To Biax or Not To Biax...

Installment XIII in the reconstruction blog of a 1973 Mako center console

In this next step we will wet out and layup one layer of biaxial cloth inside the transom before bonding in new transom core. Our thinking - the 1708 biaxial will bolster strength and makeup for any minor deficiencies in the old transom skin.

We mixed up several batches of West System neat epoxy (no additives) and wet out the 1708 cloth on the table. There is a science to estimating the amount of epoxy needed to wet out cloth. We don't make any pretenses to be scientists; our formula= mix many small batches til we don't need anymore. We gently squeegeed it over the surface with a single edge spreader and let the cloth soak in the epoxy. Typically the cloth will become translucent as fibers soaks up and bond to the resin. This did not hold true for the biax, but more on that later.

Transfering the wet-out cloth to the transom is most certainly a messy two person procedure. An alternate method is to bring dry cloth over and wet it in place.



Once aligned, we chased out all air from under the cloth using laminating rollers and spreaders.

Notice the Biaxial cloth does not wet out completely clear. In hindsight we learned that styrenes in this "mat type" fiberglass cloth do not bond with epoxy as they do with polyester resin. (Biaxial is woven cloth one side and mat on other side.) Engineers at West System acknowledge this quirk between mat type fiberglass and epoxy in their literature. They also state it does not negatively impact the finished composite strength (from extensive testing). Yet any mat based fiberglass wetted out with epoxy will be less conformable (i.e. harder to drape over a shape) than the same mat cloth using polyester resin. Wow, who knew? Certainly not these two guys. Also, if this were a critical application where it needed to be clear, biax and epoxy should not be your mix of choice (i.e. a cold molded hull where beautiful mahogany shows clear through). Lucky for us, biax and epoxy are fine in this hidden and flat application. Henceforth though, I think we'll stick to regular 4 to 8 oz cloth compatible with epoxy.

With the biaxial cloth in place, we're now ready to bed in the core.

Mixing It Up

Installment XII in this Classic Mako rebuild blog

In our post from the previous week, TJ and I completed prep work for replacing the transom core. Now it's time to put this core to bed. Our first order of business is to fill in all gaps so we don't leave any moisture trapping pockets behind.

We've moved the boat over to a nice clean and warm workshop for the epoxy application. 60 degree or better room temp is what we're after. For this whole process we've got a wealth of resin and hardener onhand. We also have a cache of mixing pots, mixing sticks, single edge spreaders, fillers, latex gloves and tyvek suits.
The transom area received one last go over with wire brush and vacuum. (See "Preparing Laminates" for more info.) Then we wiped it clean with acetone, let it flash off, and wiped dry with a clean dry cloth. We measured out the resin and hardener into separate graduated mixing pots, and then mixed them together. (Isolating resin and hardener during the measuring process allows us to recapture overpours. An alternative is pumping from the calibrated measuring pumps, but this proves tiresome and slow for large volumes.) We added a 50/50 mix of high density filler and coloidal silica until it was thickened to a peanut butter consistency.We layered on several pairs of gloves so we could peel off a layer without interupting workflow as the epoxy becomes tacky and tools stick. We then used spreaders to fill the epoxy mixture into all sharp angles and hollows, with a fillet joint bead lining the hull to transom joint. Our goal is to ease any sharp angles for the biax cloth, which we will layup in our next step.

This first step will fill voids so that in our final bonding step, when the core is clamped in under pressure, there are no voids. Clamping will squeeze out any excess epoxy which will then be removed. We mixed several small batches to complete this step. Large batches hold heat from the resin/hardener exotherm and will surely kick before you can apply it all. Should the epoxy "kick" in the pot, you'll feel the heat building in the mixing pot, and the epoxy balls up like chunky cookie dough as you spread it - not good. To forestall the heat buildup we could chill the resin beforehand or disperse the mixed epoxy in a flat pan, but it's easier- with less waste- just to mix in smaller portions.

We focused attention under the transom top edge and in the complex top corner where gunwale, transom and hull meet.

Once this step was completed we moved immediately to wetting out the biaxial cloth.

Core Strength

Installment XI in the Mako rebuild project.
TJ and I have reached a major turning point - so far it's all been destruction to this old Mako. Now we're ready to build! After a fair amount of elbow grease cutting, grinding and chiseling out the old rotten core from the transom, we'll begin preparations to bond in new core.
Step one - we need a template for the new core. A large cardboard box worked well for this. Matching the exact core shape is a challenge since we destroyed the old core. We approximated the shape by first clamping the cardboard sheet lightly to the outside of the transom. We traced the exterior contours for a rough, oversized shape, too big to fit inside. Then we pared away excess to fit the inside the boat. An easier approach would be to drape a simple cloth over the inside and trace it with a sharpie, but we used what we had. Matching every intricate imperfection would be extremely tedious. We didn't sweat the details too much, any voids will be filled with thickened epoxy. We will ultimately bed our core into place with thickened epoxy, like a brick in mortar.

With a satisfactory fit of the template, we traced the shape to new core. We selected a sheet of 3/4" Penske board. Penske, properly known as Airex PXc board, is high density urethane with glass fibers mixed in for reinforcement. A sheet of Penske is structurally strong, shapes easily and is impervious to water. Unlike traditional plywood, this core will never rot! We managed two transom cutouts onto one 4'x8' sheet. Sandwiched together it will give us a total core thickness of 1.5 inch. To scribe the cut, we used a Festool Jigsaw. The jigsaw style seen in the picture is the barrel grip. I prefer this style to the traditional top handle for lightness and scribing blind underside cuts.
We dry fit our core cutouts to the transom. A little grinding was required on high spots. We also trimmed the top to approximate the old core height - slightly less than the outside skin.

Now, I'd like to say removing the old core came off without a hitch- but I can't. We punched through the outside a time or two in the process and those need some touching up. A
proper patch would suffice, but we opted to bullet proof the transom with one layer of biaxial cloth. This will go in place just before the core installation. Biax, also called 1708, is great stuff for structure applications. But before you go ahead and jump on the biax train too, you may want to read on in future post to see how it worked with epoxy. These pictures show our cut of the cloth.

We used these electric shears to cut the cloth and man they're awesome. One of those simple things that makes life so much easier. A little battery powered gadget that sounds like an electric toothbrush. You can lay the cloth over a shape and cut it out without manipulating it and distorting the weave. If you have alot of cloth cutting to do, it's worth it's weight ten times over for the savings on damaged cloth!

The old Mako made a trip for the the next phase. We were blessed with access to a heated building in winter! The unheated warehouse posed a problem for resins to cure. Cold temps can be overcome by very fast hardeners in a pinch, but it's a give and take. Very fast hardener will accelerate cure in cold temperatures, but epoxy cures much stronger in temperatures above 60 degrees (the warmer the better). The cure strength is much happier in a warm workshop. Come summertime mother nature will take care of the heat for us, but for now this workshop is a blessing.


Before calling it quits for the night we did a few last odds and ends. We tried out the biax cloth for fit and made some diagonal slices to fit it into the odd corners. Next we etched the whole bonding area with a wire brush and course sandpaper to add a little tooth for bonding. We then cleaned the inside thoroughly with the shopvac. Placed a few bandaids with painters tape to show our sympathy for the wounds we had inflicted, then we filled all those old screw holes and damaged spots with thickened epoxy. Content with how things looked, we retired to our favorite pastime - standing back for a gam session with a beer in hand, talking about "what we should really do next is..."

Dissecting the Transom

Installment 10 of TJ and Mike's Mako rebuild saga



In our last segment, we cleared out all the belowdeck flotation foam. This boat looks and feels lighter and drier. It was noticeable when we jumped aboard. The weight loss changed the trailer balance enough that we had to re-adjust the poppets bracing the stern. Now it's time to address the transom.






Our goal in this next phase is to cut out the wet core in the transom. We need to leave the outside skin intact to keep the hull shape - draw upon those old frog dissections skills from science class. After some discussion, we decided to also cut out the corner fish boxes. The old style boxes aren't really big enough to store anything, they're mostly shinbusters eating up valuable deck space. Admittedly, the heavy layup of the fish boxes added tremendous structural strength to the transom. But the stress cracks in the corner joint needed addressing regardless. We'll build a little extra reinforcing to make up for the loss, but the end result will be alot more usable deck space.


For precise cutting we used a Fein Multimaster with HSS Saw Blade. This tool is great for digging in odd corners and angles. But for this task it lacked the muscle to power through efficiently. The beefier Fein Supercut might have torqued through this material easier. Some advantages of the multimaster you just can't beat- it is light, easy to use, and will do things other tools simply cannot. But this isn't one of em.

After what seemed an eternity of carving out the score lines to pop off the inside laminate, the blade of the tool heated up too much and shattered. Perhaps there is a better tool for this work.



We grabbed a tool with muscle. The Dynabrade Cut-Off Wheel cuts a 1000 times faster than any reciprocating tool, but still is easy to control. The shape is more challenging to squeeze into contours, and impossible in some tight spots. The cutoff wheel also requires a compressor with a little bit more umph to power her (21 SCFM max). Because our compressor was a little shy on airflow, the tool would bog down once the cutting edge was buried. Frustrating, but still much faster than the multimaster alone for this job.

With our cuts completed and the March night mercury dropping to hand numbing levels, we retired for the week.


The following Wednesday we began the most tedious of all tasks- removing the inner fiberglass skin and every last bit of plywood bonded to the outside skin. The inside laminate came away without too much effort. Removing the newly exposed plywood was tougher, much tougher. The wringing wet plywood was still heavy and dense. The plywood must have been a good marine grade to have held up intact while soaking wet all these years. The challenge was to remove the wood without destroying the outside of the hull. We set to the task with chisels, pry bars and some heavy hammers. But prying would certainly crack through the outside of the transom. Instead we chiseled and chipped away. It took what seemed an eternity, like "sailing from tedium to apathy and back again, with an occasional side trip to monotony." Plywood is made of wood pieces or veneers bonded together in layers. It did not respond well to a chisel. Separating plywood bonded to fiberglass made for slow going. This job took us two complete nights of work.