Wetted surface area (AMYA MY #183)

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Wetted Surface Area

by Lester Gilbert


One of the perennial issues in designing and building hulls is to minimize wetted surface area, because we know that a significant component of hull drag is due to wetted area. Should we make our fin or rudder narrow and short rather than wide and deep? Should we lean towards a deep V rather than a skimming dish? Should we find some depleted Uranium for our ballast? (Just kidding, folks, just kidding…!) What we don’t usually know is how much extra drag a little extra area gives us. Graham and I ran some tests in the towing tank, and one of the outcomes was some insight into the relationship between extra wetted surface area and resulting hull velocity.


Towing tank testing

A couple of issues ago (#174 to be exact), I described how we ran some experiments in the Lamont towing tank of the University of Southampton. We continued to use a refined version of Graham’s falling weight device (FWD), comprising an axle and a drum, to tow our hulls. A line is wrapped around the axle several times and attached to a weight. A second, towing, line is attached to the drum at one end, and to the hull at the far end of the towing tank at the other. Release the weight, and the drum rotates, reeling in the line and towing the hull.


For our latest series of tests, we have developed improved timing accuracy and repeatability. Previously, I timed the models by filming them with a video camera. The video frame, which showed the model bows just crossing a sight line, gave me a time which was accurate to within 1/30th of a second, because the camera video operated at 30 frames-per-second. But as might be imagined, I had problems with camera shake, focus, sight alignment with high-speed runs, and general nervousness running alongside the tank with several thousand pounds of camera equipment in my hands.


This time I used remote triggering of timers when laser beams were interrupted. The general arrangement is illustrated in Figure 1. We had three laser beams and associated triggers. The first trigger started a pair of simple laboratory timers, accurate to 1/100th of a second. The second trigger stopped the “A” timer to record the acceleration period of the model run, between 4 and 14 seconds over around 5 metres, and the third trigger stopped the “T” timer to record the full-speed period of the run, between 6 and 20 seconds over around 7 metres, depending upon the towing force generated by the FWD.



Figure 1. Towing tank set-up.


Each laser beam comprised a laser pointer and a remote light sensor, shown in Figure 2. The sensor was wired to the trigger, which detected when the beam was interrupted by the passage of the hull’s towing post. The laser pointer was accurately aimed using thumbscrews in a carriage clamped to the side of the tank. The red dot seen in Figure 2 on the other side of the tank is the reflection of the beam from the lens of the light detector, itself clamped to the tank exactly perpendicular.



Figure 2. Laser beam and trigger set-up.


The trigger, shown lightly clamped to the tank side, has a green LED that tells us that the laser beam is correctly set. The exceptionally helpful folks at “VersaTrigger” (http://versatrigger.co.uk/) made a small change to the detector circuitry so we could set a larger than usual time-out. Usually, the trigger resets almost immediately and anything that then interrupts the beam will send another trigger signal. We didn’t want another trigger signal until the run had ended, the timings had been recorded, and the model had been returned to its starting position ready for another run, so we were given an extended time-out of around 40 seconds. Perfect.


We found that timing accuracy was indeed within 1/100 s, and was repeatable to within 1/100 s, both quite exceptional outcomes given the relative unsophistication of our equipment and set-up.


RG65 fins

To look at wetted surface area effects, we towed an RG65 hull with two different fins at three different speeds. The long fin had a surface area approximately 270 sq. cm, and the short fin area was around 245 sq. cm. The RG65 hull, rudder, and bulb together comprised a wetted surface of around 950 sq. cm, so the difference in wetted surface area between the two fins was around 2.3% of the total.


We used three FWD weights resulting in three towing speeds, low, medium, and high, such that the hull reached velocities of approximately 0.45 m/s, 0.65 m/s, and 1.2 m/s respectively.


Now, just before you see the results, you should have a guess about your expectations. Do you think that an extra 2.3% wetted surface area would slow the boat down by a similar amount, around 2 or 3%? Do you think the boat would be slowed down equally across the different towing speeds? Or do you think the extra area 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, heck, don’t peek, give it a go. OK? Here are the somewhat surprising results…


Effect of extra wetted surface area

The timed run took somewhere between 6 and 20 seconds, depending on the FWD towing force. Three runs were made for each towing speed for each fin, 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 %” from the lowest 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 0.2%. That is, using classical statistics, we can be 95% confident that a delta of, say, 2.5% in average velocity is indeed somewhere between 2.1% and 2.9% (plus and minus two standard errors). The resulting graph is shown in Figure 3.



Figure 3. Change in hull velocity at three towing speeds with long and short fins.


We can see from Figure 3 that, at medium towing speed, for example, the additional wetted surface area of the long fin slowed the hull by 3.8% relative to the short fin. This difference is highly statistically significant, as may be seen by the fact that the error bars are much shorter than the differences they calibrate. At low towing speed, the difference in hull velocity is around 4.7%, while at high speed it is much less, around 0.7%, but still significant. Did you guess that?


Discussion and conclusions

We see that a modest additional wetted surface area, 2.3%, has disproportionately slowed the hull by 4.7% at low speed, and equally disproportionately had a much smaller effect at high speed, slowing the hull by only 0.7%.


Why is that? The simple answer is that at high towing speed a large proportion of the hull’s drag is wave drag, and the frictional drag component of the additional fin area is masked. At the lower towing speed the wave drag is not dominant, the drag due to wetted surface area is proportionately higher, and so the short fin hull with lower wetted area shows as having significantly lower drag and hence higher velocity.


If you think your day’s sailing will be in light or very light airs, well, a smaller rudder or a smaller fin will help you. On an RG65, a 20% smaller rudder will give around 2% reduction in wetted area. Other things being exactly equal, hull velocity upright in a straight line (no waves!) might be expected to increase by around 4% respectively.


If you think your day’s sailing will be in heavier airs, well, you can increase your fin length (if your class rules permit) and not suffer excessively. The hit on hull speed when stability is of little value, as when sailing downwind, is proportionately much lower, and performance would probably be easily recouped by improved stability, that is, increased righting moment, when sailing to windward.



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


©2023 Lester Gilbert