Rigs and Rigging Weight vs Drag?

One of the things that we have discussed/debated more than once is the aerodynamics of sails. What I can't find much information on is the aerodynamics of the rig as a whole and the effect of stays on the L/D ratio.

There have been any number of posts that show that increasing the L/D ratio of a rig, improves it's performance. Tom Speer even went so far as to say that there are greater performance gains to be had from reducing drag than increasing lift.

There have also been claims to the effect that free-standing wing rigs have only 10% of the drag of a mast and rigging.

Until very recently (in the overall history of sailing) the trend in rigging has been to reduce the diameter of the rigging. An example taken from Brion Toss's book shows a progression from 1-1/2" diameter tarred hemp in as late as the 1850's, through 6x7 iron wire rope (11/16" dia), 1890's 5/16" 1x19 galvanized steel wire, to yesterday's 3/8" 1x19 SS wire and 9/32" solid SS rod.

Today I inspected some of Navtech's new PBO (Zylon) rigging. for the first time in the history of sailing (that I know of) there is a change to a larger diameter for the same stretch/strength.

The new PBO rigging is about twice the diameter of the rod it replaces.

The argument is that it reduces weight aloft. What then is the trade-off? Can reduced weight aloft more than compensate for the increased parasitic drag of doubling the rigging diameter?

I have no idea how to go about estimating the effect of weight aloft other than reducing it will lower the CG. The lower CG should increase the boat's RM? The increased RM will allow it to carry more sail or a higher aspect ratio rig? Would the gain from reducing induced drag be greater than the loss due to increased parasite drag?

I'm sure someone has worked this out and concluded the gains outweigh the penalty. I'm just curious about the design spiral that lead to the conclusion.

 

did some reading on PBO (Zylon) its the newest aramid right?
looked at specs, rope or rod twice the diameter of SS rod?
see also sails of it, price may come down after police get their new vests
planes impact isolated and production established, hmm by that time...
the poll drag against weight is intersting but i dont know

 

A free-standing Wing-Drive have 10% less drag then a mast and rig. This has been tested in wind tunnel.

 

A very tricky question, RH.

I guess it all depends on the design objectives and desired trade offs.

I can imagine at least five:

1.) Windward vs Non windward performance,
2.) Combined (Windward and Non windward) performance vs cost,
3.) Windward vs cost,
4.) Windward vs reliability, and
5.) Combined vs reliability.

There are other considerations as well. If standing rigging is being used, is it rod or rope? With rod, it is theoretically possible to give the riging an aerodynamic cross section and thereby cut its upwind drag quite drastically.

My guess is that the answer is always somewhere in between. For upwind sailing, some standing riggging, but not a lot, is the best compromise. This may be the reason for the popularity of fractional sloops.

If you give the mast a nice airfoil section then park a small jib (somtimes a very small one) in front of it, you can keep the section size and wall thickness of the mast within reason and still have something strong enough and versatile enough to stand everyday use.

It seems cat rigs (single sail and ketch and schooner rigs without jibs) just aren't taking off. Whenever I see one, It is always either custom or very limited production. My guess for for this is that there are cost/reliability issues involved. Racing rules may also play a huge part part (maybe because there are cost/reliability issues)

I would imagine most of the designers who appear on this site could design a jib less rig that will get reasonable widward performance for reasonable costs. They could also design one that would go upwind like a rocket but require a NASA budget to build.

So now. Where does that put us?

It seems that that puts us where the question, by itself, is simply not relevent. More has to be added to it to make a reasonably accurate answer possible.

One problem about jibless rigs that I forgot to mention is that it is often inconveniet to add light wind sail area to them. For this reason, their working sail rigs tend to be larger than that of their jib headed cousens. For instance. The fractional sloop I used to own had a large 'drifter' that went up in place of the working jib. It boosted the sail area from 145 sft to 180. That was an increase of over 24%. All of this was doable, single handed, with the boat staying underway and me not having to leave the deck.

Bob

 

Some claims about wing sails:

"The wind resistance of the wings in Zero position is less then a normal rig without sails. This has been verified in wind tunnel tests."

"When in neutral the wingsail has only around 10% of the drag of a conventional mast and are much safer up than traditional rigs."

"Scaled models of these 2 boats were tested in wind tunnel to compare the drag of the bare rig and the Wing-Drive in neutral position. The Wing-Drive has 10% les drag then the bare rig."



Claims only ... no data. Not fact, until supported.

Who did the test? What wind tunnel? What were the dimensions of the models? What Re?

Dates and numbers please.

 

the question is simple, the answer (if anybody has one) will be tricky.

The objective is to improve performance of a sailing rig.

Reducing parasitic drag always improves performance. At one time boats were rigged with teardropped shaped "lenticular" rod. It was developed for 12 Meters in the America's Cup and found it's way on to other racing boats.

Reducing weight aloft also improves performance. Reduced pitch and roll moments? Lower CG (higher RM)?

What I'm most curious about is the relative importance of the two.

Cost is not a concern (at this point). I'm just interested in the theory.

The trade off is that the new PBO standing rigging is twice the diameter of the rod it replaces. Is it possible to gain more performance due to the reduced weight than is lost due to increased drag?

There are several "It Depends" answers ... please share your thoughts on what it depends on?

I have a few "it depends on" ideas, but I'd like to hear what others (that are not selling PBO rigging) have to say.

 

As the saying goes, "If you're hunting elephants, you have to go where the elephants are." It's not just the drag of the rig that counts - the topsides are very important, too. After all, drag is drag, and it matters little where it comes from, except with regard to what you have to change to reduce it!

A really good article on the importance of reducing drag is John Shuttleworth's "Beyond the Tektron 50 - the design of the new Dogstar 50". He has a very interesting breakdown of the lift and drag on this exceptionally clean cruising cat going to windward:

WIND
Sail lift - lbs 3610
Total air drag 1013
Sail and rig drag 361
Air drag of hull 652
WATER
Keel lift 3610
Total drag 687
Keel only drag 176
Hull drag (from tank testing) 511

The air drag of the hull is more than the water drag of the hull! And the total air drag is much higher than the total water drag.

The irony is, the higher the performance (higher the L/D), the more important drag reduction is. If the L/D is 3:1, a one lb or N decrease in drag is 3 times more important than a one lb or N increase in lift. But if the L/D is 10:1, a one lb or N decrease in drag improves the L/D 10 times more important than a one lb or N increase in lift! That's why sailplane pilots are so scrupulous about reducing the tiniest source of drag, even down to cleaning fingerprints from the wings. When the L/D is 40:1, the impact of drag is magnified even more.

 

In absolute terms this question can only be answered on a case by case basis, but there may be a trend for the majoity of cases. The only way to know for sure is to do the calcs for a broad sample. This may not be that difficult. The variables are:

- Change in hull drag due to change in displacement. This can be estimated.

- Change in aerodynamic drag and its effect on total drag. This can be estimated.

- Change in righting moment due to weight aloft. This can be estimated and only effects boats that sail heeled over (non canting keel boats).

- Change in boat handling charactoristics due to weight aloft. This is probably indeterminate, but could be ignored in all but extreme cases, e.g. Moths.

Note that I am not about to do this excersize

Mal.

 

Thanks Tom.

Here's what he says about the rig:

Rig.
The rig is a double diamond swept spreader carbon wingmast of short chord. The jib, which is sheeted at 6 degrees is not self tacking because a small overlap is needed to obtain optimum effect from the slot. On a boat with such narrow sheeting angles, a large chord wingmast rotating near the forward end of the mast tends to close the slot between the jib and the mainsail. Keeping the chord small and rotating the mast aft of the centreline will keep the slot open and will improve sail power upwind. The rig is very tall and a carbon mast will be the lightest option to reduce pitching, and all the rigging apart from the forestay and diamonds will be synthetic fibre instead of stainless steel to reduce weight. Diamonds will be stainless rod for minimum stretch and lowest windage.

This sort of thing drives me bonkers!

Here is a designer that looks at every little detail and goes to great lengths to reduce drag, yet he chooses fibre rigging to save weight on some parts of the rig and SS rod for reduced windage on another part. I would think that if drag reduction is the highest priority He would have chosen SS Rod for the rest of the rigging?

I can quantify the effect of reducing drag. How does one quantify the effect of reducing pitching (or rolling)?

I also don't quite understand the vector diagram he has that shows the forces. He shows the driving force equal to the total water drag of 687 pounds. Doesn't the drive have to equal total drag? Shouldn't the drive requirement be equal to the vector sum of both the air and water drag? There is a 1013 pound air drag force that is not canceled in the diagram.

 

Its definitely one of those 'it depends' questions and you are right, it is almost impossible to quantify the trade off between windage and weight. The stability and motion response of the boat, how 'draggy' the boat is (does it have a large superstructure etc), the wind strength, the sea state etc etc.
I imagine that if the boat is fairly heavy, then you won't save much weight (as a percentage of total displacement), so stick with the thin rigging. I'm not sure it works in reverse though. That is, I'm not convinced that a light boat should use the lighter, but dragier, rigging, becasue, as Tom says, high performance boats need to minimise drag.
The question is analagous to masts. Do you go for a wide diameter, thin walled section which is light, but has high drag, or a slimmer, thicker walled section that is heavier but more aerodynamic? Or you could go for a really thick section that doesn't need diamonds at all!

 

It appears that the heavier, less draggy riging is being used where the rigging is closest to the sails and can interfere with the airflow over the them the most.

The thicker but lighter stuff seems to be used either in front of the sails, behind them, or far enough from them that, despite its tendency to create turbulence, it is generally far enough away from the sail to not effect the airflow over it that badly.

So now its lighter weight can become the sole criteria in choosing it.

Bob

 

In the diagram, the total air drag (hull + rig) and the total lift or crosswind force (hull + rig) have been resolved into a single force with the value 3610 lbs. Then this force has been broken down into a thrust force (687 lbs) and a heeling force (3548 lbs) which are respectively parallel and perpendicular to the direction of travel. These are the force components which are balanced by the hull drag and lateral resistance forces. So the diagram is correct.

Another way of doing it would be to subtract the hull air drag component from the total air drag and add it to the hull to the hull water resistance, to give a total hull resistance force. The numbers would look different, but the end result would be the same.

Mal.

 

Yup, I had to read that 5 times and stare at the diagram some more. I get it now. Thanks!

 

That's a way to look at it that I hadn't thought of. And of course the further the stay is from the CG the greater it's effect on roll and pitch will be.

My hidden motive here is to arm myself with enough knowledge so I can properly advise my rigging customers. I'd like to have a good feel for when I should recommend or encourage the use of PBO rigging and when I should recommend more traditional solutions.

I haven't priced it out yet, but Navtec is saying the PBO is a 2-3 year replacement item. The rod it replaces has a service life of 40,000 sea miles to major inspection, and possibly another 40,000M after the rods are re-headed.

This seems a very high price to pay for reduced weight aloft! Particularly when it comes with a windage penalty. Granted the windage of the standing rigging is small compared to the total aero drag, but as Tom points out, every little bit helps!

It is an interesting question to be sure.

 

Weight aloft is widely considered to be more detrimental to perfornance than weight added at the c.g., because it increases the moments of inertia and reduces stability when heeled. However, considering just weight of the rigging as weight, here's a possible way to make the tradeoff.

Take the Dogstar 50 as an example. The aerodynamic lift/drag ratio is 3.5, the sail area is 160 m^2 (1700 ft^2) and the displacement is 4,000 kg (8,800 lb) for a Bruce Number of 2.0. Just eyeballing from the sailplan, it looks like the shrouds are 72 ft (22m) long.

I don't know the diameter of the rigging Shuttleworth specified, but let's go back to RHough's original question and ask, "Which is better for the Dogstar 50, 9/32" (7mm) Nitronic rod [weighing 0.201 lb/ft (0.3 kg/m) with a breaking strength of 11,800 lb (5,350 kg)] vs 7/16" (11 mm) Kevlar (since I don't have data for PBO) [weighing 0.087 lb/ft (0.130 kg/ft) with a breaking strength of 13,541 lb (6,144 kg)], on a pure weight vs drag basis?"

Assuming a drag coefficient of 1.0, the two Nitronic stays have a drag area of 3.3 ft^2 (0.31 m^2). At a lift/drag ratio of 3.5, this is equivalent to 12 ft^2 (1.1 m^2) of lift area, or (assuming a lift coefficient of 1.2) a sail area of 10 ft^2 (1 m^2). The weight of the stays is 29 lb (13 kg). These stays effectively cut the Bruce number by 0.4%.

The two Kevlar stays have a drag area of 5.3 ft^2 (0.49 m^2), corresponding to a sail area of 15 ft^2 (1.4 m^2), and weigh 12.5 lb (5.7 kg). These stays effectively cut the Bruce number by 0.5%

So it appears that the Nitronic rigging is the way to go over Kevlar on a drag vs weight basis for this case. The Nitronic will have a greater effect on the gyradius. At 18 Msi (129 Gpa) for the Kevlar vs 28 Msi (193 Gpa) for the Nitronic, the Kevlar equals or betters the Nitronic in stretch because of its larger diameter.

Naturally, you could substitute some other rating formula for the Bruce number. But I think the idea of equating the drag with an equivalent sail area, based on the L/D, is a reasonable way to make these trades because designers have a pretty good idea of the tradeoff between sail area and weight on performance.