sail aerodynamics

I'm studying square rigger sails and how they work. I assume the same theories of low pressure on the leeward side "pulling" the ship along are the same on a square rigger for the various points of sail as the yards are turned amidships (close hauled, close reach, beam reach, broad reach). However, what are the aerodynamics of the square rigger sails when the ship is running before the wind? Thanks beforehand for any help anyone can give.

 

Brian, while I understand the theoretical efficiency of the headsail due to its cleaner leading edge, I'm still puzzled.....if mainsails are so slow, then why do the most efficient boats of all (speed windsurfers, YPE, foiling Moths, C and A Class cats) all use cat rigs?

If jibs are more efficient, why have A Class and C Class stopped using them? Why is a guys like the world's fastest Canoe and C Class sailor so convinced of the speed of cat rigs that he's experimenting with them in Canoes?

Why do fractional rigs generally go faster than masthead rigs when they have been tried together on the same hull (apart from perhaps in light wind areas when the fractional carries a fractional kite?).

 

Stanford University is working on a modern Clipper Ship Rig.

Here is a link to what they have published so far. They only went to AWA = 80, so that does not answer your DDW question.

Like any sail, when DDW the drive from the sail is CD x area x air density x velocity^2. For a flat plate of width = height x 2 (20 x 10), the CD is 1.18 or so.

Clipper Sails

Hope this helps.

 

There is more control over shape with sails set on elastic spars. Better area distribution is also possible, you wont see many square top jibs.

Fractional rigs always win when total sail area is measured. Class after class has ended up with the fractional small jib and large main for course racing.

If the rig can be sized so that CL over about 1.6 is not needed (upwind), there is no reason for a multiple element sail plan. Windsurfer, foiling Moths and Cats all have very high righting moment to sail area ratios. There is little need to control the heeling moment with low aspect ratio rigs, or splitting the area between main and jib.

On single-handed dinghies the added workload of trimming two sails to work properly probably slows the boat more than any extra power from the rig gains. On boats that always sail with the apparent wind forward of the beam wing-sails are a logical choice.

One of the worst sail plan shapes is a triangle. The added efficiency of a clean luff does not make up for the losses from the tip shape.

I don't know that any one of these reasons works for all the boats you mentioned, but they all apply to some degree.

 

The main inefficiency of the mainsail is the turbulence caused by the mast disturbing the laminar flow on the leeward side of the sail in the luff region. This is overcome by the pocket luffs of sailboards and most Moths and rigid wingsail of YPE and C class cats. With the A cat I believe that very high aspect mainsail (almost the identical planform to a high performance glider wing) overcomes any advantages of sloop rig due to sufficient RM to support the greater heeling force.
There is a point when the apparent wind moves behind the beam when the jib starts to become too full and twisted when it is eased (it has no boom). If at this point of sailing the area in the jib was transferred to mainsail the overall efficiency would be greater and result in a L/D increase of the rig as a whole.
The situation with fractional vs masthead rigs is more complex than it appears on the surface as usually many other variables are also present in the comparison. For starters only, most mastheaders have a simple triangular mainsail often with short battens and a non-flexing mast.

 

Before the Wind

Thank you for the reply and the help. You see, I have been pondering and thinking of the aerodynamic wind principles that the triangular cut sail is based on. High pressure areas, low pressure etc., but......, when the square rigger sails on a ship are set to run before the wind (basically a somewhat flat shape with the coming from the stern), isn't this more of a rearward force pushing on an object and moving it and the aerodynamic wing principles of somewhat null? Am I off course here?

 

Frosh and Rhough, I agree with you and actually I'm up on that stuff - I just wonder what Brian, the man who is most involved with pushing the mast-aft rig, has to say about all the good reasons you have given.

 

in Power Mode

Sorry CT, I've just not had any time to respond at this time. I would have to go back and find a number of other references to help substaniate my claims. I will eventually. Meantime a few others are taking up the argument.

Besides my real income, I'm working on a new RIB concept, and the water-jet propulsion, and rim-drive propulsion that goes along with it, and the fishing design. Guess you might say I'm in the 'power mode' at this time.

Real short reference on uni-rig capabilities....I would reference you back to #3 posting in this subject thread:

 

I think you have it. Dead Down Wind is dead slow for that reason. No amount of sail area will make a boat faster than the wind when sailing DDW.

Force = CD x density x area x velocity^2, as you sail faster the velocity drops and the force gets smaller. The aerodynamic forces that create lift (circulation and momentum) are not at work when the sail is square to the wind. On a square rigged vessel with more than one mast the forward sails are blanketed by the aft sails so V^2 is lower on the forward sails. Turning the boat up to unblanket the forward sails should allow more of the full sail area to work at full value of V.

As soon as the boat turns up a bit more so there is spanwise flow on the sails, lift is generated and CL replaces CD in the force equation.

 

Perfect sir, thats what I needed to know. Greatly appreciated.

 

Study

I note that on this study the streamlines, when comparing 30degrees of sail angle up to 80degrees, the streamline shows a reach to be the most efficient point of sail, while DDW on the clipper ship the least. I am understanding this right?

 

I think you are.

When there is spanwise flow across the sails, as is the case on a reach, aerodynamic lift creates a greater force than the pure drag when sailing DDW.

 

JavaFoil

I found a post that had a reference to JavaFoil, Martin Hepperle's 'relatively simple' inviscid foil analysis program that will do multi-element airfoils. It is great for illustration purposes here.

Clearly it shows the affect of the whole system- actually, as a cascade of foils. Further Aft foils must have more incidence- but when incidence is added, they help increase circulation around the whole system, increasing the forward foils effectiveness by inducing more incidence and accelerating more mass about the whole mess.

Brian Eland will find this of great interest I am sure, it also shows clearly the slowing of flow in the slot and the resulting tendency to backwind the main. The middle sail does not suffer quite as badly. I tried to simulate an aft mast rig here by using a cutter rig and smaller chord (50%) 'main'. By opening it up to a reach, it is very similar to a square rigger on a reach as well, since this is only a 2D simulation, though Martin has included aspect ratio effects on the last tab, and they are shown to affect the lift coefficient!

Impressive program.....

Rick Loheed

 

Yes, there's one guy who reckons his boat would be improved with a jib - that's not surprising, since he didn't mention cutting down the mainsail we can assume he was going to add area.

But we still have one one-off J/90 where it is assumed a jib is faster, and on the other hand we have every competitive C Class, A Class to show us that years of development have demonstrated that cat rigs are faster.

I haven't got any problem with the idea that some - maybe the vast majority - of boats are faster with the added SA of a headsail. Headsails have lots of advantages, we all know that...seems to me they become more effective on heavier boats.

But it still remains proven by decades of experience that cat rigs are faster in A Class and C Class cats, and there's plenty of evidence that many of the times when similar monos have been rigged as frac or mastheader of equal SA, the big mainsail boats have been faster. Given all that, surely it's reasonable to say that mainsails cannot be all that slow.

Experiments with big genoas and small mains have been going on at least since Sherman Hoyt's 6m "Atrocia" of the '30s or so....big mains still tend to win most of the time AFAIK.

Isn't it possible that some testers have exxagerated the deleterious effects of a mast on a main? Bethwaite, for example, disagrees with Marchaj about RAF type sails and pocket luffs. I don't know who is right, but practical experience shows that "cleaning up the luff" via the use of wing masts is a distinct performance improvement, but NOT the sort of earth-shattering improvement that is worth giving up lots of things like gust response and handling qualities for.

Classic case comes in OD racing in wingmast boats. If you jam a Tasar wingmast the wrong way around, you'll drop back from the lead to back 1/3 of the fleet or worse in an interclub - that's a significant drop but only of the same order as (say) pulling on too much vang. The whole mainsail goes pear-shaped, the flow to lee is terrible, but the loss is only a fraction of a knot.

If you sail a Taipan 4.9, which has a very large wingmast, you'll find that some top sailors de-rotate in a breeze but others over-rotate more. Since one of those options would (one assumes) really mess up the airflow over the mainsail, yet both options actually perform similarly and both are faster than leaving the mast in the normal "clean" position, once again it would seem (to my inexpert mind) that the "cleanliness" of the flow over the luff is no more important than correct gust response, correct draft, correct twist etc.

In the light of all that, to go for a rig that tends to be less self-tending than a mainsail would be a hard choice to make.

 

Square riggers downwind

On the old clippers the wind went over the yard and down the sail when the ship was sailing downwind. This gave two advantages:

1.) the low height to width aspect ratio now became the higher width to height ratio, and

2.) It also added a slight upward lift component which was very welcome by these hard driving clipper masters.

The clippers were intended for mostly down wind sailing anyway, so there was not a lot of incentive to improve thier upwind performance. Also, by the time most of them kissed the water, there were steam tugs available to drag them into and out of port.

Hence, especially on the later ones, you see very low height to width ratios on thier sails.

It is also interesting to note that the 'wind jammers' that followed tended to have higher height to width ratios on thier square sails. Maybe that was because lower operating cost was supposed their primary virtue. And tug fees just did not work out in that equation.

Bob