Hull Defects

By Paul Williams.
Article posted 16th February 2009.

Rocker.

If the area of the hull bottom immediately forward of the transom is curved, the hull is said to have rocker. Rocker is a problem because it causes the boat to porpoise, or continually pitch up and down as it rides through the water. Porpoising destroys performance, and also makes the boat much more liable to capsize.

Rocker: a curvature of the bottom of the hull forward of the transom. Causes porpoising which destroys performance and makes the boat more liable to capsize.

Rocker can be an intentional design feature of cheap plastic kit boats to maximise speed by reducing wetted area, but is most often a manufacturing defect. ABS plastic moulded boats formed over a male tool will have a small radius where the transom meets the hull bottom; this is normal for this manufacturing process, but can be a problem as the lack of a sharp edge at the bottom of the transom can allow water to flow up the transom instead of breaking cleanly from the bottom of the boat.

Vacuum-formed plastic hulls formed over a male tool have dull, radiused edges. A fast boat needs sharp edges which shed water and reduce wetted area to a minimum.

Rocker can be cured by reprofiling the bottom of the boat with car body filler. A plastic hull with a radiused transom can be fixed by gluing a plastic doubler plate to the transom, then filling the radius with car body filler.

Radius from forming.

The edges of the planing surfaces must be sharp to shed water. Take care not blunt these edges when building, or through excessive amounts of paint, otherwise water will creep up the sides of the planing surfaces instead of breaking off cleanly.

Hollows.

Hollows - also known as hook - is a depression in the hull that acts like a large trim tab, sucking the boat onto the water and causing it to run very flat, and thus excessively wet, with consequent loss of performance. Hollows can be caused by warping or shrinkage of the laminate on a moulded glassfibre boat, or simply from bad tool making or design. Mild hollows can be cured by filling with car body filler. If the problem is very bad, consider returning the hull to the vendor and getting a replacement.

Hollows: a depression in the bottom of the hull that acts like a large trim tab, sucking the boat onto the water.

Warps and Twists.

A hull with any significant amount of warp or twist can be a nightmare to try and correct. It doesn't need much of a warp or twist in a hull to totally ruin the boat's handling, and no amount of tinkering with offset or angled drives, torque wedges etc. will ever cure the underlying problem.

Warps and twists.

Before you do anything with a new hull, check it over carefully with a steel rule or on a flat surface. If you detect what looks like any amount of warping or twisting, consider asking the vendor for a replacement - you might just save yourself a lot of grief and wasted time building a donkey instead of a thoroughbred.

Thankfully, the majority of boat hulls made for fast electric racing (as opposed to fun running kit boats) are made by reputable suppliers using epoxy-glass in preference to the cheaper, and infinitely inferior, fibreglass. Fibreglass is notorious for warping as it cures, especially if the laminate is pulled from the mould too soon after lay-up. Given the superiority of epoxy-glass in terms of rigidity, resilience and overall quality, the best advice is to avoid fibreglass boats altogether if possible. One good, hard thump in a crash can completely wreck a fibreglass boat, whereas epoxy-glass would bounce back with minor damage.

If you're building a wooden boat from plans, for example a scale hydroplane, outrigger hydroplane or catamaran, continually check your work as you build. Once you find you've built something out of true, if you cannot make it good, discard it and start again. It's simply not worth continuing to build something that is warped or twisted.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:06