Re: I would like to introduce myself, and talk about my rebuild.
Since we're discussing hard spots a little, I thought I'd add some info here. Boy, am I getting long-winded in my old age.
Here's some questions and answers about it:
* What's a "hard spot"?
A "hard spot" is a place where a fiberglass hull flexes in response to stresses unevenly with respect to the surrounding glass. IE, the supporting structure does not permit the hull to give, so the effect is similar to placing a coin on a cushion, putting a sheet of paper over it, and pressing down on it. The coin doesn't give, so the paper is embossed with a circle where the coin lays.
* Why are they bad?
Because uneven flexing of cured fiberglass usually creates even greater stress at the interface between the flex sections and non flex sections... remember the outline of the coin? That usually means the resin at that spot flexes beyond its ability to resist cracking, and it starts to form a crack that weakens the hull. Repeated flexing creates more cracks, and weakens the existing ones. Once the resin cracks it isn't holding the glass in place. Without reinforcing resin, fiberglass has little strength and the hull eventually forms a leak, hole, or fails entirely.
* Can non fiberglass hulls have hard spots?
Sure, but most non fiberglass materials don't crack or break like resin does when flexed, eg. metal or solid wood, so the hard spots don't matter as much. It's still best to avoid creating spots of great stress on the hull for other reasons, of course.
* Can a cored hull have hard spots?
Yes, and in fact this is usually worse than a solid hull having them, because the core is usually water absorbent one way or another, and water getting to it will cause rot and delamination, causing a hull failure sooner than a solid hull, and forcing a larger repair to get all the water out.
* Can stringer wood touch the hull and NOT form a hard spot?
Sure. As long as whatever support structure the hull presses against supports it evenly, no hard spots will be formed, IE the whole hull will flex together.
* But ideally, you shouldn't have wood touch the hull, right?
It's the best way to make sure you don't have hard spots, especially if you're going to stress your hull a lot.
* So what's the best way to keep the wood off the hull?
You don't actually have to keep the wood off the hull so much as you have to make sure it touches evenly. The easiest way to do this is putty the stringers in place... use a poly adhesive to bed the stringers, or putty made from resin, or any other appropriate thixotropic (non flowing) adhesive. Whatever you use, it needs to form a bridge between the hull and the stringer. Since it matches the contour of the stringer on one side and the hull on the other, it transfers force evenly and prevents hard spots.
* What about using foam?
That's the ideal way to keep the stringer off the hull, but it's harder to build. Basically you use a trapezoidal piece of foam (almost any type works, but it must be resin compatible) to suspend the stringer in the air, but in place in the hull. You then glass the stringer in place. The foam stays in the layup, but performs no function after the stringer is glassed in. The fiberglass between the stringer and the hull transfers all the forces of motion from hull to stringer and back very well, and for extreme uses permits more flexure without breakage than any other method. Think of it as similar to the old eastern proverb about an old tree breaking in a strong wind, but a young tree bending and surviving.
There's a variant on this method where you use blocks, clamps, or whatever you like to suspend the stringer in place while you fillet the bottom edge and glass, but this requires more adhesive than other methods.
* Where else can hard spots form?
They can form elsewhere in any place a non yielding item sits directly against the hull. The corner of a fuel tank that's not padded, a frame or bulkhead, basically anywhere there's a spot on the fiberglass that has something non yielding behind it. In non hull glass it doesn't matter as much, since it's not under as much stress as the hull, but hard spots abound.
* Where should I never get hard spots?
There should NEVER be stress cracks is the keel.. on most hulls this is the strongest area, and if this area is stress cracked, something is very, very wrong with the hull. This area of the hull also shouldn't flex except on a microscopic level, since it's supported by most of the rest of the hull, so avoiding wood touching the hull here isn't much of an issue (unless the wood that's there is rotten and not supporting the keel). It's more likely there's a major structural problem with the hull, or there was a high stress single event like a hard stranding. Likewise, the transom on most powerboats should not flex, for many of the same reasons.
* If I have a hard spot that's cracked, can I fix it by grinding out the cracked glass and glassing over it?
Yes, but the hard spot will still be there, and your repair will crack at the least, and might fail catastrophically depending on your repair technique (since it's not part of the original hull). Repairing cracks or holes in ANY piece of fiberglass requires first understanding why the problem is there. Cracks or holes are almost always symptoms of something else, stress on the glass from somewhere, and not fixing that first means you're just doing a cosmetic repair.
* What if I have stress cracks that aren't near a stringer or any other wood?
Look carefully at the crack. Usually it's at the center of the stresses involved, like a line in the center of a fold in paper. A horizontal crack means something squished the glass above and below it vertically somehow, a crack in a corner means usually something tried to "spread" the corner, etc.
As mentioned above, you need to figure out what's causing the crack, or at least what's permitting it to happen. Remember that whoever designed your boat, if they did a good job, knew the stresses that would be placed on it and designed it not to crack, so if it's cracking then either some support member is broken or failing, or you've placed the glass under an unusual amount of stress, something the boat wasn't designed for, like being dropped from a forklift or running aground at 80 knots.
Look for what stiffens and absorbs force for the glass that's cracked. The deck or sole is supported by stringers, which transfer weight from the deck to the hull. The hull bottom is also supported by stringers which transfer force to the rest of the hull, the transom, and deck. The sides of the boat are supported by the bulkheads, top cap, occasionally some molded seating or a cockpit, etc. Sometimes even injected or pourable foam may play a part in support, and when degraded will cease doing that job.
Sometimes you'll see cracks form that are secondary symptoms. Like for example cracks forming around the bolts on a seat pedestal. The pedestal itself might not be at fault, nor the deck under it weak, but your whole deck may be sagging due to a bad stringer, or worse your frames and bulkheads are weak and permitting the hull to "fold" slightly lengthwise.
Finally, you may see cracks that aren't from hard spots or repeated stress, but that don't appear after any obvious incident. For example, a crack around a circular area on a boat top cap, or a large series of cracks around a cleat. Gelcoat is good at hiding cracks since it's a solid color all the way through, so you may not see the problem until dirt collects in the crack. The fix is still the same... figure out what happened, fix the structure, then fix the glass. If you can't figure out what happened, post pics on iboats
Take care not to form hard spots in your boat repairs, but don't stress if you find one, you can fix the problem and go on using the boat safely. Just check your hull on a regular basis when you do your other maintenance.
Erik
