Aspect ratio on headsail vs shroud angle?

I counted recently that I have 80 pages on my website at the moment. I rarely delete any, mostly I just add more. You are not the first one to get sucked in--people tell me I am a pretty engaging writer, and I like to tell stories.

When sails are set together, they form a sort of synergistic whole--the lift caused by the air circulation around the pair is usually more than the sum of the lift of the sails individually. So really, you have to look at the whole sailplan to judge its effectiveness. There are idiosyncracies in various parts, like the head of the jib close to the mast, that kind of get ignored. Square-top mains are always better than triangular topped mains. And a fractional jib keeps its head low so that it won't influence the tip vortex of the main head which is shaped as it is to take care of itself. Generally, on a free-standing rig with a rotating wingmast, any jib (which should be fractional) will help improve speed and pointing in light winds, and so that is when they are used the most. In moderate winds, the jibs are needed less, so they are reefed or taken down.

Eric

 

Thanks Eric. That makes sense.

 

Hmm... we found virtually no difference in performance upwind between a regular and a square top main, in a CFD study.

The triangular sail plan is not so bad because sailboats are usually heeling moment constrained. Square top is only beneficial if mast height is limited - else a taller mast with a more triangular sail but same area is more efficient.

 

That is a very odd finding. It is counter to the study and findings of aerodynamics for the last 100+ years as proven in wind tunnel studies and on the race course. If triangular rigs were more efficient than square top rigs, then sailboat design of the last 20 years is way off track. Same with airplane design since its inception. Certainly, we know that a sail of higher aspect ratio, which you describe, has higher lift/drag ratios than lower aspect ratio rigs. But a properly designed square-top rig, which has more uniform airflow distribution, uniform downwash, and lower tip vortex drag, should be more efficient than a triangular rig, according to all practical knowledge. Such a rigged craft should sail faster and point higher than the same boat with a triangular rig.

So, can you explain your findings more fully? How good is your CFD program? What are the specifics of your two models, and what parameters did you use to compare performance? Can you offer sample polar plots or other form of results that we could see?

Thank you,

Eric

 

This is a bit off-topic of the original thread, maybe we should start a new one.

When looking at the optimum planform, you have to consider that there is more wind aloft, heeling moment is often a limiting factor, and triangular sails are designed flatter in the foot, fuller in the head. Tom Speer would be the right person to comment on mast height & planform & induced drag.

Our CFD is state of the art, but it is still just a simulation. And this was only for close upwind sailing, I haven't looked at full polars, reaching and running the square top is beneficial as there is more wind aloft and heeling is not such a problem.

I reckon the ORCi measurement rule (ex. IMS) has come to pretty much the same conclusion since the influence of the square top main on rating is minimal, if sail area remains the same. ORCi rule changes were based on windtunnel measurements in the Milano twisted flow tunnel, by Fossatti, if I recall.

 

Aspect ratio to a sailor is usually the luff hoist divided by the foot.

The REAL aspect ratio formula is Span Squared divided by Area.

with this formula about 6-1 works out well.

FF

 

I expect this is one of those situations where what is best depends very much on the constraints, setup and criteria used for judging. For example size of the sails:

Equal sail area
- or -
Equal rated area for a given rule
- or -
Equal mast height
- or -
Equal heeling moment

Equal area for a given rule is what many designers and researchers take as the given for the constraint.

Other items which could lead to different conclusions:
- What was the assumptions for the wind velocity (speed and direction) profile with height?
- Heel angle if any?
- Camber and twist of the sails?

 

These subjects being discussed appear to be gravitating towards those numerous ones in 'Sail Aerodynamics' There is a lot of good material interdispersed thru-out this subject thread.

In fact I was back there searching for "fractional" and ran across the link to your webpages Mikko. I'll have a few other questions for you.

 

Fractional rig vs Masthead Rig

Here is that webpage link
http://www.wb-sails.fi/news/Ad_aerodynamics/index2.htm

Just makes me ask again and again, why such an overwelming use of fractional rigs over mastheaded ones? There appears to be a more harmonous interaction of headsail and mainsail in the mastheaded rig. Plus, it can carry a similar size sail area with a shorter mast.

 

Fractional vs Masthead

I have previously asked about this subject:

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-12.html#post94004

 

Aspect Ratio on Cruising Vessels

I knew I had made a number of postings on this subject, but didn't find them right away. Here's one...#168
http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-12.html#post93500

I would remind those interested in CRUISING vessels vs racing ones;
"In general we have three basic sailing directions we need to consider, upwind, reach, downwind. And for the cruising sailor the upwind 1/3 of the total is not even an equal partner (as many cruisers often chose not to fight upwind work). As Marchaj and many others have reported, high aspect ratio is principle beneficial for upwind work."

...and
"What I found rather interesting here is something I’ve commented on before, the influence of ‘rating rules’ on designing boats and interpreting sailing science, ie; 'interesting to see how the penalties imposed, affect the efficiency.' We need to keep this in mind as we refer to observations (usually our reinforcing ones) based upon particular classes of racing sailboats."

 

Very interesting.

Is it just me, or does the most simple interpretation of Mikko's posts lead to the conclusion that masthead rigs with a smaller pinhead main are better than what is usually found today? IE, that had it right in the early IOR days and have been going backward since with squaretop mains and fractional rigs?

 

I don't think there is a simple explanation. Speed ratios have changed. Leeway angles have changed. The premier venues have changed. Rig tech has changed. Some economics have changed. The size of a boat sailed by a given crew has changed. Navigation and instruments have changed. Cruisers figured out what worked for them sixty years ago. Their requirements have been pretty stable and so has their preferred rig.

 

Here's from a private discussion a few years ago:

"My specific question is: Do a fat-head main alter the flow-dynamics of the headsail and the gennacker in a good way or is it just the above mensioned effects?"

We have not simulated the fathead main with a gennaker, but I don't think it will do much more than exactly what you mention above. We did an upwind simulation, where a small fat head mainsail betters a normal triangular sail by some 3% (3% more drive), and a nearly square top main again betters the small fat head by 0,5% only (all sailplans have the same area). This is specific to the rig configuration & sail shapes inspected here only, and not to be generalized to all square top mains, but the gains indeed appear to be smaller than expected. Especially surprising was that the square top was hardly any better than the fathead - the reasons for this is too complicated go to in depth here, but the bottom line is there is little to be gained, when the mast height (span) and sail area remain unchanged (aspect ratio spanˆ2/area unchanged).

I would not convert my triangular main unless I feel I need more sail area.

 

Yet another one: Two mainsails, same area. The ORC VPP thinks the Fathead is 0,02 kn faster in TWS 6 kn (5,09 kn vs 5,11 kn), and 0,02 kn slower in TWS 20 kn. The break even point is 12 kn, pretty much when the boat reaches its "design wind" and stability becomes an issue.