I figured why my speed was not what I expected.

[Bugsbunnyboater]

That nice comfy Bimini top acts like a nice drag chute at speed. Duhhh I feel slow for not realizing that.
I noticed a 5 plus mph with it down gain.

[sunedog]

Bimini's are funny. At first blush, I would expect them to behave like yours did -- like a drag chute. But I recall someone on here said he runs faster with his open. I think it adds lift and helps pull the boat out of the water more. I think you have to be running higher speeds than I do to get the advantage, though.

[rancherlee]

mine makes a difference of only ~1mph, 2mph if I'm pushing a big headwind. Mine is mounted with the front a hair lower than the rear, if the front is higher than the back it can "trap air" and slow you down a big more along with the "buggy style" that have the fabric that goes from the top to the handrails. And make sure its TIGHT, loose fabric has the same resistance as a mattress on top of a mini-van.

[Bilboinsa]

I believe it varies with the design of the top. Some top have almost no vertical sections. Others have vertical corners in the rear that actually would act as a minor air brake.

[smoker62]

mine runs the same , up or down. I cant believe it adds any lift . The bimini fabric itself is not strong enough to do much up lifting on a 3000 # boat, jmo. Mine says don't run above 30 mph with bimini up. I have had it to 46 with it up but the stitching looked a little stretched lol.

[illinoid]

Mine has straps front and back so it could probably be adjusted to either be a drag chute or airfoil to lift a bit.