Why does a cutter rig point higher & sail faster?

Thanks for that Brian, I've only got Arvel's stuff on hard copy.

Mr Hugenot, the url Brian gave provides the perfect answer for the reason that, as you put it, rules committees no longer know about the slot effect and acceleration; it's because the accelerated flow doesn't exist. Criticising committees for not knowing about accelerated flow through the slot is like dissing them because they don't believe in the Easter Bunny.

The knowledge that you claim generations of sailors to have had is incorrect. The flow is SLOWED between the sails, as proven by modern aerodynamic testing.

 

Here's another reference to Arvel Gentry's work that I had put on my website, but its a little more difficult to understand http://www.runningtideyachts.com/articles/SailScience1.html

A better explaination of sail aerodynamics can be found here by Paul Bogataj http://www.northsailsod.com/articles/article6.html It is well worth the little inconvience of registering for the PDF file. In fact I would suggest one download it and make a copy to read.

I can't totally fault Alan Hugenot. While he mixed the 'succeeding sails' explaination up , I've actually made that mistake myself when I was writting a posting ...sort of reversed the order. The point is I do not think many modern sailors realize the correct explanation for the slot effect, nor its implications in making a very powerful sail out of the headsail(s)...considerable more than the mainsail. And I still believe that the sloop configuration with its uneven (triangular) slot between the headsail and the mainsail has some considerable improvements that might be made.

And cat rigged boats do not outpoint a headsailed boat. Ask Mr Hall at Hall spars. And I've seen what a mizzen staysail can do for a ketch rig on a Morgan OutIsland.

 

"They ran out of time. Steam powered boats were begining to appear on the ocean. Hi-tech of the day did them in. What is the speed of the Tall Ships in the present day?"

damn those modern 'canting keel" raceyachts that run diesels to tack etc, can we regulate them to STEAM, may help prove a point that there NOT sailing anymore !!!!!

 

> And cat rigged boats do not outpoint a headsailed boat. Ask Mr Hall at Hall spars.

I should have qualified that, I think. I certainly don't mean that if you simply rip the headsail off a sloop it points better!

It's possible that something like this is true: When all design stops are pulled out, and you're building a sail-powered devices for no purpose except to point as high as possible, you'll do better with 1 sail than with more than 1.

But before I get much further, let me confirm whether or not it's TRUE that windsurfers do in fact point higher than just about anything? I have certainly heard this to be true, but I'm no windsurfer, and I've not paid much attention to them.

 

multiple head sails

Dreamer:

Cutters are great preformers. That multiple slot allows you to ghost along in very light airs. A schooner, which can rig numerous slots can actually move along at 50% faster than a sloop in light airs. For example, picture two vessels with a waterline of 45 feet, one is a sloop and one is a schooner. If the schooner skipper is experienced and well versed in apparent wind, sail dynamics and proper sail trim, he can get his schooner up to hull speed on a reach in a 10 knot breeze. That means that he will be bowling along at 8 to 9 knots, while a perfectly trimmed sloop in the same breeze will be unable to develop enough horsepower to keep above 6 knots on the same reach. That three knots is a 50% increase.

That schooner will simply be using the multiplying effect of the multiple head sails to achieve sufficient horsepower to push along at hull speed. Similarly, your cutter will experience about a 25 to 30% better efficiency than a sloop when reaching along in light airs.

Unfortunately, this advantage disappears as the wind reaches 20 knots when all the hulls can acheive maximum horsepower, even without the slot effect.

Again, the sloop sailors only like to race in strong breezes and tend to call the race if the wind does not reach eight knots. But, you will find that your cutter will kick along at 3 knots in a 4 knot air.

I operated a traditional gaff ketch with a 45 ft waterline for many years and waxed numerous sloops in the light morning breezes.

But be careful, science has not figured out how the slot effect works. The results are undeniable but former theories of why it works have been disproved in controlled experiments. Many sailors believe that this also disproves the results. Suffice it to say that the multiple slots have the effect of moving boats faster, but we still can not explain exactly why in terms of physics.

You might want to get the book entitled "Auxiliary Sail Vessel Operations" by G. Andy Chase c. 1997 Cornell Maritime Press, which treats this subject fairly clearly in chapter 3 and 4. However, he does not make any comparisons of cutters to sloops. His treatment of improving apparent wind will give you a good feel for the forces involved in the discussion.

Alan

 

You are correct that the speeding up of the air flow in the slot has been proven to be an incorrect explanation for why the slot effect improves boat speed. Because the air in fact has been found to not speed up.

However, your conclusion that this then disproves the theory that increased slots improve boat speed, is a trifle hasty.

Although, the application of Venturi and Bernouli to sails may have been disproven. The fact remains that we still have lift occurring with all sails and foils, and no way to explain it witout Venturi and Bernouli.

Just because we can no longer explain it adequately, does not mean that it went away.

But, It still happens.

The slot effect is an empircally observed phenomena that has been measured and proven to actually occur. The result is that multiple slots do improve boat speed. Unfortunately, at this juncture we just don't know why.

It is up to physics to figure out why it happens. And while it may feel smug to believe that it does not happen, merely because the latest attempt at explaining it has failed. Such a course ignores proven truth.

With regard to cat rigs, windsurfers etc. and who points the highest. Again let's not get to hasty. If you want to compare the results of different sail rigs, you must compare them on similar hulls. The underbody hydrodynamics control how the "hull" responds, and how high it can point. A windsurfer's board is a very different animal under water that a spade keeled sloop or a full keeled cutter.

Also A windsurfer may point higher with a single sail, but many of these are not cat rigged, but are instead lateen rigged. If the sail overhangs forward of the mast, there will be an entirely different aerodynamic at play. Arab Dhows use lateen sails with no headsails. Again you need to compare apples to oranges.

That ignorant fool in SF

 

Efficiency on a reach has not been the issue for this thread. The reason a cutter is more efficient and can be faster on a reach is glaring and obvious and does not have anything to do with the mystical slot effect that defies physics explanation.

 

I honestly don't know how to begin. So how about I use my boat as a real-life example of a sloop and how your claims don't seem to match real-life.

I have a 45' boat. It's a fractional rigged sloop, and a racer/cruiser at that. In 10 true I see a velocity of 8.7 when the true wind angle is 90.

So I say that to your claim that sloops can't reach their hull speed in 10 knots of breeze is a bit off.

A sloop's inability to move in light air? Tosh. For my boat we consistently see a velocity equivalent to the windspeed or faster in true winds under 8 knots. In 4 knots of breeze we do 5.5-6 knots. So by your math that would be a 75%-100% improvement in efficiency by reducing the number of sails.

 

Generations of sailors knew about the improved performance caused by multiple slots between multiple head sails.

Physicists, tried to use Venturi and Bernouli to explain what was happening. But the physicists explanation has been proven wrong.

Just because their theory of WHY was incorrect, did not get rid of the fact that the slots result in improved boat speed.

None of the sailors knew anything about increased speed of air flow in the slot. Nore did they care about it. They just ovserved emprically that the boat went faster with more head sails.

What they observed can still be observed.

Unfortunately, most racing rules committees are ignorant of this, and continue not to incorporate it in their planning.

I have only been stating facts, not theories.

Alan

 

Your "facts" are clearly not facts as demonstrated by the previous post by Tripp Gal. Interesting that you now call upon generations past. Generations of observers of the heavens swore by everything they knew and truly believed that the Earth was the center of the universe. They and their empirical observations were wrong. Slots have nothing to do with horsepower for cutters. No one has observed the "slot effect" in action. It was a model used to explain something because people couldn't explain why two sails were better than one. Since those days we have advanced and we know quite a bit about how two sails and more interact. Several references has been given here already. There are volumes more. Physics and aerodynamics offers some very specific observed, calculated and tested systems that show the effects in play. Bernoulli has its place and the "slot effect" just like the Bohr model in chemistry has a place in building a simple model to explain to laypersons so that they can have a simple if not clinical understanding of what is happening. Please don't claim "facts" based on flawed information it puts you into the mighetto category. Although it may be fun to see which one of you would win the arguement about whether a cutter or the Mac26 is faster.

 

Best thing I've ever seen is A.M.O Smith's 1975 Wright Brother's Lecture, "High Lift Aerodynamics" (Journal of Aircraft, Vol. 12, No. 6, June 1975, pp 501-530). Smith was Chief Aerodynamics Engineer for Research at Douglas Aircraft and a Fellow of the AIAA.

Smith lists 5 effects of properly designed aerodynamic slots:
"1) Slat efect - in the vicinity of the leading edge of a downstream element, the velocities due to circulation on a forward element, for example a slat, run counter to the velocities on the downstream element and so reduce pressure peaks on the downstream element.
"2) Circulation effect - in turn, the downstream element causes the trailing edge of the adacent upstream element to be in a region of high velocity that is inclined to the mean line at the rear of the forward element. Such flow inclination induces considerably greater circulation on the forward element.
"3) Dumping effect - because the trailng edge of a forward element is in a region of velocity appreciably higher than freestream, the boundary layer "dumps" at a high velocity. The higher discharge velocity relieves the pressure rise impressed on the boundary layer, thus alleviating separation problems or permitting increased lift.
"4) Off-the-surface pressure recovery - the boundary layer from forward elements is dumped at velocities appreciably higher than freestream. The final deceleration to freestream velocity is done in an efficient manner. The deceleration of the wake occurs out of contact with a wall. Such a method is more effective than the best possible deceleration in contact with a wall.
"5) Fresh-boundary-layer effect - each new element starts out with a fresh boundary layer at its leading edge. Thin boundary layers can withstand stronger adverse gradients than thick ones."

Note that #2 is actually the opposite of some of the statements earlier in this thread. A main operates in a header created by the jib, but the jib gets a lift from the main - the net effect is greater than either sail by itself.

#5 is the typical explation for the benefit of slots, but #1 is the opposite of conventional wisdom. The slot does not accelerate the air at the leading edge of the main, it decelerates it compared to what it would be without the jib. And the slot does not really act like a venturi, because there's not a fixed massflow through the slot. As you close down the slot, the air can flow around the leading edge of the jib instead of through the slot, so instead of consistently raising the speed through the slot as the slot is closed down, the speed can actually decrease because of reduced massflow through the slot.

 

Hang on Alan, your first post attempted to tell us that the slot effect works by acclerating flow over sails further aft....now you're trying to tell us no-one knows how it works. Why not stick to one story at a time?

"A windsurfer may point higher with a single sail, but many of these are not cat rigged, but are instead lateen rigged."

What? A lateen rigged windsurfer? Gee, in all these years, racing windsurfers to world championship level, I never saw a lateen rigged windsurfer. Please tell me more.

"If the sail overhangs forward of the mast, there will be an entirely different aerodynamic at play."

Why? References please.

"The slot effect is an empircally observed phenomena that has been measured and proven to actually occur. The result is that multiple slots do improve boat speed. Unfortunately, at this juncture we just don't know why."

So...multiple slots increase boatspeed upwind, yet no grand prix sailors use multiple jibs upwind. So you, Mr Hugenot...you know more about going fast in sailboats than Paul Elvstrom; you must therefore know more than North, Connor, Cayard, Coutts, Scheidt who don't use them.

Gee, I'm honoured to have met you (in hyperspace at least)...the man who knows more than the world's most succesful racers. Wow.

So...we KNOW that sailors like America's Cup winners Connor and Bertrand and Blake were very familiar with twin headsail rigs when reaching, because there are pics of their boats doing so and they have written about it. YET they did not use them upwind; even when there was no penalty at all under rules for doing so.

So what happened? Did Connor, Bertrand, Tabarly, Blake et al suffer a simultaneous brain tumour that robbed them of this knowledge?

It's one hell of a call you make, sir...to say that almost every top racer and designer is wrong....even those like Tabarly who learned to sail on traditional boats, and those (Sean Langman of Xena, Nigel Irens) who still sail traditional boats. Somehow they've all got the same mental block that stops them from using their knowledge.

Well, let's all thank god we have you to tell us all where everyone else has been going wrong.

"Although, the application of Venturi and Bernouli to sails may have been disproven. The fact remains that we still have lift occurring with all sails and foils, and no way to explain it witout Venturi and Bernouli."

Yes there is, try Newtonian physics.

"Again, the sloop sailors only like to race in strong breezes and tend to call the race if the wind does not reach eight knots."

What???? Sorry???? Gee, we're definitely racing in different places. Ah yes, I can remember now...there we were, three days and 450 miles into the Sydney-Hobart and the race committee came by to tell us the race was off because the wind had dropped. NOT!

"If the schooner skipper is experienced and well versed in apparent wind, sail dynamics and proper sail trim, he can get his schooner up to hull speed on a reach in a 10 knot breeze. That means that he will be bowling along at 8 to 9 knots, while a perfectly trimmed sloop in the same breeze will be unable to develop enough horsepower to keep above 6 knots on the same reach. That three knots is a 50% increase."

But schooners rate lower than sloops under IMS and IRC. They rated lower than sloops under RORC and CCA and IOR. So if schooners rated lower AND when 50% faster, they'd win almost every light air race.

Instead, some late schooners like Pen Duick were converted to ketches (and went better). Their skippers went to ketches and then sloops in later boats. Alan, why did Eric Tabarly (a man who learned sailing in a traditional Fife CUTTER and died on it) throw away the schooner rig on Pen Duick III and IV (IIRC) and go to a ketch for his Whitbread maxi? Why did he then move to a sloop rig for his foiler tri? Was Tabarly suffering from some strange brain disease that made him move to a slower rigt? In that case, why did his boats get faster?

Uffa Fox (a man whose first boat was a cat ketch and who sailed and liked multiple headsail rigs for cruising) wrote in 1935 about the way that the J Class were moving from triple headsails, to twin headsails, and towards "the single headsail rig of the future". This was in reference to the Herreshoff J "Whirlwind".

Watson and Nicholson moved to fewer jibs in their succesive re-rigging of the Watson designed Brittania, perhaps the most succesful boat of all time. See the accounts in "The Kings yacht Brittania" and Ian Dear's "Enterprise to Endeavour".

So going for the sloop as the superior upwind performer, we have basically all oft he world's best racing sailors, as well as Watson, Nicholson, Herreshoff, Tabarly and Fox.

On the other side, we have Alan....and probably Mighetto.

Gee, hard choice.

 

It depends enormously on the board and even more on the fin, rig and conditions. I haven't sailed modern FW boards in open waters much although I have sailed against championship level racers on them, and from memory they were pointing slightly lower than good catamarans and definitely lower than dinghies. FW boards rely on their fins and they much reach off and get good planing speed before even thinking of pointing.

Slalom boards point low, waveboards barely point at all.

I regularly race on dinghies against longboards (Olympic Mistral, former Olympic Lechner and original Windsurfer) and vice versa. The Lechner is the only one that can point with a dinghy upwind and then it's competitive with or slightly lower than a Moth, International Canoe or Laser.

The modern windsurfer RIGS are effective upwind due to their extremely flat low drag shape. It's quite strange to be on a slalom or Formula type board and pointing 50-55 degrees or so just on a fin. But the boards are aimed more to point low and go fast.

 

Ballerina Science

Well, are we reaching a consensus that Dr. Hugo is either very misguided or a troll? No offence Dr. Hugo, but I think you're way off base. And that's the NICE way of saying it.

Sail interaction is a very important topic for anything larger than a sailboard or a cat-rigged dinghy, and I would very much like to hear about it. One decision might be whether to continue a serious discussion on this thread, or perhaps pick up the old one again? I'd hate to see Tom's contributions wasted on a food fight.

 

I don't know enough to argue the slot My first thought though is could you imagine a tacking duel upwind with a cutter rig? Maybe they didn't use one in americas cup not because it isn't faster but is simply difficult with which to race. Just a thought