I am building an aluminium boat. How do I attach the anodes?

The most sure way is to use either aluminium anodes or zinc anodes with an aluminium core and weld them in place. That way you know that they have proper electrical connection to the hull. The anodes must be ground off for replacement, which must be carefully done so that they don't grind into the hull skin and the hull coatings must be repaired after the new anodes are fitted.

The other option is to bolt them on with 316 stainless steel bolts. The anodes must be in electrical contact with the hull so the bolts must not be isolated from the hull but they must also be watertight. I suggest that the bolts have a washer, neoprene waterproofing washer and thin nut on the outside, with the bolt projecting far enough beyond the nut for the anode. The electrical connection will be from contact of the bolt in the hole and under the head of the bolt on the inside of the hull. Di-electric grease should be used between the bolt and hull to improve the connection and to protect the connection from corrosion. You can get to the head of the bolt inside the boat at any time if needed to clean the bolt and surrounding metal and add some more di-electric grease.

If you want the anodes to be flush in the hull surface for reduced drag then a short section of pipe must be welded into the hull, with the inner end closed off with plate welded in. Flush anodes are normally circular, with one attachment hole in the centre. The anode is fastened with a stainless steel bolt, the same way that a surface mounted anode is fitted.