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The St. Lawrence Seaway and Welland Canal locks have the same limiting ship size, L730'*B78'. Allowable draft varies slighty but a little over 26' in general. The allowable beam was increased slightly some years ago by allowing less clearance to the lock walls; it's common now to see Canadian lakers rebuilt with a couple of feet extra beam in the cargo section fairing back to the original beam in the engine room section, so critical are those economies of scale. The locks to get from the upper St. Lawrence to Lake Ontario are generally referred to, certainly by people who navigate them, as 'the Seaway'; the whole waterway from Duluth, MN to the mouth of the St. Lawrence is sometimes called the "St Lawrence Seaway System". Below the Seaway locks on the St. Lawrence, IOW Montreal etc., much larger ships can call.

Largest warship I can find a web reference to passing Seaway/Welland Canal is Baltimore class CA Macon, 673'-5"*70'-10"*24' load
http://www.britesites.com/macon/slidemain.htm

The Newport News class would fit though 716'-6"*75'-4"*26 load, and there was a recent (maybe still active?) proposal to tow USS Des Moines to Milwaukee as a museum.

Joe

"JBren1 actually your information is a bit wrong seeing the the Rt. Hon. Paul J Martan is 739'07" long and has a 78' beam. This is the largest laker that is able to pass the Welland Canal (I believe). "

Thanks, I see the length limit was also relaxed to 740' from 730' as beam was relaxed from 76 to 78 (the locks sizes haven't changed, and are the same in the Welland Canal and Seaway, both are judgment calls about safety, based on experience, how much clearance is needed from ship to lock walls and gates). The boatnerd site gives Martin as 739-07 and fleetmates CSL Assinboine, Laurentian and Niagara as 739-10, but the also reliable "Know Your Ships" 2005 edition says all four are 739-10, by 78'-01". But note all were originally built in the 1970's to the 730'*76' limit and have had new forebodies built which are 78' and slightly longer. The US barge Great Lakes Trader is listed at 740'*78'; it was built on the US Gulf Coast and made a one time trip (probably the only time) through the system on delivery. Algoma's Algowood was also lengthened to 740 with no increase in beam, still around 76'. Anyway most Canadian and a number of US lakers are still 730', the old limit. Note that dimensions of merchant ships are traditionally quoted molded, to the outside of the frames, but will sometimes be quoted extreme, to the outside of the shell plating (which is what the Seaway authority cares about) so that can cause minor confusion in quoted dimensions.

Joe