Closing the Gap- How your Kayak is made, The Laminating Process

So, if you are still hanging in there, this is where it all comes together. The laminating process is the single biggest factor in the final weight of a kayak. The chosen layup, resin and gelcoat will all be stuffed into a kayak shaped mold, and with luck, we'll have a kayak shaped object once allowed to harden. Or at least two halves of one.

So, remember back in the Gelcoat article I mentioned a control? The average 16' fiberglass kayak with a surface area of 5.5 square yards? Well, now is where we will use it to compare the different methods of construction and the impact on final weight. So, lets dive into the three different techniques and how they are done.

First is Hand Lamination. This is the oldest method of the three and it has worked just fine for decades. Gelcoat is applied to the mold and allowed to cure. It's still tacky as it will not fully cure when exposed to the presence of oxygen. The first layer of the layup is put in the mold and then resin is used to fully wet it out. Subsequent layers are applied in the same manner. The process is repeated until the layup is complete and then it is allowed to cure. This process is popular when Fiberglass Mat is used in the layup. The positives for this style of construction is the low cost. The labor is also the lowest cost of the three as it is the quickest method. The negatives, it is the most resin rich of the three methods. To keep weight in check, the amount of fiber in the consolidation will also be the lowest. Resin rich means it is also the most brittle of the three. Generally speaking a woven cloth layup that is hand laid will have a resin ratio of 125-150%. Mat ratios are higher at 200-250% resin to fiber. So, 12oz cloth will hold 15-18oz of resin (by Weight). Chopped strand mat will require more resin.

Next we will move on to a vacuum bagged layup. It is also hand laid into the mold, but the consolidation is put under pressure from a vacuum bag. As you guessed it, improved resin to fiber ratio is the result. The labor and materials increase in cost for this. A greater skill level and a little more time is required to pull this off. In vacuum bagging, there are two ways to to perform the layup schedule. The standard hand laid then bagged layup, and the wet bag method. In a hand laid and bagged layup, all of the fiberglass and resin is put into the mold and then a layer of peel ply release fabric and a layer of bleeder cloth. The stack is put under vacuum and then allowed to cure. The upside, better resin to fiber ratio. Approximately 60-70%% resin to fiber is a good assumption. But that is dependent on the people doing the layup. Too much resin in the initial layup can't be squeezed out into the bleeder layer. The analogy that comes to mind, is trying to mop out the cockpit of your boat with a sponge. It is impossible to get it all out with one squeeze. Too much resin from the start, it will always have too much. Just less than a traditional hand layup. The pressure from the vacuum bag does straighten the fibers which increases stiffness and ultimately strength. Wet bagging has been misrepresented as infusion at times. Ask any Composites Engineer, and they will quickly point out that fact. I ran across a video series of the build of a sea kayak and the builder kept referring to his method as infusion. He was wet bagging. So what is wet bagging? The  fabrics are put into the mold dry. A layer of release fabric goes in on top and then a vacuum bag. The whole thing is put under vacuum and then a predetermined amount of resin is put into the bag and pushed around by a laminator armed with a silicone squeegee. This is a very good method as the result is more predictable and repeatable, no matter who is pushing the resin around. Resin ratios around 50-55% are achievable using the wet bag method. It does cost more as the peel ply and vacuum bag ultimately go in the trash. The same 12oz cloth now only requires 6-7oz of resin due to the pressure applied by the bag.

"Wet bag" Lamination Process

Resin Infusion is where vacuum is the vehicle for transmission of the resin into the consolidation. Positive pressure outside of the mold (one atmosphere) and negative pressure in the mold (One atmosphere), drive the resin into the matrix. The layup is placed into the mold dry. A layer of release fabric and a carefully designed system of flow media and inlet lines are placed under the vacuum bag. The consolidation is placed under vacuum to remove excess air and any potential moisture. Once it has been under vacuum for a predetermined time period, usually two hours, Resin is pulled into the consolidation by the vacuum. The remaining air is replaced by resin and the outcome is extremely accurate. A good infusion specialist can infuse a large part and have a few milliliters left in the resin supply pot. Resin consumption is optimized for the best possible resin to fiber ratio. 40% is the target for resin infusion but it can be "tuned" by changing the timing of vacuum source and resin inlet shut off. A slightly richer resin content can have a positive effect on the cosmetics of an infused part. My target is 41-42%. So, 12oz cloth will only require 4.8oz of resin.

Lets use the "control" kayak with a surface area of 5.5 square yards for comparison.

Hand Layup using a 40oz cloth layup schedule:

• Cloth Weight 220 oz/ 13.75 lbs
• Resin Weight 330oz/ 20.6lbs
• Gelcoat Weight 6 lbs (sprayed application)

Total weight of the bare hull and deck would be 40.35 lbs

Add in another 15 lbs for hatches, bulkheads, seam and all outfitting the grand total will be 55lbs

If you use Chopped strand mat for this same boat, it would weigh 69lbs. In order to get the weight down to an acceptable level, the weight of the fiber would have to be reduced by a substantial amount. 

Wet Vacuum Bag using a 40oz cloth layup schedule:

• Cloth Weight 220oz/ 13.75lbs
• Resin Weight 121oz / 7.5 lbs
• Gelcoat weight 6 lbs (sprayed application)

Total weight of bare hull and deck would be 27.25lbs

Total weight of finished kayak 42.25lbs

Vacuum Infusion using a 40oz cloth layup schedule:

• Cloth Weight 220oz/ 13.75lbs
• Resin Weight 88oz/ 5.5 lbs
• Gelcoat Weight 6lbs (sprayed application)

Total weight of bare hull and deck 25.25lbs

Total weight of finished kayak 40.25 lbs

As you can see, the method of construction will have a huge impact on weight! Wet bagging and Vacuum Infusion yielded similar results in weight. What sets them apart is strength. Resin penetration into the strands with vacuum bagging is very good. Tensile strength and elongation of the matrix will be superior to a hand layup. Fiber Penetration in an infusion is excellent and will yield the best tensile strength and elongation.

In a study conducted for the Journal of Physics, they put this very comparison to the test. Same fiber, same layup for all three methods Five identical samples were created for each process and then were tested using a three point test method. The results speak for themselves. The Vacuum Bagged Layup showed a 5% increase in tensile strength as compared to a Hand Layup. The Infusion Layup showed a 25% increase in tensile strength over a Hand Layup and 22% over Vacuum Bagged. The question then becomes, why? It comes down to resin penetration into the cloth matrix. On a microscopic level, the resin can penetrate beyond the tow (or strands) of the fabric and down to the individual fibers that make up the tow. The mechanism for bonding the resin in a hand layup, is pressure applied by the laminator as they move the resin around the cloth and gravity. Wet bagged layups are under one  atmosphere of negative pressure and therefore the resin is driven deeper into the fibers. Infusion is a controlled transfer (done properly) where the feed is regulated and pulled into the fiber matrix. The slower the rate of infusion, the better penetration into the fiber.

Note: 1 Megapascal (MPA) equals 145psi

Below are images of the test samples after being stressed to failure. They were inspected under an electron microscope and the images reveal the "secret sauce" about all three processes. It comes down to resin penetration. The surface area of the bond to fiber is increased under vacuum and even more so in infusion. This is where the difference starts to show.

Hand Layup at 1000X and 2500X Magnification

Vacuum Bagged Layup at 1000X Magnification

Vacuum Bagged Layup at 2500X Magnification

Resin Infusion at 1000X Magnification

Resin Infusion at 2500X Magnification

Now, is all this technology worth it? It may have been to the folks who have ever had their kayak repaired. The question becomes, is it better to have a boat that is easily repaired, or one that is lighter and less likely to ever need repairs. That's a question only you can answer.



Credit for the composites process study published in the Journal of Physics:

Abdurohman, Kosim & Satrio, T & Muzayadah, N & Teten,. (2018). A comparison process between hand lay-up, vacuum infusion and vacuum bagging method toward e-glass EW 185/lycal composites. Journal of Physics: Conference Series. 1130. 012018. 10.1088/1742-6596/1130/1/012018.