Cajuns Boats is OOB so I can't contact them. I've been doing a LOT of research on Honeycomb transoms and have not been able to find any of the common MFG's that have built a boat transom using this technique. There may be or have been MFG's that built honeycomb transoms...I'm not saying your boat doesn't have one and I'm not saying it didn't come from that factory that way. I'm just saying I can't find any documentation stating that it is or did. It seems to me the cost of doing so would be prohibitive and Cajun boats were NOT the Cadillac of Bass Boats!!! But who knows:noidea:...I'm curious now so I'll keep digging.
I did find this very good article on transom construction and in this authors opinion honeycomb is NOT a recommended material for transoms...
Transom Construction
Your boat started off as just a hull, a silhouette of a boat. Structurally it would need to be strengthened and stiffened before ever seeing the water. The first step of this strengthening process was the installation of the transom. In the 50s and 60s, the process for strengthening and stiffening a transom was a foregone conclusion. A wooden core was laminated to the inside of the fiberglass transom skin and a new build up of fiberglass was then laminated onto the wooden core. This process layering fiberglass, then wood, then fiberglass is known as sandwich construction. There was some variation in the core material, but the mechanical properties of sandwich construction have never changed. To this day, transoms are largely built the exact same way. Sandwich construction is an improvement over solid fiberglass because it is stiffer compared to solid fiberglass of equal weight. The problem with sandwich construction is that organic core materials, such as wood, rots when it gets wet. Boats, as you may already be aware, spend a great deal of time in a fairly wet environment. Using a core material that is highly susceptible to water damage on a boat is a little like storing fireworks in the cabinet above the stove. Drilling holes in the transom to mount an outboard motor is like dangling the fuse next to the pilot light. Eventually the fuse is going to light. Eventually water will make its way into the core. Maybe there's a better idea.
Before we get into the myriad options available for replacement or the ?how-to? part, we have to spend some time familiarizing ourselves with some of the mathematic principles behind sandwich construction. A working understanding of the concept is vital ensuring a safe repair.
The Core Principles
Cored fiberglass, or sandwich construction, as stated above, is stiffer than solid fiberglass of equal weight. But why?
The concept is the same as I-Beam construction. (Incidentally, it is also known as ?H-beam?, which makes sense, but also cryptically known as ?W-beam?, which stands for ?wide flange?, and also the stupidly over-complicated ?Double T? construction). In an I-beam, two thin lengths of steel that are stiff in a left to right direction, but flimsy in an up and down sense are connected by a third piece of steel that is stiff in an up and down direction, but flimsy in a right to left sense. The result is an I-beam that is stiffer in every direction than a solid piece of steel that weighs the same. So how does this translate to a transom with a core? A transom isn't made out of I-beams. It's made out of sheets of fiberglass bonded to each side of a solid core. That doesn't sound like the same thing at all.
Well, it is and it isn't.
Let's do a hypothetical experiment. Let's say you have two lengths of ?? thick fiberglass that are both exactly 10' long. When you lay them on top of each other, the outer edges line up perfectly. Now imagine you've taken a pencil and marked the edges of each piece in one foot increments. If you then bent them into a U-shape. The edges of the fiberglass would no longer line up with each other. That's because the inside of the curve is a tighter radius than the outside of the curve.
If you looked at your pencil marks, you would see that they line up perfectly in the middle of the bend, but the further away from the middle you go, the bigger the gap between pencil marks. That gap is called "shearing". Now if you repeated the experiment, but this time you screwed the two pieces together at each pencil mark, you would find that it's much harder to bend them at all. That's because the fasteners won't allow the two pieces to shear away from each other.
This is essentially what a core does. It prevents the two panels of fiberglass in a transom from bending by limiting their ability to shear away from each other. The thicker the core material, the stiffer it is. To illustrate this point, I'm going to steal some numbers from Nidacore's website (Nidacore is a manufacturer of composite core material. We'll discuss their products as well as other's a little later). A sandwich construction of fiberglass-Nidacore-fiberglass was 7 times stiffer than the same amount of fiberglass without the core. (The thickness of the Nidacore was equivalent to the total thickness of the fiberglass) When the amount of core material was doubled, this number jumped to 37 times stiffer than fiberglass alone.
Stiffness vs. Strength
As you can see, adding a core between two fiberglass skins is a great benefit to stiffness. But stiffness and strength aren't the same thing. Confusing the two can be disastrous at sea. In the previous example, 1? of core material was sandwiched between two ?? fiberglass skins, resulting in stiffness 37 times greater than just ?? of fiberglass alone. But how much was the strength improved? The answer may shock you.
The increase in strength was hardly even noticeable. The overall increase was limited to the strength of ?? fiberglass plus the strength of the core material itself. It still takes a roughly equivalent amount of force to break through solid fiberglass as it does to break through cored fiberglass, the difference is that solid fiberglass is going to deflect, (bend), long before it fractures.
Now here's where it gets tricky.
When you hang a 250 lb outboard from your transom, any flexing will amplify the amount of force caused by motor. Let me use another example here. If you get on your bathroom scale it may say you weigh 175 lbs. But if you hop up and down on the balls of your feet while standing on the scale, you'll see the needle jump all over the place. You may only weigh 175 lbs, but on the downward motion, the needle might register 300 lbs. In this analogy, you are the outboard and the scale is the transom. When you're hopping up and down, that's the amount of force amplified by the deflection. So, while a cored transom may not increase the overall strength, it does mute the amount of deflection and thus, the amplified force put on the transom. The result is a transom that isn't stronger, but instead, a transom that, because of its stiffness, lessens the force against it.
Types of Cores
In the beginning, cores were almost entirely organic material. Transoms were mostly cored with plywood. Other materials included end grain balsa and solid wood. These organic materials inevitably have shorter lifespans than the fiberglass itself. These materials are also probably the reason you're reading this in the first place.
Honeycomb Cores
Honeycomb cores are the next generation of core materials and, even though they are both costly and not always readily available, I think these are the first evolution of fiberglass construction that is, hands down, a distinct improvement over standard organic cores. The honeycomb material is made from polypropylene which has good points and bad points. Polypropylene is plastic. It's the same plastic used to make extruded parts and cheap children's toys. It's also spun, braided, and used to make two ropes and safety lines for boats because it's stretchy and it floats. Understanding these qualities of polypropylene should help you understand it's qualities as a core material. First let's look at the mechanical aspects of these cores. They are honeycombed, but why is that good or bad? Well, a hexagonal support structure between two fiberglass skins makes for a rigid sandwich that is protected from shearing in three directions. The small hexagonal voids also become individual pockets in the structure. This means that if the boat should become punctured or water should penetrate the outer skin through improperly bedded hardware, the moisture will be confined to the compromised cell or cells and won't wick through the entire core like it would with organic materials.
Because these cores are mostly air surrounded by a latticework of I-beams, they are also very light. Sandwiching a honeycombed core material to the backside of an unsupported solid fiberglass panel, such as the fore deck of a small runabout, will greatly improve the structure without adding much weight.
Honeycombed cores have excellent impact resistance too. Because it's made out of a semi-elastic plastic, a sharp impact to a cored material will first cause the core to compress, then spring back to its original shape. These are the reasons this core material is so prevalent in many hull constructions.
How about for transoms? This is where they're not recommended. A transom, as reiterated countless times so far, requires high shear strength and high compressive strength. Polypropylene cores have multi-directional shear strength but it's not particularly high. It also has fairly low compressive strength.
Another factor at play with these materials is the layering necessary for excessively thick panels. A transom is often the thickest panel on a boat because it requires the greatest strength and rigidity. Honeycombed cores over an inch thick can become weak and collapse. When using these cores on thick panels, it's necessary to layup two successive layers of core material separated by a central layer of fiberglass, essentially turning the sandwich construction into something closer to a club sandwich construction; outer skin ? core ? central skin ? core ? inner skin.
This does provide and extra layer of protection if the panel should be punctured, but it is labor intensive and, in the end, over-engineered and no better than using an alternative core material.
