Sailing theory questions

[Mark Prouty]

I understand the Bernoulli effect where the shape of the sail causes wind speeds around the sail to create a low pressure on one side. The sail shape causes a low pressure where the wind has to go faster around the side that is curved. The boat therefore moves from high to low pressure on a close-haul.

I somewhat understand the vector science behind true and apparent wind.
True wind is the wind strength and direction you'd feel if your boat were standing still. Apparent wind is the combination of the true wind and the wind caused by your boat's movement (boat-speed wind). Apparent wind can be faster than real wind causing the boat to sail faster than true wind alone. If I understand it correctly, the longer the dashed line below, the faster the apparent wind.



As the sailboat accelerates on the same point of sail, I will gradually feel more and more apparent wind coming from the front of the boat even though the "true wind" is from my side.

A boat sailing close-hauled toward the wind will feel a much stronger apparent wind than a boat sailing on a broad reach, even though they are in the same true wind.

What this phenomenon indicates is that the apparent wind decreases from close-hauled to run. Yet, I seem to go faster as I head off from a close-haul to a broad reach.

I having trouble understanding the dynamics of it all. My experience seems to contractict the facts here. With more apparent wind close-hauled than on a broad reach, would'nt the fastest point of sail be a close-haul. In addition close-haul has the Bernoulli effect that would work in its favor and add speed.

to rephrase:
Close-haul has true wind + boat speed = apparent wind (because of vector science is faster than true wind alone). My boat will actually sail faster than the wind speed.
In addition, close-hauled has the Bernoulli effect in its favor.

From practical experience, a broad reach with a surfing wave action is the fastest I've sailed in a planing sailboat. I've been able to scream on a broad reach surfing the waves.

Questions:
1. Why isn't it close-haul fastest point of sail?
2. Isn't a broad reach the fasest point of sail?
3. How much can the speed of waves help improve boat speed"
4. What is the fastest combination of wave, Bernoulli and apparent wind?
5. What is the fastest a sailboat has ever gone? What was the boat design?
6. What hull configuration is the fastest?
7. Isn't a planing hull inherently faster than a displacement hull.
8. Could the theory of having an exceptionally fast sailboat that doubles as a good powerboat ever work?
9. The Mac26X/M can plane; it does under power. Why is it unable to perform this under sail? You would think that combining true wind plus boat speed plus its flat hull and possible wave action in its favor, it would routinely be able to do this with proper sail configuration. I'll bet when it was being designed, this was a goal.

[Don T]

Hello:
First, I am no expert but I will give my opinion.

1. Theoretically, sails at 45* will produce equal wind to boat speed (not considering drag, slip, apparent wind). 50% of the force is lateral and 50% is forward. 30* should be faster but our rigs are inefficient, our ballast will not resist high enough side force to benefit the from the forward force. At 30* we get 75% lateral force and 25% forward. The lateral force heels the mac, drag increases, rig efficiency drops.
If we could get our macs to stay verticle we could go fast. All it would take is removing the swingboard and bolt in a fin keel with 700 lbs of lead at the bottom, recut the sails, lengthen the mast and strengthen the rigging.

2. For us and a lot of other boats, yes, broad reach is faster. Consider ice boats. 2~3 times the wind speed is possible because of all the + factors added to no side slip, good form stability and very little drag.

3. If you are running it is possible to surf the waves. I have seen video of Maxi's surfing above 20 kts. Little boats like ours would probably speed down the wave and nose in at the bottom of the trough.

4,5,6,7 = n/a

8. A dual purpose boat is a compromise. The laws of physics dictate that it will not be able to do both in exceptional form. I would have to say that the Mac is one of the best examples for overall performance in a dual purpose boat. Not to say it won't be improved upon in the future (cats?).
9. See above, the mac cannot get enough force with enough stability to operate in that mode for any length of time. Not to say I wouldn't like to try (with an expendable boat.......not mine)

[Mark Prouty]

Hello:
8. A dual purpose boat is a compromise. The laws of physics dictate that it will not be able to do both in exceptional form. I would have to say that the Mac is one of the best examples for overall performance in a dual purpose boat. Not to say it won't be improved upon in the future (cats?).

All right were talking catamarans. Why hasn't someone developed one? What about this boat for starters. It is more of a tri-hull and looks too lightly built but does this design have any potential as a powersailor? I could see that such a design might provide a better planing ability.




Or is it just a Glastron powerboat with sails?

Too tame Scat -27? (mentioned in previous thread)



LOA - 26.9'
Beam 8.2'

[Mark Prouty]

The record is currently 46.52kts. The record is held by a Trifoiler called yellow Pages. It's all about overcoming drag, and hydrofoils are great at it! After that, speed is almost limitless! Thats why iceboats are capable of triple digit speeds.

This boat went much faster than the true wind speed:
The new mark was set at 46.52 knots (86.52 km/h) in only 19 - 20 knots of wind.

The land lubber in me:

53.57 mph in 22 - 23 mph of wind.

Yellow Pages

[Don T]

Hello:
Actually, don't hydrofoils suffer from the same problems airfoils have as speeds reach a compressive limit? Much like the speed of sound for air foils, hydrofoils experience it at around 75 mph(?). Since water won't compress I'm not sure if it can be "pushed through" like the airfoils can. It is the same thing that stops jet drives from spinning up too fast. At some point the water would rather go around with the impeller instead of passing through.

[Mark Prouty]

Great questions! Let me take a stab at them...

How do you guys spit this stuff out. You're like walking reference manuals.

[Phil Marriott]

Is this our Mr. Therapy? looks like a nice yacht.

http://www.schock34.net


Maybe he (possibly Jeff) can comment....

Then again, looks like there are a few Schock yachts called Schock Therapy....

[Tom Root]

Schock Therapy wrote:

" It is true there are alot of Schock Therapys out there. My boat had that name when I got her, and i've always liked it, so I kept it. Incorporating the builder or model into the name of a boat is kind of a racing tradition."


Well, if that's was the case, I'd have to call my boat....."X adgerated"



But, I still love her, and "Great White" was sorta a tribute to the clorox bottle!

[Mark Prouty]

But, I still love her, and "Great White" was sorta a tribute to the clorox bottle!

"A tribute to the a clorox bottle"

Is that because the most seaworthy object is an empty bottle with the lid screwed on - a clorox bottle. Also, a Great White shark is very seaworthy.

You named your boat because like a clorox bottle or a Great White shark, the the design of your seaworthy Mac has come pretty close to the concept of being totally capable of sailing in off shore or rough water conditions.

[Mark Prouty]

JUST KIDDING!! JUST KIDDING!!

I "X adgerated"

[Tom Root]

Mark.....now that was funny...and loud and clear!

[Dimitri-2000X-Tampa]

Yea, that is a really cool link Steve. Btw, in flying school, I was taught that only 75% of the lift is generated by the Bernouli principle and the other 25% is from the angle of the wing (ie, just deflecting air downwards creates lift). Of course, this may not be as true with a sail since it passes through the air much slower than an airplane wing.

Here's a question about sail trim, I was trying to pay attention to my telltales yesterday when just sailing with the genoa. I usually trim the jibsheet in just until the inside (windward) telltales stop fluttering and straighten out. Yesterday, I noticed that although the outside (leeward) telltales were flying horizontally, the inside telltales are actually pointing somewhat upward. I didn't have enough time to play with it much but now after the fact, I'm wondering what it meant. Maybe sheeted in too tight? I can't remember where the car was but maybe it was too far aft? Maybe I need more downhaul tension?

[Mark Prouty]

....just an observation and something I learned this past year...

The commonly taught theory of aeronautical lift and of Sail Force based on the Bernoulli principle turns out to be all wrong. The fallacy lies in the assumption that the two airstreams separated by the wing/sail have to arrive at the trailing edge/mainsail leech at the same time.

Google on theory of flight or check out

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

What!! The Bernoulli theory is fundamentally wrong! The only part of the theory that is correct is the difference in pressure across the airfoil produces the lift! That it is only the angle of the airfoil that makes much difference! What does this do to my sailing! Based on the part of the theory that is correct, wouldn't you sail as fast with a flat sail as apposed to one with a slight curve; that only sail angle would determine speed on a given point of sail?

I imagine people in the know chuckle now. Ya Ya, tee, tee he' talking that ridiculous "Longer Path" theory, or the "Equal Transit Time" theory what does he know. He believes in the Bernoulli theory.

You don't know how strongly I trusted that theory. I had wondered why a balsa wood toy airplane with flat wings seemed to fly just fine. I was just too much of a wimp to think about it much or ask why. After all, the Bernoulli theory was everywhere; it had to be correct. I explained it to my son and talked about it to who knows how many people.

If you can't trust the Bernoulli theory, what can you trust!! What are you going to tell me next that apparent wind has always just been a figment of my imagination! I'm just not going to trust anything anymore. How can I look my son in the eye and tell him I was wrong - the whole theory was wrong. He'll never trust anything I tell him again. I can see him now as I'm explaining something: "oh yea dad, just like the Bernoulli theory." Life used to be so simple.

[Mark Prouty]

Presuming Ed thanks for your input the sailing vids were awsome!

http://www.mastersofspeed.com/
http://www.skiff.org.nz/movies.htm
http://www.moth.asn.au/pictures.html

Those Skiff videos remind me when I was in the Hoofers Sailing Club here in Madison, WI. I used to sail 470's. We'd get out on that trapeze and really fly!

[Dimitri-2000X-Tampa]

Nice physics lesson Ed, although your reasoning of why a Mac can't plane under sail (based on too little sail area and lower aspect ratio efficiency) seems to imply that if there is more wind, that wouldn't be an issue. I still think if you sailed a completely empty Mac with no ballast in winds of 30+ knots on flat water, you would be planing nicely...these boats are light and flat so they plane at about 9.5 kn. A Melges 24 is a fine racing boat and can plane in much less wind (of course, you can buy 4 Macs for the price of one). An empty Mac probably only weighs a few hundred pounds more but of course also has a much better chance of losing the mast in those kinds of conditions.