October 2011 Issue
A Mathematical Decision-maker
Its easy to underestimate the amount of actual work involved in scraping and sanding old paint.
An easy way to compute the do-it-yourself labor commitment involved is by timing how long it takes to scrape clean two 1-square-foot patches. The first is in the center of the least well-adhered paint; the second is in the midst of an intact portion of the bottom. Dry scrape each section with a thin-bladed putty knife and a sharp drag-type scraper, noting the time it takes to remove about 90 percent of the coating.
Next, estimate the percentage for the bottom that should be dubbed easy to scrape paint versus areas where the antifouling is stuck like glue. Finally, you need to roughly calculate the square footage of wetted surface area (WSA) that comprises the underbody. This is by no means an ordeal that requires advanced geometry. It’s a rudimentary calculation that requires only three bits of information (waterline length [Lwl], maximum waterline beam [Bwl] and draft [T]). The user-friendly formula (WSA = Lwl x (Bwl + T) applies to most heavy displacement cruising hulls. Multiply by 0.75 for medium-displacement vessels and 0.5 for light-displacement boats. This oversimplification of wetted surface calculation would provoke a scolding from David W. Taylor, William Froude, and other dons of naval architecture, but for a paint removal estimate, it works just fine.
Finally, take the results of the calculation and divvy it up according to the proportion of easy versus difficult regions of paint removal. For example, if only 15 percent of the bottom is cracked and peeling, and the removal time per square foot in this region is 30 seconds—as compared to four minutes per square foot in the well-adhered section—the ratio is by no means encouraging. Applying the ratio to the wetted surface calculation drives home the level of paint scraping required.
Let’s assume that the vessel has a 30-foot waterline, 6-foot draft, a waterline beam of 10 feet, and is a medium-displacement vessel. Our fuzzy math for a medium-displacement sailboat fills in the WSA equation as follows: WSA = 30 x (10 + 6) (.75), and the result is 360 square feet.
Next comes the all-important apportionment of easy versus difficult paint removal. In this case, 85 percent falls into the tough-going regime that equates to four minutes of toil per square foot (0.85 x 360 x 4 = 1,224 minutes of misery). Add in the easy scraping (0.15 x 360 x 0.5 = 27 minutes), and the result is 1,251 minutes, or just under 21 hours of serious scraping. This is unequivocal arm-in-action time, and the ride to the boatyard, coffee breaks, and chats with sistership owners, stops the clock.
Chemical paint removers, a large crew, or true friends can soften the blow, and so can a more deteriorated surface. If 70 percent of the bottom paint has a cornflake-like look, it will allow a thin bladed putty knife to effortlessly separate it from the hull skin, and a very different labor picture arises. Run the numbers, and you’ll find that in this scenario, 9.3 hours of toil would strip away the residue. In short, it’s not the bad spots you need to worry about, it’s where the paint looks good that causes all the problems. This is why procrastination rules, and we tend to postpone bottom paint removal in favor of spot prepping and feathering in the bad spots. At some point, it’s time to bite the bullet and tackle what’s been pushed down the road.