Stress Cracks
06-17-2009, 11:12 AM
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Stress Cracks
Has anyone ever seen stress cracks on the transom? On both sides of the transom between the Trim Tabs and Hull there are cracks. For right now I'm assuming they are stress cracks and nothing else.
06-18-2009, 01:58 PM
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Gel coat is very brittle. The structure is much more flexible than the gel coat. Therefore, its common for any boat finished with gel coat to develop cracks in the gel coat.
A good reason to finish a boat with 2-part paint is that the paint is more flexible than the structure, so you don't get the cracks.
The cracks are "mostly harmless." On deck, you can consider them to be simply appearance blemishes, fix them when you get around to it. Under water, such as on the transom where you are seeing them, its a different story. Fix them sooner rather than later.
The reason gel coat or paint is needed is because the structural resin is not perfectly "waterproof." Its not that drops of water gets in, its that molecules of H2O get in. Of course, the molecules work their way back out too if the boat is out of the water.
But if the boat is kept in the water, then over time, the H20 can cause problems with the bond between the resin and the fibers. The gel coat, or paint, provides a barrier coat that prevents the water vapor (H20 molecules) from migrating into the structure.
Its easy to do local fixes to gel coat: grind it a tiny bit, paint it on, sand it smooth, and you won't be able to see the repair at all if the color is matched. That is why fiberglass repair is (or always should be) so inexpensive (or high profit!).
If you are seeing the cracks outboard of the tabs, between the tabs and the side of the boat on the transom, then you are seeing the effect of the boat itself flexing slightly between the high forces of the trim tab and the very stiff transom where it is supported by the hull sides. Its a good example of how brittle gel coat is: that transom is very strong, very stiff, but the extremely small amount of deformation is enough to crack gel.
Awlgrip and similar paints are flexible enough that they won't crack in these situations. But its a LOT more expensive than gel coat.
A good reason to finish a boat with 2-part paint is that the paint is more flexible than the structure, so you don't get the cracks.
The cracks are "mostly harmless." On deck, you can consider them to be simply appearance blemishes, fix them when you get around to it. Under water, such as on the transom where you are seeing them, its a different story. Fix them sooner rather than later.
The reason gel coat or paint is needed is because the structural resin is not perfectly "waterproof." Its not that drops of water gets in, its that molecules of H2O get in. Of course, the molecules work their way back out too if the boat is out of the water.
But if the boat is kept in the water, then over time, the H20 can cause problems with the bond between the resin and the fibers. The gel coat, or paint, provides a barrier coat that prevents the water vapor (H20 molecules) from migrating into the structure.
Its easy to do local fixes to gel coat: grind it a tiny bit, paint it on, sand it smooth, and you won't be able to see the repair at all if the color is matched. That is why fiberglass repair is (or always should be) so inexpensive (or high profit!).
If you are seeing the cracks outboard of the tabs, between the tabs and the side of the boat on the transom, then you are seeing the effect of the boat itself flexing slightly between the high forces of the trim tab and the very stiff transom where it is supported by the hull sides. Its a good example of how brittle gel coat is: that transom is very strong, very stiff, but the extremely small amount of deformation is enough to crack gel.
Awlgrip and similar paints are flexible enough that they won't crack in these situations. But its a LOT more expensive than gel coat.
