Sunday, July 5, 2015


The last few days I've been going back and forth on several design options for my hard top bimini replacement. I didn't realize that there would be this many decisions to be made for such a simple sounding project.  It's not rocket science, just a mostly flat fiberglass panel.

I've been in touch with a couple of friends who have Leopard 38's with hard tops to get pictures and design ideas.  One had a tubular aluminum frame, and plastic panels were laid on top to create the bimini.  The other was a molded fiberglass top that only utilized metal for a few struts at the front of top to hold it above the cabin roof (it may actually be one of the $15K tops built in Florida).  They are both nice tops and served their purpose, but the decision on which direction to go was an easy one there.  I'm no welder, so the more I can do out of fiberglass the better.  I guess I'm no expert at fiberglass either, but at least I've done small fiberglass projects with some success.

The next part of the design was how I wanted the top to cover the cockpit.  The soft top comes straight forward from the arch and then curves down as it gets to the helm, and I don't particularly like that.  All of the winches, except for the dinghy/traveler one, are also out in the weather, and it would be nice to have access to them from a dry location while sailing in rainy weather. So it made sense to try to create a 3-D model of the cockpit and see how various top configurations might look.

The top with an example 230 watt solar panel 

I downloaded a program called SketchUp and began working on creating a model of our cockpit. I wasn't looking for a detailed replica of our cockpit, just a rough approximation except for a few critical areas like the dimension and curve of the arch and cabin top as well as the location of the two sheet winches.  It took me a while to fight with learn to use the program but I finally gave up got the model close enough to be useful. Angled and curved surfaces were a pain to model.

I tried a variety of different ideas.  Should I embed the solar panels into the top thereby reducing the amount of materials I may need and allowing for better cooling of the panels?  Do I want to integrate a water collection system into the top? How far can I extend the top and brace it while allowing sufficient clearance for the winch handles? Can I integrate wiring chases into the design for the solar and cockpit lighting?  Can I integrate storage into the top?

The side profile of the top...I think it looks decent.

So far, I've decided that the best approach I can take is one of relative simplicity.  I'm adapting simple designs that I know have worked for other people, adjusting only to simplify the build and as needed to fit the boat.  I've had a difficult time finding solid engineering data so I'm leaning on the existing working designs as well as the advice of a couple of aerospace engineers I've met here at the marina (yep, I did in fact ask a couple of rocket scientists). The pictures you see in this post are what I have come up with.

An underside view of the top.  A couple minor drawing glitches.

The plan is that most of the top will consist of a foam cored fiberglass panel.  I'll use either 3/4" or 1" foam that will then have two layers of 12 oz biaxial cloth glassed on each side.  I may add a third layer to the bottom side for added rigidity as a deck surface.  The best information I have indicates the panel alone should be more than enough to support the weight of people and gear.

The handhold edge, bolt rope and rear tab detail.

U shaped channels will be created that run from front to back on the underside of the top to provide for wire chases as well as add a little additional rigidity across the length of the structure.  At the front and around the window cutout at the helm seat, a foam doubler may be added for additional rigidity in those areas,  Around the edges I'll create a rounded surface to act as a hand hold and to provide a bolt rope slot for attaching fabric dodgers.  The rear tab for mounting the top to the arch will be solid fiberglass around all mounting locations. In all locations where there will be penetrations, holes will be over-drilled and filled to seal the core.

So, what do you think?  I know some of you out there have some experience with what have I missed?  I think I'm going cross-eyed staring at this drawing on the computer screen.


  1. Hi Mike,
    These drawings look good. Foam thickness and # layers of fiberglas looks about right
    Have you decided on Corecell or Divincell or ?
    Are you looking to have hardtop attached to the bottom of the targa or top of targa? Top would be best as it relies on strength of hartop resting on targa vs attachment to bottom of targa relies on strength of bolts and epoxy surrounding the bolts.
    Is clearance of boom an issue. Actually read a blog where mainsail was cut in order to have hardtop bimini over the drivers head cutout.
    Solar- might consider walk on Solara or Aurinco panels- boy are they expensive and saw one blog where Solara covering went opaque in 3 yrs in southern sun. Maybe better off with Kyocera panels perhaps spaced 6" apart if you need stepping space. A honeycombed Poly stepping strip for air and water movement may work to bring stepping height up to height of panels. There are some non stepable panels at lower cost that might eliminate fear of sheets somehow catching a solar panel corner. Saw one blog (out chasing paradise) where one MPPT controller used for each panel to maximize output during shading. Might be worthwhile for panels mounted on bimini.
    Doug from VT

    1. Hey Doug,

      I'm actually looking to attach it to the side of the arch, similar to how the other two tops I have for reference are attached. In the detail drawing you can see the end "tab" that will bolt on to the arch. Unfortunatley the traveler mechanism would prevent being able to attach it to the top of the arch.

      The dimensions of the example solar panel rectangle are from the Kyocera...I think 230 or 235 watt model. The idea is to have them as far out on the edges of the top as possible to leave a walking space along the center that will also allow the boom to be centered to reduce shading at anchor. While sailing...well...there is no place you can really put panels that won't be shaded from time to time...unless you are dragging them from a raft a few hundred feet behind the boat. :-)

      Interesting about the multiple controllers. Wonder how they wire them to the batteries such that one in charge mode doesn't fool the next one into thinking the battery bank is charged. Must be some sort of communications interconnect. Will have to look into that...but I can imagine how expensive multiple MPPT controllers could be.

    2. Hi Mike
      See what you mean by fastening to side of targa
      It was outchasingstars dot com that got 1000 watts of Solara panels, spread them around cat and used 8? Genasun mppt controllers. The Genasun 140 watt controller is $109 each but they only handle 140 watts max and designed for max 18 V input. They can be used on Kyocera 140 watt panels but at $2+ per watt before shipping, but I assume you want higher wattage panel with higher voltage output to cut down on size of wiring. I see the name Kyocera a lot on marine solar panels and forums indicate no real dif in reg Kyocera vs ones sold by marine installers/stores. Outchasingstars website has a whole post on it with dozen pics and author does answer inquiry email/comments. There are other higher rated mppt controllers. I would think individual controllers only needed for bimini mounted panels if you went that route. Other panels on dinghy davit would need only 1 controller between them.
      Doug from VT

    3. My boat doesn't have a davit system that would support solar. I have seen a couple boats that have cantilever mounted panels on the aft edge. You are correct I would like the large voltage panels...for wiring and the amount of power generated per unit size of panel. With MPPT, it seems to make sense. But this is down the road...first, need to build the top. :)

  2. I like the hand hold/grab rail idea. If you put that "fiddle" concept all the way around the top, it will double as a stiffening rib and serve as a gutter for water collecting.
    When working with foam, the inner and outer glass skins should be joined. Make lots of small holes through the foam to allow the resin to of the two sides meet. We use to use a piece of plywood with lots of nails driven through and protruding the other side. This is then simply pressed onto the foam and helps bond the glass to the foam.
    When laying up the biaxial, make sure you have approx. 2" overlap of the glass sheets. Offset the over laps between the layers to make sure you don't have a big bulge. Putting a layer of 300g (sorry don't know the imperial measurement) Chopped Strand over the biaxial will greatly reduce the amount of sanding and filling needed. Glassing is much easier and quicker than sanding and filling :)
    Looking forward to seeing how this project progresses.

    1. I've heard of perforating the foam to increase surface area for better adhesion. Didn't get the impression they were holes all the way through though. Doesn't that increase the weight of the panel by a lot? And doesn't a hole filled with un-reinforced polyester resin result in a more brittle substrate...not good for the shear forces? Sorry...just trying to understand the recommendation.

      And 300 grams is 10.5 oz, so I think I can figure that out. But how much sanding and filling is needed when you are going to apply a non-skid coating.

  3. Only need to use small panel nails. It is only the resin that flows into the holes, but it isn't significant amount. It helps bond/lock the glass onto the foam. If the glass delaminates from the foam, then all strength is lost. We use to use 8mm foam to build small racing skiffs, and vacuum bag it to squeeze out the excess resin from the glass reducing weight (the resin in the small nail holes was insignificant compared to what is in the cloth).
    The areas where there are multiple layers of the biaxial glass (edges, stringers, grab rails) will end up a bit "lumpy", even a single strand of glass will stand out. The chopped strand with the smaller fibers will blend and smooth over the bumps. If applying the non slip then you may wish to just put the chopped strand over the edges and stringers.

    1. Ah, Ok, was thinking a larger nail.hole. Definitely want to avoid delamination. Thanks for the info!

  4. Mike,

    Love the design. I used to work with Sketchup in my last job and loved it. You're right, it's not very intuitive for a new user but it's a fantastic tool for graphics people like I was. Can't wait to see it finished!

    S/V Kintala

    1. Thanks Deb. Compound curved surfaces seem to be nearly impossible to model from what I could tell with Sketchup. I had to do a few cheats to get some of the stuff to render at all.

      So, are you guys in the water yet?