Bulb size & shape

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Gary Cameron (e-mail gcameron@satlink.com.au) offered some detailed comments a while back on bulb design.  Here they are.

These are some notes I have made over the years on bulb shapes. The bulb on the original TS2 was bought from a local R/C sailor who made it to a NZ "Gopher Gold" design. The bulb was similar to the old Red Wine length although very different in shape (approx. 45 mm max dia).  When Craig Smith went to make his own leads for the production TS2 I wanted a longer thinner bulb for no reason other than I did not like thick fat airfoils.  The TS2 bulb became approx. 300 mm long and 41mm max dia (13.67% t/c ratio) and was a NACA four digit section.  Testing with six very competitive TS2s showed that in 10 knots of wind the boat with the first of the new bulbs was between 4 to 6 boat lengths faster down a run.  As each boat was converted to the new bulb design over a few weeks, they all became competitive again with the boats previously converted.  The choice of bulb design was not scientific at all.

I then looked at a number of other shapes in 1995 and felt that a longer bulb approx 360 mm long with a 36.4mm max dia was the best trade off between increased wetted surface and reduced sectional drag.  This was based on using the wind tunnel data generated by Michael Selig's group at the UIUC and coming up with a scheme of non-dimensionalising the drag data based on the bulb volume rather than the test section's plan form area, as is typical in wind tunnel tests.  This approach allowed me to evaluate different shapes at our Reynolds numbers.  We have not used a bulb of this shape on the TS2 as we did not want to change the boat and hence potentially super-cede any of the boats already out there even though I have had a mould of this shape for 3 years.

This shape -- long and thin -- does seem to go against the trend from full sized yachts. Why? This has to do with the Reynolds numbers we sail at compared to our full size counter-parts. At our Reynolds numbers we are in danger of the laminar boundary layer separating (forming laminar separation bubbles or fully separating) from the surface of our foils and bulbs, resulting in substantial drag.  Thick sections are particularly susceptible to this as are the typical laminar flow sections seen on full sized gliders and yachts.

As an example.  The section that I have felt is the best for an IOM, if put on a 40 foot keel boat, would generate 32% more drag than if the bulb was shaped to an Eppler E520 profile.  (This is for a 40 footer I designed, sailing upwind at 7.4 knots).  As the bulb only provides a fraction of the overall drag of the total boat, this 32% equates to a variance in the overall drag of only about 0.6% but is still significant when racing.  However, the same comparison performed on an IOM bulb sailing at 1 knot indicates that the long thin bulb produces 41% less drag than an equivalent E520 shaped bulb.  (The raw drag data was generated using Mark Drela's Xfoil program at the representative Reynolds numbers).

As far as the discussions about pitching moment goes, the long thin bulb increases the pitch, yaw and, very marginally, the roll moments of inertia.  Full size yachts have proven for many years that reducing the pitching moment of inertia generally results in substantial improvements in the speed through waves. The bow buries less on approaching the wave and the stern drags less as the wave leaves.  There are a few occasions when increasing the pitching moment of inertia helps but this usually relates to very small waves with respect to the wind speed (a building breeze).  Some dinghy sailors will position the forward hand far forward and the skipper well aft in these conditions to advantage.  These conditions normally only last for a short time (my experience is a maximum of one windward leg) and the same sailors quickly resort to centralizing their weight as the seas build.  My personal view is that the increase in pitching moment of the long thin bulb is more than offset by the drag reduction for an IOM as the pitching moment increase relative to the overall boat is small.


2011 Lester Gilbert