Stern extensions - Phase II

The next phase of the project was the actual build of the new stern.  This involved laying up the fiberglass into the mold to create a fixed shape of the final hull extension.  

Fiberglass is strand-silica (glass) in a sheet/mat format.  

Chopped strand mat consists of random short threads pressed into sheets and yes, it is itchy!

A specific type of mat designed to add longitudinal strength is called stitched biaxial fiberglass.  You can see the long strands of continuous glass that are laid together in parallel layers with the layers at 45 degrees to each other and stitched in place.  One side of this mat has a thin layer of chopped strand to aid in bonding.

Our stern plan was based upon the layup schedule for our boat, which designates the number of glass mats laid out to form a given hull strength and thickness.  It is surprising how thin the glass can actually be ~ in particular when cored ~ to create a multi-ton, floating structure like Reach.  

For example, Reach's hulls and deck have sections of 1/2" or 3/4" nidacore sandwiched between ~1/8" triaxial glass.  The hull below the waterline is 3/8" solid fiberglass.  

Something interesting to ponder when you're blue water sailing and a long way from the heavy 1" solid glass hulls of 60's & 70's-era boats.  

Technology and chemistry never cease to amaze!

Our stern extension layup schedule was as follows:


Upper hull

  • Layer 1:        Gel-coat
  • Layer 2:        Chopped strand mat
  • Layer 3-4:   Stitched biaxial mat (x2)
  • Layer 5:         1/2" nidacore 
  • Layer 6:        Stitched biaxial mat


Bottom hull

  • Layer 1:         Gel-coat
  • Layer 2:        Chopped strand mat
  • Layer 3-6:    Stitched biaxial mat (x4)

Now to build this thing, you first must wax the hulls to allow for removal of the structure at the end (remember this important step for later).  The first layer going into the mold is a layer of gelcoat (white resin) that will soon be the outer hull.

Then you line-up jugs of resin and MEKP activator (methy-ethyl ketone peroxide).  

The resin is made of polyester dissolved in styrene that when active, cross-links chemical bonds between the layers of glass.  

Pieces of glass mat for the layup were cut and fitted into the molds, numbered and removed.  

Next, resin is applied to the surface and then layer by layer the sterns were glassed with continuous polyester resin application.  This was fun to watch, as most boat-building in countries that use launchas as a main mode of transportation are used to pouring in the materials to create a strong and very thick boat hull.  

However, Mark wanted to try to maintain a reasonable resin:glass ratio (60:40 would be good for a hand-layup and 50:50 or less can be achieved with vacuum bag and infusion layups) to avoid pouring in all of that added weight.  On a related note, this is another reason that lightweight core is used in fiberglass boats, to add strength without extra weight.  As we know, weight is the enemy for catamaran performance.

The result was an assembly line formed of Saul pouring in thick coats of resin with Mark following behind to flatten the glass with a roller.  He then went on a mission to squeegee away all of that extra resin and pour it out.  Then another would come in behind to touch up with more resin, followed by more culling by Mark, and so on... and so on... 

In the end, the desired result was achieved of a lightweight stern with some room to spare for an added bulkhead, plus assembly glass and layup.  

The upper hull was a bit stubborn to release from the mold and raised the blood pressure of many of us (me) in the yard at this moment of anticipation. 

We ran out of PVA mold release solution, so the plywood steps were not properly treated (only waxed) and were stuck in the middle, while the rest of the hull was jumping out of the sides.  With plenty of wedging, ripping, pounding and cursing the mold finally surrendered the hull.

Once it finally popped out and was turned over, all of the effort for perfecting the mold was justified.  The stern was absolutely beautiful, with sexy curves and fair edges that should make the final sanding stages relatively easy.  

We were all so excited and couldn't resist a sneak peek at what our final product might look like ~ smiles all around!!

© M&M 2019