IOM bulb drag (AMYA MY #184)

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IOM Bulb Drag

by Lester Gilbert


In recent years there has been a revolution in our use of longer ballast bulbs, if not quite matched by any revolution in our understanding of why longer and thinner is better than shorter and fatter. Shorter and fatter might have a somewhat higher cross-section, but longer and thinner has a proportionately much higher wetted surface area. Graham and I ran some tests in the towing tank to explore the effects of differences in bulb shape on hull velocity.


Figure 1 shows Graham at the far end of the tank preparing his falling weight device (FWD). On the left side is the first of the three laser-beam arrangements clamped to the tank side (see MY #182 for more details on the tank and timer setup). Some old towing models are hung on the right wall, while the structure on the left-hand wall is part of the low speed wind tunnel—they were a little short of building space some years ago!



Figure 1. View of the towing tank.


IOM bulbs

We towed an IOM hull, a SAILSetc Fraktal, with three different bulbs at two different speeds. The “fat” bulb, length around 250 mm, dates from circa 1992 and is one of the Young streamline body forms. The “medium” bulb, length around 300 mm, is a design from circa 1998, while the “standard” bulb, length 350 mm, is the current SAILSetc IOM pattern as of 2015.


We used two FWD weights resulting in two towing speeds, “low” and “high,” such that the hull reached velocities of approximately 0.5 m/s and 1.0 m/s respectively.


What are your expectations? We all know that the old “fat” bulbs are slow, but how much slower? 1%? 2%? 4%? Do you think the 300 mm bulb would be measurably slower than the 350 mm bulb? Do you think the differences between the bulbs would be seen equally at the different towing speeds? Or do you think the shorter bulbs would slow the boat down more at low towing speed than at high speed? Or the other way around? And if you don’t have a thought right now, well, try for a little thought, perhaps 10 mm long and colored pink (smile). OK? Here are the results…


Effect of bulb form

The timed run took around 7 and 14 seconds depending on the FWD towing force. Three runs were made for each towing speed for each bulb, giving a total of 18 runs. The time for the three runs was averaged, converted to an average velocity, and the percentage change in velocity for that towing force was calculated—a “delta %” with reference to the slowest velocity. The measurement error for each delta was estimated as a standard error, and it is worth noting that the standard error of the delta (velocity change) was of the order of 0.15%. That is, using classical statistics, we can be 95% confident that a delta of, say, 1% in average velocity is indeed somewhere between 0.7% and 1.3% (plus and minus two standard errors). The resulting graph is shown in Figure 2.



Figure 2. Change in hull velocity at two towing speeds with three bulbs.


We can see from Figure 2, when using the 25 cm fat bulb as a reference, the medium 30 cm bulb was 0.6% faster at high towing speed and 1.4% faster at low speed. The standard 35 cm bulb was similarly faster at high speed, but showed a significant further improvement at low towing speeds of around 0.5% over the medium bulb, being almost 1.9% faster than the fat bulb. Did you guess any of that?


Discussion and conclusions

Bulb geometry is a trade-off between cross-sectional area, responsible for pressure drag, and wetted surface area responsible for frictional drag. We can see that at both low and high speeds, an IOM hull is faster with a lower cross-section bulb. There may be limits, of course, and it turns out that the benefits of reduced cross-sectional area reduce at high speeds.


If you think your day’s sailing will be in light or very light airs, there is some evidence that an exceptionally slender bulb might continue to pay off. If you think your day’s sailing will be in heavier airs, well, the hit on hull velocity is proportionately higher with increased wetted surface area, and so a bulb around 300 mm or 350 mm seems to be the current sensible limit. We’d say there is clear evidence that the longest is best, especially taking into account the additional stability from the lower vertical centre of gravity of the keel. The interesting question is, “How much further can we go?”



These experiments would not have been possible without Graham Bantock’s enthusiasm and knowledge or without the support of the University of Southampton.


©2024 Lester Gilbert