Sail trim quiz question
- Thread starter Jackdaw
- Start date
I was focusing mostly on non-overlapping (mostly 9/10s) boats as well, because that's what I sail... First 260, FarEast28r, J/70, Pogo 40... Also that's where all the current modeling is being done. On these boats the starting point for main trim is boom to the centerline... in optimal conditions slightly above. I'll agree that each boat will be different and even more different in changing conditions. The point of the original question was more based the chord angle difference between the headsail (8-ish) and the main (0-ish), and not the exact angle of the boom.
But the follow-up conversations are just as good!
OK, another follow-up for you. When close-hauled and overpowered, one technique is to ease the main and let the luff of the main soften while keeping the trailing edge flat. How does this fit with that single jib/mail foil model?
Not just soften but get backwinded, as in this photo.
Not just soften but get backwinded, as in this photo.
For sure, the time-honored 'sailing the back half of the main' model. Speed bubble and a trimmed leech. The key here is that you are over-powered. All sail trim models are designed for fully powered up scenarios. Once you get more pressure than that, your job is to deal with it without creating more drag. Sailing the back half does that.
I once had a main trimmer would would let the whole sail luff. I told her 'That's all drag. You keep that up and I'm going to tell them to drop the sail'.
That’s an interesting point. In their upwind configuration the VOR65 is fractional rigged, so the main would be the largest power source. With multiple headsails you would think each sail would be headed by the one in front of it, so the main is headed not just by the Genoa but also the staysail. Further complicating things it looks like the the code sails are carried at the masthead, making them a larger power source and presumably compensating for the loss of power from the headed main.
So, in terms of the jib/main system I think of it as another case of "trimming" the combined foil that they comprise. Easing the main reduces the camber of the overall foil, reducing both lift and drag. Like flattening the flaps on a wing as the speed increases. I think its a good trim example.
The drawing you supplued has a different boom angle. That might have thrown us off track.
Turns out the math was done in 1933
https://ntrs.nasa.gov/search.jsp?R=19930091501
Turns out the math was done in 1933
https://ntrs.nasa.gov/search.jsp?R=19930091501
One more cant resist. Note how AoA increases with each surface. The composite of all surfaces is one upper/leeward curve. The lower/windward side is much less important. Thats why weapons and engines are on bottom.
This is a 747 coming in for a landing. Note the smooth condensation trail tracing out the laminar flow. Is there also a bubble at the 2nd slot?
This is a 747 coming in for a landing. Note the smooth condensation trail tracing out the laminar flow. Is there also a bubble at the 2nd slot?
The two booms were meant to have different angles. One expected(0), the one matching the jib (8).
This statement is often made about lift on a wing or sail but, not exactly accurate. Lift is the result of the pressure difference between the outer surface and the inner surface. Therefore, the inner surface is just as important in generating lift. However, that pressure difference is achieved by constricting the flow over the outer surface. This causes that flow to accelerate in order to equalize between the leading edge and the trailing edge, thus reducing the pressure on that surface. Pressure across the inner surface is higher because flow is slower and force vectors are less directed from fore to aft. It happens that the inner surface is more forgiving with regards to drag because of the slower flow. It is possible to interfere with lift by changing the inner surface. It's just that adding a few protrusions to the underside actually may increase pressure by slowing the fluid (air) even more. In the end, according to Bernoulli, they must all equal out at the trailing edge.
By the way, I first read this in Nathanael Herreshoff's book Common Sense of Yacht Design when I was in high school. My physics books put it into modern terms.
BTW. Awesome post, Jackdaw. I have been thinking there might be room for a forum dedicated to seamanship for just this kind of discussion. Also anchoring, knots, navigation.
- Will (Dragonfly)
This is a pretty cool video i fall back frequently. Its a silent film from the 1930's demonstrating wings and plates and flaps. The complex wing begins at about 8 minutes. So much in there for rudders, centerboards, mainsails, headsails.
The slot extends laminar flow and provides more lift at lower angles of attack. That sounds pretty good for going up wind.
The ramifications are do it right fly upwind, do it wrong you go slow.
Since everyone has a different wind with different sails its a tough call to say one set up is the best for everyone. I dont have a traveler on my tub so ill grab tbe mainsheet lines once and a while to finish the leeward curve.
The slot extends laminar flow and provides more lift at lower angles of attack. That sounds pretty good for going up wind.
The ramifications are do it right fly upwind, do it wrong you go slow.
Since everyone has a different wind with different sails its a tough call to say one set up is the best for everyone. I dont have a traveler on my tub so ill grab tbe mainsheet lines once and a while to finish the leeward curve.
Jackdaw,
One thing occurred to me regarding your original diagram...
Let's assume, in the drawing, that the boat is sailing mainsail only and no jib. We've established that without the 'header wind' caused by the jib, the main is over trimmed. If that is the case, then the boat, in theory, should be able to point 8 degrees higher where the wind flowing over the sail would be optimal and the sail properly trimmed. Yet we know that sailing without the jib often requires us to foot off rather than sail higher.
How can you explain that?
One thing occurred to me regarding your original diagram...
Let's assume, in the drawing, that the boat is sailing mainsail only and no jib. We've established that without the 'header wind' caused by the jib, the main is over trimmed. If that is the case, then the boat, in theory, should be able to point 8 degrees higher where the wind flowing over the sail would be optimal and the sail properly trimmed. Yet we know that sailing without the jib often requires us to foot off rather than sail higher.
How can you explain that?
It seems that it would be the case that by adding a jib to that equation you get the lift of the jib, which performs better than the main due to an improved leading edge, and the slot helps increase laminar flow over the after surface of the main by directing more air into the slip stream. That would increase air speed over the outer surface on the back side as it exits. The pressure variance would improve between the inside and outside of the curve. The limit would be in the angle to the air flow. You wouldn't be able to pull the main past center without causing it to come into the wind and stall.
It would have a tendency to increase weather helm except the increased pressure on the inside of the jib would increase Lee helm even more.
I'm making this all up, of course. It just makes sense based on what I learned in school and see when sailing. I'm curious to know what real experts think.
- Will (Dragonfly)
Sailing on the just the main, the main will be eased out and the boat will be bow-down (sailing at a wider angle) than it would with a jib up. That's expected
Putting the jib up, the main gets trimmed in, and the boat will point a few degrees higher. That's expected as well.
There is a upper limit. At some point, any boats foils (sails, keel, rudder) will start to stall as the AoA gets too tight. While for most boats the optimal angle is close to 45 true wind, race boats can easily cut 10 degrees off of this.
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.
What do you think of this sketch?
The jib if you connect from tack to clew it makes a line with an angle, and if you look at the main and connect the tack and clew it makes a different angle. I propose this...
If you imagine the jib as a single sail that extends until it touches the max depth of the main tangent you have a line from the jib tack and main tangent that is pretty close to parallel to the main tack to clew(boom).
The jib stops before the mast so the angle looks different, but the angle of attack might actually be tbe same for both sails. The jib just has a larger camber.
The jib if you connect from tack to clew it makes a line with an angle, and if you look at the main and connect the tack and clew it makes a different angle. I propose this...
If you imagine the jib as a single sail that extends until it touches the max depth of the main tangent you have a line from the jib tack and main tangent that is pretty close to parallel to the main tack to clew(boom).
The jib stops before the mast so the angle looks different, but the angle of attack might actually be tbe same for both sails. The jib just has a larger camber.
