How To Repair Stringers In A Fiberglass Boat

by Brian Knight—GBI Technical Advisor

Comprehend Photo: Fiberglass boats tin exist repaired with West Organisation Epoxy.

Fixing damaged or delaminated stringers is i of the about mutual repairs associated with fiberglass boats. The usual causes of stringer failure are disintegration of the stringer core fabric, impact damage from slamming and grounding, and fatigue from normal use. Although each repair state of affairs has its ain unique problems, the following techniques are central to stringer repair. These guidelines will assist you lot repair almost any damaged stringer. Remember, stringers are structural support members. As you repair or supersede damaged material, use your best workmanship.

Typical stringer construction

Stringers are support members bonded into gunkhole hulls, usually oriented parallel to the long axis of a boat hull. They are there for a variety of reasons. They stiffen unsupported apartment hull sections, they support cockpit and motel soles, and they distribute high load concentrations from engines and other mechanical systems. Often they perform several of these functions simultaneously.

In fiberglass boats, yous volition find that most oftentimes, stringers are equanimous of a core material overlaid with a fiberglass skin. The skin usually extends a few inches on either side of the stringer. This pare extension, or tabbing, ties the stringer to the hull or bulkheads and spreads the load of the stringer over a larger area. Tabbing may be a simple piece of glass tape across the stringer/hull joint, or an integral structural role of the stringer. Some cores are structural, or active, and some are inactive, used primarily to provide a form for a structural fiberglass skin.

With active core stringers (usually solid wood or force per unit area treated plywood), the core material is providing the stringer with almost of its structural strength. Generally, the more dense the core cloth (like wood or plywood) the more than of the load it is expected to conduct. The fiberglass skin covering an active core is primarily used to protect the wood and to adhere information technology to the hull. It is by and large thinner than the skin on inactive core. When you replace structural cores, you have to use proper scarf bevels or other proper ways of piecing the new core into the old.

Occasionally, y'all will find a material that at first glance appears to exist plywood but on closer exam, you volition observe that all the veneers are oriented in the same direction. You cannot repair this unidirectional material with plywood. Plywood has half the grain running at right angles to the face veneer. Information technology does non have the same strength equally unidirectional fabric. A common example of unidirectional plywood cloth is called laminated veneer lumber (LVL).

Low-density core cloth is nonstructural, or inactive. A depression density cadre depends on the fiberglass pare to carry the loads. Inactive cores are made of low-density foam, cardboard tube, or in the instance of molded stringers, no cadre at all. Occasionally, stringers are pre-built in a mold and tabbed in after the hull is built. This type of stringer has no core material, just fairly heavy fiberglass skins to provide the structural strength.

Appraise the harm

Before you start any repair, it is a practiced idea to know what you are getting into. Looking at the suspected surface area of damage may be every bit easy as opening a hatch, merely don't count on it. Hull liners are mutual and usually fastened to the very stringers you are trying to gear up. It is also difficult to see under engines, water tanks, and the similar. You may take to cut access holes in the hull liner or cabin sole to see the expanse in question. Fortunately, you can purchase access covers to fill the hole.

Once you have resigned yourself to cut holes in your boat, utilise a mirror and flashlight and look for the post-obit:

Affect damage—Look for obvious fractures in the stringer. Also expect for delamination of tabbing and core away from the bear upon point. Inspect the tabbing where the stringer attaches to a bulkhead or transom.

Rot impairment—Woods cores rot from water leaking around fasteners and from h2o collecting where the fiberglass pare has delaminated. You tin can oft tap the suspected area of stringer with a small hammer. The impact of the hammer has a definite "expressionless" sound where the core is not firmly attached to the fiberglass.

Repairing local core damage

For small areas of rot, dry and inject epoxy. While this is a mutual method of wood stringer repair, it is not well-nigh as effective as replacing the damaged surface area with wood. Without removing the skin from the forest, it is frequently hard to decide the extent of the rot. Also, the degree of penetration of the injected epoxy cannot be accurately adamant and so yous do non know how expert your repair is. If however, you choose to utilise this method, nosotros recommend the following procedure:

1. Drill a pattern of three/16″ diameter holes over the rotted area. Infinite the holes 1″ or less from middle to eye in all directions. Drill each hole deep enough to laissez passer through the rot, but into solid forest.
2. Dry the area thoroughly. If necessary, use heat guns or fans to advance drying.
3. Inject or pour resin/hardener mixture into the holes while the core is warm. Epoxy will exist warmed by the cadre. Information technology will become thinner and penetrate more than securely into the exposed terminate grain. 206 Slow Hardener should penetrate more deeply than 205 Hardener before information technology begins to gel.
4. Proceed to add epoxy to the holes until the wood tin no longer absorb more.
5. Fill remaining voids with thickened epoxy subsequently the injected epoxy reaches its initial cure, if necessary. Apply an epoxy/low-density filler mixture for cosmetic fairing of the surface.

Repair guidelines

For more serious repairs that involve removing and replacing stringer material, try to duplicate the original construction. Unless the harm is straight owing to an undersized stringer, assume that the stringers were structurally adequate and properly located when the boat was originally built. Making a repair that is significantly stronger than the original pattern can cause hard spots which may misconstrue or crack the hull crush. A repair that is lighter than the original may fail prematurely. When removing and replacing stringer cloth, observe the following guidelines:

Duplicate the shape and dimensions of the original stringer. Where the stringer is supporting a cockpit, or cabin sole or engine, the height of the repaired or replaced stringer must be the same as the original. If not, you will have a great deal of difficulty reinstalling the equipment.

Duplicate the original cadre fabric or observe an equivalent material. Use wood where wood was used, plywood for plywood, foam for cream, etc. Effort to duplicate the species of forest used in the stringer equally well as the dimensions of the woods. Yous can use a more cavalier approach to replacing depression-density core materials than you lot can for active cores.

Measure the thickness of the fiberglass peel and duplicate it. On stringers with an inactive core or molded stringers (with no cadre), watch for variations in the pare thickness. Occasionally, the top pare of the stringer is thicker than the side skins. This "cap" can significantly increase the forcefulness and stiffness of the stringer. If the extra thickness is present, try to duplicate it.

Locate new stringers every bit shut equally possible to their original position. This is especially truthful of engine stringers or stringers that support other equipment.

Support the hull. If major stringer replacement is necessary, exist sure to support the hull well so the original shape is maintained. Stringers that are removed or have broken away from the hull may permit parts of the hull to sag.

Replacing active core sections

Ofttimes damage to the cadre of a stringer is limited to a small department, or the stringer may be likewise difficult to remove. You may be able to supersede only the damaged portion, restoring the strength of the stringer while leaving it in position in the boat.

Considering the woods in woods cored stringers is structural, whatsoever repairs yous make to information technology have to be joined with a proper scarf. If you lot are replacing a section of plywood stringer, use a minimum of an viii-to-1 scarf bevel. For a ¾"-thick slice of plywood this equates to a 6″ long bevel. When repairing hardwood or highly loaded core areas, utilize a longer (12-to-1) scarf angle. When cutting scarfs, keep in listen, the longer the scarf angle, the greater the joint surface surface area, the stronger the articulation. All joints in fiberglass skins should have a 12-to-1 bevel or overlap.

Forming the scarf bevel on the new slice of wood is adequately easy. You tin utilize typical cutting tools with the piece of forest supported on a work bench. Cutting the matching bevel on the wood that remains in the gunkhole is non as easy. You volition need to be creative. Yous will use chisels, disc grinders, hand planes, hand saws, and anything else you can call up of to cut forest and fiberglass. The surface of the bevel does non have to be perfect. The epoxy that you use to glue the joint is an first-class gap filler.

1. Cut out the damaged section of the existing stringer. Remove as much pare as necessary to remove all of the damaged core. Trim the exposed core ends to a minimum 8-to-1 scarf angle.
2. Grind the edges of the skin to 12-to-1 scarf angles to prepare for the peel replacement.
3. Trim a new piece of core material to fit the size and shape of the void in the existing core. Use the aforementioned species of wood as the existing core. Cut a matching scarf angle on each end of the new cadre section. Dry fit and trim the new slice and existing core ends equally necessary for a skilful fit.
4. Set the surfaces for bonding. All surface should exist clean, dry out and sanded.
5. Install the new cadre section. Wet out all contact surfaces of the new and existing core. Apply a liberal amount of thickened epoxy/406 mixture to i side of each contact area.
6. Clamp the section in position. Clean up excess epoxy before it cures. Remove clamps subsequently epoxy cures thoroughly.
7. Supervene upon the fiberglass skin as described afterwards.

Replacing stringers

Completely replacing a stringer is often easier than replacing a section. For case, engine stringers commonly run from the transom to a bulkhead. They may non run the unabridged length of the boat. Complete replacement of the damaged stringer may be much easier than attempting to supersede a section of it.

1. Mark the location of the outside surfaces of the stringer. It is often critical that the stringer gets replaced in exactly the same position it was previously located. When you remove the erstwhile stringer, yous will need reference points to locate the new one. Locate the reference marks far enough away from the repair area and so they will non exist disturbed when you prep the area.
2. Remove the stringer and core. Save whatsoever big pieces of cadre yous remove. They brand nifty patterns and will speed fitting the new cadre. Measure the thickness of the fiberglass peel so you can duplicate information technology.
3. Using the same species of wood as the existing core,trim a new piece of core material to fit the size and shape of the core in the removed stringer. Dry fit and trim the new piece for a skillful fit.
4. Prepare the surfaces for bonding. All surfaces should be make clean, dry and sanded.
5. Wet out all contact surfaces of the hull and core. Apply a liberal amount of thickened epoxy/404 High-Density or 406 Colloidal Silica mixture to one side of the contact area.
6. Push button the stringer in position with business firm mitt pressure. The epoxy mixture should clasp out of the articulation. Brace or tape the stringer in position as necessary.
7. Shape the squeezed out epoxy into a fillet. Apply additional thickened epoxy to the joint if necessary for a smooth ½"-radius fillet. Clean upwardly backlog epoxy before information technology cures. Remove clamps after epoxy cures thoroughly.
8. Supersede the fiberglass skin as described later.

Replacing or repairing the fiberglass skin

Afterward repairing or replacing core material, information technology is necessary to supercede the fiberglass skin. To duplicate the force of the original skin it is important to duplicate the thickness of the original pare and to properly prepare the surfaces for a expert bond.

Preparing the fiberglass fabric

Measure the thickness of the pare on the original stringer. Go along in heed, the superlative skin may exist thicker than the sides and the tabbing. Refer to the chart to determine the number of layers of a item weight fabric necessary to achieve the thickness required.

Cut the necessary number of strips of fiberglass fabric the length of the stringer. Cut the beginning piece large enough to extend equally far every bit the original tabbing from each side of the stringer. Cutting each of the remaining pieces one″ (½" each side) narrower than the previous one. When laying out the layers of fabric, do not allow the tabbing edges to end at the same place. For stress reduction, step the edges of the textile to create a tapered edge. If you fail to exercise this, all the load the stringer is carrying will be transferred to the line on the hull surface where the tabbing ends, and the hull may crevice at that point. If however, you lot step the tabbing edges, the load from the stringer is gradually distributed to the hull. Where stringers terminate at a bulkhead or the transom, wrap the glass tabbing onto them in the same way.

W Organisation 738 Material is ideal for stringer repairs. It yields about 0.040″ per layer in a hand lamination, so you will need fewer layers of cloth to achieve the necessary thickness for about stringers. Fewer layers of textile translates into less labor to install it. There is however, nothing wrong with using a lighter fabric. It volition require more layers per unit of measurement of laminate thickness and thus more time to install information technology. Structurally, at that place is little departure between 5 layers of 24 oz. material or ten layers of 12 oz. fabric.

Preparing surfaces for bonding

Surface preparation for bonding is a disquisitional office of whatever repair. The bilge of a boat can be very difficult to prepare for bonding, considering it is likely to be contaminated (especially around engines) and many areas may be inaccessible.

Use a degreaser or detergents in areas that may exist contaminated with gasoline or oil residue before wiping with solvent. Utilise a stiff brush on heavily textured surfaces like roving. Remove any traces of contamination by wiping the surface with solvent and drying with paper towels before the solvent evaporates.

Apply a fifty-grit grinding disc to set the surface. l-grit cuts quickly with footling heat build-up. If gelcoat is present and information technology is soundly fastened, you exercise not demand to remove it. Grind it to create a fresh, no-gloss surface. Brush the area gratis of dust or loose material. Use a wire brush to abrade heavily textured surfaces. The bonding surface should appear dull.
A 12-to-1 bevel must be footing into any existing fiberglass left on a stringer. The new fiberglass will run onto this bevel attaching the new cloth to the original material. A 12-to-1 bevel provides adequate surface area for the transfer of loads beyond the repair area. For case, if the pare on the original portion of the stringer is ¼"-thick, the bevel needs to be 12 10 ¼" or 3″ wide.

It is hard to class fiberglass cloth around a sharp xc° bend. You have to create radius at the top edges of the cores and fillet at the core/hull and core/bulkhead inside corners—a 3/8 – ½" radius is a good starting place.

Applying the fiberglass skin

1. Prepare fiberglass fabric and bonding surfaces as described to a higher place.
2. Wet out the entire bonding surface, including the stringer, with a mixture of resin/hardener. Squeegee a sparse layer of thickened epoxy over the exposed panel bonding area if the surface is heavily textured. Mix epoxy/404 Loftier-Density or 406 Colloidal Silica filler to the consistency of mayonnaise. The thickened epoxy will fill up voids on the surface and provide better contact with the commencement layer of fabric.
3. Center the largest piece of fabric over the stringer and reinforcement surface area and moisture it out with the resin/hardener mixture. Squeegee any excess epoxy from the surface, making sure the unabridged piece of fabric has been saturated.
4. Apply each successive piece of fabric in the same manner. Successive pieces may be applied immediately subsequently the previous slice or whatever time earlier the previous piece reaches its final cure (ideally while it is notwithstanding tacky). The cloth edges should exist stepped, with the last piece extending most 1 ¾" to ii ¼" from each side of the stringer (depending on the number of fabric layers). Let the lay-upward to reach its initial cure.
5. Apply ii or three coats of epoxy before the lay-upward reaches its final cure. To avoid sanding between coats, use each glaze before the previous glaze reaches its final cure. Allow the final coat to cure thoroughly.

Note: The final two or three coats may exist tinted with West SYSTEM 501 (white), 502 (black) or 503 (grey) paint or with 420 Aluminum Pulverisation (gray) . If you desire a smoother cosmetic terminate, the lay-upward may be faired and finished.

When your repair is complete, you will take a little additional finishing work to practise. Fiberglass repairs inevitably have some abrupt edges or sharp "hairs" sticking out. These make cleaning the bilge difficult if non downright dangerous. It is a expert idea to employ some eighty-grit sand paper to eliminate the imperfections that might cutting you.

You have a couple options for final finishing:

1. Do nix. Since most of the work is in the bilge area, you lot do not need to utilize a final end. UV deposition of the epoxy volition non be a trouble and in many circumstances, the appearance of the repair does not matter.
2. Paint the repair. If the appearance of the repair matters, select a paint color that matches the rest of the area and paint the repair. Proper surface preparation of the repair includes washing with water and thoroughly sanding the epoxy surface. Apply a paint primer or apply the topcoat straight to the prepared epoxy.

As ever, when you lot're installing any hardware, use epoxy to seal all holes you lot drill. If you fail this step, you volition likely have another repair job in a few years when the core material rots.

Source: https://www.epoxyworks.com/index.php/stringer-repairs-in-fiberglass-boats/

Share this post