On my boat I've noticed that sometimes when I'm close hauled, wind and waves from the same direction, that starboard tack is faster than port tack, sometimes by a whole knot. I can't figure out why.
My rig is balanced, straight mast, I've tuned the shrouds at very nearly the same tension. I do have a bit of a rake, about 2-3 inches. Rudder isn't bent, keel isn't either.
At dock there is a slight list to the port side as there's more weight there, but I can't imagine that's the reason. If anything, shouldn't that make me faster on a port tack?
Am I missing something?
Reading the words I bolded from your post, I take it that the effect is not always there and when it is there not always in the same amount. There are a number of things that could cause your boat to be slower on one tack than the other. Start making notes when you go out, and keep track of what the conditions (wind, current, sea state, trim) are when you experience the effect and note the level of the effect as well. Also make note of the wind direction and your heading for each tack. Are you pointing higher and slower on port, lower and slower, or equal pointing and slower. Try also to determine the amount of leeway on each tack by comparing GPS heading vs magentic heading (adjusting for variation) and speed over water with your knotmeter vs GPS speed. That kind of information would be very helpful in diagnosing the problem.
You also state that the shrouds are very nearly the same tension. How did you measure this? If it was by hand and not with a Loos guage, your estimate could be surprisingly wrong. If you have not already, put a Loos guage on it. Also, make sure that the mast is truly centered. This can be a surprisingly difficult thing to measure since boats are rarely perfectly symmetrical, but is important to get right. Measure to the rail, and not to any deck hardware, and make sure the point you are measuring to is equal on both port and starboard by measuring the distance to the bow and transom from both points. Once you know the mast is centered, check to see if the deck hardware is placed in the same exact location on both sides of the boat. It is not unheard of for a boat builder to work with a hangover, leaving your sheeting angles on one side less than ideal.
Lastly, don't be surprised if your keel is actually not on straight or if the shape is not symmetrical (many boats came from the factory this way), either of which could cause a difference in speed and/or pointing from one tack to another.
I am going to make a wild guess that it is actually rig tension, and that the lowers and/or uppers are different tension on one side vs the other. you may not notice this in 8 knots, but when the wind picks up to 15 or 20, the mast is probably bending different on one tack vs the other. Could be that the uppers on stbd are lower and so the top is bending off, and keeping the boat flatter when the wind kicks up, or maybe the port lowers are not as tensioned and so the center sags to leeward on port tack giving you a not so fast shape. Of course this is just a wild guess, and I am just as likely to be wrong as right having never sailed on your boat.