It sounds like its a permanent dock. If so the ice will crush those aluminum pipes. Since you mentioned custom you may want to get a seasonal lift dock. Can't damage the dock if its out of the water. You may want to check Raise-A-Dock, Wallmark, Center Harbor dock for those. I have a 6X33' dock using galv post with a galv base on each post. Mine just sits on the bottom. Granted I have 2 feet on you for more stability but if you have cross pieces at the end and the 3rd section of the dock it should stabilize the dock. I do not have whips have little boat traffic although when boats come in the waves are in line with my dock and I'm at the mouth of a cove which get the weather. I have a 20' runabout and the dock does not move. I would suggest getting a 6' wide dock, it does make a difference on room. I had a 4' previously.
Also when I got my aluminum dock the installer said not to drill holes in the frame for whips, etc. I can't put whips on my decking since its is not attached. My only choice is to put them on the galv posts.
Dave M
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