A Painted Ocean

Is there an issue with backed sail producing a turning moment when making way astern which is opposite in sign to that expected.

(in the eye of the wind, with backed mizzen alone, set a full stack of sail - I'd expect drag to take the stern downwind, I'd also expect the pressure on the backed sail to drive the stern away. from the wind. Currently the stern will draw up under that condition, even with rudder set for the desired coming through the eye while making sternway). This seems to warrant investigation, as it is counter my expectation.

I'm not sure what you mean by driving the stern away from the wind. Surely when you start off in eye of the wind, then whichever way the stern turns, port or starboard, it's into the wind?

This is the behaviour I see now. If I start off in the eye of the wind, and set just the mizzen topsail and topgallant, braced up on the port side, then as expected this pushes the stern over to port (so the head of the ship turns to starboard). However if, after the ship has gathered some sternway, I then brace the mizzen sails up on the starboard side, the stern continues going slowly to port, rather than coming around the other way.

What's happening in this second part is that there are two forces fighting against each other. The hull going backwards has some angle of attack on it, so the lift force is generating torque making it want the stern want to turn even further to port; while the sail is trying to push it the other way. I think the hydrodynamic torque is way too strong now, because the lift force is acting too far aft. Essentially it's the same issue as the "griping" control, but when going backwards. Effectively the backwards griping is set at 20. A quick test shows the sails having more effect if I set this to 10, but I'm not sure what the correct behaviour is.

https://imgur.com/a/lqFpt5d

A bare hull should turn to port when driving forward with the rudder set to turn to port. It should also drive to port when the rudder is sent over (and we seem to miss an explicit command to reset the rudder midships) to turn to starboard but with sternway on. This should be at roughly the same radius regardless of speed in the absence of strong trim from the rig.

When you set sail balance ahead of the centre of lateral effort the head should fall off the wind, with the balance behind the lateral effort the head should come to.

"Eye of the wind" is listed over a range from roughly the bracing angle of the yard through the wind in your reporting. This is the usage I made - and with the rudder and mizzen stack and spanker all working together to bring the head around I cannot see the head falling off being a reasonable outcome.

The scale of drag from backed sails seems much lower than the drawing of a driving sail, but this is contradictory to my understanding, where the CD max of a cloth airfoil at 90 degrees is closer to 2.0, with the 'filling and lifting' portion of the drag and lift curve being only important at the relatively low angles of attack. CD being relatively flat function of AOA over the 'draggy' portion of 'non lift'.

ppitm2 years ago (1 edit)

So when I teleport-turn the ship into the position below, the bow falls off the wind. I believe this is the scenario Lieste is describing as counterintuitive. However, if I repeat the experiment with all fore-and-after canvas doused, the ship turns bow to wind and even tacks itself. So the aerodynamic behavior of the square sails seems to be correct. This raises the interesting question of whether it is possible to tack the frigate while stationary...

Just stopped by to say that I finally managed to circumnavigate (with manual sail selection as well)! 294 days starting September 18th in Plymouth and ending in the same spot on July 9th.

The default sail trim looks a bit off - with topsail and topgallant and on a beam wind, I need to have all three headsails and rudder 'off the wind' - The mizzen needs to be shivered or furled to give even vaguely neutral trim with only 1-2 headsails.

If I set the Griping to 3 or so then I get a more controllable trim which allows all the square sails to draw, and the rudder to be trimming the course, rather than fighting the sails, small variations around here give a slight tendency to bear off the wind or come to it, but the default value (6) is too extreme IMO.

Thanks for looking at this. How do you find tacking, if you reduce griping to 3? Does it become much harder?

I carried through a tack without making sternway or performing any adjustment to the sails set (headsails eased, spanker hauled in on the approach to the wind, then reversed after backing the main and mizzen and passing through the wind). I could have carried even more speed through the tack with a moderation in canvas in places where it opposes the motion and causes merely drag.

I'll have to try from a lower speed as well, this was from a fairly high speed initiation.

Lieste2 years ago (1 edit)

So. Within the harbour scenario. Came off the anchorage to face south, under topsails jib and spanker. Tacked to the north gaining 4 lengths to windward, pivoting under sternway in under a ship length and gaining a course to the north with a net gain of roughly one length to windward. (Speed around 4 kts on entry). Wind on the beam.

When at 4 kts to the north, again tacked this time under reefed topsail, lost way after the turn backed the fore topsail, but continued after reversing rudder getting around to the south within around 1.5 lengths - lost more room 'gathering' under stronger leeway, but the net loss around one length, and the recovery to the beam wind had the ship pointed to the upwind side harbour exit.

I'd describe this as comfortable tacking under light sail in "Fresh Breeze (Royals etc), though not nearly as sharp as under more sail and with more way on.

Trim on a beam wind or higher can be performed with reef on the mizzen and by trimming, easing and hauling the spanker and jib, with the rudder able to be kept neutral or a point or two to favour gathering way, or staying up on the wind. This gives a minimal leeway and less trim drag when accelerating.

In 17 kts of wind and good headway, I find the ship to maintain course with a neutral helm and all sail set except for the spanker and mizzen staysails. This strikes me as a very reasonable result.

It may well be that at lower speeds the ship is unbalanced with this sailplan.

For some reason in the new version there does not seem to be any AA. I took pictures in both builds side by side and the difference is quite pronounced:

That's weird, for me this build doesn't seem to change anything about AA. If you hold down shift while double clicking on the game icon to run it, Unity opens a graphics quality menu. This is supposed to default to "fantastic" but maybe I did something to mess this up. Anyway, you can change it and I believe Unity remembers the change.

I have replaced the uploaded build with a new one, still 5b but marked "February 2022" which I hope has the right default.

OK, for some reason my graphics were set to Fastest, but your trick worked. Probably has something to do with my GPU being dead and using an integrated chip?

BTW the main menu says January, not February

I have a double wind arrow on the compass in the new build

The double arrow is deliberate. It's one arrow for light winds, two for medium, three for strong. Is it a problem now to see where the wind is coming from?

Make sense, I assumed it was a bug. Still easy to read.