I'm building a gantry crane to pull the engines out of my boat. I need to know what size aluminum I beam I should use for the top beam? I have 8 feet in between my 45 dergree angle.

That will depend on the aluminum alloy. I don't have any engineering data on aluminum. A steel 35ksi W8x10 at 8'-0" span simply supported at each end will hold a 7500 lb point load without exceeding the allowable stress of 24ksi. A W8x10x8'-0" would be relatively inexpensive, and will only weigh about 80 lbs. A W6x12 would hold a similar load and be a little shallower, but it will also be a little heavier.

Get a steel beam.

Any local steel shop will be able to calculate the load.

I gona use aluminum to save on weight so I can take it a part.

I'm not sure how much weight you'll be able to save by going with aluminum. The section will have to be deeper and beefier than its steel counterpart which means that it will not be significantly lighter. I would guess around 15% - 20% lighter max. That isn't going to make that big of a difference.

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