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 testingA 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.
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.
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.
RG65 finsTo 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.
Effect of extra wetted surface areaThe 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.
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 conclusionsWe 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%.
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
AcknowledgementsThese 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