The N17 Mk. 2 and F20c from Nacra both have substantial inside reinforcements where the beams meet the hulls and at the center sleeves. Upgrade kits are available, and Nacra have standardized some of this across the range (i.e the F18 Infusion gets the same beam as the N17 Mk. 2 nowadays, though the stock beams work fine and haven't given me any issues). There were a handful of 10' beams broken on the early F20c's, perhaps those are the beams the dutch importer had lying around? I would definitely advise the O.P to go for substantial reinforcing though with a F18 rig I don't think you have the power that the F20c has and frankly the 10' beam is overkill (it just means you'll be in the trapeze in 12-13kts of breeze instead of 10 kts as an example).
Last edited by samc99us; 01/11/18 10:43 AM.