluminum backing plate 150mm x 300mm (6" x 12"), nuts, and washers. If this sounds over kill, it is. In theory, the surface area of the base of each leg should be able to hold the loads just with epoxy gluing it. The swivel blocks are mounted through the arch with stainless backing plates. Each bolt hole was drilled out and filled with 25mm (1") G11 rod the thickness of the arch. These rods were epoxied into the core before glassing. Once glassed, I drilled the plugs for the bolts. This way, the plugs are bonded to the core and fixed by the glass. The plugs take the compression of the bolts and ensure that the balsa is well protected.

The dodger top was contrusted using 2 layers of 20mm (3/4") Nida Core for a total thickness of 40mm (1.5") and yes I'm aware that it can just be bought in thicker sizes but this was not available in my area. It has a single layer of 1200g/m2 (32oz/yd2) triaxial e-glass with the 0 degree orientation running athwardship. These curved structures are very easy to build. I built this top and the hard bimini in a weekend each. Fairing could take a month...

The dodger sides are built from 20mm (3/4") divinycell with a single layer of 1200g/m2 (32oz/yd2) triaxial e-glass on each side. The 0 degree fibers run vertically here.

Windows are 6mm (1/4") polycarbonate. To avoid showing fasteners on the inside, I made G11 plugs and embedded them into the foam sides before glassing. These plugs were then drilled and tapped to except flat head machine screws.

The carbon handrails were built by forming 3/4" PVC pipe around a mold with hot sand. Once set, the sand is removed and the pipe is skined in 2 layers of 8 oz carbon braid sleeve. To keep the carbon look and protect the epoxy from UV, I put about 10 layers of polyurethane varnish on them. I'll probably have to varnish these yearly to keep them looking nice. These grab rails are epoxied into the structures with G11 rods. I can do pull ups on them but they do flex a little bit, FYI.

I can jump up and down on the dodger and it doesn't flex at all. All in, the arch and dodger weigh ~58kg (130 lbs) so, less than a crew member. It took me about 500 hours and I had help from my wife and friends at times.

The boat is a 1976 C&C 38-2. You can look up the specs if you're curious.

Feel free to ask questions. I'll tell you what I did and why but keep in mind, I'm not a naval architect.

Another warning, before attempting structural modifications like a mainsheet arch, you should make sure you know damn well what you're doing or consult someone who does! I'm just another guy on the internet!

I'll try to post some better photos in the next few days.

 

[RMA]

Here are some more photos throughout the construction.

 

[Milehog]

Clean work!
Too many homemade dodgers look like hell.

 

[gonzo]

This is a fully cored composite structure, engineered to take all of the force from the main sail (plus a hefty safety factor of 5x).

There are several threads asking about the structural design of arches like yours. Could you post the calculations you used to engineer it?

 

[RMA]

Gonzo, I agree that this would be useful information but as I'm not a NA, I'm not comfortable putting out what I've used for calculations. My calculations are at best an oversimplification based on theoretical material properties. I'd hate to put out a half answer or worse, misleading information. My numbers are based on some formulas from Larsson & Eliasson as well as simple beam theory. Sebaseba, on this forum has put together a simple beam calculator for cored composites. Here is the link: https://plesko.si/beam/s.html. However, this useful tool should really only be a first step and sanity check in the design process. It does not account for various laminate types and their orientation in the layup, nor does it consider fiber ratios. As you likely know, the construction process also greatly influences the strength of the final product, and mistakes in the build can seriously compromise the theoretical properties.

My process was to get a feel for laminate thickness and core dimensions. Then calculate the strength of my proposed laminate schedule and use these values in Sebaseba's calculator. This was an iterative process until I was satisfied that I had a substantial margin of safety and that the arch would fit and look right dimensionally. I combined these values with my personal experience in cored composites and considered it quite reasonable. I am by no means saying that I did it right. I am only offering what I did for my boat. I hope that I have stressed this enough.

 

[scherzoja]

RMA,

Nice dodger. Where did you get the opening window on the forward face?

 

[RMA]

RMA,

Nice dodger. Where did you get the opening window on the forward face?

I built all of the windows from 1/4" polycarbonate. The forward opening window is supported by 3 Seadog SS hinges and two SS gas struts. It is locked in place my Lewmar hatch handles. There is a 1/4" hollow rubber seal glued on the dodger side, which when latched shut creates a pretty good seal. This design is not at all up to the task of replacing deck hatches. It is not as strong or watertight at those products. I simply could not find a proper hatch in the dimensions I wanted and even if I could, it would likely be over budget. I had to get creative. The lack of complete watertightness is a non issue, the lines all run through cutouts in the lower forward sections of the dodger and will permit water to pass through anyway. The point of a dodger is not to remain completely dry. It helps to keep some of the water out. If you want to be totally dry, get a boat with a pilothouse or don't go sailing at all!

Cheers,