DDW faster than TWS

[Steele]

I'm not sure if this has come up here before, but it is very interesting and hard to wrap you mind around. The video is longer than it needs to be, but I think it is worth a look,
https://www.youtube.com/watch?v=jyQwgBAaBag

[BeauV]

RIck built this thing to answer a question posed by Stan (yup, that "Stan"). They worked together at ETAK and again at SportVision. Rick is an ace windsurfer, sailor, and wonderful mechanical engineer. At the company, each month Stan would pose a question that had nothing whatsoever to do with the company's key purpose or direction. The goal was to get these ferociously talented folks to think about something different, blow the cobwebs out of their brains, and then come back to work refreshed. Rick took it to an entirely different level. There are numerous videos claiming to expose Rick's machine as a fraud, thousands of posts on various board claiming it's a fraud, and it goes on and on even today.

What most folks don't grok is that while the vehicle is going straight downwind, the prop blades are not.

That simple observation matches directly with how we sail fast boats downwind faster than the wind speed; or, said another way our VMG downwind is faster than the measured true windspeed. Our sails, like the blades on Rick's vehicle, are gathering up the wind on a broad reach, the boat goes faster and the apparent wind goes forward, the boat bears off and while planing or foiling it easily achieves a VMG that is greater than the wind speed. The America's Cup cats went about 2.3 times the wind speed on runs. Foiling kiteboards often achieve 3x the true windspeed downwind; with the apparent wind well forward. What most folks don't grasp is that a foil moving through the air captures the energy in all the air it redirects. This swept area of wind is much larger than a spinnaker on a boat sailing directly downwind. The key is swept area, not sail area.

I love that this simple puzzle still gets people to think hard about what's going on.

Now, imagine working for a guy who poses one of these questions each month.....

[Charlie]

A similar thing happens with iceboats, which are always sailing upwind -even when sailing downwind -because the apparent wind is so far forward due to their speed. It’s a weird feeling to push-start an iceboat and suddenly feel it pull away from you as the apparent builds. Windward mark roundings are especially fun, because as you turn down, the apparent suddenly shifts aft and overpowers the now over-trimmed sail, causing the boat to heal onto two runners ( “hike” in iceboat parlance). At that point you can ease the sheet as you turn to counteract the hike, but the faster technique is to turn down harder, using the centripetal force of the turn to counteract the hike. The result is an accelerating turn balanced between the two opposing forces. It’s a really fun experience. Highly recommended if you get the opportunity.

[BeauV]

Charlie,

The desert sand sailors have exactly the same characteristic. However, limits on the traction of the rubber tires on the sand, dirt, or salt can result in some pretty crazy maneuvers. The front-wheel steers and when it starts to go sideways across the surface it's time to prepare for a crash.

When the foiling AC boats were in San Francisco we got to watch as some of the best sailors in the world tried to work this out for windward mark rounding in strong winds. It seemed to mostly work. What seems pretty clear is that a foiling sailboat has a real problem when you take everything you've described above and then add a vertical component caused by chop for smaller boats and swell for larger ones. The French are finding that (so far) they need to drag the transom in the water to keep the offshore boats stable enough to drive. I think they'll figure out the 3-D problem soon and we'll have these monster water-bug-like boats flying around the globe. Of course, they also need to figure out how to get all those foils out of the water for light wind sailing. It is astounding how slow these foiling boats are when there isn't enough wind to foil.

[Charlie]

I’ve never tried a landsailer, but I know some of the Class C Skeeter guys rig their boats to “go both ways”. Skidding in sand must be freaky.

As for foilers in displacement mode, the shortcomings were obvious in the last AC. Falling off the foils, or failing to get on them while the other boat did, often resulted in an unrecoverable deficit. Speed deltas could be 20 knots+.

It will be interesting to see the development in the next few years - for all types of boats, not just the AC.

[BeauV]

Charlie,

I can hardly wait for powerboats that foil well. There are two uses that would really work well for me. First, if we can get foiling powerboats that can stand a 10' chop, then I could simply drive up the coast to SF in my foiling powerboat to meetings, rather than sitting in traffic for 2+ hours. Yes. I know it wouldn't be environmentally terrible compared to the Tesla, but I'd rather drive a boat. Second. I could drive to Redwood City and hop in a foiling powerboat to cover the last 40 miles of the trip. That would save a lot of time.

[kimbottles]

Makes complete sense to me because I have spent the last 6.5 years watching FRANKIE pull the wind WAY forward.
The B&G wind instruments show we are close reaching to the apparent wind angle while the true wind angle is DDW.

[Tucky]

I followed the original DDWFTTW thread on SA, and have one of the little carts for a treadmill that were sold as part of that- I never did the work to get it running but loved the thread- reminded me of the Lets make a Deal thread on switching when the host opens a door- fun to see people get awfully invested in their truth:-)

[Jamie]

I’ve never tried a landsailer, but I know some of the Class C Skeeter guys rig their boats to “go both ways”. Skidding in sand must be freaky.

As for foilers in displacement mode, the shortcomings were obvious in the last AC. Falling off the foils, or failing to get on them while the other boat did, often resulted in an unrecoverable deficit. Speed deltas could be 20 knots+.

It will be interesting to see the development in the next few years - for all types of boats, not just the AC.

It reminded me of racing windsurfers in slogging conditions. Puff roulette and trying to go upwind by going down wind first.

[avramd]

I ordered one of these little wind-powered cars on Amazon after learning about DDWFTTW:

wind-car.jpg

It is nearly impossible to use b/c it needs an extremely smooth surface or it capsizes on the bumps of 1/8" irregularities, and it must be used at some elevation b/c due to wind-sheer there is typically no wind in the bottom 8" above a parking lot surface.

However, once you find the right conditions, it is shocking how fast it goes. In just a few seconds it seems poised to destroy itself.

Sadly, the Amazon page for it is gone, I could only see the attached thumbnail of it in my order history.

[kdh]

To my simple mind I just think of it as translating up-wind-style lift into propulsion down wind.

[BeauV]

To my simple mind I just think of it as translating up-wind-style lift into propulsion down wind.

Yup, that's exactly right. You'd be amazed how many folks don't understand this.