Sail trim quiz question

[danstanford]

The boat is probably sailing into 25 degrees of true wind. The apparent feels like it is on the nose.
View attachment 144109

I have never managed to point much more than 30 degrees apparent which I think is about 45 degrees true. If I have the main on the centreline at that point does this mean it is way over trimmed? Is it sheeting angle on the jib that prevents me from getting anywhere within 20 degrees of that boat or is it a ton of other things all added together?

 

[DayDreamer41]

Appears there is (2) jib tracks on this boat one is further aft and runs athwartships as well, tracks are specific to jib sizes?

 

[Jackdaw]

Appears there is (2) jib tracks on this boat one is further aft and runs athwartships as well, tracks are specific to jib sizes?

There is just one track; the boat can only fly jibs. They can fly smaller ones for big breeze but the clew is cut higher to match the existing track.

 

[Jackdaw]

I have never managed to point much more than 30 degrees apparent which I think is about 45 degrees true. If I have the main on the centreline at that point does this mean it is way over trimmed? Is it sheeting angle on the jib that prevents me from getting anywhere within 20 degrees of that boat or is it a ton of other things all added together?

Boom on the center-line is universal starting point for upwind trim on all boats, regardless of what their actual TWA into the wind will be. That angle (as you guessed) is made of of many factoring components, but jib sheeting angle is clearly a big one. Optimally 9-8 degrees on most production boats, it is often wider due to construction constraints... like cabin walls or windows. On these boats, inhaulers can often buy you an extra 3 degrees or more of point.

 

[Will Gilmore]

Appears there is (2) jib tracks on this boat one is further aft and runs athwartships as well,

It does appear to have a track, by the cockpit combing.
- Will (Dragonfly)

 

[JRacer]

Follow on to that last point, look at the trim on a TP52 raceboat going upwind in optimal conditions. The jib chord angle is about 6 degrees, and the main is on centerline. Notice that the jib tracks run athwartships! The boat is probably sailing into 25 degrees of true wind. The apparent feels like it is on the nose.

View attachment 144109

And, it would appear that the crew is all puking over the side.

 

[BobbyFunn]

The jib has an imaginary line to near the deepest draft of main or trailing edge of main.

If jib had a boom that extended to imaginary trailing edge kissing main it would be pretty close to angle of main boom. Even thought the imaginary boom from the jib tack to jib clew would be greater.

 

[Jackdaw]

It does appear to have a track, by the cockpit combing.
- Will (Dragonfly)

I know. But its not. ;^)

Remember the boat has rigging that goes out to the gunnels. So a head sail cannot be trimmed around the shrouds.

 

[Jackdaw]

And, it would appear that the crew is all puking over the side.

Hero hiking. Whenever we get pictures like that, the crew examines to see who is hiking hard, and who's sitting on the rail taking a ride. You don't want to be seen just taking a ride.

 

[BobbyFunn]

Looking at the samples from everyone and thinking about the jib as if it always should be trimmed as if the jib ended by the main clew got me thinking.

The jibs shouldn't be cut as a complete naca profile. The jib depending on how far back it goes should be cut so that from tack to clew is a part of naca profile of the entire sail plan.

That would almost entirly eliminate the back winding on the main and certainly improve the slot.

The jib on the hunter170 is a complete naca profile. Its very hard to get rid of backwinding.

I made some jibs for my rc boat and they too were modeled on complete naca profiles. Looking forward to testing these ideas out.

 

[jwing]

Boom on the center-line is universal starting point for upwind trim on all boats, regardless of what their actual TWA into the wind will be. That angle (as you guessed) is made of of many factoring components, but jib sheeting angle is clearly a big one. Optimally 9-8 degrees on most production boats, it is often wider due to construction constraints... like cabin walls or windows. On these boats, inhaulers can often buy you an extra 3 degrees or more of point.

I decided to thoroughly read one source on sail trim, apply what I read, then compare my experience tto the book-learnin', and then contemplate other sources of info. I don't remember why, but I chose: Illustrated Sail & Rig Tuning (Illustrated Nautical Manuals) by Ivar Dedekam.

Mr. Dedekam's explanation of the question posed by Jackdaw has been a bit of a head-scratcher for me, but it works (I think), so it's good enough for me, so far. He says that the main sail creates an air "upwash" that affects the foresail in such a way that the jib must be let out to a higher sheeting angle than the main for optimal performance. In other words, the main lifts the jib. That implies to me that the main boom should be trimmed first, in accordance with Jackdaw's statement.

Dedekam goes on to instruct that the optimal jib sheeting angle is 7-20 degrees, depending on the boat, and offers that inhaulers can reduce the angle if the jib lead track is too far outboard. He adds that increasing the jib angle is good for heavy conditions, but I've yet to experiment with that.

 

[BobbyFunn]

Heres another sketch only now the boat is closer to a reach.

Ive added an asymetric. Does this fit with your experiences?

Note the jib angle is still

different than the asymetric and the main, but if the line extends to the full sailplan the angles are parallel. This looks like a couple VOR images.

Note the short jib is a mess to sheet. The ideal attachment point is over water. Any sheet trailing off the jib has to terminate over water or pull the clew closer to centerline. That ruins tbe shape and begins main backwashing. Thought i saw some raceboats with outriggers for jibs, no?

 

[walt]

Finally found the picture below. I saw this in some forum and its linked from boatdesign.net.. but dont know where.

I believe that pressure is inversely proportional to velocity so red would be lowest pressure and blue highest pressure. This is a simulation but you can see about the angles in the original post of the main with almost no angle of attack and some angle of attack on the jib (is that 8 degrees.. dont know but maybe ballpark). .

Some interesting observations.. the highest lift is going to be where you see lots of red next to lots of blue and that is happening on the jib. Lift is normal to the sail so the lift vector of the jib is also much better for a forward force.

It looks like the velocity away from the sail is green so I would take this to be apparent wind speed if the sail wasn't there. The color is green to yellow between the two foils so there really isn't much acceleration in the slot.

Also interesting.. if the jib was not there, the main would need to generate all the lift and also operate at an angle of attack closer to what the jib is doing - other wise you would have no forward vector in the lift generated. Its likely that the simulation now would show a huge interruption due to the non rotating mast. But add the jib and the non rotating mast would seem to not matter as much now. Non rotating mast especially not so important for upwind and downwind racing.

 

[Owen Thistle]

I believe that pressure is inversely proportional to velocity so red would be lowest pressure and blue highest pressure.

The pressure is actually inversely proportional to the square of the velocity. That doesn't take anything away from your points, but is good to know when looking at these graphs. If you double the velocity delta you quadruple the pressure delta.

 

[walt]

Owen, thanks.. that part was a little over my head.. I thought is was governed by the ideal gas law or something like that.. https://en.wikipedia.org/wiki/Ideal_gas_law P*V = nRT (ie, Pressure times Velicity is a constant for the existing conditions). But all the lift and drag equations have velocity square term in them similar to what you mentioned..

 

[Owen Thistle]

Owen, thanks.. that part was a little over my head.. I thought is was governed by the ideal gas law or something like that.. https://en.wikipedia.org/wiki/Ideal_gas_law P*V = nRT (ie, Pressure times Velicity is a constant for the existing conditions). But all the lift and drag equations have velocity square term in them similar to what you mentioned..

You're very welcome, I'm glad it didn't come across as nit-picking. It's a pretty significant thing though. No wonder it's so hard to get just the right amount of wind - it "dies" when it drops in half and it knocks you over when it doubles. It's usually called Bernoulli's principle, and is a consequence of conservation of energy in a closed system (the first law of thermodynamics). Potential energy in the fluid is proportional to the pressure and kinetic energy is proportional to the mass times the velocity squared, so when the energy changes form the pressure change has to respond to the square of the velocity change to balance out. Then it gets more complicated with viscosity, boundary layers and circulation, which I won't pretend to fully understand. When it comes to sailing and lift forces it seems like practical knowledge is still a step ahead of mathematical predictions.

P.S. The V in the ideal gas law is volume, not velocity. If you reduce the volume by half and keep temperature constant the pressure will double, etc. You in Denver and I in Calgary experience the ideal gas law every now and then when a Chinook wind pushes air down the mountains - P goes up, V stays about the same, and T goes up, sometimes a whole lot.

 

[Jackdaw]

With advances in technologies like CFD I find looking at the digital sailplan models instructive. The rub is, the theorists don’t fully agree, and even the best models are full of imperial coefficients that they use to allow the models to come close to actual real-world testing. But it’s clearly the way forward!

To me, the best course of action is to know what the models suggest will optimize trim, and then, with repetitive and detailed testing, go see what makes your boat fastest on the water.

 

[Jackdaw]

Heres another sketch only now the boat is closer to a reach.

Ive added an asymetric. Does this fit with your experiences?

Note the jib angle is still View attachment 144116 different than the asymetric and the main, but if the line extends to the full sailplan the angles are parallel. This looks like a couple VOR images.

Note the short jib is a mess to sheet. The ideal attachment point is over water. Any sheet trailing off the jib has to terminate over water or pull the clew closer to centerline. That ruins tbe shape and begins main backwashing. Thought i saw some raceboats with outriggers for jibs, no?

Lots of questions here!

With three sails, each one will be trimmed tighter than the one in front of it. Your picture shows them in equal trim.

Your picture of upwind trim shows the boom really far off the centerline. That’s never the case for optimal trim.

Yes, when Reaching with the jib clew has to move outboard for proper trim. Most modern boats now have a mechanism to allow this, either with out haulers or with three-dimensional clew rings.

Yes the Volvo boats use reaching poles to Hold the spin sheet outboard, to allow the clew to stay outboard. Greatly improves sail shape with the code 0.

 

[Will Gilmore]

To me, the best course of action is to know what the models suggest will optimize trim, and then, with repetitive and detailed testing, go see what makes your boat fastest on the water.

There couldn't be a better way.

Even with centuries of experience, study and modeling, we still only have theory that appears to work but not much in the way of irrefutable laws of physics. And, there are so many variations and variables that averages and statistics are the best we can do for now.
Just get to know the theories and get to know your boat. Even one designs vary with things like weight distribution, materials, individual air and water currents, drag above and below the waterline, ... the list is endless.
- Will (Dragonfly)

 

[BobbyFunn]

Simulations are cool but they are sketchy. The math is pretty far advanced, but there is too much in chaos (real fractal randomness) that accounting for sail thread pattern and batton stiffness, among other things that cannot be accounted for other than looking at telltale for real and looking at speed guages or competitors.

In the 1930's they did a bunch of wind tunnel tests and came up with most of the naca profiles we use today. The P31 airplane was a top secret design based on the video posted here earlier. Based on the wind tunnels the P31 should have perfect laminar flow along lifting curve. In practice it still has almost none. It was a better wing no doubt, but the real world didimt have striahht wind flow and even the paint on the wing ruined the laminar flow. Even today it is impossible to have the laminar flow on a real world surface. Its just less turbulence.