Skip to main content

Vacuum Bagging


When building or repairing a substantial laminate, it helps to have pressure to compress the air from a stack or hold the core in place until a cure is reached. Frequently when doing hull and deck repairs, we have to work upside down, and applying pressure is difficult.

At sea level, the atmospheric pressure all around us is just under 15 pounds per square inch, and we can use that pressure to exert a tremendous clamping force with little actual weight. We do this by sealing out the atmosphere on all sides and then removing the remaining air (atmosphere) inside the sealed area. Because the atmosphere is trying to get into the lower-pressure area, it exerts this 15 PSI evenly over the whole sealed space, squeezing down everything under the weight of the air outside the vacuum bag.


The elements of a successful vacuum bag operation consist of an airtight seal around the perimeter of the repair (or, at a boatbuilding plant, sometimes the entire mold) and a vacuum pump. The vacuum bag material is typically a tough, clear plastic.

For repair areas, once the core or laminate (fiberglass or a veneer) is in place, sometimes with a temporary holding device, a seal (typically a soft butyl sealant) is put in place around the perimeter. The laminate needs a medium on top to make sure air can flow out of the bag in all the corners, and some plastic or fabric that is glue- resistant. The type of non-stick material is key in preventing accidental incorporations into the laminate stack.

Next, we need a sealed exit from the vacuum bag. That exit can be a piece of rigid hose or, in a repeat operation, a self-sealing vacuum fitting made for the job. A resin trap to catch any excess before it reaches the vacuum pump is good insurance, and it can be shop-made or a commercial item. The vacuum pump is sized for the volume of air to be evacuated, usually measured in cubic feet per minute. I have used small refrigeration vacuum pumps, large industrial vacuum cleaners and specific laminate pumps to do various jobs.

Roger Hellyar-Brook

Once you have set up and seen the results of a bagged repair, you will use the technique more often. The laminate stack is dense and air-free, and large areas of core material can be easily glued, even upside down. n

Roger Hellyar-Brook runs a marine consulting business, repairing and upgrading boats of all types. He has spent more than 40 years in the marine industry and is the former manager of the systems program at The Landing School in Arundel, Maine.

Paul Mirto is a digital illustrator, longtime boater and former Coast Guardsman.

This article originally appeared in the March 2018 issue.



Repairing Composites

When a single-skin fiberglass boat has a hole or severe gouge, the damaged fibers have to be replaced with laminates that match as closely as possible the construction of the original.


Charger Wiring

The size of battery-charging equipment has grown to keep pace with larger and more sophisticated batteries.

Illustration of Typical OEM alternator setup

High-Output Alternators

Most engine manufacturers deliver the  electrical system so that the installer just has to hook up a cranking conductor and engine block ground cable.


Gasoline Venting

Just as automobile technology is changing to match new environmental standards, so must our boats.

Alternator Pulley Side View Illustration

Alternator Upgrades

If the size of a boat’s battery bank is  increased or the type of battery is changed, the engine alternator might have to be upgraded to a higher-amp model.


Sizing Batteries

Amp-hours on board are measured just like kilowatt-hours in your home. How much current we consume and for how long are the factors that determine the overall capacity needed in a boat’s house bank of batteries. Starting batteries are easy to size since


Exhaust Separators

When we mix exhaust gases with an engine’s discharge water, we cool the water enough to use rubber and plastics in the exhaust system.


Battery Types

The world’s most common big batteries are called flooded or wet cell, as they have enough liquid acid to cover the battery plates.