Keeping one boot in the boat...

Marchaj’s test foil, which was used to produce Fig. 83, was rigid, and no doubt just made of a sheet of metal, so would not collapse even with the AoA set to 0°. Still, the lowest AoA he tested here, was with it at 5°, and that on all three foils.
This was probably because he knew that real sails, made of sailcloth, would collapse well before that.

I have now drawn in the AoA = 5° case of sail 2; the sail with 10% camber. I found that  the drag angle (Epsilon) drops from 8.9 to 5.3°. However, this comes at a cost  -  the lift drops with 32%.
In next diagram (Fig 84, not shown) Marchaj tilts Fig. 83 to show three sailing angles, 30, 60 and 90° to the apparent wind. To my surprise, sail no 3 (12.3% camber) comes out with the highest drive force when close-hauled, with AoA at 15°. Sail 2 comes in just behind, also with AoA at 15°.
The price to pay for sail 3 is higher side force, so not every boat could make use it.

What is sure from Marchaj’s diagrams here, is that the  5° AoA, with the lowest drag angle, is not the setting which will propel a boat fastest to windward. One needs some brute force as well. Don’t forget the boat.

Telltales at the leech  -  useful or not.

My leech telltales are cheap and not that accurate way of warning against a stall.

• ·         First of all, they are helpful in shouting to me when a wind shift or bad helming have stalled the sail completely.
• ·         Sailing upwind in light winds, I tend to sail close to stalling, with these telltales flying –  mostly. This clearly gives the best speed, at the cost of pointing angle.
• ·         As the wind picks up and the toerail approaches water, I usually head up a little, to point higher. Keeping an eye on the wind indicator helps. As the wind increases more, I end up flirting with luffing angle, until I find it wiser to drop a panel.

Whether the leech telltales warn me too late or not about stalling, I dunno. Maybe that is true, but it cannot be that bad, and anyway, that’s what I have.

(Btw. when watching videos on Youtube, showing stall-tests of aeroplanes, the tufted wings clearly indicate that separation starts at the trailing edge...)

Arne

Anonymous wrote:

The Cl/Cd approach is interesting to work on the flow but is not representative of the thrust and drug on a boat as the angle of attack is ofset of the boat speed vector from about 45° (at best).  – Eric A.

Eric, I do not quite understand what you mean. Are you referring to True Wind Angle (TWA) and Apparent Wind Angle (AWA)? The simulation I ran have an Angle of Attack (AoA) towards an incoming flow. Transferred to a boats coordinate system, that AoA from the simulation translates to the angle of the sail towards the AWA – not the TWA.

The Cl/Cd approach has been invented in the aeronautical world, but it is not at all relevant in boating.

In aeronautic, the speed vector of the aircraft is almost in line with the zero lift cord of the wing (forget stalling aerobatics).

So the thrust of the engine (or the horizontal gravity force in a glider) must be almost equal and opposed to the drug of the wing (the aircraft in fact). So the Cl/Cd value is relevant to evaluate the efficiency of a wing. 

On a sailing vessel it is not at all relevant. At first, the drug of the sail... is rediculous in regard to the drug of the hull in the water. segondly, the drug vector of the wing is ofset from the boat speed vector from about 40° headwing. Downwind it is just a non-sence; the drug of the sail is the powering force.

For a sailing vessel, the important decomposition of the thrust vector of the sail is the powering thrust and the heeling force. The heeling force should be sufficiently low to allow the boat to sail in good condition in order to maximize the powering thrust.

As such, it might be interresting, for example, to ease the sheet to increase the powering thrust while reducing the heeling force and, incidentaly, the thrust vector. It is what is done on racing boats when you ease the mainsail sheet to reduce the heeling moment.

Eric

Hi,

As always the firsts cases are rather trivial and the results are obvious. – Eric A.

Yes, Eric, that is exactly what I intended to show. Before looking at improvements/innovations, the simulation needs to be capable to reproduce trivial and obvious results first, to prove it is reliable. Which it does, read on!

The Cl/Cd approach is interesting to work on the flow but is not representative of the thrust and drug on a boat as the angle of attack is ofset of the boat speed vector from about 45° (at best).  – Eric A.

Eric, I do not quite understand what you mean. Are you referring to True Wind Angle (TWA) and Apparent Wind Angle (AWA)? The simulation I ran have an Angle of Attack (AoA) towards an incoming flow. Transferred to a boats coordinate system, that AoA from the simulation translates to the angle of the sail towards the AWA – not the TWA.

Also, Eric, I had that photogrammetry idea, too. As far as I know, a modern Iphone Pro has LIDAR-sensors built in, which are capable to do a 3D scan. As you proposed, doing this in a berth, with the sails set and sheeted in properly (leech telltales flying, luff not deflating), should give some pretty accurate results. A 3D-model of at least one panel would be sufficient, it could be sliced into 2D and here we go. However, I do not own such a device, but am having a sharp lookout within my friends in Kiel.

Doing multi-physic coupling, that is combining flow simulation with textile strength simulation, as you thought of, Eric, is in another league. Possible, but really a tremendous amount of work and quite costly regarding computational power. If time was unlimited, I would go for it, but unfortunately it is budgeted as anything else. And one would need to ask oneself: what is the advantage? If we get the actual 3D shape of an inflated panel from i.e. photogrammetry, what better can it get?

what about if instead of chipping in money, we chipped in computer cycles - can the calcs be run in a distributed manner, a la Seti@Home, or Bitcoin blockchain, or something along those lines? – Kurt R.

Interesting approach, Kurt. However, CFD uses multi-core distribution in a way, that the “virtual wind tunnel” is divided into smaller segments, one segment for each core. On my 6-core laptop, the virtual wind tunnel is divided into 6 segments. Now at the end of each timestep, those segments need to communicate with each other so that the flow is able to advance downstream, so to say in a simple way. There are about 1.5 million timesteps necessary for each run. If you imagine different computers all over the world to communicate via internet such oftenly, that would be quite slow. I think in that case, unfortunately, my one-core smartphone would even be faster in the end. There are applications that are prone to do what you suggested, like i.e. SETI, but CFD calculations are working differently. Still, interesting approach. CFD is highly interesting for the industry, so there is a lot of improvement going on. Who knows what's next... I know of some very clever people working right now on using GPU-cores instead of CPU-cores. Looks promising, but still no miracle should be expected…

… the purpose of these basic models is to find comparative data, on just an arbitrary section of a junk sail panel. – Graeme K.

The numbers themselves may not be absolute, but the comparison is still well made, and we have learned something – or at least verified something. – Graeme K.

Graeme, you are spot on with your explanation! I am not aiming at actually calculating absolute values of windward performance of an actual yacht. The approach I take here is not intended to do so, and definitely not able to do so. However, instead of absolute results, it gives relative results: How is the port tack compared to the starboard tack. What effect has 10% mast balance to 25% mast balance? How are split junk profiles working compared to flat cut profiles compared to cambered profiles compared to flat cut profiles (watch out, I intendedly wrote “profiles” - not rigs or sails!). Those results can be transferred to a real sail/rig, but in reality are probably blurred by other things like 3D-effects (yard angle, batten angle, sail plan form), environmental influence (seastate, gusts) and yacht related issues (hull shape, weight, mast weight, mast stiffness, …). However, the effects from the 2D view are still there. They are simply blurred. (sorry for my unaccurate English!)

I hope Paul will put things right, and not be afraid of telling me. – Arne K.

Right now there is too big difference between Machaj’s and Paul’s results – Arne K.

Actually, Arne, the difference is marginal. I’m not afraid of proving it to you – but in a separate post ;-)

Thanks for all your interest and input so far! It is fun and valuable to not do this alone in my lonely corner, but discuss it with you and thus improve it.

Cheers,

Paul

Graeme wrote:

Hi Arne, I am not sure if we are maybe talking past each other here.

What I meant was - I was referring strictly to “comparative” data. For example, for a given AoA the model can make a comparison between the numbers arising from port tack and starboard tack (the effect of the mast). The numbers themselves may not be absolute, but the comparison is still well made, and we have learned something – or at least verified something.

That was all I meant, but it is only a small point, I suppose.

Having made that one small point, and assuming the AoA figures are not so far out that the conclusion would be reversed, I don't have enough knowledge in theory or practice to question what you are suggesting.

(By the way, just to correct the written record, should you not edit your previous post referring to Marchaj's AR=3 ? Or, am I missing something? I know it's not an important point).

In the last line of that posting I wrote:

The next diagram shows a more realistic sail with AR =3.

What I meant with that was that the sails on Fig 99 show worse results (L/D) than the reference sail with AR = 5 (fig. 83)-

Maybe I have missed something of importance. I bet I will then be told so rather soon.
Maybe my writing here looks a bit blunt, but I can assure you, I am just technically focused, and then finer diplomacy must give way.

I hope Paul will put things right, and not be afraid of telling me.

Arne

Anonymous wrote:

Right now there is too big difference between Machaj’s and Paul’s results

Arne

Bonsoir

What Paul provided is a first step of a process that could last for years !

It is very theorical and has no aim to represent anything real !

It is a very interresting approach but we should not wait for any short terms operatioanal result.

Marchaj work is also very theorical in the way it supposes, for example, that the flow is steady and that, for example, the hull is not pitching or rolling!

Science is progressing step by step and each step is a long and tedious time and ressources consumming process. At first results are accademic and trivial and step by step the result are improving and beconming more and more realistics.

So please, don't be on any hurry and let the science the time to progress !

Eric