sail aerodynamics

[powerabout]

I see all Finns have wing masts now with a limited amount of rotation as in fixed with the boom

 

[powerabout]

Whats the aerodynamics of a Star downwind where they let the mast rake go forward
as a DDW boat I thought they would be vertical or even just raked aft?

 

[SukiSolo]

Finns and OKs' have had short wing masts for quite a while, both use effectively bolted through goosenecks to make the boom rotate the mast. Not sure this is max efficiency but obviously you need a 'tolerant' section shape to get some benefit. I've had a play with full size short length sections to see if you can get attached flow earlier, on a mast size limited class, interesting little side project.

The Stars seem to get a better tip roll over vortex from extreme forward rake on a run, it does look extreme especially with the crew on the foredeck, but it works for them. Most classes definitely go better on a run in light air with a lot of twist getting the top of the leech really open and forward to get a similar spill off the top of the sail. In fact a mainsail that is slow on this point of sail (in light air) is too tight on the leech...... go get the sailmaker to add a few mm probably around the lower batten (assuming you have one) approx 1/5th height up. A good trick for speed (on a run) is to hold the boom up vertically slightly so the sail has only it's own weight to support. In certain very light air, it can be noticeably quicker to do this whilst everyone else is just waiting for more puff...

 

[powerabout]

can you explain spilling the air out of the top of main on the star when ddw?
I thought you would be trying to make like a spinnaker?
what angle they actually go downwind?

 

[brian eiland]

Slot considerations with increasing mast height

Slot considerations with increasing mast height.

By now I believe we all agree that the slot phenomena (between headsail-mainsail, or between 2 headsails) does exist,....And that generally speaking it states that the mass of air encountering such a 'restriction' between the two sails gets split up into portions that choose to flow around the edges of the 2 sails, rather than necking down to flow thru the slot.

The slot that most of our sailboats have (fractional or masthead sloops) is a triangular area between the headsail and the mast. This slot significantly decreases in 'size' as we move up from the deck to the masthead. Meantime the velocities and mass of air increases in size as we move up from the water's surface (nature's wind gradient).


I have in the past suggested that 2 parallel headstays/headsails, as designed into the front portion of my aftmast rig, that might be a more favorable configuration,...one such posting
Double Headsails vs Fractional & Masthead Sloops


View attachment 102583

Recently I was made aware of another double headsail arrangement (no mainsail), that tested out quite promising.
Wind tunnel and CFD investigation of unconventional aftmast rigs
But in this arrangement the 2 headstays both terminated at the masthead, thus forming another triangular slot area.
View attachment 102582

So my primary question with this posting:
1) Is this decreasing slot size (with height) compatible with the increasing wind velocity/mass?
2) And/or are there better configurations to process this airflow,....more effectively (more driving force, better pointing, less drag, etc) ??

Which of these 2 slot configurations should prove superior by aerodynamic theory?

(note: For the moment forget the rest of the rig's construction holding things up, and just consider the 2 leading headsails)

I still have not seen anyone attempting to answer this question??

 

[powerabout]

add this thought
boats without jibs go to windward better if we are talking angle

 

[markdrela]

Which of these 2 slot configurations should prove superior by aerodynamic theory? I still have not seen anyone attempting to answer this question??

Any answer attempt without seeing aero calculations will be largely a guess, and certainly not conclusive. And before any calculations are done, it is necessary to design each configuration as best as possible, within the constraints of sailcloth construction (no wings). Comparing a bad config 1 design with a good config 2 design, or vice versa, is not conclusive or even informative.

Regardless...
Having both stays attached at the head makes more sense from a sail airfoil viewpoint. You have two elements with a constant chord ratio and a constant slot/chord ratio all across the span. So with good twist control you can have a good spanwise loading shape AND a good cl or stall margin distribution, simultaneously. You can't beat that combination.

Attaching the rear jib partway up the mast will tend to make a mess of the spanwise loading shape, and will mess up the cl distributions as well. The only saving feature might be the two separate tip vortices like in a biplane, which can have less induced drag than a monoplane. But here, one tip is well inboard of the other, which is generally unfavorable, and might make things worse. Overall, I don't see any favorable features of this configuration.

 

[Mikko Brummer]

Brian,

It appears my definition for "slot" is different from yours. You seem to call "slot" the area between the luffs of two adjacent sails - or the area between the forestay and the mast, for a traditional sloop rig.

To me, the slot is the area/opening between the jib/genoa leech and the mainsail surface… this could differ from the definition for airplanes, but when sailors/sail trimmers speak about the slot, they refer to this area.

I think the reason could be that sail trimmers regulate the power of the rig by adjusting this opening between the jib leech and the mainsail. Tighten the sheet, you close the leech, air is driven over the windward side of the main and you add power. Ease the jib sheet/twist the leech/open the slot, and power is dumped - the slot is like the safety valve of a sloop rig.

Actually, even if the jib trimmer thinks he is in control, it's really the main regulating the power (like prof. Drela states in post 576): Tighten sheet/close slot, and it allows you to crank a little more in the main without stalling its leech. Ease the sheet, and you can let the main out more without excessive backwinding.

Attachments: The First 36.7 genoa provides a perfect slot for a higher maximum lift than the jib on the X-35. On the jib rig, the slot is between the jib leech and the mast - with the jib, you adjust the slot by moving the lead sideways (in-hauling), and by tightening the leech so it is nearly parallel to the mast.

 

[Mikko Brummer]

Attached, the spanwise distribution of lift & drag of the complete boat, including hull & superstructures (the crew on the rail as well, although not shown).

There a kink to zero lift at approximately the heeled deck height - not sure where this comes from, there's a considerable positive pressure on the fore deck inside the jib, from lift carry over. Maybe that has a component against the lift of the hull side, as well as vortices along the cabin sides, and inside the cockpit.

A positive kink due to the edge vortex under the boom, and a buldge in the drag distribution above the jib - this probably more due to flow separation in the mainsail head than a tip vortex there.

Anyone wishing to comment about why lift drops to zero towards the gunwhale height?

 

[brian eiland]

Slot Definition

Brian,
It appears my definition for "slot" is different from yours. You seem to call "slot" the area between the luffs of two adjacent sails - or the area between the forestay and the mast, for a traditional sloop rig.

To me, the slot is the area/opening between the jib/genoa leech and the mainsail surface… this could differ from the definition for airplanes, but when sailors/sail trimmers speak about the slot, they refer to this area.

I agree with you Mikko that we have slightly different definitions. I had mostly thought of the 'slot' in terms of the 'opening mouth'/'the leading edge' that the incoming wind would see, and have to decide how it would negotiate.

As you so rightfully pointed out in a previous posting the air slows down in this opening section of the slot, yet speeds up later down the slot.

To say that sailors/trimmers refer specifically to the aft portion of the slot (the leech area) as the true slot, then goes a bit contrary to what Marchaj and Arvid Gentry talked about when they originally explained the slowing of the air speed thru the slot?

I would agree that the volume of air and the speed of the flow in the slot is governed by BOTH the opening available and the variation in trim of the 2 sails at the aft end.

 

[brian eiland]

Anyone wishing to comment about why lift drops to zero towards the gunwhale height?

Didn't Shuttleworth do some considerable studies and diagrams on the gunwhale effect,...I seem to recall he was very interested in streamlining his gunwhales even on multihulls.

 

[markdrela]

I would agree that the volume of air and the speed of the flow in the slot is governed by BOTH the opening available and the coking-off presented by the aft end.

From what I gather, there's a common view that narrowing the slot (the rear opening) by sheeting in the jib somehow "narrows the venturi" and thus makes the slot air blow faster over the main. This is a is a myth. In reality, the velocity through the rear opening is mostly independent of the size of the opening. If anything, the slot velocity actually decreases slightly for a smaller opening, but assuming it doesn't change is a good approximation.

So sheeting in the jib doesn't load up the main. It loads up the jib, and unloads the main, such that the total lift an drive are largely unaffected (assuming no effect on separation anywhere).

One definite effect of sheeting in the jib is that the rig's center of pressure moves forward. So if the helm isn't adjusted, the boat will tend to bear away from the wind, and the rig will load up to some extent. I don't know how strong this effect is on actual boats. Maintaing heading avoids this in any case.

 

[tspeer]

There's a really great book that came out in 2013, written by a retired Boeing Technical Fellow, Doug McLean. The book is Understanding Aerodynamics Arguing From The Real Physics. This book is about the in-depth physics of fluid flows without going into the mathematics. He does have a few equations, but only in the context of discussing how the physics are abstracted in the mathematics for those that are dealing with the math, and these can be skipped without missing any of the physics.

He also discusses many of the most common misconceptions and shows why they incorrectly represent the actual physics.

There's a whole chapter on airfoils, including slotted sections, and he has a paragraph devoted to sails. Here's what he has to say about headsail/mainsail interaction:

According to older folklore among sailors, the slot directs a "high-velocity jet" of air along the lee (suction) side of the main (aft) sail to "energize" the boundary layer. ... The correct view of the slot effect in the case of typical sail configurations recognizes that the velocity through the slot is only modestly elevated above the freestream, and that the main benefit the slot provides for the mainsail is suppression of the leading-edge suction peak. Gentry (1971) provided the sailing community with one of the earliest correct interpretations of the slot effect.​

The reason suppressing the mainsail's leading edge suction peak is important is because two-dimensional section aerodynamics is all about the care and feeding of the boundary layer. If it weren't for the boundary layer, section shape wouldn't matter at all - the performance of all sections would be equivalent (similar lift curve slopes with no profile drag).

 

[Remmlinger]

Anyone wishing to comment about why lift drops to zero towards the gunwhale height?

To me it seems that the jib is not sealed to the deck - no foil, no lift.
Uli

 

[Doug Halsey]

Originally Posted by Doug Halsey :

Tom : I beg to differ ...

Since it's getting late here, I'll confine my comments to one simple counterexample & save my longer explanation for later.

Consider a boat sailing dead downwind. The driving force from the sails consists of their aerodynamic drag only. There is no aerodynamic lift or hydrodynamic sideforce. The larger the aerodynamic drag, the faster the boat goes. That's why symmetric spinnakers were invented & why low-aspect ratio sails can be very fast downwind....

Yes, you're right that if drag is all that's driving the boat, then the more drag the better. In that case the apparent wind angle is 180 deg, and the lift/drag ratios of both the hydrodynamics and aerodynamics are zero.

As the boat heads up from DDW, lift does start to come into it, and there will be a point of sail at which an incremental addition of drag has no effect on the performance. Above that point, any additional drag will be detrimental. Where that crossover point of sail is has to be determined by a VPP. It can't be determined by aerodynamics alone, because the lift/drag ratio of the hull is involved.

I suspect the crossover point of sail is pretty deep, even for a heavy keel boat. It would be interesting to see if it's as wide as the course for best downwind velocity made good (Vmg). Only for the case where the crossover point exceeded the best Vmg angle would there be a regime where additional aerodynamic drag would be beneficial.

This exchange inspired me to run a few more cases in the VPP, paying attention to the Vmg, in addition to the absolute speed. My general conclusion is that adding aero drag can definitely improve the maximum Vmg of a boat (as well as the speed at a given heading), but whether it does or not depends strongly on whether it's a fast or a slow boat. (I confess that this was my bias to begin with, but the calculations really do support it.)

The calculations are summarized in the attached graph, which shows curves of Vmg as a function of heading, for 3 variations of hulls & 3 variations of sails. The baseline hull (labeled A in the graph) was the International Canoe, with experimental data for its hydrodynamic drag reported by Marchaj.The slower hull (labeled B) had 5 times the drag of the Canoe at any given speed, while the faster hull (labeled C) had 0.4 times the drag. The baseline sail characteristics (black curves on the chart) were taken from a paper by Otto Scherer. The other 2 variations were obtained by multiplying the aero drag by 0.0 (red curves) and 2.0 (blue curves), respectively.

As explained before, the crossovers happen when the apparent-wind angles reach 90 degrees. At larger angles, increasing the aero drag increases the boatspeed in the direction it's headed (& vice-versa). If the boat is slow enough, the maximum Vmg will also be increased, but if it's fast enough, it will not.

In my opinion, Case A is the most interesting of the 3. The crossovers happen at very nearly the same point as the angle for maximum Vmg. Setting the aero drag to zero hardly changed that optimum angle & had a very small detrimental effect on the magnitude of the max Vmg. Doubling the aero drag moved the optimum heading angle to dead downwind, but only gave a modest improvement in the max value of Vmg. So, in a sense, the Canoe is very close to the dividing line between a fast boat & a slow one. (In fairness, I should point out that this is a very old canoe; I'm sure modern ones are considerably faster. Also, my calculations were only done at a windspeed of 10 knots, so the results could be much different in better planing conditions.)

The slower hull (Case B) already has its max Vmg at 180 degrees, and adding to its aero drag simply increases its max speed at that angle.

The faster hull (Case C) has its crossover point at a much larger true-wind angle than its angle for max Vmg, so adding aero drag is not beneficial.

Frankly, I'm much more interested in the faster boats where aero drag is never beneficial, but this has been an interesting exercise for me anyway.