You might have some other problem. The shroud length seems ok with 18´ 11". Three sides make a triangle, check if the shrouds enter the hulls at the same position as on other cats. Check the length on the mast from bottom to the shackle where the shrouds are attached and compare with other H16. With the 18´11" shrouds you definitely should get quite some rake when using the bottom hole. My shrouds are about 1 inch longer and I use the second hole from bottom to get the "block to block" mast rake.