We mark things for a few different reasons. One is to be able to replicate settings - they give us a reasonably exact breadcrumb trail to be able to get back to settings we found to be fast. They also allow us to gauge how much we are moving stuff - going "one stripe more" on jib halyard tension is much more exact than "adding a little halyard tension."
Some marks are what I'll call hard references. On the M32, we mark the daggerboards at the point two which we raise the windward board. We're not trying to learn about the shape of the daggerboard at that point or anything, we just want to know when it's up as much as we want it to be, so we mark that point and that's how high it gets raised. You should mark your centerboard so you know how high it is.
Other parts are settings references. Where you put your shroud pins is a good example of this. Also, where your jib halyard block is on the mast is another. The combination of those two will tell you your rake and tension settings, because if your shroud pins are at position "X" and your halyard is at position "3," there is a rake and tension combination that they will make.
This picture shows how you would mark your jib halyard. Make a set of marks like the pink ones, that cover the range of where your halyard block would be in different conditions. You need to know where your reference point is on the halyard block, and use the same reference point all the time.
Then there are trim references. Trim references are sort of kind of like training wheels. They are good to use, but eventually what you want to be able to do is look at the sail and be able to adjust it to make it look how you want it to look. I'm not saying that these trim references are wrong at all, and they are efficient to use even for very advanced sailors - for example I might know that if my jib lead is at position "C," I have to be careful about how tight I trim the jib in order to prevent stalling. Even if I can't see the jib leech to know when it's stalling, I can look at other clues to help me know where I am with the trim - how curled the foot is, how many stress wrinkles the clew has, etc.
Here are some of the types of trim references you can use:
Having clew stripes like this allows you to know the trim axis of your jib sheet. If the jib sheet is lining up with the orange/yellow line, we know that we are trimming the foot quite hard and the leech not so much. If the sheet lines up with the red line, then we are trimming the leech very hard and the foot will be round. The main trim line should point to a spot on the jib luff that is slightly less than halfway up the luff from the tack. On a boat that doesn't have adjustable jib leads, your rake setting will be the only adjustment you can make to this.
Stripes like this orange ones on the splash rail or deck allow you to see how far inboard your sheeting is. Windward sheeting, leeward sheeting, and rake/rig tension (to a smaller degree) will affect how your jib lines up with this set of marks. Windward sheeting has the biggest effect. Notice proper use of "affect" and "effect" in this caption.
Marks like these on your spreaders will tell you the inboard/outboard position of your jib leech. The clew stripes (which are most affected by rake and rig tension, remember) and leeward and windward sheet tension will affect how your jib leech lines up with these stripes. If we had our jib sheet lined up with the yellow/orange clew stripe in the picture above, the jib leech would line up with a far outboard spreader stripe like the yellow one pictured here. If we lined up the jib sheet with the red clew stripe, the jib leech would move inboard, to the blue line maybe. Having your jib leech further inboard can help your pointing, but it makes the boat harder to sail (especially in chop) and can hurt boat speed. Fast sailing is all about balancing different things.
Marking the vang is helpful. Mark it like this, in the purchase system, instead of on the tail. On I420s and Z420s, you have two tails and they move around relative to one another, so marking the tails is useless.
The last picture I'll put up today is this one. You could use a sticker like this for your jib halyard markings, and you can also use it for your outhaul. Marking the outhaul is good.
The point of using all these marks isn't to become robots and set things to specified marks and away we go. Instead, they help us quantify different settings. "Outhaul at position 4" is more exact than saying "medium outhaul." If you were really really overpowered and you had your shroud pins at position 3, so try moving them down to position 5 next time and see if that works better. If position 5 gives you great speed but your pointing is bad, then you can try position 4. Just remember that your jib halyard and shroud pin positions are always interconnected - you can't think of one without referencing the other.
Unless you have your marks exactly calibrated to another boat's marks, they are only really useful when sailing your boat - to talk about your jib leech lining up with your third spreader mark is useless if the other boat's third spreader mark isn't precisely where yours is. About the best you can do in that situation is if the other boat is going better and you all think that their jib leech is positioned more inboard than yours, you might try to set your jib so that the leech lines up one mark farther in on your spreader. If you are going to tune with another boat a lot, it is worth calibrating all of your marks to be exactly precisely the same. That allows you to move one variable at a time, to an amount and to a position that both boats know, and if it works for one boat then the other one can replicate it quickly and try it.
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