High-Speed Sailing

Vessel Name: Sailien
Vessel Make/Model: Experimental/custom
Recent Blog Posts
28 September 2013

The America’s Cup

I watched, with interest, the videos of the 34th America’s Cup. At last we have fast sailboats engaged in a competition that is fun to watch. The virtual images (such as course boundaries, distance grid lines, separation between boats, etc.) overlaid on the real images really helps to keep the race [...]

31 August 2013

100 Knots for Hydroptere?

The latest news from Hydroptere is that they have plans for a 100 knot sailboat. This was posted on 26 Aug 2013, so look for that date at this address. http://hydroptere.com/en/the-news/last-news/

17 November 2012

Aptly named Sailrocket, blasts off!

While yet to be ratified, Sailrocket posted an average speed over 500 meters of 59 knots. I will not be surprised if they increase their record into the 60 knot range during this record attempt.

02 January 2012

More drag for VSR2?

I want to start by pointing out that the whole VSR2 team has done a stellar job and has demonstrated conclusively that the forces that drive a sailboat can be aligned for roll stability without using ballast and without using any down-force. (Trifoiler achieved roll stability by using down-force, but [...]

18 December 2011

My analysis of Sailrocket

I copied a diagram of VSR2 (wing doesn’t show well) and added in the major force arrows that apply. Be aware that these arrows are not correct in terms of scale (length) and some of their locations are guesses, however I believe I’m correct enough for us to learn something about what VSR2 has been [...]

23 October 2011

60 is within reach, what’s next?

I have been watching Sailrocket’s progress with great interest and there’s no question they have a winner. I fully expect to see them reach 60kt in the near future. Sailrocket has now demonstrated what I first learned with my models and again with my full-sized prototypes, that if you get the forces [...]

More observations on Sailrocket

09 December 2008
First, there is a good news item on why the IASF decided to grant kite boards the outright sailing speed record at the following address; be sure to check it out.
http://www.sailing.org/26447.php

I want to add some more observations about Sailrocket. If we go back to Paul's blog of Sun, 11/02/2008 - 13:51 and look at the photos, we notice the nose of the craft has come off the surface in the first photo. In the second photo note that the lee pod is quite high and the craft is rolled to windward a few degrees. Paul also writes that when he decided to abandon the run, the pod was "way high". With the set-up Sailrocket is running there are two ways to lift the lee pod like that, one is with the airfoil on the cross arm and the other is by the inclination of the wing (airfoil).

Be aware that I have no information about Sailrocket's set-up, except for that which is posted on their website. I do not know the exact positioning of the wing relative to the hydrofoil but the only correct positioning is for that airfoil and the hydrofoil to be parallel and also positioned such that if a line were connected from the center of effort of the airfoil to the center of effort of the hydrofoil, that line would intersect both those foils at an angle of 90 deg. to the length of the foils. Any other positioning will be unstable, although if it's close, the instability will only show up at high power levels.

If the airfoil is tilted further to windward (no longer parallel to hydrofoil and no longer 90 deg. to "connecting" line) it will lift the lee pod (roll the craft to windward). If it is set with less tilt, it will put down-force on the lee pod and with enough power, lever the windward hull up.

The airfoil on the cross arm is designed to fly the lee pod at speed and has been doing this. I assume that the foils were originally set up correctly (parallel/90 deg.) and that the cross arm (with its outboard flap) lifted the pod. I do not know if this assumption is true or not because I do not know the actual set-up details.

If we look at the hydrofoil, it will be stable at the parallel/90 deg. position described above. If it has less tilt (more straight-up) it will force the hull up, while with more tilt, it will force the hull down. Note that in Paul's post of Sat, 12/06/2008 - 20:53, he says that they think the hydrofoil is flexing more than anticipated. If it flexes, it will be bent to less tilt at the lower end and this will force the nose of the craft up. Adding tilt will compensate for this but only at one power level. The correct solution (time permitting) is to make it stiffer. While flex or too little tilt of the hydrofoil will raise the nose, it will not flip the craft. If the nose rises, the force on the hydrofoil is decreased and it will start to drift, slowing the craft and sliding it down wind. Note that Paul (11/02 post) said the nose lifted and the craft yawed sideways. This is correct; anytime the hydrofoil looses power (by lifting out, ventilation, cavitation, etc) it will drift; this is a stable situation, providing yaw can be handled without being forced into a jibe.

In Paul's post of Sun, 11/30/2008, he says "we have raised the rig around 1.4 degrees..." (to take lift off the nose). But, as I've stated, this will put down-force on the lee pod and tend to lever the nose up. The next big event was described in Paul's post on Wed, 12/03/2008 - 22:40 and included Sailrocket's back-flip. I am quite certain that the sole cause of the flip was the airfoil on the cross-arm, although hydrofoil inclination/flex and airfoil inclination may have contributed to raising the nose a bit initially.

Finally, in Paul's post of Sat, 12/06/2008 - 20:53, we find the following: they plan to "...angle the (hydro)foil a few degrees more so it is pulling the front planing surface down harder..." (this is correct) but they also plan to "...stand the rig up so it is pulling up less." (this is incorrect as I've described above). Paul continues with: "We had already begun moving in this direction and you can see that the leeward pod isn't flying quite as high as it had in some of the previous runs." (I believe the less inclined airfoil forced the lee pod down, although if the outboard flap was brought up a bit, it would have lowered the lee pod too.) They are also considering "...adding lift-spoiling devices on the inboard end of the beam." (This is correct, but should be done to the whole beam as a temporary fix - when time permits the whole cross-beam should be re-examined, since anytime the nose lifts, the angle of attack on the cross-beam increases, potentially launching the craft again.)

By now I'm sure some of you are saying "Bob's nuts, it's obvious that an inclined rig with its upward lift component will provide lift to the craft and by standing it up a bit, that lift will be reduced." My reply is: You can't look at the airfoil's forces separate from the hydrofoil's forces; they are totally dependent on each other. Refer to my earlier posts about "squeezing the soap bar", you can't squeeze a bar of soap with the tip of one finger, nor can you generate any force on your airfoil without the water's resistance. Fly a child's kite, as long as you hold the string, the kite will fly but if you release the string the kite will drift down-wind and fall to the ground. The kite represents Sailrocket's wing, you holding the string, represent Sailrocket's hydrofoil, if we attach the kite's string to a hydrofoil, you've got a sailboat with an inclined rig; this is what Sailrocket is doing, this is what Paravane is doing (and they really do have the hydrofoil on a "string") and this is what I've been doing (since 1985).

I want to add that all of you who are interested in high-speed sailing are an inspiration to me. My readers have posted some great comments, which help me get a bigger picture. And the Vestas SailRocket and Hydroptere teams have been great to follow.

Bob
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