Thanks you all again for the very valueble help. It's very good to know that sometimes it's not us, but the gear. For now this spinnaker is good to learn, but indeed in time we can invest in something better.

We certainly will check the tack line, and replace it by something less elastic.

@ Jake: Thanks for all the lessons. This is much appreciated. Indeed the jib is in our team often neglected. We should pay more attention to it. Often it starts to rattle a little, and therefore we sheet it in. We experienced very recently (after some comments from another sailor) the benefits which it could have.

About the sheeting in part: Last year we decided to go for a newer sail on our Inter 18 when we needed to replace it. Due to the large top, the foot of the sail is shorter, and we had to find a way to connect it to the boom. The sail works out quite good, however the connection could be a little better (shorter) Now I have the feeling we loose about 50 to 90 mm there (2-4 inches). And indeed sometimes I have the feeling that I can use it on the sheet. So we have to look for another modification there (maybe replace the boom)