I've picked up a Chinook boom over the off season and have a question regarding the markings on the extension in relation to what they actually mean.
My sail has a 188cm boom setting...on the Chinook boom 188cm is marked but is it point to the pin hole or just a point on the extension. Sound strange? I'll explain.
Last couple of times out I set the extension to have the pin go into the hole marked 188cm and outhauled to the end of the boom, but the sail...(with right downhaul set) was difficult to control. I experimented this morning and set it to 188cm and measured from the inside of the front clamp where the front of the mast is (which other posts in this forum indicate is the correct place) to the inside of the end of the extension. It was only 186cm.
So if I pull the extension out so the next pin (190cm) is engaged and the 188cm marking is now showing and pointing at the end of the clamp and measure it is now 188cm.
I checked the Chinook website and there seems to be no information about what is correct or not. They focus on how to lace the tail piece.
Anyone know where the markings are meant to line up on the boom. With the pin or with the end of the clamp?
Setting any extension by the numbers is just basic maths unless it's 20 knots and then it suddenly becomes an Einsteinium set of calculations that can never be figured out.
You are setting it too short. The 188cm is to the end of the outer tube not the holes. Like sean said the length marking you want should be seen.
I think you answered your own question with "So if I pull the extension out so the next pin (190cm) is engaged and the 188cm marking is now showing and pointing at the end of the clamp and measure it is now 188cm. " 
That 2cm can make a big difference
Thanks everyone for the responses. I'll take that as a rule of thumb.. "The length you want should be seen".
I should find the sail a lot more user friendly next time I'm out since I've been under outhauling the last two times out and getting smashed.
