How you could (but probably should not) make a

bimini hardtop.

I will make my blog post in english again, because it seems there are many foreign readers.  

Most modern cruising catamarans have some kind of bimini installed. On a catamaran this is a great and cost effective way to increase living space on board, as the cockpit area is often spaceous. The main manufacturers realized this many years ago, and now integrate hard biminis on their cruising models.

The composite hard bimini has many advantages. It is very durable, strong and completely water proof. It is fairly easy to attach many solar panels and also water collection systems to it. On the underside lamps and gear storage equipment can easily be mounted almost anywhere. A composite hardtop bimini can be left in place permanently with minimal effects from weather. If built strong enough, the hard bimini can be walked upon, rendering easier and more safe access to the full lenght of the boom. In addition, by building a curved (convex) surface, water pools that can easily form on large canvas biminis will be avoided.

The downside is added weight and cost/labour. For some, the loss of the canvas removal option for storm preparation might also be a barrier. 

We (my poor wife is involved in all kinds of wild projects) have changed the original bimini cloth only once in 14 years. But the advantages of a hard structure are tempting, so when a long distance trip of several years was planned, an effort to build a hardtop was initiated in the winter of 2013/2014.

The Norwegian winter is not ideal for home building of large composite structures. Especially in the area north of Oslo. Temperatures are mostly below freezing, and a spaceous heatable garage area will be needed. Luckily we have a quite large garage, and part of it was sectioned off with tarpaulin walls to minimize the heating requirement.

The first challenge was to dismount the bimini stainless steel frame and trailer it home. It passed through the marina pontoon gate only using advanced twisting motion and hardly an inch clearance. On our tiny car trailer it looked a bit oversize.

The somewhat ambitious plan was to build a plug on top of the frame and then lay up a fiberglass mould on top of that. So why did we want the same size and shape as the old bimini? For one thing, the old stainless steel frame or main parts of it could be used. Fitting would be easier that way. And I actually did not want to build the hardtop overly strong due to weight considerations. It would be fine only to be able to walk reinforced areas along the boom. Kind of the same concept as a canvas bimini. Many would have made the hardtop a bit larger than the original to have better sun protection. Unfortunately it is very probable that our time on the catamaran will be longer in northern latitudes where we need the sun, than in the tropics. We also have a winterization problem. I like to have the main parts of the boat under a tarpaulin which is laid over the boom, and the wider the hardtop, the more exposed it would be to snow loads. The final argument for my choice is esthetic. I think the flatter shapes looks better and works better in the warm climates.

 Starting building the plug.

A plug can be made of sheets of fiberboard. If you do it the right way. The right way requires much time and effort. For us neither was abundantly available. We had a lot of other projects going because of the planned departure late in May 2014. Another consideration was that this was a one-off project, so the mould would be used one single time and not for further production. Being pressed on time, it seemed sensible not to put more effort into it than absolutely necessary.

The plug skin almost finished. The fiberboard must be thick enough to compensate for the layers that will later be built up in the mould. This to ensure that the hardtop will fit on top and outside the stainless steel  frame.

Nevertheless it was quite some work to shape the plug. And make it ready for the special tooling gelcoat. Reading about such gelcoat, one is warned that the plug surface must be painted with many layers of polyurethane paint to avoid it adhering to the plug. 

Makers of such instructions usually exaggerate to be on the safe side, don't they? And do they know how hard polyurethane gets and how much work it is to sand down the wave pattern from the paint roller? And furthermore, how easy it is to sand through to the fiber board around edges? Probably not....

To avoid that the mould would stick to the plug, many layers of Carnuba wax was put on the plug.

Then came a hard days work . Laying out the mould gelcoat. Laying up layers of fiberglass chopped strand mat. Cutting and shaping 20 mm Bonocell foam (for stiffness), and laying up the top layer of chopped strand mat.

Resulting in this top view of the mould at the end of the day.

The  inside of the mould did not turn out satisfactorily. More like a disaster, actually. The fiber board stuck to the gelcoat along edges, and the inside of the mould was more or less a mess because of bonding to the plug. But it generally had the correct shape, so giving up was not an option. 

By filling and sanding the mould was smoothened, and then painted with spray paint. Epoxy is much more forgiving than gelcoat when it comes to not dissolving the underlying surface, right?

I mentioned added weight as one of the drawbacks of adding a hardtop bimini. I have wanted to limit that drawback as much as possible. And have tried to find ways to save some weight. We have all read about the fantastic strength and weight savings of vacuum bagging techniques. So in that lies a possibility. Youtube videos explain how easy this is, and information sources tell you the opportunity must not be missed if you want to save weight and gain strength.

Unfortunately bagging equipment is not so readily available for retail in Norway. I envy the americans, their domestic market is so huge that just about anything is easily purchased, and at a reasonable price. 

Turning to that market is a solution, even though our VAT and the shipping cost increase prices significantly.

New words now came into my vocabulary. Peel Ply, Vacuum Bag, Bagging Release Film, Bleeder Breather Cloth, Tacky Tape Vacuum Bag Seal, Nylon Vacuum Bag Bagging Infusion Film, Resin Trap, etc.  

A vacuum pump is also needed. These are in no way meant for a single occasion use, they are quite expensive. But again, internet is the saviour. I found information that a refrigerator compressor unit can be used. Easily taken from an electric appliance dump.

The process of acquiring all the necessary gear took some months, But at last we were ready for a very hard and stressing day. All the fiber glass weave had been pre-cut to shape, likewise the 10mm Divinycell foam and the vacuum bagging equipment. The vacuum pump was tested to be working and ready. 

The epoxy resin hardener was specified as slow, still we knew that we had to work fast. But laying out epoxy, top and bottom layers of weave and the foam on such a large area took a lot of time. In addition the rim and hatch area had to be set up. But worst of all, the hardener was not very slow. Already after the first 2 layers of weave, I felt failure creeping in over us. The resin started to get viscous. Even frantic work could not save us. When all the layers were in and the vacuum bag sealed, it was already too late. It did not help us a lot that the homemade vacuum pump was way too weak to do its job, but it could not have saved the day anyway.

As suspected, after having a hard time getting the bimini out of the mould (release problems again), it was clear that it could not be used. It delaminated easily because of insufficient bond between the layers. Departure date for the Caribbean  was coming close, so there would be no hardtop bimini for that voyage.

The product had to be cut in pieces and taken to the dump.

Lesson learned is that if vacuum bagging a project of this size, infusion technique should be used. In addition I suspect that the epoxy resin and hardener may have been old, and therefore curing too fast. When using leftovers of the same stuff now, it starts curing really fast.

None of us thought that we would ever want to try such a project again, but the mould was anyway hoisted up under the garage roof for storage before departure.


The canvas bimini did its job fine during the two year cruise. But the weather and use wore on it. And after returning home the thoughts of a hardtop again surfaced when repairs were necessary. In the autumn of 2017 the mould was taken down from the roof and rigged on the garage floor. It was going to be difficult to use it, as the gelcoat inside was damaged after the first use. So I expected to have to repair the final product extensively. But a new mould was not an option, way too much work.

Like the first project, this would be epoxy and divinycell based. I have learned that the common gelcoat topping can not be used  in such projects, it is polyester based and does not match well with epoxy. Paint is thus the usual top coat. And in that case some surface work will be needed anyway.

Divinycell comes in sheets, precut into small squares and uncut sheets. I like the precut. The 3 x 3 cm squares are glued on one side to a thin glass fiber weave with a small gap between each square. Thus the sheet takes the shape of the underlying mould much more easily, and does not have to be weighted down so carefully. Another advantage is that resin fills the gaps in the sandwich construction, so no specific holes or areas to bond the upper and bottom layer to prevent layer skidding are necessary. On the other hand this concept may require more resin and be a bit heavier.

When buying glass fiber weave this time, I also learned of twill weave. Again, this type of weave is much softer and will shape around spheres and corners much better than ordinary weave. The only small disadvantage is that after cutting out a shape, it must be handled carefully because it easily twists out of shape.

Epoxy resin is more expensive than polyester. My reason for using epoxy was again weight considerations. Epoxy/fiberglass is a very strong mix, stronger than building with polyester or vinylester.  I do not think it is lighter per se, but one can use less materials for the same strenght, thus there is a weight saving potential.

Before the first attempt I had made a couple of test sheets, and found 2 layers of 300 grams glass fiber weave each side of 10mm divinycell very promising. I did use the same in the second attempt.

Here is the hardtop plan.

Without the stress of the vacuum bagging setup, the casting of the hardtop was a bit easier. One should however remember that if too much time elapses between layers, the surface must be rinsed before continuing. This is because of the so called Amine Blush, the next layer might have problems sticking to the previous.

Laying epoxy, fiberglass and divinycell in the mould.


With my substandard mould inner surface layer, I knew I had to expect problems with getting the product out of the mould. So I did follow the advice of attaching a tube to the underside before casting. With a piece of tape on top to prevent resin escaping that way. Good move, without it the job of getting the hardtop out would have been a nightmare. Now I could pump in pressurized air from the bottom. Which made the difference.

And no, the product surface was not perfect. It was repair time in sevaral ways. In some smaller areas I had to re-bond the upper surface due to too little resin used. ("weight saving" is not always a success). And the whole surface did of course have to be sanded and smoothed before painting.

Polyurethane paint was used as the top layer. Using a paint roller will not give the same finish as sparaying, but spray painting polyurethane in the garage was not worth the trouble for me.

Our original plan included a sliding hatch, but when I discovered that the front and back of the hatch opening inclined at very different angles, I realized that it would be complicated. So the solution was a folding hatch from canvas and composite planks. Luckily my wife is now quite experienced in sewing canvas.

When the hardtop was mounted, it became clear that it was quite stiff and strong, at least when using it atop the stainless steel frame.  (Also, below the gangways the thickness was doubled). I had beforehand cut away some obviously superfluous sections of the frame, and plan to cut more. But first we would like to observe how the hardtop works the next season and winter.

In our home climate canvas sidewalls is a necessity. For mounting these. we chose  to use a band of canvas around the edge, supporting YK 10 zippers. The canvas was caulked onto the inside, We will find out if it has to be supported by screws.

The original dream for the project was that it would be all composite and that the stainless steel frame could be taken away. It weighs 40 kg. I believe it is absolutely possible, but it would require  a lot more work and time. Having the boat an hour drive from home would also complicate such a solution a lot. Not to speak of the long season of temperatures when it is too cold for outdoor composite work. In the summer we want to cruise, not work.
So the hardtop now adds approximately 33 kg (73 lbs) of weight, hatch included. ( It is a bit heavier, I deduct the old canvas and the cutaway stainless steel parts).

But as everyone who has added one, we think it will be worth it all.

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The great day of mounting it.