Things to consider is getting in your way when up, proper height to function as you choose when it's up and where/how you going to store it. Next the sun never rests and neither does you boat on the water. So about the only time you can calculate your shade pattern is at high noon...sun vertical.
Take some time, think about it, get in the boat with others and pretend you had the top, go through the what if's and you should do alright.
I have had several and I thought I wanted a heavy duty one once. It was about 7' or so as I recall. I had it made from heavy rubber reinforced canvas and used ss bows. It was nice and tough alright, but it took two folks and a bear to do anything with it. I never saw an aluminum with the bows torn up so I'd make it alum and if you want it tougher, increase the diameter of the bows.
HTH and I've had half a dozen over the years.
Mark
