Mainsail Trim

[chucklesR]

There's a tad of ambiguity in your (original) post so pardon me for wandering.

3-4 = knots or Beaufort Scale?

Assuming 3-4 knts of wind speed upwind you can get more speed (in a typical 33 ft cruiser, maybe 2.5 - 3 knts VOG) in such light air because the apparent wind will be higher and that will help, you say you have owned several boats so I'll assume you know that.
Wind off the beam, and especially behind the beam with no apparent wind to lift you, of course you will 'slow right down' - the best you make is wind speed minus drag - probably all of 2 knots tops in a typical production boat.

Then again, with wind that low and (for us readers) unknown current - the act of changing from upwind to offwind could mean the current/tide caused the boat speed change.
It's really hard to guess all the factors - so I mention them in passing, no insults intended if it seems a simplistic response.

You don't mention your jib or what it is doing so I'm assuming it's set correctly and wind ahead of beam telltales are flowing aft, wind behind beam the forward telltales are limp, inside are dancing independentally.

Luffing the back of main with the wind on the beam implies the front of the main is full - which means the draft of the sail is moved forward by releasing the sheet more than you feel is appropriate, yet not enough to luff the front of the main, admittedly strange, but why your boat speed improved - your forward driving force improved. You can get the same effect (draft shifted forward) by leaving be on the sheet and easing your outhaul and creating a fuller mainsail (which is the correct thing to do in light air). Leech line tension should also be checked as others have indicated, it should not cup and trap air, it should let air flow smoothly off the back of the sail while still remaining 'attached' to both sides. Incidently, in air that light it also helps to ease the main's halyard just a bit and fall off a tad more than normal when going to wind.

If I got anything screwy I assure it because of multiple edits

[jobark01]

All of the preceeding is great advise and good analysis on the subject of mainsail trim. I, we, are lucky to have this venue for exchange - lord knows I learn something new from everyone here.

One thing - ChucklesR - that is the ugliest 4 legged thing I have seen outside of the world's ugliest dog contest. I love it! Thanks for sharing.

[sailingdog]

How dare you insult our friend Chuckles...that's an accurate and recent photo of him.

Originally Posted by jobark01

All of the preceeding is great advise and good analysis on the subject of mainsail trim. I, we, are lucky to have this venue for exchange - lord knows I learn something new from everyone here.

One thing - ChucklesR - that is the ugliest 4 legged thing I have seen outside of the world's ugliest dog contest. I love it! Thanks for sharing.

[jobark01]

Dog, no offense was meant - just glad we all have (or had) mothers to love us! still love the photo.

[bambam]

there are a lot of questions I have such as wind speed and direction is there weather helm what are the tell tails doing? but from what I have read and if I am correct you may want to ease the out haul. It was stated that you pick up speed as th main is let out if this is correct it would seem that the sail might be too flat. it is not uncommon to over trim the main if you know any one with boats that have similar rigs go out with them and see how they trim and how their boat performs.
Other valuable source would be the dealer or a Rigger. These are a best guess on the info in your post but as always there are too many variables to consider

[jbondy]

Also, the type of rig and size of the jib may factor into what is happening here. I've heard that with a masthead rig and a large genoa, it is not uncommon for the jib to backwind the main a little. With no battens in the main, the extra curvature may contribute to this. The majority of the main, though, is now in a better position to contribute to forward motion. With telltales on the leech of the main, you can let the main out until they all stream in line with the sail. If there is a bit of flutter in the luff, so be it. The main should usually be eased more than one thinks, hence the common mistake of over-trimming. Just a thought.

[sailingdog]

Easing the outhaul might help in light winds..but will make problems in heavier winds. Overtrimming is probably the most common mistake, and letting the main out until you see the sail start to luff and then trimming just until it stops... is generally a good idea.

Originally Posted by bambam

there are a lot of questions I have such as wind speed and direction is there weather helm what are the tell tails doing? but from what I have read and if I am correct you may want to ease the out haul. It was stated that you pick up speed as th main is let out if this is correct it would seem that the sail might be too flat. it is not uncommon to over trim the main if you know any one with boats that have similar rigs go out with them and see how they trim and how their boat performs.
Other valuable source would be the dealer or a Rigger. These are a best guess on the info in your post but as always there are too many variables to consider

[chucklesR]

Originally Posted by sailingdog

How dare you insult our friend Chuckles...that's an accurate and recent photo of him.

I'll admit the photo is a photoshop of my face on my cat's body, and the photo is from before I lost weight and earned the skeletor nickname.

[sailingdog]

That is certainly not a cat.

[Bellamar]

My thought is that the sail has no belly. The wind is not that great in the example you set, and I would think that loosening the Vang and moving the traveller up is a possibility.