Roadsters aren't supposed to be practical. They have two seats, hardly any spare room, and a soft top. That shouldn't stop you from using one to transport a 17 foot kayak, though.
When I first started building a kayak about a year ago, I thought I would get a hitch for my Z3 and transport the kayak with a small trailer. Hitches aren't cheap, though, and would add a significant amount of weight (plus, I'd have to buy a trailer). It occurred to me that I might be able to bridge the gap between the roll hoops and suspend the kayak between that point and the top of the windshield. So, I drew this to get started:
Next, I cut pieces of dense foam out of an old laptop box and trimmed, sanded, and scraped until I had two pieces that would fit over each of the the roll hoops.
To bridge the gap, I used fiberglass and epoxy to bond a wooden beam between the two halves. For appearance's sake, I wrapped fiberboard around the crossbar (forming an airfoil) to make the rack look more aerodynamic. Fiberglass was then added to the whole structure to smooth the surface and strengthen the rack. I cut six or seven thin wooden strips and laminated a cradle to match the kayak's hull. This is a picture of the rack before receiving its final coats of paint:
To load the kayak, you start by slipping the rack over the roll hoops and tying each half to its respective hoop. I then attach two loops to the top of the windshield (where the roof connects with the windshield) to use as tie-off points for straps. The kayak can then be lifted and slid across the top from the rear pretty easily. I use two ratcheting straps to tie the kayak down in the middle. A piece of foam placed between the top of the windshield and the hull protects both surfaces from bumps and scratches.
Apart from off-center-mounted tow hooks, Z3s do not have strong attachment points on the front and rear bumpers for securing the ends of the kayak. The removable tow hooks are positioned on the passenger's side for the front bumper and on the driver's side for the rear bumper. Tying the ends to these points works, but it does tend to cause the kayak to want to shift diagonally along the top of the car. Fortunately, the straps used to hold down the kayak in the center do a pretty good job of limiting motion in all directions. I tie the ends down with the very useful trucker's hitch knot.
No, the Z3 is not the ideal car for transporting a kayak, but you can make it work. Just drive slowly (<55 mph) and hope it doesn't rain. I've carried my kayak this way several times now and will continue to do so until I get a vehicle with a proper roof.