High-Speed Sailing

I'd like to thank Walter (Wiebel Sailing) for a tip about some videos on You Tube showing the model Monofoil sailing. Go to You Tube and enter "Monofoil" in the search box and take a look. While these videos were posted only a couple of months ago, I suspect they come from a few years back (their website hasn't been updated for some time).

We can learn some interesting things from the video clips. The clips show both starboard and port tacks, and I like that because they're working toward a practical craft. Also shown is the craft in planing mode and in "flying" mode, so we can see its performance potential. There is, however, an extremely interesting clip of the model flipping; we can learn a lot from that. This clip is titled "Monofoil, wind gradient crash" and I'll explain why the craft flipped a bit differently than the text which starts with: "A model testing problem" (be sure to click the button to get the complete text).

Here we go: the explanation given for the model's flip is that as it rose up into "flying" mode, the stronger wind at the higher elevation generated more force, causing the flip. The explanation is correct, but it is looking at the problem from the wrong direction and it will get you in trouble. Note that the explanation also states that the force increase is more significant for a model than for a full scale boat. While that may be true, it still shows that there will be a stability problem with a full size craft.

I'm going to be referring to the diagrams on Monofoil's website (use my link). On their site, go to the page "Design", which has the diagrams. (Either work off your monitor or print a copy of the diagrams.) Extend the "lift" arrow and the "foil reaction" arrow back toward each other until they intersect. (The edge of a piece of paper works great against your monitor's image.) At the point of intersection (just below the airfoil) is where the resultant force will act. Represent this force with a small arrow, which points up and left toward the wing tip. This resultant force is the force that lifts the hull of the craft out of the water into "flying" mode; it has to be greater than the weight of the craft to do so. However, while the weight of the craft does not change when sailing, the wind force is constantly changing, therefore the lift force, the foil reaction force and the resultant force are also constantly changing. This is an unstable condition that the crew must constantly correct for (in strong winds) or the craft will flip, as we see in the video. Note that this is essentially the same as Sailien force diagram #2 (post of 12 Dec 08).

On Sailien, I align the "lift" force and the "foil reaction" force, using them only to drive the boat, and I use hydrofoils or planing devices to lift the craft reducing the water's drag. Since the craft always remains stable, it can take "any" amount of wind force (within the limits of structural strength and sea state).

Bob