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Depends on the extent and location of the damage. If the boat has been wrapped and has multiple tears or cracks, the process can be much more involved and complicated. With extensive damage you may have to remove or replace the gunwales of a canoe to relieve any distortion to the hull. Your intent in repairing your hull will be to restore stiffness and strength to the hull.
Working with cloth and resin always has the potential to be challenging and unless you approach it in an organised manner, it can get messy in a heartbeat. It's best to get all the necessary components together and sorted out before starting. It may also be a situation where the repair is best performed over a series of days, rather than at one fell swoop. Generally, you will perform the structural repair on the inside of the hull to restore integrity while repair to the exterior of the hull will be more cosmetic in nature. If you are repairing a kayak hull sometimes it is unavoidable to perform the structural repair on the hull exterior if the interior surface is unreachable.
The actual process is pretty straightforward in that you wet out the prepared surface with resin, lay patching fabric in place, wet out that patch, and repeat the process until you have restored the hull to its original stiffness and structure. However, fibreglass resin is challenging to work with. It's sticky when wet and then you have a limited working time with which to apply it. As long as you're organised and have the materials and conditions at hand it is a realistic process to undertake.
It can be tough with many boats, especially those with gel-coated exteriors and painted interiors. Just about any damage to a fibreglass hull will show up as white, whether it be a scratch or a fracture. Surface scratches, painful as they may be, are relatively easy to identify and quantify. Regardless of the color of your gel-coat, scratches coming as a result of beaching the boat or sliding over a midstream ledge or rock will show as white. If the gel-coat is still intact and you can't see any damage to the underlying fibres or cloth weave, most likely this isn't cause for concern. Eventually, you will want to restore the gel-coat in these areas to maintain the protection the gel-coat provides to the hull structure.
The key to determining structural damage is to keep an eye out for any new or excessive flexibility in your hull. If you've taken an impact but don't see any exterior signs of damage, take a minute and push against the hull in the area of the hit and compare how flexible the hull is in that area compared to a corresponding point on other side of the hull. If the impacted area is more flexible, then you may have an issue to deal with.
Sometimes structural failure is a cumulative thing that results from use over time and can't be attributed to one instance or one rock. Signs to look for are flexibility in the hull while paddling. Take a minute and drive your boat hard and watch the interior of the hull. Any up and down movement in the bottom of the boat is not a good sign. Unlike "rubber" boats (polyethylene & Royalex) where such "oil-canning" is expected and desirable to a degree, such flex in a composite boat indicates a problem. Another area to look at is to inspect the exterior of boat along the chines (where the hull transitions from bottom to sides) and look for any pattern of parallel cracks in the gel-coat. If there is a series of cracks on one side or both, this can be an indication that the internal layup has been damaged and needs to be stiffened.
Literally, if you have the time and patience, you can restore a boat that's been broken into multiple pieces. It'll be heavier than it once was but it will be functional.
Generally speaking, damage to composite hulls falls into several categories. The first is cosmetic wear and tear. Scratching the hull is inevitable and the first scratches are the most painful. Regardless of the gel-coat color, composite hulls will scratch out white, making this type of damage look worse than it is. If the gelcoat is still intact and there is no sign of corresponding whiteness on the interior of the hull, its most likely that all you're facing is a gel-coat repair and this can easily be accomplished with the materials provided in Harmony's gel-coat Repair Kit.
Most damage that requires structural repair is the result of impact or from excessive flexibility in the hull as the result of fatigue or unrecognised damage. It is true that continued severe abrasion can necessitate a structural repair if the gel-coat has been removed and you are seeing the white fibres of the cloth exposed or lifting up. However, what we're most concerned with here is fixing punctures and cracks or fractures. It's not necessary to have exposed fibres to indicate a fracture. A white line on the interior of the hull could indicate what is called a resin fracture where the fibres have not actually been torn but the bond between resin and fibres has been broken. It's important to be proactive. If you see a point where the hull has been damaged, it's best to repair it as quickly as possible to prevent damage to the lay-up from water getting inside the lay-up and causing further deterioration.
First step will be to clean the hull thoroughly. This will make the extent of damage that more evident and allow you to assemble the necessary materials. Next, you will want to remove the damaged fibres and resin from around the area to be repaired. This is accomplished by a considerable amount of sanding and cutting of 'loose" fibres from around the injury. It is key to remove any raised fibres as these have already been resin impregnated and will not absorb any new resin and you'll find them extremely stubborn in terms of trying to get them to lay flat under a new patch or reinforcement. This step actually resembles making the damage worse in that you are opening up the crack somewhat by removing material that is still present but no longer of any structural value.
The next step will be to cut reinforcing patches of cloth to lay in over the damage. Plan on laying in multiple layers of fabric to restore stiffness and integrity. Usually this can be done at one time. Once the patches have cured, then you've got a bit of sanding to be done to smooth down any sharp edges. As structural repair is done on the inside of the hull, its up to you how far you want to go in restoring the original appearance of your hull. On the exterior of the hull, with enough time and energy, you can make the damage invisible by using the appropriate colored gel-coat kit.
No, that's one of the beautiful things about composite construction. You can repair your hull to the point where it is as strong as originally built or you can take it a step or two further and buttress the area by adding additional layers or specific types of fabric. The only drawback to building up an area is the additional weight involved.
Harmony gear offers prepackaged repair kits for Fibreglass or Kevlar® repairs. The kits contain enough material for most repairs of cracks or tears up to 30cm (12") long. You can also obtain additional resin and cloth from your local outdoor specialty store, marine stores, auto parts stores or from boat building supplies on the Internet.
As for resins, your choice will usually come down to polyester, vinylester, or epoxy. Most performance boats are built of either vinylester or epoxy. Both are significantly stronger than polyester (and more expensive). Polyester resin is adequate for the majority of repair work and bonds well to whatever resin may have been used originally. For extensive repairs in critical areas of the hull you may want to consider vinylester (has the advantage of being slightly more elastic and is less threatening to work with) or epoxy (stronger and lighter but more hazardous).
When it comes to fabrics, avoid fibreglass mat. Mat can be identified by fibres running in random directions with no semblance of a weave or pattern. Mat has the advantage of being able to easily conform to tight curves and corners but provides little or no strength, as the random fibres are not linked to each other. It is serviceable to use as a filler on the outside of a hull beneath the gel-coat or if you're trying to build up a repair right up in the nose of your boat but other than that you'll be better off going with a woven fabric.
Woven roving is characteried by a coarse weave consisting of large fibres. It has the advantage of providing substantial stiffness over a large area but tends to be heavy compared to more tightly woven fabrics as it collects substantial amounts of resin within its large weave. Its best application is to restore stiffness across the interior of the bottom of the hull. It does not conform to curves and corners very well.
"True" woven fabrics come in a variety of weights, usually measured in weight per metre. For most repairs, a 170gm cloth or similar will work fine. Its relatively fine weave provides a good resin-fabric ratio, it conforms to hull curvatures well and has good strength to weight characteristics, especially when used in multiple layers. Lighter weight cloth may be called for when you're working in an area with a tight curvature and need a more flexible cloth. Heavier weights such as 300gm or similar can be used where stiffness is desired or where you've got a fairly large area to restore. Many builders use a mix of different weights for best results.
In addition to weights, you'll also encounter different types of fabric. Your basic fibreglass fabric is called "e-glass" and is perfectly adequate for most applications. "S-glass" fabric does provide a higher level of abrasion resistance but is harder and stiffer to work with. If you're repairing the end or keel line of your boat and you feel like you have to reinforce the exterior of the hull, s-glass would be a good choice. Many manufacturers use it as the outside layer on their Kevlar® or graphite canoes for this reason.
Beyond fibreglass, one encounters the more exotic fabrics such as Kevlar® and graphite. Kevlar® has a well deserved reputation for providing lightweight durable canoes. It can be more difficult to work with as it is lighter than fibreglass cloth and may tend to "float" a little more in the resin, making it harder to get it to lay down in the center of a curvature such as the stem of your boat. Kevlar® also does not take to sanding very well as it tends to "fuzz" as you sand it. Graphite is rarely used in hull repair. It's a very stiff fabric that doesn't take well to being bent or forced to follow a hull's curves. It's also difficult to wet out with resin and hard to tell when it is sufficiently impregnated.
No. Since it’s the resin that bonds fibre to fibre, it's certainly feasible to use fibreglass cloth to repair localised damage on a Kevlar® hull. As mentioned above, many Kevlar® boats are built using fibreglass layers for good reasons such as added abrasion resistance. If your damage is relatively concentrated and doesn't require a lot of material, you'd be unlikely to notice the few ounces of added weight incurred by the use of fibreglass cloth. On the other hand, if you're repairing a wider expanse of the hull, you'd probably be better off using Kevlar® fabric to maintain a more consistent flex pattern throughout your hull.
When sanding the fibreglass or other fabric, always wear a dust mask at the minimum, along with eye protection, and long sleeve shirt and long pants. Better yet would be a regulator approved respirator with an organic vapor cartridge. The primary dangers involved with composite boat repair are the dust from sanding and the vapors emitted by the resin used. Take necessary steps to minimise your exposure to these influences.
Fibreglass fibres are no fun to get onto your skin. They itch like mad so keep covered and when finished, wash the clothes worn while doing the work separately from other laundry or simply dispose of them outright.
The best location for doing this type of work is outdoors with the temperatures in the 18-24°C range and a light steady breeze blowing. If that is not possible and you must work indoors, make sure there is a positive ventilation flow by turning on a fan or two. Vacuum up all dust generated and dispose of used materials properly. Particularly make sure that any unused resin completely cures to a solid state before disposing in any closed or sealed container.
Follow instructions provided with repair materials carefully. All resins have a "working time" and repairs should be adjusted to fit within that time frame. Working time will vary with temperature, becoming shorter the warmer it is and longer when it is cooler. Do not undertake repairs when the temperature is 10°C or less unless you take steps to warm the resin mix and the curing resin/fibre patch with a hair dryer. Do not use a heat gun or open flame under any circumstances. If temperatures are warm, plan on mixing smaller and more frequent batches of resin to avoid a larger batch kicking over to a solid before you are finished with the work.