I'd like to learn more about the advantages and disadvantages of uni vs sloop rigs. F16s use both, and one design will be uni only. In talking to Mark Talla, he says his Stealth sails great without the jib and he often just leaves it off. Obviously, the Taipan must work well uni/cat rigged; otherwise, I doubt A-cat world champs would fiddle with it. And now the Jav 16 is forthcoming as a uni only design. Sloops with big jibs are great for reaching, but much of cat racing is on windward-leeward course, so reaching speed alone is not that important. We see the F18HT class emerge as a uni class, yet the vast majority of all sailboats are sloops. (In keel boats, I can only think of the Nonsuch monohulls as cat rigs.) So...

Which rig is most efficient upwind, considering VMG?

Which rig is most efficient downwind?

If you had 2 rigs of equal total sail area, which would be more advantageous?

I realize there are a lot of variables here, like mast height, heeling moment, high vs low aspect sails, etc. But surely some science has been done on this. Yes, I've read Bethwaithe. He only really talks about sloop rigs, and I didn't really understand everything anyway. So I'd like to learn here. Not about boat handling--sloops definitely have a lot of advantages there--I'm interested in speed and efficiency.

Thanks,

This is one question up my ally.

I will give answering it my best shot before I begin I would like to underline one very important thing. More efficient does not always equal being faste. For one great story on this please read the Miss Nylex C-class article at :

http://wwwstaff.murdoch.edu.au/~tlamont/miss_nylex/page2.htm

It is more efficient in rig design and sorts but it still lost to US build Aquarius because Miss Nylex was more efficient in the wrong set of condtions, the races turned out to be in light winds. But also very much because going conservative allowed Aquarius to be some 90 kg's lighter. And that is a weight advantage that must first be corrected out over before and gain is made, no matter how more efficient the new technology was.


>> Sloops with big jibs are great for reaching,

And therefor great for distance races which often have significant portions of (shy) reaching and in them.

>>but much of cat racing is on windward-leeward course, so reaching speed alone is not that important.

Just like the designers of miss nylex discovered. Mind you guys there is a limit to plaining craft like the ventilo here. These may have great top speeds but how will that translate into "round the bouys" speed ?


>>Which rig is most efficient upwind, considering VMG?


And this is the difficult question. Okay lets say that the design with the highest VMG upwind is more efficient. Can we answer your question now ? No, we can't. Why, because Efficiency times the amount over which the efficiency is taken is the determining factor. Lets look at a practical example:

Say a uni-rig has a sail area of 20 sq.mtr. and is able to fly that without depowering. This system has, lets say, an efficiency of 95 %. Ergo is effectively has an active area of 19 sq. mtr.

Now the sloop. The sloop has a main of say 18 sq. mtr. with an efficiency of 85 % = effective area of 15,3. Noticably less than the uni rig. But because it's heeling moment is alot less as a result of this shorter rig with less mainsail area, this boat can fit a noticably large jib before it arrives at the same heeling moment as the uni-rig. Lest say 5 sq.mtr. with the efficiency of 75 %. This will lead to an effective jib area of 5 * 0,75 = 3,75. With this the total effective area becomes 15,3 + 4,25 = 19,05 sq. mtr. effective area = more than the 19 sq. mtr. of the uni rig. So despite the fact that both the main and jib have much worse efficiency the sloop can expect to be just as fast or faster depending based on these named %'s

But ofcourse when the jib has an efficiency of 70 % than the sloop has just less total effecive area tha the uni-rig.

So one can not simply say that one rig is better than another, it dependents on the way the other rig is designed and even then because the example above is still too simple. Because it is a well know fact that sloops may point a little lower than unis upwind but also that they broad reach (downwind) alot better. Depending on the individual design the one outweights the other and visa versa. The spi goes a long way of negating this.


>>Which rig is most efficient downwind?

Without a spi the sloop is faster downwind, not more efficient.


>>If you had 2 rigs of equal total sail area, which would be more advantageous?


And here you get to the core of the problem. When two rigs are of equal size than the advantage moves a significant way towards the uni-rig setup. When looking at upwind -downwind courses. With increasing shy reaches and shy broad reaches the advantage shifts back to sloops again. Mind you a windshift on a windward-downwind course could be enough to shift the balance significantly.

But even this question is more complex than it seems. Which one of the two will determine what the "equal area" is ? The Tornado features 18 + 5 sq.mtr. of sailarea = 23 sq.mtr. of area. What will happen if you fit that area to a uni-rig. In order to get a significant aspect ratio which is needed for a high efficiency % the masts needs to be tall. Some 10,5 to 11 mtr.. It was said in the F18HT class tha F18HT's went down to 10 mtr. mast because 10,5 mtr. wasn't faster. Overpowered? Or will the most efficient area of a uni-rig be used to limit the sloop area, in this situation is actually holding back the sloop as that craft will than be underpowered. Neither of this hypothetical situations is fair to one of the two rigs and therefor such a comparison is usefull to extend understanding but is not a practical methode of comparing rigs.

In effect, the F16's can never feature a 18,5 sq. mtr. uni rig and be faster, the platform won't be able to harnass the power. But the 18,5 sq.mtr in the sloop configuration does make the craft alot faster than the jibless F16 setup with a 14,85 sq.mtr. main even though overall efficiency may be less in that setup. So more efficient but slower and less efficient but faster. It's a strange world and than we aren't even discussing the dynamics of different windstrenghts.

And ofcourse we are talking aerodynamic efficiency here. And than there is that other thing about aerodynamic efficiency. Surely high lift to drag ratios at small angle of attacks is important upwind. So there the uni rig is aerodynamic more efficient than a sloop. But what about non-upwind courses where the angle of attack is often alot greater and the drag of rig has alot less impact on the overall drag of a craft ? Typically sloop rig can operate efficiently over a wider span of angles of attack. This makes sloop rigs more aerodynamic efficient on non-upwind courses.

And there is the boat speed devided by windspeed ratio. Below a certain ratio large angles of attack are encountered and sloops are aerodynamic more efficient even when pointing high (mono's) and above a certain ratio only uni rigs are efficient enough to achieve these speeds (ice-yachts). Where is this ratio and on what side of that ration do catamarans as we know them lie ?

Some have argued that because ice-yachts with speeds typically 3 times the windspeed use uni-rigs that it means that catamarans with typically speeds of 1 to 1,5 times the windspeed are better off with a uni-rig too.

I say this difference between 3 and 1-1.5 times the windspeed is the same as the speed difference between WW2 prop aircraft and supersonic jet figters. Clearly a big stretch to base comparisons on, lets not forget that those WW2 props were very efficient in their operational conditions !


>>I realize there are a lot of variables here, like mast height, heeling moment, high vs low aspect sails, etc. But surely some science has been done on this.

Like I said read the Miss Nylex article and notice how it failed to defend the little america's cup title.

And please notice that a regatta is won by having the best average results over multiple races under different conditions. Surely Miss Nylex is a better design in the top windspeed ranges and Aquarius is better in the ligter conditions but this doesn't give an outright advantage to either of them. It is totally dependent on the conditions and even the spread of conditions of who wins the regatta.

Another example (analogue this time) : what if a foiling craft is alot faster in winds over 12 knots but slow under those 12 knots. it will win all 12 knots and over races with a non foiling craft coming second. Now the first race below 12 knots the foiling craft will bump out and come in say 10th while number 2 of the other races comes in 1st. The chances of the foiling craft of winning the regatta are now pretty much nullified.

So next to being fast and efficient as good regatta boat has one other requirement and that is to minimize its maximum speed difference to the average of the fleet. A craft can be ver fast but if that is achieved by sacrificizing alot of speed under a different of conditions then it will still not be a winning design. In regatta's often the craft that does best on average without being exceptional end ups high on the list or on the first spot.

>>Yes, I've read Bethwaithe. He only really talks about sloop rigs, and I didn't really understand everything anyway.


To bad because that book is really something. And you should read it again and again and again. As often it's nuances are only understood after several times of reading and meditating. It is all there but it is indeed an excersize of letting go of preconceptions and analysing the data in teh book.


>>So I'd like to learn here. Not about boat handling--sloops definitely have a lot of advantages there--I'm interested in speed and efficiency.

I hope I've helped you in this respect by this post.

Regards,

Wouter

Eric

You've asked a hard question.

Firstly the boats you mention, Stealth, Taipan and Bim are not so diferent, They all sail as uni rigs with the same rated sail area, the Taipan and Stealth have 8.5 m masts with about 15 sq m of sail and the Bim has a 9 m mast with about 14 sq m of sail, both rigs will produce about the same power and will perform about the same.

The difference is that the Taipan and Stealth have jibs in addition and as such will produce more power and be faster in all conditions.

Next to comparing speeds ofunis to sloops with the same sail area.

The dificulty is that all cat rigs are a compromise, you have to use the same sails in 5 and 25 knots, look at a swing wing fighter plane, at high speed it need short fat wings to go fast but at low landing speeds it sweeps its wings out or it can't fly, likewise at high wind speed a short fat rig is faster and at low wind speeds a tall skinny rig is faster.

Lets take a fully compliant F16 with 15.25 sq m main 8.5 m mast and 3.5 sq m jib, 18.75 sq m in all, just over 6% less than an 18HT sail, so we'll put it on 9.4 m mast, that is a big sail for a 16 ft boat! We'll put the f16 17.5 sq m spinnaker on.

How will it perform.

Light winds <8 knots

The uni will blow away the sloop upwind, downwind it will be a little faster.

Light- med wind 8 - 12 knots

As the wind increases the sloops speed becomes closer to the uni, by the time the sloop is twin trapezing (9-10 knots) it will be slightly faster upwind, downwind the uni will be slightly faster although as the wind increases the speed will get closer.

Med winds 12 - 16 knots

From here on the sloop is going faster and faster upwind compared to the uni, at 16 knots it will blow away the uni, by now the downwind speed will be pretty much the same although if its choppy the sloop may well be quicker, at some point around this wind speed the taller unirig with a much greater pitchpoling moment will have to start backing off downwind and then the sloop will become quicker.

Med - strong winds and above

The sloop is quicker all round, the stronger the wind, the bigger the difference.

Efficency

The thing here is that the uni rig produces more power, the problem is that this power is generated half way up the mast, and most of the power is pushing sideways with only some of it pushing forwards.

So once you get to the point where the power is more than the crew can hold upright (either capsising sideways or pitchpoling forwards) then you have to de-power the rig, either by flattening the sail or by running deeper, either way you are not usng all the power available so the efficiency is going down.

So the unirig is more efficint at producing power, but produces it in a way that less of it can be harnessed.


Hope this helps

john

Thanks John and W.

I understand you're saying. To get a uni rig with the same power of a sloop, the rig would be too tall to control in all but light air. Here's my next question:

Why is a uni rig more efficient upwind? Why does it point better? With a sloop, the jib helps attach air to the main better and you can sail upwind travelled all the way in. But with a uni rig, seems like I've got to travel out a bit to get the leeward tells to flow.

Eric,

As I understand it the jib bends the wind around and 'funnels' the wind across the back of the main but it also has the effect of causing the main to see a header. So that the direction of the wind as it meets the main is headed somewhat on a sloop but of course on a cat rig the main sees the wind without the header caused by the jib. This has some effect on the pointing ability, I suspect, of a sloop v a cat.

Rob.

Eric

The simple answer here is that one is not more efficient or points better than the other. Its the height of the rig whether its uni or a sloop, that makes the difference.

Some of the reasons for this are:

1.) Most of the power is generated at the front of the sail so longer sails produce more power, this is the same for jibs and spinnakers.

2.) Taller rigs have more sail area higher up where there is more wind.

3.) Because of the wind shear in point 2 the apparant wind at th top of the taller rig is offset so the sail is not so hard on the wind.

Its the height of the rig that generates the extra power not the lack of jib. If you had a uni rig with the same height mast and sail area as a sloop but just with a longer boom, there would be no advantage to the uni.

Most sloops have shorter rigs than most unis and this is the main difference.

The reason that unis travel out more than sloops is because they get overpowered much quicker because of the taller masts and the only way to get the mainsheet tension on is to downhaul very hard and travel out, but if the sloop had a mast of the same height you would have to do the same.
(see the thread about I17 and pointing.

As Wouter said its a very complex question without a simple answer I'm afraid.

Hope this helps a bit

The way I see it, the uni can point higher because the main can be sheeted to centreline of the boat, whereas a sloop has a jib which can only be sheeted off centreline, otherwise it would stall the main.
The fact that the jibs angle of attack is different to that of the main limits your ability to point. If you move your jib fairleads inboard (without closing the slot) you should be able to point higher on a sloop.
Then again, I`m no Hydro-aero-cosmo-nologist, so don`t quote my theory in your book.

As are the others.

The jib will produce a header with respect to the main. This also mains that a sloop rig as a combon can operate to larger angles of attack of the apparent relative to centreline of the boat. The maximum angle of attack of a uni rig is smaller and for a given course this means that the uni main needs to be travelled out more.

But some may argue that due to this smaller maximum angle of attack that unis can point higher and have a better VMG. Sadly even this situation is more complex than it seems. Yes a uni rig can point higher, but this will not always translate itself in a higher VMG. Why ? Because to achieve a certain speed the rig needs to harvest a certain amount of energy; the amount that can be harvested is also influenced by the angle of attack. Less angle => less energy harvested with the same rig. Also this proces is a battle between positive and negative effects. In the beginning pointing will contribute more to VMG than other factors take away but after a certain point the effect is reversed.

But what if boat boats stay on the same course ? Than a unir rig can have another advantage that many over look. Without the jib the uni -rig can operate a smaller angles of attack. For high performance boats this is the last barrier which needs to be overcome for truelly high speeds. Or rather high boat speeds devided by windspeed ratio's. Something this effect is dramatic enough that a uni-rig on the same course as a sloop by virtue of higher top speed relative to windspeed can have a higher VMG than the same boat on a higher course than a sloop.

Sadly this "go faster because your rig can still operate at smaller angles of attack" is mostly influenced by the need to supply a absolute amount of power and this is closely linked to the overall drag of the craft which on a upwind course is mainly determined by hull drag and centre board drag.

So by making a boat alot lighter one reduces wave making drag but more importantly wetted surface friction drag. This in turn reduces the required sail force which will reduce the amount of force excerted on the boards which will lower the centreboards induced drag which results in even lower requirement of rig power.

This reduced rig power allows a crew to sail with less camber in their sails and have a rig that operates efficiently at smaller angles of attack ergo higher boatspeed to windspeed ratio's.


But for now the cats are still in the region of speed ratios where the angle of attacks are still large enough to have sloop rigs and not be limited in top speed to much. Only after designs with even less drag are designed will the smaller angle of attack benefit of uni-rigs really kick in. That is as long as these rigs can be harnest by the platform which till now is the more limiting factor.

Ofcourse this explains why Formula boats are often not very good reachers as their jibs and mains are cut to operate efficiently going upwind at small angles of attack. By thsi the sacrifize the big angle of attack efficiency which are found on reaches. On broad reaches this disadvantage is offset by flying a spinnaker.

And ofcourse this spi needs to become flatter with increasing downwind speeds which is of course the new trend in designing.

And so on and so on.

Wouter

Wouter-
I understand the "A" cat Flyers advantage upwind is due to their ability to make a better VMG by "footing" more than the "other" designs and so they must be sailed differently than the "conventional" (ie non-wavepiercing) designs. This is even given the same rigs. Could you comment on why this might be the case (only reasons I could come up with is that other boats when trying to foot "lose" too much to leeward due to wave drag/resistance- ie trying to plow over rather than through waves at an increased angle and decreased windage of the Flyer bows).
What do you think?

Kirt