Why a Yawl or Ketch instead of a sloop

Paul,

I love the story about the guy with the tape! I'll remember that.

I also know it's way easier to sail a TP52 or any multi-sail-choice boat with instruments. I also agree that the good sailors only use the instruments to augment what they're feeling. My point was that it might be even more interesting if all the TP52s had to sail without instruments, sort of like big complex lasers.

Beau

 

Are the wind tunnel models fabric or solid?

I can imagine a sheet metal foil doing what you show, but a fabric sail is going to luff if it is sheeted that far off the CL (connecting the leading edges of both sails) while sailing at 29 degrees from the True Wind. At 29 degrees to Ture Wind the main leach would line up with the luff (leading edge) of both foils, and the leech of the forward foil would be at less than 8 degrees from the CL. Even then it would probably be luffing.

However, since the wind tunnel model is static I suppose the wind in the tunnel would be more like Apparent Wind. So 29 degrees would be just outside of upwind, about 2 to 5 degrees in foot mode. So maybe the main would be a degree or so off CL and the jib might be out to 10 or 11 degrees from CL.

Maybe there is something I am not seeing.

 

The angle is to the apparent wind. That is all JavaFoil can work with. You have to do the vectors to get the true wind and that will be based on windspeed and boatspeed of course.

The angle of the streamlines in the image is because the image is near the foil. The foil affects a huge zone around it. Many times greater than its length. If I zoomed out the steamlines are running horizontal on the extremity of the image.

Rick W

 

OK, that is what I thought.

So, what is the sheeting angle for the headsail? It looks to be more than 10 degrees. I think it looks too open for that point of sail.

What is the sheeting angle of the mainsail?

For most boats out there 29 degrees Apparent really is upwind. For some more modern boats it is just cracked off. Your sail trim doesn't look that way to my eye. I'll take a look and measure myself when I get home.

Does the program allow you to "trim" the sails as I suggest? I'm sure the overall L/D would drop, but it might be more representative of the real world.

 

You should have a go with it. It is the best thing I have found for getting hard data on any foil you choose:
http://www.mh-aerotools.de/airfoils/jf_applet.htm
This will start as a Java applet in explorer.

You select a foil. I usually use a NACA 4-series with a 1% thickness and 10% camber to model a sail. You need to set the options page to suit the AR of the sail. This is typically 4 to 5 but you can work it out for the particular sail.

You then orientate the sail. I have found it is easier to comprehend if you leave the angle of attack at 0 degrees and fiddle with the arrangement of the sail or sails.

Playing with the angles and orientation is just like trimming sails and you can get some useful data on lift and drag coefficients.

The streamlines tend to deceive the eye when it comes to angles.

You can play around with more complex foils to see how they perform.

Rick W

 

"...Does the program allow you to "trim" the sails as I suggest?..."

Javafoil is a very limited program for classic 2D flow for infinite lengths, within a discrete set of parameters. To suggest that the software can model complex interactions of sails and obtain meaningful qualitative results is fantasy world.

A normal sail for instance ranging in camber from around 1~10% from the boom to the mast head. It is not a standard aspect ratio (in terms of aeronautic definition) and rectilinear shape. As you have also noted too:
"..I can imagine a sheet metal foil doing what you show, but a fabric sail is going to luff if it is sheeted that far off the CL .."

Javafoil was developed for fixed wing aircraft designers to get a quick rough idea of lift/drag. It only assists those that do not have the 2 classic books on obtaining lift/drag data, not cheap, and those that have no formal training in aeronautics and to an extent hydrodynamics.

 

Give it up ! Mr. just my Name.. will not ever leave his point of view. He has a mighty software backing him.
And we poor idiots have to folllow his figures.
Be sure Willoughby, when ever you arise, I will have a comment!

 

The model represents a rigid shape. If the shape distorts under pressure then the shape needs to be remodelled to that new shape. The reason for modern sails using very stiff fabrics is to avoid the shape changing under load. They can hold more to the ideal shape. Of course the best sails for performance are rigid foils but this brings in a whole new range of issues like reefing that have been solved for fabric sails.

The model is not accurate once you lose streamline flow such as experienced in a luffing condition. The foil performance will drop off anyhow under these circumstances. You can also see by the pressure fields if the sail is luffing.

Thin foils only operate over a narrow range of angles so you have to trim them as you move one in relation to the other. This is no different to what you do with sails.

Rick W

 

OK this is what I have been trying to say.

If you have a boat sailing upwind the sails will be trimmed in. The forward foil in your document seems to have the line between the tack and clew parallel to the (apparent) wind direction. That's not how you sail upwind. The sail would be flogging.

If you have a boat sailing at 40 degrees off True Wind (tacking through 80 degrees, not a bad boat), sailing uphill at 7 knots speed in 12 knots true wind (Farr 40-ish), then you would have an apparent wind angle of about 25.5 degrees. So your 28 degrees is only about 2.5 degrees cracked off from real point mode. So instead of sheeting at 8 degrees from CL you might be at 10 degrees, as I am showing here in the attachment.

The main would also be slightly cracked off from CL, unless you were a slower boat and then you would be going to weather with the main basically on CL (dashed red line).


There must be something I am not understanding.

 

Paul

Perhaps some real data would help clear up the issue for you, see attached?

(ops forgot to add the other file-done)

 

I own that book. I don't see how that attachment answers my question.

 

People as Greg Elliott? http://www.elliott-marine.com/primo.html

 

You are correct about the sheeting angles. The only way you will achieve this and have streamline flow is to push the camber further forward on the sail. You can do this with JavaFoil. It allows a higher angle of attack with each individual sail. The maximum camber occurs at more like 20 to 30% than the 40% of a NACA 4-series.

There is also the possibility of pushing the sail closer to stall (this is when you see the tufts lift) and this could very well be the best sail trim for the best VMG. JavaFoil has various options to select for the stall condition but these are empirical relationships that are based on observations and not analytical in derivation.

Attached shows the leading foil with camber further forward that is still operating in streamline flow.

You can spend a long time modelling the exact shape of the sail to get it close to what is happening in real life. The aim would be to see what adjustments are made to then get better performance.

Rick W

 

Paul
"...The forward foil in your document seems to have the line between the tack and clew parallel to the (apparent) wind direction. That's not how you sail upwind. The sail would be flogging..."

fig.107, with increasing angle of separation/trim between the sails, the higher AoA promotes the onset of separation of flow and leads to your 'the sail would be flogging".

If i understand you correctly.

 

No. It has nothing to do with interaction between sails. It would be an issue if we were talking about a catboat.

If a sail's tack and clew are aligned with the wind direction it will not fill, it will flog.