It’s not the meat, it’s the motion…
When a sailboat makes a turn in Second Life (and real-life), three major things happen that a skipper needs to think about:
- the boat changes heading in response to the hydrodynamic forces generated by the rudder deflection;
- the boat travels along an arc until it establishes the new heading; and
- the boat speed usually drops due to a loss of momentum generated by the turn.
How well a boat accomplishes these points often has a big effect on the user’s sense of fun and realism.
For a while now I’ve been trying to come up with a few simple ways to measure the turning properties of SL boats. That information might be useful when comparing different boat classes, and it may also help assess whether a given boat is a reasonable emulation of it’s real-life counterpart. Anyway, I admit I don’t have any big conclusions at this point, but I wanted to show a few charts here to see if any sailors have better ideas how to approach this issue.
For large US merchant transport vessels, there are detailed performance standards published by the American Bureau of Shipping. The ABS is a good resource for maneuverability test requirements and discussions about the principles involved.
Probably the most common performance test is a Circle Test that monitors a boat’s ability to move through a full 360° turn. That works well for a powerboat, where the engine can maintain a constant thrust during the exercise. However, it doesn’t work very well for a sailboat, since the wind is constantly changing during the turn. A full 360° turn is pretty much never a “circle.”
It makes a lot more sense to test a sailboat using a 180° half-turn, as the boat flips into the wind from one beam reach to the other.
To do that in SL, I’ve been using FRAPS to generate screenshots each second while a boat does a standard 180° turn. I then plot the boat position data sequence on an X-Y matrix. Each data point also includes the instantaneous boat speed and heading.
Rene Marine 12
Let me give a quick shout out for the Rene Marine 12 (Tofinu). It’s a great boat for this kind of test. It’s built by Rene Underby, who has a long track record as an accomplished boatbuilder in Second Life. Her Rene Marine boat yard is filled with a full line of sail craft that emphasize both authentic styling and realistic performance.
The RM 12 is her newest creation. Apropos of this discussion, it’s designed to give a realistic response to rudder deflections, it has an option to hold the rudder at a constant angle, and it has a full info-HUD that displays the boat’s status.
Here’s a chart of boat position for the RM-12 as it does a half- turn.
click to enlarge
It shows a plot of the X-Y map location at each second for an RM 12 with RWS= 15kn. The boat begins on a starboard tack with RWA 90°; it then turns through 180° to a new heading of RWA 270°.
The boat initially sails in a straight line with a constant boat speed of 8.6 (green arrow). The skipper then swings the wheel hard over to initiate the turn (indicated by “TILLER” above) and locks the rudder at maximum deflection until the turn is complete.
If you then follow the dots, you’ll see that within a few seconds the boat responds to the rudder deflection by turning into the wind and losing speed. At the point indicated by a red star (*), the sails suddenly flip over, changing the boat to a port tack. That actually happens quite early, when the boat is only beginning the actual turn. It then takes approximately 15 more seconds to complete the 180° course change. By that time the boat speed has dropped to 4.2, half the original. In addition, the arc of the turn moves the new heading approximately 30 m further windward (Red Arrow).
A boat’s initial speed and consequent momentum has a big impact on how wide a turn the boat will carve in the water. This is nicely shown in the figure below.
The green curve to the right shows the sequence of positions each second for a RM-12 as it goes through the half turn powered by a 15 kn wind. The dark red curve shows the same boat, but this time powered by an 8.0 kn breeze. Under the lessened wind, the boat travels more slowly (the dots are closer together), and the boat cuts a much sharper turn.
The fact that the slower boat has the sharpest turn deserves an extra comment or two.
First of all, if a boat is not moving through the water, the rudder is useless and the boat can’t turn. The force that causes the boat to change direction is generated by the deflection of the water flow passing the angled rudder. This point is sort of obvious, but it’s worth mentioning since most SL builders add a small ‘kick’ to their boats so skippers can still maneuver them even without sail power or headway. The RM-12 and a few other SL boats are more realistic, and you’ll find the RM-12 won’t turn unless it’s moving.
Second point: Once a boat is underway, the turning force produced goes up with the square of the water velocity over the rudder. In other words, the turning force at 4kn is four times the force at 2kn; the faster boat turns more quickly. However, if you look at the figure above, it shows that the faster boat actually cuts a wider turn. That’s due in large part to the greater residual forward momentum at faster speeds. The turning force may be stronger, but it has more work to do to reverse the boat direction.
A Few Comparisons
Why is any of this worth worrying about? Well actually it’s not worth worrying about, but it is interesting when you start to compare the turning ability of different boats powered by the same wind speed.
Here’s the same chart I posted above, showing the RM-12 at 15 kn (green) and 8.0 kn (purple). I’ve now superimposed two more curves. The orange curve is for the popular Melges-24 racer.
Although it’s quite a speedy boat, the M-24 can cut one of the sharpest turns of the entire sailboat fleet in SL as shown above in orange.
So if you love the maneuverability of the Melges-24, the chart explains why! The Melges is a spry, high tech, and compact raceboat. It’s designed to slice a turn as sharply as possible and the SL data backs it up!
Now look at the other boat I’ve added to the chart above in red. It’s the Wildwind Open 60, a new, very fast ocean racer that’s slated to replace the JMO-60 very soon. The shape of the turn in the Open 60 falls right on top of the RM-12. However, don’t let that fool you. If you look at the distance between each of the dots in the curve, you’ll see that the Open 60 is moving easily twice as fast as the more traditional and reserved RM-12. At those speeds, the Open 60 gets around race marks pretty well, but you do need to leave a lot of room!
OK, here’s another comparison to the right. This time it’s for three pretty similar ocean racers: the Wildwind Open 60, the Mesh Shop VO 70, and the Mesh Shop OD 65.
The three boats end up with very similar turning properties. In fact, although they are totally different designs coming from boat yards in Japan and Netherlands, the Open 60 has a turning curve that nearly exactly overlaps the VO-70. I think that indicates both builders came up with designs that reflected real life performance, and it looks like they both hit the mark.
The last chart for today is shown below, and I apologize it’s really ugly; it looks like clumps of seaweed, or Lindsay Lohan’s hair after a particularly rough night.
The chart overlaps turn plots from a variety of different boats, to give you a flavor of the diversity in the fleet.
There are some interesting findings. For example, the Mesh Shop OD-65 has the same turning radius as the Trudeau New York 30. However, once again the OD-65 has a much faster boat speed that likely explains the apparent similarity.
Two boats have a surprising overlap on the chart that I can’t easily explain. Motor Loon’s Loonetta 31 is an absolutely delightful, fully appointed cruiser. It’s not intended for competitive sailing, and in fact Loon went out of his way to make it clear the boat was not a racer. Well kids, here’s another reason to love your Loonetta: it turns out to have the same turning radius as the Mesh Shop Laser One, and it does it at the same boat speed!
Anyone up for a Loonetta Regatta?
I admit I’m still not sure what to make of these curves, if anything. For the moment, I think they just provide another way to display some performance characteristics of virtual boats we all sail. There are certainly no “good curves,” or bad ones. The results are just interesting, and maybe some are fun.
A few might even have something to do with sailing in SL.