What’s Resin Infusion? (And Why Should I Care)

Many boatbuilders bandy this term about when promoting their boats, but exactly what is resin infusion, and should it mean to me anyway?

Read through 10 Signs of Top Notch Boat Construction, and you’ll come across several items that directly relate to how the major parts of the boat are “laid up.” This is to say, how the fiberglass cloths and fabrics are saturated with resin and catalyst, then harden into a FRP (fiberglass reinforced plastic) part. Just how this process takes place dictates a number of things about a boat: how heavy it is, how sturdy it is, how rigid it is, how thick the pieces-parts are, and so on. And many modern boats are laid up via a process called resin infusion. In fact, you’ll often see that term used in marketing brochures and hear it at boat shows. So, just what is resin infusion? And, should you really care if a boat you’re looking at is resin infused?

28 Bertram
The Bertram 28, weighed about 12,000-pounds dry.

Old fashioned fiberglass molding techniques did result in strong boats, but also exceptionally heavy ones. Most of today’s similarly-sized boats weigh between 20- and 40-percent less than this old Bertram 28, which was about 12,000-pounds dry.

How to Build a Fiberglass Boat

To get a handle on just how important resin infusion is or is not, a quick retrospective is in order. In the good ‘ol days of production boatbuilding, after the gelcoat was sprayed into a mold the fiberglass cloth was laid down or chopper-gunned in, sometimes both in alternating layers, and then the resin was mixed with a catalyst and used to saturate the cloth by hand with rollers and brushes. When the stuff dried, you had a chunk of fiberglass in the form of a hull or a deck. Simple enough (and yes this is an over-simplification, but you get the basic picture). Some common issues with this system were that air pockets could form if too little resin was used parts of the glass could go un-saturated and if too much resin was used the boat’s weight grew without providing any additional strength. (Remember: resin in and of itself is brittle and doesn’t really add much strength to a fiberglass part; if you molded a boat out of resin alone it would be pretty easy to crack, shatter, and smash).

fiberglass boat building
Modern core materials have revolutionized boat building.

Fiberglass boatbuilding tech progressed rapidly, especially from the 90s on, including the use of materials like lightweight cores that increased stiffness while reducing weight.

As builders began getting more advanced, they tried techniques like adding lightweight cores between layers of glass, using more advanced fabrics that provided more strength, and the use of two-part molds that provided finished surfaces on both sides. But one of the biggest leaps came when builders began experimenting with vacuum-bagging.

In a nutshell, vacuum-bagging meant wrapping the entire part, be it a hatch or a hull, in thick plastic wrap. Then a vacuum pump sucked out the air, compressing the wetted fiberglass and drawing out excess resin. As a general rule of thumb, this process would reduce resin amounts to the tune of 40-percent as compared to hand laminating, without losing any strength in the finished part, producing a much better strength-to-weight ratio.

boat building
Vacuum bagging cuts a lot of extra resin out of a boat and especially those that are weight-sensitive, like flats boats, can benefit greatly from the process.

Vacuum infusion is the next natural step in the progression. Using this technique, the entire part gets bagged while still dry, the vacuum pump evacuates all the air, and then tubes feed the resin into the bag. The resin then gets sucked through the fiberglass, infuses it, and any excess is sucked right back out. Net result? A reduction in excess resin to the tune of 60-percent or so as compared to hand laminating.

Edgewater boats
This Edgewater boat hull is prepped for vacuum infusion; Edgewater calls their particular method SPI, or Single Piece Infusion.

Sounds cool, right? But the question remains: what does all this mean to Joe Boater? First off, when you’re looking at a boat built entirely via resin infusion you know that it should weigh less—but be every bit as strong—as a boat built via vacuum bagging, much less one laid up with more traditional methods. In many cases, you’ll see boats that are built with a mix of these techniques used for different pieces-parts, which lay somewhere in-between old and new tech. And just how extensive the builder’s use of vacuum infusion gives you a lot of insight into what sort of builder they are. A manufacturer constructing entirely via vacuum infusion, for example, is likely to be up to date on and utilizing other techy features like digital switching and gyroscopic stabilization.

Intrepid boats
Some high-tech boats like this Intrepid 345 have entire hulls that are resin-infused, while others have pieces-parts that are infused.

So, buying a resin-infused boat is a no-brainer, right? Not so fast. Yes, having a lighter, stronger boat means faster speeds, better efficiency, easier towing, and all the advantages that go along with shedding weight. But on the flip side of the coin, you’ll come across boats like the Steiger Craft 28 DV Miami, which are built with more traditional methods and carry more weight—and as a result, have an easier time bulling waves out of the way. Heavier boats are less likely to launch (and we all know what re-entry can be like), and in some cases may even have a stability advantage. As is always the case when it comes to boats, what person “A” prefers isn’t necessarily what person “B” will prioritize, and there are trade-offs involved with every advantage. But when it comes to resin infusion the advantages are quite clearly numerous—and in today’s’ world of boatbuilding, it’s one of the most advanced techniques around.

Check out more great boating information and tips from BD’s Lenny Rudow!

Lenny Rudow
Lenny Rudow …has been a writer and editor in the marine field for over two decades, and has authored seven books. He is currently the Angler in Chie...